Evolution, 4th Edition

■■Animals, plants, and microbes have evolved

diverse forms of sexual reproduction, which is

the mixing of genetic material from different

parents. Almost all organisms on Earth engage in

some kind of sex.

■■Males and females are distinguished by the size

of the gametes they make: males make small

gametes, and females big ones. Many species

are hermaphroditic. In species with separate

males and females, sexual dimorphism ranges

from minimal to extreme.

■■Sexual selection, which is selection caused by

competition among individuals of the same sex

for mates, leads to the evolution of exagger-

ated secondary sexual traits that increase mating

success but usually decrease survival. Sexual

selection acts on males much more often than on

females. Males can often increase their fitness by

mating with more females, but females typically

do not benefit from mating with more males.

■■Some unusual species show sex role reversal.

Here sexual selection acts on females because

the operational sex ratio is female-biased: more

females are available to mate than males.

■■o ne of the two major modes of sexual selection

is male-male competition, which occurs by male

combat, sperm competition, infanticide, and

other mechanisms.

■■The second major mode of sexual selection is fe-

male choice. Female mating preferences evolve

as the result of direct benefits that females

receive from their mates, pleiotropic effects of

preference genes, and the good genes mecha-

nism. Some preferences result from perceptual

biases and apparently did not originate by either

direct benefits or good genes. A final mecha-

nism for the evolution of female preferences is

Fisher’s runaway process.

■■Sexual selection on plants favors flowers that in-

crease pollinator visitation, production of greater

quantities of pollen, and pollen that outcom-

petes other pollen in fertilizing ovules.

■■The sex ratio can evolve in many species. In most

situations, selection favors producing equal

numbers of males and females. Exceptions occur

in organisms such as fig wasps where selection

favors those families that produce the largest

numbers of daughters.

■■ The rarity of asexual reproduction is a puzzle be-

cause several factors give it an evolutionary ad-

vantage over sexual reproduction. The biggest

of these is the twofold cost of males suffered by

sexual species. other advantages to asexual re-

production include reproductive assurance and

escape from sexually transmitted diseases.

■■ Recombination gives sexual reproduction sever-

al advantages that compensate for its disadvan-

tages and thereby explain why it is so common.

The Red Queen hypothesis suggests that sex is

favored in changing environments. Recombina-

tion is also favored because it reduces selec-

tive interference, which is a general term that

includes clonal interference, the ruby-in-the-

rubbish effect, and Muller’s ratchet.

■■ Inbreeding depression frequently causes the

evolution of mechanisms that prevent self-fertil-

ization and mating between close relatives.

TERMS AND CoNCEPTS

alternative mating

strategy

anisogamy

clonal interference

direct benefit

direct selection

environmental sex

determination

Fisher’s runaway

good gene

haplodiploid sex

determination

hermaphroditic

inbreeding

depression

indirect selection

infanticide

lek

male combat

male-male

competition

Muller’s ratchet

operational sex ratio

parthenogenesis

perceptual bias

pleiotropic effect

primary sexual trait

Red Queen

hypothesis

reproductive

assurance

ruby-in-the-rubbish

effect

secondary sexual

trait

selective

interference

self-fertilization

self-incompatibility

sex ratio

sex role reversal

sexual selection

sexually dimorphic

sperm (pollen)

competition

two-fold cost of

males

SuMMARY

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