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5: New Species Arise Through the Evolution of Reproductive Isolation

5: New Species Arise Through the Evolution of Reproductive Isolation

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Population and Evolutionary Genetics

Modes of Speciation
Speciation is the process by which new species arise. In
regard to the biological species concept, speciation comes
about through the evolution of reproductive isolating mechanisms—mechanisms that prevent the exchange of genes
between groups of organisms.
New species arise in two principle ways. Allopatric
speciation arises when a geographic barrier first splits a
population into two groups and blocks the exchange of
genes between them. The interruption of gene flow then
leads to the evolution of genetic differences that result in
reproductive isolation. Sympatric speciation arises in the
absence of any external barrier to gene flow; reproductive
isolating mechanisms evolve within a single population.
We will take a more detailed look at both of these mechanisms next.

Allopatric speciation Allopatric speciation is initiated
when a geographic barrier splits a population into two or
more groups and prevents gene flow between the isolated
groups (Figure 17.10a). Geographic barriers can take a
number of forms. Uplifting of a mountain range may split
a population of lowland plants into separate groups on
each side of the mountains. Oceans serve as effective barriers for many types of terrestrial organisms, separating individuals on different islands from one another and from
those on the mainland. Rivers often separate populations of
fish located in separate drainages. The erosion of mountains may leave populations of alpine plants isolated on
separate mountain peaks.
After two populations have been separated by a geographic barrier that prevents gene flow between them, they
evolve independently (Figure 17.10b). The genetic isolation allows each population to accumulate genetic differences that are not found in the other population through
natural selection, unique mutations, and genetic drift (if
the populations are small). These genetic differences eventually lead to prezygotic and postzygotic isolation. It is
important to note that prezygotic isolation and postzygotic
isolation arise simply as a consequence of genetic
divergence.
If the populations come into secondary contact
(Figure 17.10c), several outcomes are possible. If limited
genetic differentiation has taken place during the separation of the populations, reproductive isolating mechanisms
may not have evolved or may be incomplete. In this case,
the populations will remain a single species. A second possible outcome is that genetic differentiation during separation leads to prezygotic reproductive isolating mechanisms;
in this case, the two populations are different species. A
third possible outcome is that, during their time apart,
some genetic differentiation took place between the populations, leading to postzygotic isolation. If postzygotic isolating mechanisms have evolved, any mating between
individuals from the different populations will produce

(a)

Population
An original
population…
Geographic
barrier
…is split into
two populations
by a geographic
barrier to
gene flow.

(b)

The populations
acquire genetic
differences over
time owing
to selection,
genetic drift,
and mutations,...

Genetic
differentiation

…which lead to
the evolution of
reproductive
isolating
mechanisms
(RIMs).
(c)

Secondary
contact

If the populations come
into contact again, RIMs
prevent gene flow
between them.

Selection for
prezygotic RIM
If postzygotic RIMs
have evolved, selection
will strengthen
prezygotic RIMs, leading
to different species.
Species
A

Species
B

17.10 Allopatric speciation is initiated by a geographic
barrier to gene flow between two populations.

hybrid offspring that are inviable or sterile. Individuals that
mate only with members of the same population will have
higher fitness than that of individuals that mate with
members of the other population; so natural selection will
increase the frequency of any trait that prevents interbreeding between members of the different populations. With
the passage of time, prezygotic reproductive isolating
mechanisms will evolve. In short, if some postzygotic
reproductive isolation exists, natural selection will favor
the evolution of prezygotic reproductive isolating mechanisms to prevent wasted reproduction by individuals mating with members of the other population.
An excellent example of allopatric speciation can be
found in Darwin’s finches, a group of birds on the Galápagos
Islands; these finches were discovered by Charles Darwin in
his voyage aboard the Beagle. The Galápagos are an archipel-

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Chapter 17

17.11 The Galápagos Islands are relatively
young geologically and are volcanic in
origin. The oldest islands are to the east.
[After Philosophical Transactions at the Royal
Society of London, Series B 351:756Ϫ772, 1996.]
South
America

Pinta

Galapagos
archipelago

Genovesa

Marchena
San Salvador

0

Bartolomé
Genovesa
Daphne Major
Seymour
Baltra

Rabida

Fernandina

Pinzon
Isabela
1

Santa
Cruz

Los
Hermanos

Plaza Sur

San Cristobal

Santa Fe

Tortuga
Champion
0

50

Gardner

Santa
Maria

km
91

Española
90

ago of islands located some 900 km off the coast of South
America (Figure 17.11). Consisting of more than a dozen
large islands and many smaller ones, the Galápagos formed
from volcanoes that erupted over a geological hot spot that
has remained stationary while the geological plate over it
moved eastward in the past 3 million years. Thus, the islands
to the east (San Cristóbal and Española) are older than those
to the west (Isabela and Fernandina). With the passage of
time, the number of islands in the archipelago increased as
new volcanoes arose.
Darwin’s finches consist of 14 species found on various
islands in the Galápagos archipelago (Figure 17.12). The
birds vary in the shape and sizes of their beaks, which are
adapted for eating different types of food items. Genetic
studies have demonstrated that all the birds are closely
related and evolved from a single ancestral species that
migrated to the islands from the coast of South America
some 2 million to 3 million years ago. The evolutionary relationships among the 14 species, based on studies of
microsatellite data, are depicted in the evolutionary tree
shown in Figure 17.12. Most of the species are separated by
a behavioral isolating mechanism (song in particular), but
some of the species can and occasionally do hybridize in
nature.

