Library of Excerpts

Heterochony: Part 2

This page contains a collection of excerpts from sources used to support Shift Theory, an alternative theory of human evolution. Click here for an introduction to this new and unique theory of evolution.

"The cranial paedomorphosis and neoteny in P. paniscus may be related to reduced sexual dimorphism in morphology and behavior since reduction or truncation of a structure's growth yields reduced sexual dimorphism. (24), and P. paniscus exhibits less sexual differentiation of behavior and grouping patterns than P. troglodytes (25). Gould (7) and others (26) have suggested that analyses of heterochrony may provide a vital link between studies of morphological evolution and regulatory genetics. The seeming desparity between separating humans and chimpanzees has led many to conclude that small changes in the genetic material controlling regulatory systems might account for significant organizmal change in evolution (7, 26-28). Although the morphological differences between pygmy and common chimpanzees do not approach those characterizing a human-to-chimpanzee contrast, the above results show how substantial morphological differentiation may result from shifts in developmental timing and rates of growth among various major body regions, with little or no change in the ontogenetic patterns within each region. The data are consistent with the idea that "features of an organism are bound...In covariant sets, and these sets are often dissociable as blocks" (28). This "shuffling" of the developmental trajectories of various body regions may provide new adaptive morphological configurations with minimal genetic changes." (Shea, B.T. (1983) Paedomorphosis and neotony in the pygmy chimpanzee. Science 222: pp. 522)

"Thus, the results of this study suggest that when the growth rates and growth periods for measurements of length, width and angular orientation of the cranial base are compared in man and the great apes, differences in the size and shape of the adult cranial bases (described in detail in Dean and Wood[1981a]) can be attibuted to a combination of decreases and increases in growth rate as well as to differences in size and shape that are present at, or soon after, birth. No single heterochronic change underlies the observed differences in the cranial base shape of the adults. General differences in the growth curves, as well as local changes in growth rate, are responsible for the pattern of the adult cranial base in Homo Sapiens. It would then be an over-simplification, even within the broader definition of Gould [1977], to regard the human cranial base a 'neotenous', or to consider the modern human cranial base pattern as a paedomorphic form of the infant great ape, or infant primitive hominoid, cranial base pattern." (Dean, M.C. & Wood, B.A. (1984) Phylogeny, neoteny, and growth of the cranial base in hominids Folia Primatol 43 (2-3): pp.178)

"Theorists of human evolution, such as Bolk (1926), Gould (1977), and Montagu (1962,1989), have listed a number of physical and functional neotenous features of humans. ... Bipedality requires a change in the angle at which the spine connects with the skull. The opening in the skull where the spine connects to the skull is referred to as the foramen magnum. In all embryonic mammals, the foramen magnum is located at the bottom of the skull, so that the spine enters the skull at a right angle to the top of the skull and parallel to the plane of the face. During prenatal development, the location of the foramen magnum shifts toward the back of the skull, so that in most species of mammals the spine is essentially parallel to the top of the skull and perpendicular to the plane of the face. However, in humans, the position of the foramen magnum does not change appreciably beyond this embryonic stage. Development is retarded, so that at birth and into adulthood the sharp angle of the spine to the skull is maintained. That is, the foramen magnum maintains its embryonic position, with the result being that the skull sits atop the spine, thus permitting one to look forward while standing upright. Because the foragmen magnum shifts toward the back of the skull in other mammals, forward sight is more easily accomplished when the animal is on all four feet. Thus bipedality results from retention of an embryonic characteristic---development is retarded, setting the stage for major evolutionary change (see Gould, 1977; and Montegu, 1989). (Bjorklund, D.F. (1997) The role of immaturity inhuman development. Psychological Bulletin 122(2): 155)

"...because of the youthful tendency toward play and curiosity in animals, it is likely that new innovations will be introduced by the young rather than the adult." (Bjorklund, D.F. (1997) The role of immaturity inhuman development. Psychological Bulletin 122(2): 158)

"In general discussions of the matter, it is sometimes implicitly assumed that scores of generations of selective breeding are required before heritable differences in social behavior appear. Just the opposite seems to be the case. The present findings are consistent with those in the available studies of selection for social behavior characteristics in general (e.g, Fuller & Thompson, 1978) and aggressive patterns in particular (e.g., Lagerspetz, 1964; van Oortmerssen & Bakker, 1981)." (Cairns R.B., MacCombie, D.J. & Hood, K.E. (1983) A developmental-genetic analysis of aggressive behavior in mice: 1. Behavioral outcomes. Journal of Comparative Psychology 97(1): pp. 87) [note: this is the experiment that selected for two paths of heterochronic development by choosing most and least aggresive mice]

"A brief overview of the "endocrine growth axis" (Rimoin and Horton, 1978; Palmiter et al., 1983) is a prerequisite to consideration of these alternative views. Upon receiving appropriate neural signals, the hypothalamus produces either growth hormone-releasing factor (GHRF); see Fig 5) or somatostatin (SMS). These substances act to stimulate or inhibit, respectively, the production of growth hormone (hGH) by the pituitary. Circulating GH acts on the liver and other organs to produce somatomedins (SMDs), such as insulin-like growth factor I (IGF-I). These are the primary direct mediators of growth and activate receptors on peripheral tissues that stimulate cell division and growth." (Shea, B.T. & Gomez, A.M. (1988) Tooth scaling and evolutionary dwarfism: an investigation of allometry in human pygmies. American Journal of Physical Anthropology 77 (1): pp. 126)

"Merimee and Rimoin (1986) also report that comparison of IGF-I and IGF-II levels in neighboring African villagers and in a group of pygmies following a high-protein, high carbohydrate diet for 2 weeks revealed similar differences in IGF-I levels, making it very unlikely that this variation results primarily form dietary and nutritional factors. This strongly suggests that the small size of the pygmies is the result of genetically based differences in the ability to produce IGF-i." (Shea, B.T. & Gomez, A.M. (1988) Tooth scaling and evolutionary dwarfism: an investigation of allometry in human pygmies. American Journal of Physical Anthropology 77 (1): pp. 126)

