Allopatric Speciation
Allopatric Speciation
Allopatric (“in another land”) populations
of a species are those that occupy
separate geographical areas. Because
of their geographical separation, they
cannot interbreed, but would be expected
to do so if the geographic barriers
between them were removed.
Speciation that results from evolution
of reproductive barriers between geographically
separated populations is
called allopatric speciation or geographic
speciation. The separated populations
evolve independently and
adapt to their different environments,
generating reproductive barriers between
them as a result of their separate
evolutionary paths. Ernst Mayr (Figure
6-18) has contributed greatly to
our knowledge of allopatric speciation
through his studies of speciation
in birds.
Allopatric speciation begins when a species splits into two or more geographically separated populations. This splitting can happen in either of two ways: by vicariant speciation or by a founder event. Vicariant speciation is initiated when climatic or geological changes fragment a species’ habitat, producing impenetrable barriers that separate different populations. For example, a mammalian species inhabiting a lowland forest could be divided by uplifting of a mountain barrier, sinking and flooding of a geological fault, or climatic changes that cause prairie or desert conditions to encroach on the forest.
Vicariant speciation has two important consequences. Although the ancestral population is fragmented, the individual fragments are usually left fairly intact. The vicariant process itself does not induce genetic change by reducing populations to a small size or by transporting them to unfamiliar environments. Another important consequence is that the same vicariant events may fragment several different species simultaneously. For example, fragmentation of the lowland forest described above most likely would disrupt numerous and diverse species, including salamanders, frogs, snails, and many other forest dwellers. Indeed, the same geographic patterns are observed among closely related species in different groups of organisms whose habitats are similar. Such patterns provide strong evidence for vicariant speciation.
The alternative means of initiating allopatric speciation is for a small number of individuals to disperse to a distant place where no other members of their species are present. The dispersing individuals may establish a new population in what is called a founder event. Allopatric speciation caused by founder events has been observed, for example, in the native fruit flies of Hawaii. Hawaii contains numerous patches of forest separated by volcanic lava flows. On rare occasions, strong winds can transport a few flies from one forest to another, geographically isolated forest where the flies are able to start a new population. Sometimes, a single fertilized female may found a new population. Unlike what happens in vicariant speciation, the new population initially has a very small size, which can cause its genetic structure to change dramatically from that of its ancestral population. When this event happens, phenotypic characteristics that were stable in the ancestral population often reveal unprecedented variation in the new population. As the newly expressed variation is sorted by natural selection, large changes in phenotype and reproductive properties occur, hastening the evolution of reproductive barriers between the ancestral and newly founded populations.
Surprisingly, we often learn most about the genetics of allopatric speciation from cases in which formerly separated populations regain geographic contact following evolution of incipient reproductive barriers that are not absolute. The occurrence of mating between divergent populations is called hybridization and offspring of these matings are called hybrids (Figure 6-19). By studying the genetics of hybrid populations, we can identify the genetic bases of reproductive barriers.
Biologists often distinguish between reproductive barriers that impair fertilization (premating barriers) and those that impair growth and development, survival, or reproduction of hybrid individuals (postmating barriers). Premating barriers may cause members of divergent populations either not to recognize each other as potential mates or not to complete the mating ritual successfully. In some cases, female and male genitalia of the different populations will be incompatible. In others, premating barriers may be strictly behavioral, with members of different species being otherwise nearly identical in phenotype. Different species that are indistinguishable in organismal appearance are called sibling species. Sibling species arise when allopatric populations diverge in the seasonal timing of reproduction or in auditory, behavioral, or chemical signals required for mating. Evolutionary divergence in these features can produce effective premating barriers without obvious changes in organismal appearance. Sibling species occur in groups as diverse as ciliates, flies, and salamanders.
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| Figure 6-18 Professor Ernst Mayr, a major contributor to our knowledge of speciation and of evolution in general. |
Allopatric speciation begins when a species splits into two or more geographically separated populations. This splitting can happen in either of two ways: by vicariant speciation or by a founder event. Vicariant speciation is initiated when climatic or geological changes fragment a species’ habitat, producing impenetrable barriers that separate different populations. For example, a mammalian species inhabiting a lowland forest could be divided by uplifting of a mountain barrier, sinking and flooding of a geological fault, or climatic changes that cause prairie or desert conditions to encroach on the forest.
Vicariant speciation has two important consequences. Although the ancestral population is fragmented, the individual fragments are usually left fairly intact. The vicariant process itself does not induce genetic change by reducing populations to a small size or by transporting them to unfamiliar environments. Another important consequence is that the same vicariant events may fragment several different species simultaneously. For example, fragmentation of the lowland forest described above most likely would disrupt numerous and diverse species, including salamanders, frogs, snails, and many other forest dwellers. Indeed, the same geographic patterns are observed among closely related species in different groups of organisms whose habitats are similar. Such patterns provide strong evidence for vicariant speciation.
 |
| Figure 6-19 Pure and hybrid salamanders. Hybrids are intermediate in appearance between parental populations. A, Pure white-spotted Plethodon teyahalee; B, a hybrid between white-spotted P. teyahalee and red- legged P. jordani, intermediate in appearance for both spotting and leg color; C, pure red-legged P. Jordani. |
The alternative means of initiating allopatric speciation is for a small number of individuals to disperse to a distant place where no other members of their species are present. The dispersing individuals may establish a new population in what is called a founder event. Allopatric speciation caused by founder events has been observed, for example, in the native fruit flies of Hawaii. Hawaii contains numerous patches of forest separated by volcanic lava flows. On rare occasions, strong winds can transport a few flies from one forest to another, geographically isolated forest where the flies are able to start a new population. Sometimes, a single fertilized female may found a new population. Unlike what happens in vicariant speciation, the new population initially has a very small size, which can cause its genetic structure to change dramatically from that of its ancestral population. When this event happens, phenotypic characteristics that were stable in the ancestral population often reveal unprecedented variation in the new population. As the newly expressed variation is sorted by natural selection, large changes in phenotype and reproductive properties occur, hastening the evolution of reproductive barriers between the ancestral and newly founded populations.
Surprisingly, we often learn most about the genetics of allopatric speciation from cases in which formerly separated populations regain geographic contact following evolution of incipient reproductive barriers that are not absolute. The occurrence of mating between divergent populations is called hybridization and offspring of these matings are called hybrids (Figure 6-19). By studying the genetics of hybrid populations, we can identify the genetic bases of reproductive barriers.
Biologists often distinguish between reproductive barriers that impair fertilization (premating barriers) and those that impair growth and development, survival, or reproduction of hybrid individuals (postmating barriers). Premating barriers may cause members of divergent populations either not to recognize each other as potential mates or not to complete the mating ritual successfully. In some cases, female and male genitalia of the different populations will be incompatible. In others, premating barriers may be strictly behavioral, with members of different species being otherwise nearly identical in phenotype. Different species that are indistinguishable in organismal appearance are called sibling species. Sibling species arise when allopatric populations diverge in the seasonal timing of reproduction or in auditory, behavioral, or chemical signals required for mating. Evolutionary divergence in these features can produce effective premating barriers without obvious changes in organismal appearance. Sibling species occur in groups as diverse as ciliates, flies, and salamanders.
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