Evolution__3rd_Edition

314 PART 3 / Adaptation and Natural Selection
Sex poses a problem ...
. . . because it seems to be less
efficient than cloning
Figure 12.1
The 50% cost of sex. A
population initially contains
equal numbers of asexual and
sexual females. The females
have identical survival and
fecundity (two offspring per
parent). Asexual reproduction
rapidly takes over, because
it doubles the rate of
reproduction.
12.1 The existence of sex is an outstanding, unsolved
problem in evolutionary biology
12.1.1 Sex has a 50% cost
In asexual (or clonal) reproduction, a parent produces an offspring that is a genetic
copy of the parent. In sexual reproduction, a parent combines half its DNA with half
the DNA of another individual and the offspring is only a half genetic copy of each
parent. Sexual reproduction poses an evolutionary problem because it seems to be
half as efficient a method of reproducing as its alternative, asexual reproduction.
Figure 12.1 imagines a simple population with one asexual individual, one sexual
female, and one sexual male. (If the numbers seem unrealistically low, they can be
multiplied up by any amount. Each individual in Figure 12.1 could stand for 1,000
individuals, for instance.) We assume that the members of the two groups are identical
in all other respects: sexual and asexual individuals are equally good at finding food,
avoiding enemies, and staying alive; they produce the same number of offspring, and
those offspring have an equal chance of survival. We are considering only whether
natural selection favors sexual or asexual reproduction.
Suppose, for simplicity, that each female produces two offspring. After one generation,
the asexual group will have grown to two individuals. The sexual female will also
produce two offspring, but only one of these will be a daughter. Now we have four individuals
in all, and the proportion of asexual females has increased from one-third to
one-half. After another generation, there will be four asexual females, one sexual female
and one male; the proportion of asexual females has grown to two-thirds. Asexual
reproduction will soon take over completely from sexual reproduction. The clone of
offspring from an asexual female multiplies at twice the rate of the progeny descended
from a sexual female, and a sexual female has only 50% of the fitness of an asexual female.
In some species, the cost of sex may be less than 50%. For instance, in some singlecelled
organisms sex is not associated with reproduction. In Paramecium, two cells may
Generation 1
Generation 2
Generation 3
Asexual reproduction Sexual reproduction Frequency of
asexual individuals
1/3
1/2
2/3
..