Evolution, 4th Edition

540 CHAPTER 20

Genome size, meaning the amount of DNA in the cell nucleus, varies 60,000-

fold among eukaryotes [21, 36]. Although genome size has decreased in some lin-

eages, such as Arabidopsis, increases have been more prevalent because of mecha-

nisms such as polyploidy and the proliferation of transposable elements. However,

the number of coding sequences does not appear to be correlated with our tradi-

tional (but perhaps erroneous) impressions of phenotypic complexity; we like to

think that mammals such as humans, with about 20,000 genes, are more complex

than water fleas (Daphnia) with 31,000, or rice, with 60,000 (see Figure 14.2). T he

number of functional genes both increases and decreases in evolution; for example,

it is often much lower in parasites and in endosymbiotic bacteria than in free-living

relatives [72]. But we do not yet know if there has been a trend in the information

content of genomes throughout evolution, because the variety of functions that

reside in a genome may be greatly amplified by alternative splicing of genes, mul-

tiple binding sites for different transcription factors, and other processes.

The bottom line is that although all organisms taken together show a passive

trend toward greater complexity, no characteristic displays a consistent driven

trend among all, or even most, branches of the tree of life.

Predictability and contingency in evolution

A course of events (a history) is said to be “predictable” if it proceeds by lawlike

principles that determine the sequence of events (e.g., A, B, ..., E), each caused

by the preceding event. Physics is the epitome of a predictive science. For many

people, predictability has the implication that the realized course of events was

inevitable from the start. In this view, as the paleontologist Stephen Jay Gould

noted [35], the evolutionary history of life was inevitable, including the evolution

of humans—or at least a comparably intelligent life form (a “humanoid,” perhaps).

Among evolutionary biologists, paleontologist Simon Conway Morris [12, 13] is a

proponent of this viewpoint (see also [19]).

Historical contingency, in contrast, means that although each event (e.g., E) is

caused by a preceding event (D), the outcome of the history would be different

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_20.27.ai Date 12-16-2016

(A) Eusthenopteron (lobe-nned sh) (B) Milleretta (early amniote)

(C) Canis (modern mammal)

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FIGURE 20.27 An example of decreasing complexity during

evolution. (A) Skull of an early lobe-finned fish (the Devonian

Eusthenopteron), the clade from which tetrapods evolved. (B) An

early amniote, or reptile (Milleretta, from the Permian), similar to

the ancestors of mammals. (C) A modern mammal (the domestic

dog, Canis). The great number of labels indicates that the lobe-

finned fish had more skull bones than the early amniotes, which

in turn had far more bones than their mammalian descendants.

The reduction in the number of bones in the lower jaw (from six

labeled in A to one in C) is particularly notable. (After [91].)

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