EvoluTion And dEvEloPmEnT 371
Comparative development and Evolution
In the nineteenth and early twentieth centuries, biologists described and com-
pared the embryonic development of diverse animals (and plants, to a much
lesser extent) and described ways in which morphogenetic processes in dif-
ferent organisms result in different adult forms. One of the tasks of EDB is to
understand these processes, such as growth rates and differentiation of body
parts, in modern terms of genetic and molecular processes. As we will see, one
of the major explanations for these evolutionary changes is alteration of the
time, place, and level of expression (especially transcription) of particular genes
or sets of genes. Similar processes enable a single genome to produce differ-
ent morphologies, depending on environmental signals such as day length, or
genetic signals such as sex-determining genes. Developmental causes of pheno-
types are proximate causes, mechanisms that operate within an individual organ-
ism. These causes complement the processes that caused these phenotypes, and
these mechanisms, to evolve and to differ among species. These ultimate causes,
such as natural selection, act at the level of populations across generations; they
do not conflict with the mechanistic genetic and developmental processes. For
example, embryonic mammals and birds have webbing between the developing
digits. This remains in the wings of adult bats and the feet of ducks, but humans
and chickens have separated digits because the webbing cells are eliminated by
programmed cell death. This is a simple example of how developmental biology
helps us understand evolved differences between species. It does not answer
why ducks evolved webbed feet and the suppression of the cell death that occurs
in most other birds. A likely answer is that effective swimming enhanced fitness
in the ancestors of ducks, so that selection favored mutations in the genes that
determine the process of cell death and resulted in variant birds that retained
some webbing.
Among the first things that scientists learned about development is that spe-
cies are often more similar as embryos than as adults. Karl Ernst von Baer noted
in 1828 that the features common to a higher taxon (such as the Vertebrata) often
appear earlier in development than the specific characters of lower-level taxa (such
as orders or families) [80]. This generalization is now known as von Baer’s law.
For example, all tetrapod vertebrate embryos display pharyngeal clefts (gill slits),
a notochord, segmentation, and paddlelike limb buds before the features typical
of their class or order become apparent (see Figure 15.1). One of Darwin’s most
enthusiastic supporters, the German biologist Ernst Haeckel, reinterpreted such
patterns to mean that “ontogeny recapitulates phylogeny”—that is, that the devel-
opment of the individual organism (ontogeny) repeats the evolutionary history of
the adult forms of its ancestors, and could indicate its phylogenetic relationships.
By the end of the nineteenth century, however, it was already clear that Haeckel’s
dictum seldom holds [22]. For example, the pharyngeal clefts and associated bran-
chial arches of embryonic mammals and reptiles never acquire the form typical of
adult fishes. Moreover, various features develop at different rates, relative to one
another, in descendants than in their ancestors, and embryos and juvenile stages
have stage-specific adaptations of their own. Thus ontogeny is not a very useful
guide to phylogenetic history.
By the early twentieth century, biologists had identified several common pat-
terns of developmental differences among species—patterns that are now part of
the language of evolutionary developmental biology.
Allometric growth, or allometry, refers to the differential rate of growth of dif-
ferent parts or dimensions of an organism during its ontogeny. For example, during
human postnatal growth, the head grows at a slower rate than the body as a whole,
and the legs grow at a faster rate. Allometry thus refers to changes in the shape of
the organism or of certain of its parts, such as the dimensions of a skull or a leaf.15_EVOL4E_CH15.indd 371 3/22/17 1:30 PM