"The primary reason for the lack of size change in the skull and face of the pygmies is probably related to a combination of early growth priority and these structures (especially reflected in those measurements incorporating the skull base: basion-nasion and basion-prosthion), plus the fact that in humans overall skull size is negatively allometric to bady size and weight. This is even true of the facial skeleton in humans, so that a marked change in body weight may result in little or no alteration in facial size. Evidence to this effect can be seen in the pubertal growth spurt, during which males become significantly larger than females in height and weight, but metric aspects of facial size are still largely indistinuishable (Keen, 1950)." ( Shea, B.T. & Gomez, A.M. (1988) Tooth scaling and evolutionary dwarfism: an investigation of allometry in human pygmies. American Journal of Physical Anthropology 77 (1): pp. 129)

[see Godfrey and Sutherland for lucid interpretation of Gould heterochrony] (Godfrey, Laurie R. (1996) Paradox of peramorphic paedomorphosis: heterochrony and human evolution. American Journal of Physical Anthropology 99(1): pp. 22)

"The effects of rate differences pervade ontogenetic comparisons: at no point during ontegeny (assuming linear prenatal growth trajectories) do either of the Pan species grow absolutely faster than the gorillas. In addition, several maturation events occur earlier in gorillas (particularly females) than in Pan (especially common chimpanzees) (Table 1; Watts and Pusey, 1993). Rate differences between gorillas and Pan are achieved through fluctuating growth rates. This is summarized most effectively by pseudo-velocity curves and by nonlinearity in size trajectory plots. Variation in the timing and magnitude of growth spurts appears to be particularly important in leading to size differences among these species. Although rate differences describe variation between Gorilla and Pan for both males and females, this general pattern is complicated by variation in growth duration. Specifically, a high growth rate in female gorillas relative to common chimpanzees is coupled with early maturation in female gorillas (Leigh and Shea, 1995; Shea, 1985, 1986; Watts and Pusey, 1993). This finding implies that factors responsible for the evolution of variation in both rate and time processes distinguish these species." (Leigh, S.R. & Shea, B.T. (1996) Ontogeny of body size variation in African apes. American Journal of Physical Anthropology 99(1): pp. 54)

"On the other hand, only minimal differences in growth duration distinguish female gorillas form female pygmy chimpanzees. Comparisons among males suggest that rate differences mainly account for the size difference between gorillas and either of the two Pan species. However, extension of the male growth period in Gorilla contributes substantially to the final size differences between males of these genera. Thus, duration differences magnify intergeneric size differences among males that are initiated by growth rate differences. Contrasts in growth duration are largest when male gorillas are compared to male pygmy chimpanzees. Differences in the duration of body weight growth are mainly responsible for size differences between Pan troglodytes and Pan paniscus. This pattern is especially obvious for males. Although common sizes are observed throughout much of ontogeny, differences in the timing of growth spurts are evident." (Leigh, S.R. & Shea, B.T. (1996) Ontogeny of body size variation in African apes. American Journal of Physical Anthropology 99(1): pp. 55)

"We suggest that much of the ontogenetic variation in apes can be understood with respect to Janson and Van Schaik's (1993) model predicting negative correlations between growth rate and ecological risk. Specifically, this model anticipates evolution of low growth rates when high levels of ecological risk result form intraspecific feeding competion. Low growth rates minimize metabolic costs per unit time, and can evolve in primates because selection for rapid maturation via predation tends to be minimized by group living." (Leigh, S.R. & Shea, B.T. (1996) Ontogeny of body size variation in African apes. American Journal of Physical Anthropology 99(1): pp. 60)

"High growth rates have independently been suggested as one of several adaptations that differentiate folivores from nonfolivores (Leigh, 1994a), accompanying dental and gut specializations that facilitate consumption of high-fiber resources by folivores (Kay, 1984; Fleagle, 1988). Elevated growth rates may confer additional advantages to gorillas and other folivores, including metabolic advantages to early attainment of large size and early maturation of alimentary system, especially if the gut scales with positive allometry during ontogeny. (see Leigh, 1994a). Therefore, we suggest that high growth rates in gorillas are partly related to folivory through physiological advantages of rapid growth and reduction in ecological risks. Obviously, we cannot rule out the possibility that a derived size increase is related to increase folivory in gorillas. Ecological risk in gorillas is further reduced relative to common chimpanzees because they utilize large and ubiquitously distributed food patches (Wrangham, 1986). Enhanced growth rates might also confer reproductive advantages to females because early maturation (or early attainment of an "optimal" female adult size) is not comstrained by slow growth. (Rubenstein, 1993; Pagel and Harvey, 1993). We must note that lowland gorillas (studied here) are likely less folivorous than mountain gorillas (Tutin and Fernandez, 1985,1993), which serve as the basis for most of our information aobut Gorilla adapatations (Watts, 1984, 1985, 1990, 1994). However, levels of fovivory in lowland gorillas are probably sufficient to reflect adaptations that characterize other folivorous primates (over 40-45% of annual time spent feeding on folivorous resources seems to be sufficient to reflect this adaptation {Leigh, 1994a}. Thus, the resent analysis suggests that Janson and van Schaik's (1993) model had predictive power for explaining why gorillas grow more rapidly than other African apes, and contributes to an understanding of adult size differences between genera." (Leigh, S.R. & Shea, B.T. (1996) Ontogeny of body size variation in African apes. American Journal of Physical Anthropology 99(1): pp. 60-61)

"Boinski (1987) conducted the first field study of squirrel monkeys in which individual animals were recognized. Squirrel monkeys live in medium to large multi-male/multi-female groups )approximately 35 individuals in Boinski's study), and they are exclusively arboreal. Boinski was able to determine that females, not males, transfer between groups, That there is little male-male aggression and no discernible male hierarchy. Females are seasonally receptive and each estrus lasts only 6-8 days. Male squirrel monkeys increase in size by 20% during the breeding season. More signigicantly, Boinski determined that the largest male is responsible for 70% of the observed copulations. Females actively and assertively solicit copulations. They engage in mutal olfactory investigations with males and place their hindquarters up to the nose of males. Females apparently prefer the largest male, but when he was ocupied, or did not respond, they copulated with other males." (Small, Meredith F. (1989) Female choice in nonhuman primates. Yearbook of Physical Anthropology 32: pp. 114) [note that squirrel monkeys are one of the species used as pets suggesting tamed nature with neotenous behavioral attributes]

