Evolution, 4th Edition

268 CHAPTER 10

that none of those individuals will leave any offspring. In a sexual popu-

lation, recombination can regenerate that class of high-fitness individu-

als. But that does not happen in an asexual population (except in the

unlikely event of back mutations from a deleterious to a beneficial allele).

Each time the most fit genotypes fail to reproduce, the population’s

mean fitness is ratcheted downward. It can never recover, and in prin-

ciple this process can lead to the extinction of an asexual species.

Asexual species are thought to be so rare because some combination

of these forms of selective interference and the Red Queen hypothesis

drives them to extinction rapidly, while sexual species are much more

likely to survive. The same factors that favor sexual over asexual spe-

cies can cause selection for changes to recombination rates within the

genomes of sexually reproducing species. Recombination rates can

evolve by several mechanisms, for example changes in the frequencies

and locations of crossovers during meiosis. The details are beyond this text, but

the underlying principles are much the same: factors that favor sexual over asexual

species tend to favor increased recombination within sexual species.

Selective interference is particularly severe in asexual populations because they

have no recombination, but it also occurs in sexual populations. Within a species,

some parts of the genome have high recombination rates, while others have low

rates. Using data on the polymorphism within species and the differences between

species at protein-coding loci in Drosophila melanogaster, the rate of adaptive amino

acid substitution can be estimated for different parts of the genome. The data show

a clear pattern: adaptive evolution is fastest in regions with high recombination

rates (FIGURE 10.26) [10].

The human sex chromosomes give a graphic example of the evolutionary conse-

quences of giving up recombination. The Y chromosome originated from a recom-

bining X chromosome some 180 million years ago, and then ceased to recombine

with the X [13]. From that point onward, the Y chromosome has evolved asexually,

like the mitochondria that are passed through the female lineage. Meanwhile, the

X chromosome continues to recombine in females. As various forms of selective

interference caused the Y chromosome to degenerate, it lost almost all of the 2000 or

so genes and more than 60 percent of the DNA carried on the X [32]. As a result, we

now see dramatic differences between the X and Y chromosomes (FIGURE 10.27).

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.26.ai Date 12-15-2016 01-25-17

Rate of adaptive substitution

0 2 4

Recombination rate (cM/Mb)

0.01

0.00

–0.01

0.02

FIGURE 10.26 Selective interference in

Drosophila melanogaster decreases the

rate of adaptive evolution in regions of the

genome that have low recombination rates.

The x-axis shows the recombination rate

(in centimorgans [cM] per megabase [Mb]

of DNA); regions of the genome with high

recombination are to the right. The y-axis

shows a measure of the rate of adaptive

amino acid changes in proteins. Genomic

regions with higher recombination rates

have higher rates of adaptive evolution.

(After [10].)

FIGURE 10.27 The human sex chromosomes,

as seen in a scanning electron micrograph. The

Y chromosome (at left) does not recombine.

Selective interference caused it to degenerate

from an ancestor that was much like the X chro-

mosome (at right).

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