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Evolution__3rd_Edition

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314 PART 3 / Adaptation and Natural Selection<br />

Sex poses a problem ...<br />

. . . because it seems to be less<br />

efficient than cloning<br />

Figure 12.1<br />

The 50% cost of sex. A<br />

population initially contains<br />

equal numbers of asexual and<br />

sexual females. The females<br />

have identical survival and<br />

fecundity (two offspring per<br />

parent). Asexual reproduction<br />

rapidly takes over, because<br />

it doubles the rate of<br />

reproduction.<br />

12.1 The existence of sex is an outstanding, unsolved<br />

problem in evolutionary biology<br />

12.1.1 Sex has a 50% cost<br />

In asexual (or clonal) reproduction, a parent produces an offspring that is a genetic<br />

copy of the parent. In sexual reproduction, a parent combines half its DNA with half<br />

the DNA of another individual and the offspring is only a half genetic copy of each<br />

parent. Sexual reproduction poses an evolutionary problem because it seems to be<br />

half as efficient a method of reproducing as its alternative, asexual reproduction.<br />

Figure 12.1 imagines a simple population with one asexual individual, one sexual<br />

female, and one sexual male. (If the numbers seem unrealistically low, they can be<br />

multiplied up by any amount. Each individual in Figure 12.1 could stand for 1,000<br />

individuals, for instance.) We assume that the members of the two groups are identical<br />

in all other respects: sexual and asexual individuals are equally good at finding food,<br />

avoiding enemies, and staying alive; they produce the same number of offspring, and<br />

those offspring have an equal chance of survival. We are considering only whether<br />

natural selection favors sexual or asexual reproduction.<br />

Suppose, for simplicity, that each female produces two offspring. After one generation,<br />

the asexual group will have grown to two individuals. The sexual female will also<br />

produce two offspring, but only one of these will be a daughter. Now we have four individuals<br />

in all, and the proportion of asexual females has increased from one-third to<br />

one-half. After another generation, there will be four asexual females, one sexual female<br />

and one male; the proportion of asexual females has grown to two-thirds. Asexual<br />

reproduction will soon take over completely from sexual reproduction. The clone of<br />

offspring from an asexual female multiplies at twice the rate of the progeny descended<br />

from a sexual female, and a sexual female has only 50% of the fitness of an asexual female.<br />

In some species, the cost of sex may be less than 50%. For instance, in some singlecelled<br />

organisms sex is not associated with reproduction. In Paramecium, two cells may<br />

Generation 1<br />

Generation 2<br />

Generation 3<br />

Asexual reproduction Sexual reproduction Frequency of<br />

asexual individuals<br />

1/3<br />

1/2<br />

2/3<br />

..

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