In Japanese macaques 1) "females frequently solicit males by mounting them" 2)" Females are highly sexually assertive" 3) "Females will also transfer to new groups during breeding season, mate with extra-troop males" (Small, Meredith F. (1989) Female choice in nonhuman primates. Yearbook of Physical Anthropology 32: pp. 119)

"Barbary macaques have been of special interest to primatologists because of two interesting features. Females mate with almost all available males in the group, and yet males interact extensively with infants. In other words, males interact with infants that may not be their own. Based on observations of two separate colonies of provisioned animals, Kuester and Paul (1984) and Small (in press, b) have provided detailed descriptions of the mating patterns of female Barbary macaques. Females breed in the fall and undergo an average of 2 cycles. They exhibit extreme swelling and color changes of the perineal area that can last several weeks. Detumescence is rapid, and is followed by visible menstruation 14 days later. The second cycle is often less physiologically intense, although it may not be so behaviorally. In addition, females have a second or third estrus even if they are pregnant and solicit males at the usual rate. The most remarkable feature of Barbary macaque mating is the frequency of copulations and the number of partners. Small (in press, b) calculated an average of 2.23 copulations per hour with an average of 7 different partners during each estrus (range 1-14). Small also observed one female who copulated with 3 different males in 6 minutes. Taub (1980a) used the euphemism "consociation" in an attempt to gain some respectability for female Barbary macaque "promiscuity." In fact, females rarely stay more than a few minutes with any male, and move form partner to partner while the males seem unable to stop them (Small, in press, b). In captivity and in the field, these are the most sexually assertive of females macaques, and they seem to play the major role in determining mate with whom, and when. .... Males are somewhat related, and they also are not extensively dimorphic in size from females." (Small, Meredith F. (1989) Female choice in nonhuman primates. Yearbook of Physical Anthropology 32: pp. 119-120)

"Primary endocrine control of somatic growth is established by the secretion and circulation of growth hormone (GH), a large polypeptide molecule with multiple effects on cell division and metabolism. GH is synthesized and secreted by the somatotropic cells of the anterior pituitary gland. This secretion is regulated by the interaction of two hypthalamic hormones, growth hormone-releasing factor (GHRF) and growth hormone inhibiting factor (somatostatin), which in turn are regulated by a variety of humoral and central nervous system feedback controls. GH travels form the pituitary to all cells of the body, where it binds to specific growth horone receptors (GHR), which are widely distributed. After GH binds to the receptor on the cell membrane, it circulates with a carrier protein known as high-affinity growth hormone-binding protein (GH-BP), which appears to be the extracellular portion of the GHR dissociated from the cell. Serum levels of high-affinity GH-BP can be assayed, and are thought to provide an indirect measure of the number of GHR present and active (Baumann et al., 1989)." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 313)

"Whatever the precise hormonal mechanisms ultimately determined to underlie the reduced body size in pygmies, their effects appear to be exhibited throughout the entire span of postnatal growth (Bailey, 1991a) and not simply during or just prior to puberty, as previously claimed by Merimee et al. (1987). This is just what would be expected of an alteration involving IGF-l levels and/or distal subresponsiveness, since the predominant effects of this important mitogen are seen postnatally (e.g. Rechler et al., 1987)." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 314)

"For the morphologist dealing with terminal phenotypes, this means that complex and highly integrated patterns of proportion changes during growth and size shifts are determined by rather simple underlying controls. Consequently, a shift in terminal size will automatically yield a cascade of coordinated allometric proportions. These underlying genetic and developmental controls, and their gross phenotypic expression, must be fully understood when analyzing individuals, sexes, groups, or species which differ in body size. In such a model, globel allometric transformations will occur with body size changes through these genetic and developmental pathways, regardless of whether natural selection has driven the size change or not." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 325)

"The global ontogenetic scaling of allometries and reduced rates of body weight growth-in-time are indicative of the heterochronic transformation termed rate hypomorphosis (Shea, 1983b). Neoteny and time hypomorphosis (Shea, 1983b, or the progenesis of Gould, 1977, and other authors) will also yield a juvenilized or paedomorphic morphology. But in the first case, allometric patterns will not be coincident )or aontogentically scales) in the two groups, and in the second case, the duration of growth (and not merely the rate of growth-in -time) will be truncated, so that the transformed group will cease growth and become sexually mature at a significantly earlier age. Neither of these profiles fit the African pygmy..."(Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 326)

"The argument that the small overall body size and/or specific body proportions of human pugmies are the result of strong and sustained selection for thermoregulatory efficiency in hot and humid tropical rain forest environments is both long-standing and physiologically reasonable. However, this proposition clearly needs to be treated as a preliminary hypothesis." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 327)

"In this scenerio, small body size was specifically selected for in the pygmies in order to increase the amount of surface area available for heat loss via convection cooling. The unlikelihood of losing sufficient heat by this mechanism in the hot, humid rainforest has resulted in a shift toward the view that small size was selectively advantageous due to the absolutely (though not relatively) lower levels of heat production (e.g., Cavalli-Sforza, 1986; see also Lewin, 1991)." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 332)

"Bailey et al. (1989), Headland (1987), and Bailey and Headland (1991) have suggested that the tropical rain forest offers a very limited supply of calories, particurly in the form of carbohydrates and fat, for human foragers." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 335)

Shea definition "Heterochrony, or changes in morphology resulting from shifts in the rate or timing of ancestral developmental patterns,..." (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 70)

"It seems quite clear that early hominids potentially ancestral to the genus Homo were considerably smaller than we are (Jungers, 1988), and thus we no longer accept scenarios, such as that proposed long ago by Weidenreich (1946), of descent from giants. On the other hand, recent fossil discoveries (Johanson et al., 1987; Leakey and Walker, 1985) also clearly reject any claims for a progressive and linear increase in body size within Homo over the past 2 million years or so. If future discoveries suggest that fully modern humans exhibit decreased stature and weight compared to either Homo erectus or early H. sapiens, then we might expect some paedomorphic or juvenilized features such as gracilization, smaller joint surfaces, etc., in modern humans. But such changes in shape would represent paedomorphosis via hypomorphosis (i.e., simple allometric correlates) and emphatically not paedomorphosis presented as evidence of neoteny by Privratsky (1981) fall into this category of misinterpretation." (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 80)

