Evolution, 4th Edition

SPECiES And SPECiATion 235

several generations later [31, 54]. Typically a few of the replicate populations dis-

play sexual isolation from the parent population, although the change is often tem-

porary. There is some disagreement about whether or not to view these results as

support for founder effect speciation [15, 101].

The Geography of Speciation

A critical issue in understanding speciation is how the level of gene flow is ini-

tially reduced between two populations when they first start to diverge. This is

key because populations cannot diverge, and evolve reproductive isolation, if gene

flow is high enough to counteract divergence by selection or genetic drift.

The most common way for speciation to begin is with the appearance of a geo-

graphic barrier that partly or completely blocks genetic exchange between two

populations (FIGURE 9.22) [15, 57]. This is allopatric speciation. In other cases, spe-

ciation occurs with little or no help from a geographic barrier. The most extreme

case is when a single population splits into two reproductively isolated popula-

tions while living together, a process called sympatric speciation. An intermedi-

ate between those two situations is parapatric speciation, in which neighboring

populations of a single species that exchange genes nevertheless diverge into two

species. The critical difference among these situations is that sympatric speciation

and parapatric speciation involve speciation with gene flow, while allopatric spe-

ciation does not. The following sections look into the details of allopatric speciation

and speciation with gene flow.

Allopatric speciation

Allopatric speciation is the evolution of genetic barriers between populations that are

geographically separated by a physical barrier (for example, a mountain range).

Allopatry is defined by a severe reduction of movement of individuals or their

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_09.21.ai Date 11-21-2016

Solomon Is

Papua New Guinea New Britain

Admiralty Is

Irian Jaya

NEW GUINEA

1

(^76)
8
3
2 4
5
T. galatea
T. carolinae T. riedelli
FIGURE 9.21 Variation among paradise kingfishers in New Guinea. Tanysiptera
galatea is distributed throughout the New Guinea lowlands (regions 1, 2, 3) and
some satellite islands (4, 5). The very localized forms T. riedelii on Biak Island (6)
and T. carolinae on Numfor Island (7) are distinct species. Mayr [56] proposed
that these represent cases of founder effect speciation. (T. galatea photo
courtesy of Rob Hutchinson/Birdtour Asia; T. riedelii and T. carolinae courtesy of
Mehd Halaouate.)
09_EVOL4E_CH09.indd 235 3/23/17 9:36 AM