Evolution, 4th Edition

SPECiES And SPECiATion 241

By contrast, perhaps as many as one-third of the endemic species of plants on Lord

Howe Island may have originated there by sympatric speciation [70]. Most evolution-

ary biologists believe that allopatric speciation is much more common than sympatric

speciation. Perhaps, though, many cases of sympatric speciation have gone unde-

tected, because it is usually difficult to rule out a past history of allopatric divergence.

Parapatric speciation

An intermediate between allopatric speciation and sympatric speciation is para-

patric speciation, in which neighboring populations diverge while they continue to

interbreed (see Figure 9.22). We expect parapatric speciation to be more common

than sympatric speciation because it involves less gene flow between the diverging

populations.

Many examples have been described in which strongly selected genes and phe-

notypes differ between populations that interbreed [64]. Among these, a few indi-

cate the evolution of some reproductive isolation that reduces gene exchange. For

example, the White Sands region of New Mexico consist of dunes, formed less than

5000 years ago, that differ starkly from the surrounding dark soils. In three species

of lizards that are distributed across both soil types, the populations that inhabit

the dunes differ in head shape, toe length, and most strikingly in color—all char-

acteristics that are adaptive and are thought to be strongly selected (FIGURE 9.28)

[86]. In two of the species, a fence lizard (Sceloporus undulatus) and an earless liz-

ard (Holbrookia maculata), there are strong differences in the frequencies of genetic

markers across the boundary between the habitats. These genetic differences imply

that gene flow has been reduced by the evolution of partial reproductive isolation.

A likely reason is that the pale coloration is associated with differences in the color

of ventral blotches that are displayed in sexual and other social encounters. Studies

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_09.28.ai Date 11-21-2016

Holbrookia maculata

Sceloporus undulatus

White sand Dark soil

Genetic score

FIGURE 9.28 Incipient speciation with gene

flow: divergence of lizards on the white sand

and dark soil habitats shown in the photos at

top. Multiple nuclear genes show that geno-

types differ between habitats strongly in Hol-

brookia maculata and less so in Sceloporus

undulatus. Each bar represents the genotype

of one individual. The proportion of a bar that

is green shows the probability, based on a liz-

ard’s genotype, that the individual belongs to

a distinct white sand population, and the pro-

portion that is blue that it belongs to a distinct

dark soil population. Bars with intermediate

amounts of green and blue indicate that the

individual has a mixed genotype. (After [86].)

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