"Mongoloids are frequently cited as the most neotenous of human groups, yet a careful consideration of various creiteria of skeletal, dental, and pubertal development reveals no evidence of delayed maturity or prolonged growth in this group (Eveleth and Tanner, 1976). (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 84)

From chart in Shea extracted from Eveleth and Tanner. Age of Menarche: Europe 13.3 yrs, Descendants of Europeans abroad 12.9 years, Africa 14.9 years, Descendants of Africans abroad 13.2 years, Near East and India 13.6 years, Asiatics 13.1 years, Pacific Islanders 16.2 years. (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 84)

"Just a few of the accelerations noted by Schultz (1969), which characterize human growth and evolution and which directly contradict the predictions of neoteny, are early fusion of the central and navicular bones of the wrist, early fusion of the sternebrae, and early descent of the testes. In addition, two other non-paedomorphic features of human morphology are pelvic shape and the marked flexion of the vertebral column at the lumbosacral border (Schultz, 1950). (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 88)

"In addition, few, if any, of the pervasive morphological changes associated with our extremely specialized form of bipedal locamotion can be interpreted as the result of neotenic paedomorphism (Gould, 1977). (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 88)

"Dahl (1985) has shown that the morphology of the external genitalia in adult female P. paniscus closely resembles that of juvenile rather than adult P. trogoldytes. In addition, he argues (Dahl, 1986) that certain aspects (although not all) of the swelling and menstrual cycles of P. paniscus resemble those of juvenile P. troglodytes, particularly the length of the swelling phase and the intermenstrual interval. (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 92)

"As noted below for the human case, discussions of behavioral features in terms of neoteny and paedomorphosis are often problematic. Nevertheless, it is worth pointing out that several authors (e.g. Kuroda, 1979, 1980; Dahl, 1986) have described some of the behavioral features of P. paniscus as paedomorphic and resembling juvenile patterns of P. troglodytes. Examples here include play behavior, food sharing frequency, decreased social differentiation by sex, ventro-ventral copulation, and the characteristic genito-genital (GG) rubbing of females. Several of these behaviors may be directly linked to the morphological features of the juvenilized external genitalia discussed above (see Dahl, 1985, 1986). I have argued elsewhere (Shea, 1983a, 1984) that the most important link among the morphological and behavioral distinctions between P. paniscus and P. troglodytes is the reduced social differentiation by sex. The most notable morphological change associated with neotenic facial growth in P. paniscus is the marked reduction in sexual dimorphism of the gnathic and total facial region. Although the canine teeth of P. paniscus are significantly sexually dimorphic, they are much less so than is the case in P. troglodytes, and the same holds true for a comparison of dimorphism in facial dimensions between the species (Fenart and Deblock, 1972, 1973, 1974). While we have much to learn regarding the intriguing behavioral distinctions between these two chimpanzee species (see Wrangham, 1986, for one recent discussion), it seems likely that the reduced sexual dimorphism in the facial region of P. paniscus is related to social factors, such as lowered male-male and male-female aggression, increased female bonding, increased food sharing, and perhaps aspects of sexual behavior." (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 93-4)

"In the decade since the publication of Ontogeny and Phylogeny (Gould, 1977), there has been considerable advances in our understanding of the controls of growth processes (Bryant and Simpson, 1984). There are many control substances that affect aspects of bodily growth, but the most important immediate influence on general postnatal growth appears to be the hormone insulin-like growth fctor 1 (IFG-1, or somatomedin C). Growth hormone (GH, also known as somatotropin) regulates the local production of IGF-1 and undergo multiplication, thus resulting in growth. In turn, amounts of circulating GH secreted by the pituitary are under neurendocrine control. Any number of recent reviews and texts can be consulted for further information on the hormonal control of growth (e.g., Ludecke and Tolis, 1985; Raiti and Tolman, 1986). (Shea, B.T. (1989) Heterochrony in human evolution: the case for neoteny reconsidered. Yearbook of Physical Anthropology 32: pp. 95)

"Therefore, adults of small breeds have skull proportions like those of juvenile dogs of the same or smaller skull size. That is, all small breeds are to some extent juvenilized or paedomorphic (sensu Gould, 1977). In the extreme, some adult dogs have proportions similar to neonate German Shepherds (breeds on isometric line in fig, 6a). Large breeds fall to either side of the ontogenetic regression line and hence may be either paedomorphic or hypermorphic (equivalent in proportion to a dog that exceeds the ancestral ontogeny; Gould, 1977). (Wayne, Robet K. (1986) Cranial morphology of domestic and wild canids: the influence of development on morphological change. Evolution 40; pp. 251)

"This suggests that small domestic dogs differ from foxes because puppies of small dogs cannot grow out of their distinctive neonate morphology." (Wayne, Robet K. (1986) Cranial morphology of domestic and wild canids: the influence of development on morphological change. Evolution 40; pp. 255)

"In contrast, there are significant differences in the neonate size of domestic dogs, yet all dogs have the same 60-63 day gestation period as their progenitor, the grey wolf (Wayne, 1986). Most fox-size wild canids have gestation times approximately 52 days, and hence, relative to a domestic dog of any size, all fox-like canids are born immature or altricial." Wayne, Robet K. (1986) Cranial morphology of domestic and wild canids: the influence of development on morphological change. Evolution 40; pp. 255)

"It has often been assumed that evolution somehow rescues populations from sexual selection. Mayr (1972, p. 101) wrote that "natural selection will surely come into play as soon as this sexual selection leads to the production of excesses that signigicantly lower fitness of the species in interspecific encounters." Genetic models of the evolution of sexual selection do not confirm this belief. The notion that evolution will necessarily extricate a species from the maladaptive tendencies of sexual selection is unfounded." (Kirkpatrick, Mark (1982) Sexual selection and the evolution of female choice. Evolution 36: pp. 10)

