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WEB TUTORIAL 18.1
Allopatric Speciation
Text Sections
Section 18.2 How Do New Species Arise?, p. 291
Introduction
When parts of a population become geographically isolated, they may evolve into
different species. How does this happen, and what determines whether or not speciation will occur? This tutorial presents two examples of isolation, one of which
leads to allopatric speciation and the other of which does not.
Learning Objectives
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Understand what allopatric speciation is.
Know the two ways in which allopatric speciation can begin.
Understand the conditions that are necessary for geographic isolation to
lead to speciation.
Narration
Allopatric Speciation by Geographic Separation
This population of lizards represents a single species. The body colors of individuals vary from dark to light, which is a difference in phenotype determined by
genetic inheritance. At any time, you can press the Histogram button to see the frequencies of the phenotypes in the population. This population of lizards provides
an example of how one species can diverge into two distinct species.
Geographic barriers can come and go over time. For instance, a river can change
course and divide a population, allowing each separated group to evolve independently. The physical splitting of a population by a barrier that arises in its midst is
one of two ways in which allopatric speciation can begin. We will consider the
other—dispersal—shortly.
After 20 generations, the populations have evolved. Because of a difference in soil
color, natural selection favors darker individuals in the population on the left and
lighter individuals in the population on the right. Notice that a mutation for spotting has arisen in the population on the right.
After an additional 20 generations, the color difference has continued to evolve,
and the mutation for spotting is spreading among the lizards in the population on
the right.
After an additional 20 generations, the populations have evolved even further.
Notice that the population on the left is mostly dark and the one on the right is
mostly light, with a higher frequency of spotting among the lightest individuals.
In this example, the two populations have diverged from each other genetically.
But are these populations two distinct species?
Allopatric Speciation by Dispersal
If part of a population migrates to an isolated location, allopatric speciation may
cause the original population and the part of the population that has migrated to
diverge into two distinct species. Let's assume that lizard color is controlled largely by genetics.
In this example, a subset of the population rafts over to the island on driftwood. As
you can see, the frequency of phenotypes among the colonists is different from that
among the lizards on the mainland. The island population has already diverged
from the mainland population because it has been founded by a population with a
different proportion of genotypes than exists in the mainland population. Evolution
occurs whenever allele frequencies in two populations change over time.
After 20 generations, a mutation for spotting has arisen in the island population.
After an additional 20 generations, selection on the mainland has favored lizards
of medium-to-dark color. The island lizards are still light-colored, and more members of this population have inherited the mutation for spotting.
After an additional 20 generations, the populations have evolved further. All of the
island lizards are light-colored and have spots. In this example of dispersal leading
to isolation, the two populations have diverged from each other genetically. But are
these populations two distinct species?
When Doesn't Speciation Occur?
How do we determine if a new species has arisen. While geographical separation
is the most important factor in getting speciation going, it cannot bring about speciation by itself. Following geographical separation, two populations of the same
species must then undergo physical or behavioral changes that will keep them from
interbreeding, should they ever be reunited.
Suppose the geographic barrier separating two diverged populations is removed
and they are reunited into a single population. What will happen? In this example,
matings—indicated by lines—are common between individuals of the formerly
isolated populations.
The individuals continue to interbreed and produce viable offspring for several
generations. Therefore, speciation did not occur between the previously divided
populations. These populations did not become reproductively isolated. Therefore,
they are of the same species.
When Does Speciation Occur?
In another example, the lizards from two populations that had been evolving independently are reunited. A drop in sea level has removed the geographic barrier
between them, bringing them back together. What will happen?
In this case, individuals recognize and preferentially mate with individuals from
their former population—indicated by the connecting lines.
What will happen when a light, spotted lizard mates with a dark, unspotted lizard?
In this scenario, the hybrid offspring of spotted and unspotted lizards are at a disadvantage. The gene that causes spotting has a negative effect on fitness in darkcolored individuals. Therefore, the two populations did diverge significantly during their geographic separation.
If hybrids are at a disadvantage, natural selection may favor mechanisms to avoid
hybridization, and the populations will remain different.
Species-recognition traits, such as color and markings, may diverge to a greater
extent when two similar species are sympatric—that is, living together.
For example, the dark-colored lizards may preferentially mate with other dark
lizards, and the light lizards may preferentially mate with other light-colored
lizards.
Let's look at the next generation. Notice that the darkest and the lightest lizards are
the most abundant.
The extreme trait values that ensure mating between compatible individuals—such
as the very dark and the very light colors—are favored in these populations. The
intermediate trait values are selected against. Selection for species-recognition
traits that prevent hybridization is called reinforcement.
In these populations, matings between light-colored individuals with spots are
favored in one incipient species, and matings between dark individuals without
spots are favored in the other.
Reinforcement increases the morphological divergence between the two new
groups, ensuring that they will remain reproductively isolated. Intermediate phenotypes have disappeared, and the extreme phenotypes have increased in frequency.
Reinforcement is a specific kind of natural selection called disruptive selection.
Once this process completely isolates the two groups reproductively, we can call
them two different species.
You should now be able to…
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Compare and contrast the processes of geographic separation and dispersal.
Predict the possible outcomes of removing a geographic barrier between
isolated populations of the same species.
Define the terms “allopatric” and “sympatric.”
Explain why reinforcement is considered to be a type of disruptive selection.