The first finches to arrive in the Galápagos probably
colonized one of the larger eastern islands. A breeding population became established and increased with time. At
some point, a few birds dispersed to other islands, where
they were effectively isolated from the original population,
and established a new population. This population underwent genetic differentiation owing to genetic drift and
adaptation to the local conditions of the island. It eventually
became reproductively isolated from the original population. Individual birds from the new population then dispersed to other islands and gave rise to additional species.
This process was repeated many times. Occasionally, newly
evolved birds dispersed to an island where another species
was already present, giving rise to secondary contact
between the species. Today, many of the islands have more
than one resident finch.
The age of the 14 species has been estimated with data
from mitochondrial DNA. Figure 17.13 shows that there is a
strong correspondence between the number of bird species
present at various times in the past and the number of
islands in the archipelago. This correspondence is one of the
most compelling pieces of evidence for the theory that the
different species of finches arose through allopatric
speciation.

Population and Evolutionary Genetics

20

Geospiza fuliginosa

Islands
Species of finches
15

Number

Geospiza fortis

10
As the number of
islands increases,…

Geospiza magnirostris

…the number of
species of finches
increases.

5
Geospiza scandens
0
Geospiza conirostris

5

4
3
2
1
Time before present in millions of years

0

17.13 The number of species of Darwin’s finches present
at various times in the past corresponds to the number of
islands in the Galápagos archipelago. [Data from P. R. Grant, B. R.

Geospiza difficilis

Grant, and J. C. Deutsch. Speciation and hybridization in island birds.
Philosophical Transactions of the Royal Society of London Series B
351:765Ϫ772, 1996.]

Concepts
Camarhynchus parvulus

Camarhynchus psittacula

Camarhynchus pauper

Allopatric speciation is initiated by a geographic barrier to gene
flow. A single population is split into two or more populations by
a geographic barrier. With the passage of time, the populations
evolve genetic differences that bring about reproductive isolation.
After postzygotic reproductive isolating mechanisms have
evolved, selection favors the evolution of prezygotic reproductive
isolating mechanisms.

✔ Concept Check 7
What role does genetic drift play in allopatric speciation?

Camarhynchus pallida

Platyspiza crassirostris

Certhidea fusca

Pinaroloxias inornata

Certhidea olivacea

17.12 Darwin’s finches consist of 14 species that
evolved from a single ancestral species that migrated to
the Galápagos Islands and underwent repeated allopatric
speciation. [After B. R. Grant and P. R. Grant. Bioscience 53:
965Ϫ975, 2003.]

Sympatric speciation Sympatric speciation arises in the
absence of any geographic barrier to gene flow; reproductive
isolating mechanisms evolve within a single interbreeding
population. Sympatric speciation has long been controversial within evolutionary biology. Ernst Mayr believed that
sympatric speciation was impossible, and he demonstrated
that many apparent cases of sympatric speciation could be
explained by allopatric speciation. More recently, however,
evidence that sympatric speciation can and has arisen under
special circumstances has acculumated. The difficulty with
sympatric speciation is that isolating mechanisms arise as a
consequence of genetic differentiation, which takes place only
if gene flow between groups is interrupted. But, without
reproductive isolation (or some external barrier), how can
gene flow be interrupted? How can genetic differentiation
arise within a single group that is freely exchanging genes?
Most models of sympatric speciation assume that
genetic differentiation is initiated by strong disruptive

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Chapter 17

apple and hawthorn host races of R. pomnella and some
degree of reproductive isolation has evolved between them,
reproductive isolation is not yet complete and speciation
has not fully taken place.