"On the other hand, his sense of aethetic appreciation, based on the pleasure which man can receive from the construction and matching of musical patterns involving the interaction of rhythm, melody, and harmony and visual patterns resulting from the interaction of form and colour, has also resulted from the freeing of his association areas from the more rigid relationship with the lower centres and with the more stereotyped, amorphous symbol patterns which constitute the inner reality of all other animals (Koestler 1964). Aesthetic appreciation, therefore, is a foetalised form of the continuous search for congruity or matching between models of the environment, models which the animal constantly contructs in its brain by processing its perceptions and the stereotypes retained in its memory store." (Crombie, Donald L. (1971) The group system of man and paedomorphosis. Current Anthropology 12(2): pp. 163)

"The process of paedomorphosis has been closely involved with the evolution of human culture. The bipedalism which was partly responsible for the initiation of the whole of higher primate evolution and which followed an extension form the ecological niche in the trees to open plains did not itself owe anything that we can discern directly to paedomorphosis. Nevertheless, the modification of skull structure, and particularly the central position of the foragmen magnum, was essential if the utmost advantage was to result from bipedalism, since this allowed the head to be easily balanced in the upright position. This modification may well have been the first set of circumstances which produced increasing pressure towards paedomorphosis." (Crombie, Donald L. (1971) The group system of man and paedomorphosis. Current Anthropology 12(2): pp. 164)

"In an esay on the clinical significance of evolutionary vestiges, Harper (1962) deals with atavismand paedomorphosis as two sources of abnormality of structure and function in Homo Sapiens. Heproposed, as his main hypothesis (p.2), that 'in unfavorable circumstances.... the increased activity of the great brain of man may lead to disease based upon obsolete nervous and endocrine reflex defence patterns of a comparable date to the archaic vestiges of evolution known to exist in human skin.' These vestiges include the human pigmented naevus or mole, which has the structural organization of the tactile corpuscles of the amphibian, and is unique to the skin of man (Laidlaw and Murray 1933), and supernummerary nipples on the milk line. These are very rare in subhuman primates (Coolidge 1943), though fairly common in man himself (Geoffroy-Saint-Hilaire 1836: 170; Darwin 1874, Bruce 1879, De Cholnoky 1939, and Hamblen 1945). Harper choose naevi and supernumerary nipples because they are easy to identify and produced evidence to show an increased incidence of one or both of these features in patients in his practice who suffered form asthma and other allergies, hypertension, diabetes mellitus, or severe anxiety states." (Crombie, Donald L. (1971) The group system of man and paedomorphosis. Current Anthropology 12(2): pp. 165)

"In this sense, then, the demands of their primitive group system initially imposed on individual men via language and knowledge (which was at first only implicit in the structure of language but is now an evolutionary system in its own right) first influenced the behavior of prehominids and subsequently drastically transformed their genetic structure via the mechanism of paedomorphosis. This had secondary effects which may be expressed in certain disease processes, including the allergies, cardio-arterial disease and hypertension, diabetes, and severe anxiety states and other mental disorders. Paedomorphosis is the essential link at the interface between the structure of man and his unique behavior." (Crombie, Donald L. (1971) The group system of man and paedomorphosis. Current Anthropology 12(2): pp. 166)

"There is decreased prenatal viability, increased pre- and postnatal morbility, delayed growth, delayed maturation and bone age development and dental eruption. Decreased buffering of metabolic processes involves the well-known predisposition to hyperuricemia and increased insulin resistence, with the development of diabetes mellitus in many children and adults with Down sysdrome. Premature senescence appears to be a universal problem and involves not only the development of cataracts, but also of Alzheimer disease." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 40)

"Sixteen, or 4.5% of Down syndrome individuals had 1 or more midline anomalies, but none had more than 2. The corresponding figures in the 18 and 13 trisomy syndromes were 22 and 28, iwth 1 and 6, respectively, having 2 or more anomalies." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 41)

"Further work is required to document the status of vestigial structures in Down syndrome. ... In this connection it seems evident to us that all anomalies of incomplete differentiation represent persistence of an embryonic or fetal state, something well known to such investigators as Meckel over 160 years ago." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 41)

"In his initial studies of 5 Down syndrome bodies, Bersu found facial muscle variations in all 5, and documented the common absence and variation of many other muscles." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 41)

"B.K. Hall (1984) speaks of atavisms as the "reappearance of a lost character (morphology of behavior) typical of remote ancestors and not seen in the parents or recent ancestors of the organisms displaying the atavistic character. The phrases, 'tendency to reproduce the ancestral type'. 'reversion to a previous evolutionary state', or 'throw back'. all embody the essential features of the atavistic character, which are: 1.) its persistence into adult life; 2.) its absence in the parents or recent ancestors; 3.) its presence in only one or a few individuals within the population, and ; 4.) its close resemblance to (identity with?) the same character possissed by all members of an ancestral population." This is a rather paleontological definition which implies that atavisms can only be recognized in an organism on which we have a good fossil record and detailed knowledge of morphological change during evolution. Since this is not true of most soft structures, we have modified this definition as follows: "Atavisms are (a rudimentary) develoment of anatomic structure known or presumed to have been present in a phylogenetic ancestor and homologous to that observed in a living relative. The work of Aziz, among others, on trisomy 13 and 18 (Aziz, 1981b) has been instrumental in identifying several atavistic muscles including platysma ocipitalis, rhomboideus occipitalis, the deltopectoral complex, the latissimocondyloideus, the pectorodorsalis, the chondrohumeralis, and the peroneus digiti quinti. Aziz also documented that some muscles, such as the palmaris longus and palmaris brevis, may be lacking altogether. Aziz concludes that "the supernumerary muscles found in these aneuploid specimens are regularly found in monkeys and sometimes in great apes. It is suggested that these...muscles may be 'atavistic' structures" (Aziz, 1981b). Aziz also concluded that "careful observations show that the peculiar morphology of these muscles results from delayed developement rather than from anotomical malformation." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 41-2)

Discusses taurodontism, a tooth condition, as evidencing itself often in 57% of Down's sydrome subjects, often is Eskimo, and showing up in Neanderthal remains. Text continues..."If regular occurrence of taurodontism in the family Pongidae can be confirmedm then taurodontism must have been present in an ancestor common to modern man and the Pongidae. Thus, taurodontism probably is a true atavism." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 42)

