Evolution__3rd_Edition

588 PART 5 / Macroevolution
Summary
1 Morphological change in evolution usually occurs
by changes in developmental processes. The identification
of genes that influence development is a major
area of modern biology, and its methods can be applied
to study the relations of development and evolution, a
field known as “evo-devo.”
2 Heterochrony refers to evolutionary changes in the
relative timing and rate of different developmental
processes. For instance, the time of reproduction may
shift relative to somatic development. Also, shape
changes can result from changes in growth gradients,
and D’Arcy Thompson’s transformational diagrams
can be interpreted in terms of heterochrony.
3 Regulatory genes influence the expression of other
genes, and evolutionary change can result from changed
regulatory relations among genes as well as changes in
the sequence of genes.
4 Structures, such as the eyes of insects and vertebrates,
that had been thought to be non-homologous,
Further reading
have been found to be developmentally controlled by
the same gene. Insect and vertebrate eyes may share an
element of homology, but it is uncertain what the level
of the homology is.
5 The number of Hox genes increased from perhaps
two to seven near the origin of the triploblastic
Bilateria, and quadrupled from 13 to 52 near the origin
of vertebrates. Hox genes control spatial differentiation
within the body during development, and increases in
the number of Hox genes may be associated with
increases in developmental complexity.
6 Changes in the expression of developmental genes
are likely achieved by gains, losses, and changes in the
regulatory elements (particularly enhancers) of those
genes.
7 Some forms of life may be more evolvable than
others: that is, be more likely to undergo innovative
evolutionary change. The origin of genetic switches
may have made life more evolvable.
General developmental biology texts, such as Gilbert (2000) and Wolpert (2002)
contain chapters on evolution, as well as developmental biology background. Wilkins
(2001), Carroll et al. (2001), and Hall (1998) are texts more specifically on evo-devo.
Proceedings of the National Academy of Sciences (2000), vol. 97 (9), pp. 4424–540 contains
the proceedings from a conference on evo-devo. Gerhart & Kirschner (1997) is a
stimulating book, more about the evolution of cells, but containing much relevant
material for this chapter. Meyerowitz (2002) gives an evo-devo comparison of plants
and animals.
Gould’s (1977a) book discusses the history of recapitulatory ideas and modern
work on heterochrony. Gould (2002b) contains further material. Raff (1996) is a more
recent general book, and Levinton’s (2001) even broader book also covers the topic.
Both Gould and Raff are good on heterochrony, but see also the review article by
Klingenberg (1998), the web-page on heterochrony (and on D-Arcy Thompson’s
transformations) by Horder in www.els.org, and the think-piece by Smith (2001).
Britton & Davidson (1971) is an early work discussing gene regulation and evolution.
See also the introductory article by A.C. Wilson (1985), the recent book by Davidson
(2001), as well as the general references and some further references below.
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