Evolution, 4th Edition

250 CHAPTER 10

fishes that live deep in the world’s oceans [39]. A female anglerfish attracts her prey

with a bioluminescent lure that she dangles in front of her enormous mouth. But

even more bizarre are the small growths attached to the female’s body (FIGURE

10.4). These are male anglerfish. When young, a male swims freely until he finds

a mature female. He bites onto whatever part of her he can, and then never lets go.

Their circulatory systems fuse, and almost all of the male’s organs—eyes, gills,

digestive system—degenerate. One organ, however, grows much larger: when the

metamorphosis is complete, the male’s body is almost entirely filled by his testes.

To mate, the female releases her eggs and the male his sperm, which fertilize the

eggs in the surrounding water.

Sexual dimorphism can evolve when selection favors the expression of a trait

to be different in males and females. This situation is called sexually antagonistic

selection, because an increase in the trait’s expression benefits one sex but harms

the other. Genetic constraints (see Chapter 6) sometimes prevent a trait from

evolving sexual dimorphism, and so the fitnesses of both sexes are permanently

compromised.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_10.03.ai Date 12-20-2016 01-24-17

XY

ZW

Undifferentiated sex

chromosomes

Hermaphroditism

Environmental sex

determination

Haplodiploidy

Vertebrates

Arthropods

Mammalia

Aves

Amphibia

Reptilia

Teleostei

Acari

Coccoidea

Crustacea

Coleoptera

Hymenoptera

Diptera

Lepidoptera

FIGURE 10.3 Animals have a variety of mechanisms that determine

an individual’s sex. The pie diagrams show the fraction of species

in vertebrates and arthropods in which different mechanisms are

used. The XY system of sex determination is used in all mammals

but is not very common among other vertebrate groups. The most

common sexual system in teleost fishes is hermaphroditism. All

Hymenoptera (ants, bees, wasps, and their relatives) and a large

fraction of Acari (mites) determine sex by haplodiploidy. (After [4].)

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