370 CHAPTER 15have separate female and male flowers, but in many such species, a flower
has the sexual parts of the other sex, in rudimentary form. We see much the
same in humans: men have nipples. The bones in a mammal’s hindlimb have
exactly matching counterparts in the forelimb (femur/humerus, tarsals/car-
pals), whether in humans or bats. Evidently a developmental process is shared
between the sexes, or between limbs. But just as clearly, the developmental pro-
cesses ultimately diverge—between sexes, between limbs, and among species.
How do we account for the similarities and differences? At one level, we
know the answer: homologous similarities are often based on shared genes, and
differences on differences in the genes. But that does not tell us how a change in
DNA sequence becomes realized as a change in an organism’s form and func-
tion. Someone trained in architecture, shown a blueprint of St. Paul’s Cathedral
or the Empire State Building, might well visualize what the completed building
looks like, but in itself, the blueprint does not specify that the building must be
built from the foundation up, or provide any other information about how to
construct it.
In biology, some transitions from genes to phenotypes are simple and clearly
understood, such as transcription from DNA to RNA and translation to a phe-
notype, the protein. The far more complex transition from genes to physical
structures such as cells, tissues, and organs is the province of developmental
biology. Understanding how these transitions evolve is the task of evolutionary
developmental biology (often shortened to EDB or “evo-devo”). Especially since
the 1980s, when knowledge of molecular genetic mechanisms of development
made major advances, EDB has been an active research area that has been filling
major gaps in biologists’ understanding of organismal diversity and evolution.
Research in EDB is concerned with several large questions [85]. First, concerning
the evolution of development, what have been the changes in developmental
mechanisms that give rise to different phenotypes? A second question, closely
related to the first, is how do genetic differences among species map onto phe-
notypic differences? Third, what is the role of development in either constrain-
ing or enhancing evolutionary change in characters? That is, how does develop-
ment affect “evolvability”? Fourth, how does developmental information help us
identify homologous characters, or even define homology? Finally, can under-
standing development help us understand the origin of novel characteristics?
Much of this chapter bears on the first three questions; we will save homology
Futuyma Kirkpatrick Evolution, 4e and evolutionary novelty for Chapter 20, on macroevolution.
Sinauer Associates
Troutt Visual Services
Evolution4e_02.02.ai Date 11-02-2016FIGURE 15.1 Many species are very similar as embryos, with the distinctive features of
their clade developing only later. Here are embryos of human, alligator, and mouse.
Can you tell which is which?^1(^1) From left to right, they are alligator, human, and mouse.
15_EVOL4E_CH15.indd 370 3/22/17 1:30 PM