"Down syndrome individuals generally have retarded growth and maturational processes with retention of fetal development ("unfinished") characteristics involving brain, face, and the 5th fingers. According to Waardenburg (1932) it was Blok in 1922, who first proposed a fetalization theory of Down syndrome. From a palaeontological perspective all of these growth disturbances and developmental dysmaturities can be subsumed under the heading of neoteny. The concept of neoteny was coined by Kollmann in 1885 and refers to the retention of juvenile characters in the adult state, or to extention of fetal characteristics into childhood." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 44)

"Thus, with respect to (1) the fixation of a common pattern of major variability easily recognized in every race of humankind, (2) he invariable alteration of numerous morphometric traits and the abolition of family resemblance, (3) change in growth and of maturational characteristics with enhanced neoteny, (4) change in fertility, (5) appearance of a different behavioral phenotype, (6) change in chromosome number, and (7) changes in gene frequency ---at least with respect to genes on chromosome 21 (Goodman, 1965; Rundle, 1973; Rundle and Sudell, 1973), we can only conclude that the occurance of Down syndrome is akin to the process of speciation (albeit a sudden, rather than a gradual speciation). With respect to the relationship between speciation and chromosome abnormalities it is important to note that the types of chromosome rearangements "that occur as polymorphosisms or as fixed permanent heterozygotes invariably involve meta- or submetacentric chromosomes. Those that distinguish species and serve to isolate those species involve telocentric or acrocentric chromosomes, which are self sterilizing" (John, 1981)." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 45)

Discussion of heterochronic dynamic in Down's generation. "Thus, under normal circumstances, developmental adaptation is the canalized response of morphogenetic units (fields) to environmental and stochastic factors---i.e., adaptation is as much an attribute of epigenetic fields prenatally as it is of physiological systems postnatally." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 47-8)

"A plethora of examples attest to variability increased above normal in many morphometric traits in Down syndrome ... This classic paper [Levinson et al., 1955] in an attempt to counter the popular misconception of unusual phenotypic uniformity of Down Syndrome. This classic paper made and still makes many valuable points, namely, greater than normal variability in birth weight, psychomotor development, pattern and timing of tooth eruption, closure of anterior fontanel, speech development and handedness (normal right-handedness 95%, Down syndrome 48%, ambidextrous 18%, undetermined 20%)." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 48)

"As in Bolitoglossa occidentalis, it may transpire that a single heterochronous disturbance delaying growth and development and making the whole epigenetic system more vulnerable to stochastic factors and environmental perturbations will be a suffecient explanation of the pathogenesis of Down syndrome." (Opitz, John M. & Gilbert-Barness, Enid F. (1990) Reflections on the pathogenesis of Down syndrome. American Journal of Medical Genetics 7: pp. 49)

"Indeed, Sprules (1974) demonstrated that low temperature or reduced food causes neoteny in another kind of salamander, Ambrystoma gracile, which lives in British Columbia. Neoteny is characterized by the increase in size of the larvae and the maturity of their gonad system, while metamorphosis ceases at the larval stage (Kollman, 19=882). If the functions of the endocrine systems concerning growth, metamorphosis, and reproduction depend on the temperature of the environement, and if there are differences in the termperature sensitivity among those endocrine systems, it is possible that such differences may cause the development of neoteny." (Moriya, Tsuneo (1983) The effect of temperature on the action of thyroid hormone and prolactin in larvae of the salamander Hynobius retardatus. General Comparative Endocrinology 49(1): pp. 1)

"Even if enough thyroid hormone were present in the blood to induce metamorphosis, the hormone cannot function at temperatures as low as 4 degrees or higher; therefore the larvae can grow continually and reach normal adult size sooner or later, because prolactin accelerates the growth but retards the metamorphosis." (Moriya, Tsuneo (1983) The effect of temperature on the action of thyroid hormone and prolactin in larvae of the salamander Hynobius retardatus. General Comparative Endocrinology 49(1): pp. 6)

"This dichotomy proved particularly pertinent since positive symptoms were found to be less specific to schizophrenia, to result from a variety of (including psychogenic) causes and to be of little prognostic value. In contrast, negative symtoms seemed to encompass more accurately the deficit seen in schizophrenia, to be correlated with chronicity and to be produced by some suspected organic pathological process. Later, it was found that negative symptoms were less (if at all) responsive to medication and to indicate a poor prognosis subgroup of schizophrenics. This difference has led some to postulate two forms, if not two distinct types, of schizophrenia. Crow, particularly has utilized the positive-negative dichotomy to postulate Type 1 and Type 2 syndromes of schizophrenia. Type 1 schizophrenia represents a potential reversible form with predominant positive symtoms thought to be caused by excessive dompamine activity. Type 2 schizophrenia represents an irreversible chronic disease that results from structural abnormalities of the brain and closely associated with negative symptoms with Crow defines as cognitive impairment, abnormal involuntary movements, and behavioral deterioration." (Bemporad, Jules R. (1991) Dementia praecox as a failure of neoteny. Theoretical Medicine 12(1): pp. 48)

"The role of regulator genes in determining the developmental progression of organisms may offer a new appraoch to the study of pshychopathology. Dementia praecox, a severe, chronic illness that affects individuals as they approach adult life may be the result of the failure of a normal process of neoteny by which juvenile characteristics are normally prolonged into adult life. The failure of the neoteny process may ultimately be traced to a disorder in the activation of enzyme producing structural genes by regulator genes which control the entire course of biological unfolding." (Bemporad, Jules R. (1991) Dementia praecox as a failure of neoteny. Theoretical Medicine 12(1): pp. 50)

"Pan paniscus shows increase female receptivity, variability in copulatory position, male or female initiation of sexual behavior, differential male and female preferences for copulatory position, and association of food sharing and sexual behavior. Their sexual behavior appears to function in proximate terms as a tension-reduction mechanism. Lowered tension, in turn, facilitates multi-male, multi-female social groups. Lowered levels of aggression and increased sexual activity appear to be associated with paedomorphism, and the behavioral and anatomical/physiological characteristics of the species appear to be a consequence of a feeding ecology that promotes large groupings of the animals at preferential and comparatively rich feeding sites." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 702)

