Biology Now, 2e

Fast Lizards, Slow Corals ■ 241

mixing in the past, yet there is no evidence that

interbreeding is common; only a few rare hybrids

grow on the ocean floor between them. When

two species are reproductively isolated from

each other, we say that reproductive barriers

exist between those species. Reproductive barri-

ers are often divided into two categories: pre-

zygotic and postzygotic.

Barriers that prevent a male gamete (such as

a human sperm) and a female gamete (such as

a human egg) from fusing to form a zygote are

prezygotic barriers. Prezygotic barriers act

before the zygote exists (Figure 13.13). Barri-

ers that prevent zygotes from developing into

Figure 13.12

Sympatric speciation drives diversity

among Lake Victoria cichlid species

Scientists have described some 500 species

of cichlid fishes in Lake Victoria. Genetic

analyses indicate that they all descended

from just two ancestor species over the past

100,000 years. These four species show some

of the differences in feeding behavior and

morphology.

Q1: What is the main difference between

allopatric and sympatric speciation?

Q2: Name two events that must happen

for both allopatric speciation and sympatric

speciation to occur.

Q3: Do you think all of the 500 species

in Lake Victoria arose through sympatric

speciation? Why or why not?

Haplochromis chilotes

feeds on insects.

Haplochromis

macrognathus

feeds on other fishes.

Macropleurodus

bicolor feeds on snails

and other mollusks.

Astatotilapia elegans is a

generalized bottom feeder.

Figure 13.13

The blue-footed booby courtship dance

is a prezygotic, behavioral reproductive

barrier

This species of booby has a unique ritual dance

that must be accurately completed before mating.

Other booby species do not perform exactly the

same dance and therefore do not mate with the

blue-footed booby.

Q1: What does “prezygotic” mean?

Q2: How is the booby’s ritual dance a

prezygotic reproductive barrier?

Q3: What are some other prezygotic

reproductive barriers besides a mating

dance?

Blue-footed boobies point

their beaks, wings, and tails

upward in a mating dance

called “sky pointing.”