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Evolution__3rd_Edition

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Study and review questions<br />

1 If a descendant species, in its reproductive (adult)<br />

form, morphologically resembles a juvenile ancestral<br />

stage, what (a) is the descriptive term for this<br />

morphological pattern, and (b) are two possible<br />

heterochronic processes that could produce it?<br />

2 The eyes of vertebrates and the compound eyes of<br />

insects have utterly different structures, and almost<br />

CHAPTER 20 / <strong>Evolution</strong>ary Developmental Biology 589<br />

Gehring & Ikeo (1999) is a recent paper on the Pax6 gene and eye homology, and<br />

refers to the original papers in the early 1990s. Many authors have discussed what this<br />

and similar genetic findings reveals about homology. See Dickinson (1995), Abouheif<br />

et al. (1997), McGhee (2000), and Mindell & Meyer (2001).<br />

On the origin of Hox genes see also the material on duplications in the genomics<br />

section of this chapter. Slack et al. (1993) discuss a further topic a the “phylotypic<br />

stage.” They suggest: (i) that all animals are more similar at a certain developmental<br />

stage than earlier or later in development; (ii) the stage of maximum similarity is the<br />

stage at which Hox genes are expressed; and (iii) animals can be taxonomically defined<br />

by the possession of the phylotypic stage.<br />

Carroll et al. (2001) give references for the examples in which gene expression in<br />

development is associated with morphological evolution. On butterfly spots, see also<br />

the general review by McMillan et al. (2002) and the particular contributions of<br />

Beldade et al. (2002a, 2002b), the second paper particularly connects with another<br />

classic theme, that of developmental constraints on evolution a discussed in this text<br />

in Chapter 10.<br />

The general point about switches and evolvability is implicitly discussed in Carroll<br />

et al. (2001) and more explicitly in Ptashne & Gann (1998). The general concept of<br />

evolvability was introduced by Dawkins (1989b). It is also discussed in Gerhart &<br />

Kirschner (1997) and Kirschner & Gerhart (1998). Another, related finding concerns<br />

heat shock protein 90, which “canalizes” (Section 10.7.3, p. 276) development in animals<br />

and plants. The breakdown of canalization by hsp90 increases the range of genetic<br />

variation in a population; hsp90 could therefore normally reduce evolvability by<br />

decreasing variation but could increase evolvability in stressful times. Pigliucci (2002)<br />

introduces the topic and refers to the primary sources. Chapter 9 of this text has further<br />

material on canalization.<br />

certainly evolved independently. And yet a related gene<br />

seems to control the development of eyes in both the<br />

mouse and fruitfly. How can we reconcile these two<br />

observations?<br />

3 (a) What is meant by “evolvability”? (b) How can<br />

the evolution of gene regulatory circuits influence<br />

the evolvability of a life form?

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