Evolution, 4th Edition

264 CHAPTER 10

dandelion (see Figure 10.20C), which is very successful at colonizing new patches

of habitat, gain an advantage from parthenogenesis through reproductive assur-

ance: in the absence of other individuals, just a single unfertilized seed can start

a whole new population. Another advantage is that sexually transmitted diseases,

which are suffered by plants as well as animals, are avoided by organisms without

sex. Together with the twofold cost of producing males, these further factors would

seem to give parthenogenesis an overwhelming evolutionary advantage. Yet the

ubiquity of sexual reproduction shows that is not the case.

Since sexual reproduction is so overwhelmingly common in life on Earth, it must

have some benefits that offset these drawbacks. The most fundamental feature of

sex is that it causes genetic mixing: zygotes carry combinations of alleles inher-

ited from two parents (see Chapter 4). In most eukaryotes, the mixing results from

segregation and recombination that happen during meiosis, and then fusion of the

sperm and egg. Prokaryotes rely on various kinds of parasexuality we mentioned

earlier. The solution to the evolutionary enigma of sex must involve this mixing.

Advantages to sex in changing environments

Clues about the factors that favor sex can be gleaned from looking at species that

have both sexual and asexual reproduction. Sex seems to be more common in

those species in situations where the environment is changing. Water fleas (genus

Daphnia) are small crustaceans that live in freshwater lakes. During the summer,

the populations are entirely female and reproduce for several generations by par-

thenogenesis. In the fall, sexual males and females appear. They mate and pro-

duce eggs that lie dormant through the winter, then hatch to start the cycle again

in the spring. The chemistry and biology of a lake can change substantially from

one year to the next, and it seems that sexual reproduction is timed to happen

when the environment is most unpredictable.

The timing of sexual reproduction in water fleas suggests that changing envi-

ronments can favor sex but not why they do so. A popular argument (frequently

repeated in introductory biology texts) is that sex is favored because it increases

genetic variation. On closer inspection, there are problems with that idea. A useful

analogy here is with a card game in which the cards play the role of alleles [34]. Say

that you have just won a game with a very good hand. If you had the choice, would

you keep those cards for the next game, or would you mix half of the cards you

now have with random ones taken from the deck? If the rules of the game don't

Futuyma Kirkpatrick Evolution, 4e

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Frequency of asexuals

2 4 6

Generation

8 10

0.4

0.2

0.6

0

1.0

(A) (B)

0.8

Sexual

Asexual

FIGURE 10.21 The biggest evolutionary

disadvantage of sexual reproduction is

the twofold cost of males. (A) Each female

produces two offspring. The sexual

females are exactly replacing themselves,

and their numbers are stable. Asexual

females produce only daughters, and so

their numbers double in each genera-

tion. (B) If all else is equal, a mutation in a

sexual population that causes females to

reproduce asexually will spread to fixation

in just a few generations. The fact that

asexuality is so rare shows that there must

be strong advantages to sexual repro-

duction that compensate for the twofold

cost of producing males.

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