Evolution, 4th Edition

248 CHAPTER 10

Even bacteria and viruses have sex of a sort. Although they lack the form of

sex based on meiosis that is familiar from animals and plants, they exchange

genes using a variety of mechanisms that together are called “parasexuality”

[41]. Some bacteria pass DNA between each other by conjugation (see Figure

10.1D). Others can absorb naked DNA left in the environment by bacteria that

have died. When several viral particles infect the same host, progeny viruses are

produced that carry mixtures of their genotypes. These different processes result

in the genetic mixing that is the fundamental evolutionary effect of sex.

Flowering plants cannot go in search of partners. Many enlist help from ani-

mals such as bees, bats, and birds that pollinate the plants while visiting their

flowers for nectar. Orchids have evolved elaborate features to attract their polli-

nators, in some cases exploiting those animals’ own sexual instincts (see Figure

3.23). Euglossine bees turn the tables on the orchids: males collect the flowers’

scents and use them as their own sex pheromones for courting female bees (FIG-

URE 10.2).

Our group of animals, the primates, has its own set of sexual curiosities. Dif-

ferent species have penises that vary dramatically in shape and size. Several

have spines. In fact, our recent evolutionary ancestor had a spiny penis. That

can be inferred from a chromosome deletion found in humans but not other pri-

mates that removes a regulatory element needed for development of the spines

[31]. Across the animal kingdom, male genitalia are among the most rapidly

evolving kinds of traits [15].

These examples immediately inspire questions. Why do males and females

often look so different? Why do organisms have such diverse ways of mat-

ing? And why do some species, such as the common dandelion, forego sex

altogether?

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_10.01.ai Date 01-25-17

(A) (B)

(C) (D)

FIGURE 10.1 Sexual reproduction is remark-

ably diverse. (A) The leopard slug (Limax

maximus) is a hermaphrodite. Mating pairs

hang from a mucus thread while copulat-

ing. (B) Clownfishes (genus Amphiprion) are

sequential hermaphrodites that first mature as

male and later change sex to female. (C) The

cactus Epithelantha micromeris has flowers

that do not open. It is one of the few species

that reproduce almost entirely by self-fertiliza-

tion. (D) Bacteria, such as these E. coli, use con-

jugation and other mechanisms to exchange

genetic material.

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