"A second reported difference is that males or females may initiate copulatory behavior among the bonobos, whereas only males initiate it among chimpanzees (Savage-Rumbaugh and Wilkerson 1978). A third difference, noted by several observers (Kano and Mulavwa 1984; Kuroda 1984; Thompson-Handler, Malenky, and Badrian 1984), is that food sharing between males and females and among females tends to occur in the same context as sexual behavior, associated with excitement and tesion." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 703)

"Chimpanzees are reported to share food in two types of situations. Mothers sometimes share food with their infants (McGrew 1979), and meat---a prized food item---may be shared among adult males and females (Teleki 1973; Goodall 1986). Recent observations in the Ivory Coast (Boesch and Boesch 1989) indicate that sharing of meat by forest chimpanzees occurs several times more often than among savanna chimpanzees. Increased sharing among forest chimpanzees makes them more like bonobos, who occupy a similar environment. Bonobos, however, share food items other than meat (Badrian and Badrian 1984; Kano 1980; kuroda 1980, 1984; de Waal 1987). Sharing of food is a common phenomenon, and it occurs even when food is plentiful and when an individual already has some in his or her possession. (Badrian and Badrian 1984; Kuroda 1984). Moreover, and unlike chimpanzees, bonobo sharing food is associated with sexual behavior (Jordon 1977; Savage-Rumbaugh and Wilkerson 1978; Kano 1980; Kuroda 1980,1984; Thompson-Handler, Malenky, and Bodrian 1984; de Waal 1987). The co-occurrence of food sharing and sexual behavior among Pan paniscus appears to be structured by the social characteristics of the animals and by the type of food. Males rarely share with other males (unlike chimpanzees). Females share with other females, most often in the context of one female approaching another to beg for food and soliciting genito-genital contact and rubbing (Kuroda 1984). Badrian and Badrian (1984) also observed that genito-genital rubbing occurred most often at feeding sessions and soon before or after one of the females had been involved in heterosexual mating. Sexual interaction over food most commonly occurs, however, when females aproach dominant males to take or beg for food. Dominant males are the most likely to be in the richest part of the feeding area and to have prized foods, and they are the most likely to be approached by females. Kuroda (1984) observed that females approach males to beg for food without apparent hesitation and that they are more likely to be successful in obtaining food if they first copulate with the males. The behavior of bonobo females suggest that sexual receptivity may be a device to get access to food, and they have been reported to behave in that way (Badrian and Malenky 1984). (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 705-6)

"De Waal concluded that sexual behavior was primarily a mechanism to reduce tension and reconcile opponents after conflict and secondarily a mechanism to obtain food." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 706)

"To recapitulate to this point, chimpanzees copulate in a ventro-dorsal position, and sexual behavior seems to have principally a reproductive function. Among bonobos, sexual behavior appears, in proximate terms, to serve principally a tension-reduction function and thereby to promote sociality, and although virtually any copulatory position can be used, females appear to prefer ventro-ventral mating and males appear to prefer ventro-dorsal mating." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 706)

"Bonobos exhibit marked swelling for approximately 75% of the cycle, whereas chimpanzees show swelling for 50% or less of the cycle (Dahl 1986). Moreover, the intermenstrual interval for adult bonobos was measured to be 47 days, against 34-36 days for adult chimpanzees (Dahl 1986). In all of those regards also, they are more like the adolescent than the mature chimpanzee." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 707)

"Not only do bonobos copulate throughout the female cycle, they have a briefer period of lactational amenorrhea, returning to sexual activity sooner after the birth of an infant than do chimpanzees (Thompson-Handler, Malenky, and Badrian 1984). They are thus similar to primiparous as opposed to multiparous chimpanzees. Bonobo females have also reported to be sexually receptive during lactation (Kano 1980; Nadler et al. 1981; Badrian and Badrian 1984) and even during pregnancy (Kano 1989). The data are clear that bonobo females are sexually receptive for comparatively longer periods of time than are chimpanzees. Paedomorphism in female anatomy and physiology appears to be the basis for the divergent sexual behavior, including female preference for a ventro-ventral copulatory position." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 707)

"The aspects of Pan paniscus anatomy most readily perceived as paedomorphic are the face-jaw complex and the neurocranium. The bonobo skull resembles that of the adolescent chimpanzee (Coolidge, 1933), and in comparison to adult chimpanzees, bonobos have smaller mandibles, reduced prognathism, and a more glogular cranium (Johnson 1981)." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 707)

"The relationship between paedomorphized morphology and behavior are not entirely clear but are probably reflected in the high degree of sociability reported for the bonobos. In comparison to chimpanzees, bonobos exhibit reduced rates of aggression, especially severe aggression among males (Kuroda 1979, 1980; Mori 1984)." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 708)

"The bonobo pattern not only shows cohesion among females but close association among males and females, both atypical of chimpanzees." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 708)

"The high male-female affinity and reduced sexual differentiation in Pan paniscus social organization may be a behavioral link to the morphological characteristics (Shea 1984). Paedomorphism of a chimpanzee-like skull and consequent lessened sexual dimorphism reduce those aspects of the anatomy associated with aggressive display and activity, namely the heavier and more robust facial-jaw features. Sex as a source of aggressive competition would favor increased dimorphism, but sex as a tension-reducing mechanism would favor decreased dimorphism and promote nonaggressive social proximity. Extended receptivity and flexibility in sexual behavior, as a consequence of paedomorphism, would serve similar ends by reducing tension and promoting sociability." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 708)

"It is important to recall that the behavioral complex of begging, sharing, and copulation occurs even when food is plentiful at the congregation sites (Kuroda 1984). That suggests that it is the proximity of large numbers of bonobos at a feeding site that produces tension, and that more prized the food, the greater the tension (Kano and Mulavwa 1984). Clusters of preferred food and consequent fusion of social groups produce competition and tension, which must be reduced if relatively peaceful feeding is to ensue. In that context, a tension-reduction mechanism would be advantageous, allowing individuals to maximize access to food, and increase individual fitness. Selection pressures for more frequent, more flexible sexual behavior would favor paedomorphism, that is, reduced male robustness and more adolescent-like females with a genital anatomy that would promote ventro-ventral copulation. The proposed modification of the White and Wrangham hypothesis has the advantage of incorporating what appears to be the major tension-reduction mechasism into the feeding ecology and in the context where competition and tension are likely to occur most frequently and strongly. A fully adequate test of the hypothesis,however, will require additional phenological and behavioral studies." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 710)

