Another consequence of modularity associated with both heterometry and heterochrony is allometry—changes that occur when different parts of an organism grow at different rates (Huxley and Teissier 1936; Gayon 2000). As animals develop, their shape changes, a result of differences in the timing and duration of growth events. Indeed, morphological evolution (especially within a phylum) is due primarily to changes in body size and the relative sizes of body parts. The laws of allometric growth can be derived mathematically (Przibram 1931; Nijhout and German 2012). Such differential changes in growth rate can involve altering a target cell’s sensitivity to growth factors or altering the amounts of growth factors produced.

Once again, vertebrate limbs and heads provide useful illustrations of altered development. Local differences among chondrocytes in the limb buds cause the central toes of embryonic mammals to grow at a rate 1.4 times that of the lateral toes (Wolpert 1983). As horses grew larger over evolutionary time, this regional difference in chondrocytes resulted in the one-toed state seen in modern horses. Another dramatic example of allometry in evolution comes from whale skulls. In the very young (4–5 mm long) whale embryo, the nose is in the usual mammalian position. However, the enormous growth of the maxilla and premaxilla (upper jaw) pushes over the frontal bone and forces the nose to the top of the skull (Figure 1). This new position turns the mammalian nose into the cetacean blowhole, allowing the whale to have a large and highly specialized jaw apparatus and (not incidentally) to breathe while swimming at the water’s surface (Slijper 1962). While whale skulls depart greatly from their early embryonic form, bird skulls depart very little from theirs. In fact, bird skulls resemble the skulls of juvenile dinosaurs (Bhullar et al. 2012).

Figure 1 Allometric growth in the whale head. An adult human skull is shown for comparison. The whale’s upper jaw (maxilla) has pushed forward, causing the nose to move to the top of the skull. The mandible is not shown. (The premaxilla is present in the early human fetus, but it fuses with the maxilla by the end of the third month of gestation. The human premaxilla was discovered by Goethe, among others, in 1786.) (After Slijper 1962.)

One of the most spectacular examples of allometry is that of the human brain (Bufill et al. 2011). As mentioned in Chapter 10, the human brain maintains its fetal rate of growth well into the period of childhood and is characterized by rapid cell division, complexity of connections between neurons, and plasticity during the newborn period. One of the genes responsible for this allometry may be the truncated SRGAP2 mentioned earlier in this chapter, as well as HAR1 and GADD45G (see Chapter 10).

Literature Cited

Bhullar, B. A. and 6 others. 2012. Birds have paedomorphic dinosaur skulls. Nature 487: 223–226.
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Bufill, E., J. Agustí and R. Blesa. 2011. Human neoteny revisited: The case of synaptic plasticity. Am. J. Hum. Biol. 23: 729–739.
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Huxley, J. and G. Teissier. 1936. Terminology of relative growth. Nature 137: 780–781.

Gayon, J. 2000. History of the concept of allometry. Am. Zool. 40: 7480–758.

Nijhout, H. F. and R. Z. German. 2012. Developmental causes of allometry: New models and implications for phenotypic plasticity and evolution. Integr. Comp. Biol. 52: 43–52.
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Przibram, H. 1931. Connecting Laws in Animal Morphology. University of London Press, London.

Slijper, E. J. 1962. Whales. A. J. Pomerans (transl.). Basic Books, New York.

Wolpert, L. 1983. Constancy and change in the development and evolution of pattern. In B. C. Goodwin, N. Holder and C. C. Wylie (eds.), Development and Evolution. Cambridge University Press, Cambridge, pp. 47–57.