374 CHAPTER 15by such modules, called individualization [50, 82]. For instance, during
the evolution of mammals, teeth became differentiated into incisors,
canines, premolars, and molars, with different functions—suggesting
that some different genes are active in the developing primordia of
different teeth. Distinct tooth identity was later lost during the evolu-
tion of the toothed whales (FIGURE 15.5).
Before modern molecular biology, biologists made discoveries about
development that remain important today. First, all cells in an organ-
ism have the same set of genes, based on replication during mitosis.
Second, the differences among cells, tissues, and organs must result
from differences in the activity of certain genes. Third, different cells
have properties that affect morphogenesis (the development of form).
These include growth of individual cells, change in cell shape, adhe-
sion to certain other cells, mitosis in certain dimensions (e.g., forming
sheets or masses), cell movement (in animals but not plants), and pro-
grammed cell death (apoptosis). Fourth, many aspects of growth and
differentiation are affected by chemical signals, especially hormones.
For example, metamorphosis in amphibians is triggered by thyroxin,
produced in the thyroid gland. The axolotl (see Figure 15.3) does not
synthesize thyroxin, and it differentiates into a typical adult form if it is
injected with that hormone. (Some other salamanders are irreversibly
neotenic and do not respond to thyroxin.) Likewise, cell division and
differentiation in plants are controlled by auxins and other hormones.
By performing experiments on embryos, biologists learned, more-
over, that certain events in an animal’s development depend on preceding events,
and that the differentiation of one tissue or organ is often influenced by others.
For example, cells in the posterior region of a vertebrate’s limb bud induce the
formation of limb structures such as digits and muscles by producing signaling
molecules (formerly called morphogens). A few scientists developed mathematical
models to describe how development could emerge from such chemical interac-
tions. One of the most important models was developed by the mathematician
Alan Turing, who invented the prototype of modern computers and famously
helped the Allies defeat the Nazis in World War II by breaking the German code.
In Turing’s models, two chemicals diffuse and interact to produce a morphogen
in a spatial pattern that induces a repeated feature such as a structure or pigment.
Changing parameters such as diffusion rates produces a variety of patterns that
match those seen in real organisms (FIGURE 15.6) [32].
A key advance toward modern developmental genetics was François Jacob
and Jacques Monod’s discovery in 1960 of the bacterial operon, a combination of
a regulatory sequence and co-regulated protein-coding genes [29]. Based on this
concept, the developmental biologists Roy Britten and Eric Davidson laid the
foundations of the modern view, in which eukaryotes’ genes have gene regula-
tory elements, or binding sites, to which proteins bind that initiate or stop tran-
scription [8].^1 Changes in these interactions could result in evolution of altered(^1) This chapter is concerned only with eukaryotes, but the evolution of operons in prokaryotes is
also a subject of current research.
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_15.05.ai Date 12-05-2016
(A) Haptodus
(B) Elephant shrew
(D) Dolphin
(C) Prozeuglodon
FIGURE 15.5 The teeth of mammals provide an example of the acquisition and loss
of individualization. (A) The teeth of mammal ancestors (synapsids), such as the Perm-
ian Haptodus, are uniform. (B) Teeth became individualized during the evolution
of mammals, as illustrated by an elephant shrew. (C, D) Distinct tooth identity was
reduced in an Eocene whale (Prozeuglodon) and lost altogether in modern toothed
whales, such as dolphins. (A after [62]; B–D after [79].)
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