"What does the available evidence about Pan paniscus sexual behavior tell us about the phylogeny of the panid-hominid taxa? The primary message is that parallel evolution can produce similar anatomical and behavioral results. Selection for paedomorphism has long been thought to be an aspect of hominization, and increased sociality, sexual receptivity, and related morphological changes have been viewed as correlates. Paedomorphism "led" to striking similarities in the two species. As a further similarity, it is possible that in bonobos and in protohumans paedomorphic-derived behavior maximized access to food resources in male-female groups." (Blount, Ben G. (1990) Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92: 710)

"Evolution occurs when ontogeny is altered in one of two ways: when new characters are introduced at any stage of development with varying effects upon subsequent stages, or when characters already present undergo changes in developmental timing. Together, these two processes exhaust the formal content of phyletic change; the second process is heterochrony. If change in developmental timing is important in evolution, then this second process must be very common (if it is predominant in frequency, I will be in even better shape)." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 4)

"These classifications treat the results of changes in developmental timing (recapitulation and paedomorphosis), not the mechanisms (acceleration and retardation). (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press. pp. 8)

"Progenesis reflects the truncation of ontogeny by precocious sexual maturation (acceleration). It represents a life-history strategy for r-selective regimes, where early reproduction is highly favored. Selection is for precocious maturation or small size; juvenilized morphology is often a secondary consequence. Neoteny, on the other hand, represents the retardation of somatic development for selected organs and parts. It occurs in K-selective regimes, where morphology is fine tuned to immediate ecological conditions." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press. pp. 9)

"Neoteny has been a (probably the) major determinant of human evolution. When we recognize the undeniable role of retardation in human evolution, the data of neoteny can be rescued from previous theories that made them so unpopular. Human development has slowed down. Within this "matrix of retardation," adaptive features of ancestral juveniles are easily retained. Retardation as a life-history strategy for longer learning and socialization may be far more important in human evolution than any of its morhological consequences." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press. pp.9)

"Humans and chimps are almost identical in structural gens, yet differ markedly in form and behavior. This paradox can be resolved by invoking a small genetic difference with profound effects---alterations in the regulatory system that slow down the general rate of development in humans. Heterochronic changes are regulatory changes; they require only an alteration in the timing of features already present. If the frequency of heterochronic change were known, it would provide a good estimate for the importance of regulation as an evolutionary agent" (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press. pp. 9)

"But if axolotl represented a juvenile stage of higher salamanders, then the perennibranchiates had merely gone a step further and committed themselves to permanent youth by dispensing entirely with their adult form---an undoubted exception to recapitulation. In 1885, Kollmann designated this retention of larval features as "neoteny" (p. 391). (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 179)

"All parallels between ontogeny and plylogeny fall into these two categories: If a feature appearing at a standardized point of ancestral ontogeny arises earlier in descendants, we encounter a direct parallel producing recapitulation (the descendant repeats in its own ontogeny a sequence of stages that characterized ancestors at their standardized point - Fig. 26A). If a feature appearing at a standardized point of ancestral ontogeny arises later and later in descendants, we encounter an inverse parallel (Fig. 26B) producing paedomorphosis (early features of an ancestral ontogeny are carried forward to appear at the standardized point of a descendant). Although this simple distinction includes all parallels, it is subject to two provisos that I shall explore throughout this chapter. First, we encounter ambiguities if we do not distinguish the various criteria for standardization --- size, age, and developmental stage. I try to resolve these ambiguities with a "clock model." Second, recapitulation and paedomorphosis are results, and they can be produced by several processes. Previous studies have often classified evolutionary changes only by these results, thus mixing together distinct processes with differing evolutionary significances. I shall focus on the processes. I must emphasize that classifications based upon addition and displacement completely exhaust the morphological description of how evolution can occur........Moreover, since displacement involves no more than a change of timing for developmental stages already present in ancestors, its genetic basis probably resides in the regulatory system. The frequency of evolution by displacement rather than by introduction might provide a minimum estimate for the relative frequency of regulatory changes in evolution---a current and elusive issue in evolutionary biology." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp.215-6)

'If ontogeny includes a change in form sufficently abrupt and substantial to warrant the term metamorphosis, then heterochronic effects can be easily diagnosed either by an alteration in timing of metamorphosis itself or by the differential acceleration and retardation of morphological traits with repect to metamorphosis. Moreover, the hormonal basis of several metamorphoses has been established (Jenkin, 1970) and heterochronies can be produced and replicated experimentally (though biologists have rarely discussed this experimental research in the context of relationships between ontogeny and phylogeny). Although Willis (1974, p. 98) reminds us that "much of this scheme is still subject to controversy." the classical interpretation of hormonal control of metamorphosis in holometabolous insects involves the interaction of two substances. Molting is regulated my ecdysone (molting hormone), secreted by the prothoracic glands. The morphological results of any molt, however, are determined by the juvenile hormone produced by the corpus allatum, an endocrine organ lying just behind the brain. This hormone has been synthesized and found to be relatively nonspecific in its effects; juvenile hormone from one insect is generally effective in other species, as are several chemical analogs and mimics. Its chemistry is reasonably well understood, and it has been discussed extensively as an ecologically benign method of insect control. It engenders as many review articles each year as some popular subjects inspire in the primary literature (see Schneiderman, 1972; Truman and Riddiford, 1974; and Willis, 1974, for example). Metamorphosis depends upon the concentration of juvenile hormone. In the presence of a high titer of juvenile hormone, ecdysone will produce a larval molt; with low titers, the larval-pupal transformation is initiated, while an absence of juvenile hormons allows the pupa to molt into an adult. Thus, juvenile hormone allows the pupa to molt into an adult. Thus, juvenile hormone is not an antagonist to the ecdysones, as once believed, but acts with them in normal development. Schneiderman has contrasted the hormonal control of maturation invertebrates and insects: "Maturation in man and other higher vertebrates is promoted by the secretion of maturation hormones, the gonadotropins of the pituitary. The juvenile condidition in man hinges upon the absence of these maturing hormones. The situation in insects depends upon the continued presence of the juvenile hormones, which act on the cells themselves, and prevent them from maturing" (1972, pp. 10-11)." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belk