Evolution, 4th Edition

450-plus species of cichlid fishes evolved from 5 ancestral species in Lake Victoria

in just 15,000 years (see opening of this chapter), the average time between spe-

ciation events was less than 2000 years, which is astonishingly rapid. Molecular

clocks (see Chapter 7) can be used to estimate the time back to the most recent

common ancestor, giving us the age of their speciation. Based on this approach,

sympatric sister species of Drosophila are estimated to have taken about 200,000

years to evolve, while it requires 1.1–2.7 million years for allopatric populations to

evolve full reproductive isolation [15]. Some populations of birds that have been

diverging for 1.5–3 million years form hybrid zones, showing that it take more

time for birds than flies to evolve strong reproductive isolation [106].

In many groups of organisms, prezygotic isolation evolves considerably faster

than intrinsic postzygotic isolation (FIGURE 9.15). Consequently, closely related

species are often fully interfertile: many species and genera of birds, even after

more than 5 million years of divergence, can form fully viable, fertile hybrids when

crossed [76]. In these cases (which may be the rule in many kinds of organisms),

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_09.15.ai Date 11-21-2016

Strength of reproductive isolation

0.5 1.0 1.5 2.0

Genetic distance

0.2

0

0.4

0.6

0.8

1.0

(A) Drosophila

Strength of reproductive isolation

0.5 1.0 1.5 2.0

Genetic distance Genetic distance

0.2

0

0.4

0.6

0.8

1.0

Sexual isolation

0.20.1 0.3 0.4 0.5 0.70.6

Genetic distance

0.2

0

0.4

0.6

0.8

1.0

0.1 0.2 0.3 0.4 0.5

0

0.2

–0.2

0.4

0.6

0.8

1.0

(B) Etheostoma

Hybrid inviability

Postzygotic isolation

Prezygotic isolation

Prezygotic isolation appears at

relatively small genetic distances.

FIGURE 9.15 Prezygotic isolation evolves faster than postzygotic

isolation in flies and fishes. (A) The strength of prezygotic and

postzygotic reproductive isolation between pairs of popula-

tions and species of Drosophila increases with the amount of

time since the lineages split. The time is estimated by the genetic

distance between each pair. The strength of prezygotic isolation

was measured by observing mating between flies in the labora-

tory. The strength of postzygotic isolation was measured by sur-

vival and fertility of hybrid individuals. Comparison of the upper

left part of the two graphs reveals that strong prezygotic isolation

evolves shortly after isolation (at small genetic distances), while

strong postzygotic isolation evolves only later. (B) Similar patterns

are seen in a genus of freshwater fishes, the darters (Etheos-

toma). Thirteen pairs of allopatric species were tested for both

sexual isolation and the survival of artificially produced hybrids.

For both indices, a value of 0 indicates that the pairs are no more

isolated than conspecific individuals, and a value of 1 indicates

complete reproductive isolation. (A after [14]; B after [59].)

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