17.6 The Evolutionary History
of a Group of Organisms
Can Be Reconstructed
by Studying Changes
in Homologous
Characteristics
17.14 Host races of the apple maggot fly, Rhagoletis
pomenella, have evolved some reproductive isolation
without any geographic barrier to gene flow. [Tom Murray.]

selection taking place within a single population. One
example of how sympatric speciation might arise is seen in
apple maggot flies, Rhagoletis pomonella (Figure 17.14),
studied by Guy Bush. The flies of this species feed on the
fruits of a specific host tree. Mating takes place near the
fruits, and the flies lay their eggs on the ripened fruits,
where their larvae grow and develop. Rhagoletis pomnella
originally existed only on fruits of hawthorn trees, which
are native to North America; 150 years ago, R. pomnella was
first observed on cultivated apples, which are related to
hawthorns but a different species. Infestations of apples by
this new apple host race of R. pomnella quickly spread, and,
today, many apple trees throughout North America are
infested with the flies.
The apple host race of R. pomnella probably originated
when a few flies acquired a mutation that allowed them to
feed on apples instead of the hawthorn fruits. Because mating takes place on and near the fruits, flies that utilize
apples are more likely to mate with other flies utilizing
apples, leading to genetic isolation between flies using
hawthorns and those utilizing applies. Indeed, Bush found
that some genetic differentiation has already taken place
between the two host races. Flies lay their eggs on ripening
fruit, and there has been strong selection for the flies to
synchronize their reproduction with the period when their
host species has ripening fruit. Apples ripen several weeks
earlier than hawthorns. Correspondingly, the peak mating
period of the apple host races is 3 weeks earlier than that of
the hawthorn race. These differences in the timing of
reproduction between apple and hawthorn races have further reduced gene flow—to about 2%—between the two
host races and have led to significant genetic differentiation
between them. All of it has evolved in the past 150 years.
Although genetic differentiation has taken place between

The evolutionary relationships among a group of organisms
are termed a phylogeny. Because most evolution takes place
over long periods of time and is not amenable to direct
observation, biologists must reconstruct phylogenies by
inferring the evolutionary relationships among present-day
organisms. The discovery of fossils of ancestral organisms
can aid in the reconstruction of phylogenies, but the fossil
record is often too poor to be of much help. Thus, biologists
are often restricted to the analysis of characteristics in present-day organisms to determine their evolutionary relationships. In the past, phylogenetic relationships were
reconstructed on the basis of phenotypic characteristics—
often, anatomical traits. Today, molecular data, including
protein and DNA sequences, are frequently used to construct
phylogenetic trees.
Phylogenies are reconstructed by inferring changes that
have taken place in homologous characteristics. Such characteristics evolved from the same character in a common
ancestor. For example, the front leg of a mouse and the wing
of a bat are homologous structures, because both evolved
from the forelimb of an early mammal that was an ancestor
to both mouse and bat. Although these two anatomical features look different and have different functions, close examination of their structure and development reveals that they
are indeed homologous. And, because mouse and bat have
these homologous features and others in common, we know
that they are both mammals. Similarly, DNA sequences are
homologous if two present-day sequences evolved from a
single sequence found in an ancestor. For example, all
eukaryotic organisms have a gene for cytochrome c, an
enzyme that helps carry out oxidative respiration. This gene
is assumed to have arisen in a single organism in the distant
past and was then passed down to descendants of that early
ancestor. Today, all copies of the gene for cytochrome c are
homologous, because they all evolved from the same original copy in the distant ancestor of all organisms that possess
this gene.
A graphical representation of a phylogeny is called a
phylogenetic tree. As shown in Figure 17.15, a phylogenetic
tree depicts the evolutionary relationships among different
organisms, similarly to the way in which a pedigree repre-

Population and Evolutionary Genetics

17.15 A phylogenetic tree is a graphical

Terminal nodes represent the organisms
for which the phylogeny is constructed.

representation of the evolutionary relations
among a group of organisms.

Branches are the evolutionary connections
between organisms. The length of a branch
represents the amount of evolutionary
divergence.

Quagga

Internal nodes represent
the common ancestors that
existed before divergence.

Burchell
Zebras
Grevy

Mountain
This phylogenetic tree
is rooted, because this
node represents a
common ancestor of
all other organisms
in the tree.

Wild ass

Half ass (onager)

Horses

Domestic

8

6
4
2
Sequence divergence (%)

0
Przewalski

sents the genealogical relationships among family members.
A phylogenetic tree consists of nodes that represent the
different organisms being compared, which might be different individuals, populations, or species. Terminal nodes
(those at the end of the outermost branches of the tree) represent organisms for which data have been obtained, usually
present-day organisms. Internal nodes represent common
ancestors that existed before divergence between organisms
took place. In most cases, the internal nodes represent past
ancestors that are inferred from the analysis. The nodes are
connected by branches, which may represent the evolutionary connections between organisms. In many phylogenetic
trees, the lengths of the branches represent the amount of
evolutionary divergence that has taken place between organisms. When one internal node represents a common ancestor to all other nodes on the tree, the tree is said to be rooted.
Trees are often rooted by including in the analysis an organism that is distantly related to all the others; this distantly
related organism is referred to as an outgroup.

Concepts
A phylogeny represents the evolutionary relationships among a
group of organisms and is often depicted graphically by a phylogenetic tree, which consists of nodes representing the organisms
and branches representing their evolutionary connections.

The Construction
of Phylogenetic Trees
Consider a simple phylogeny that depicts the evolutionary
relationships among three organisms—humans, chimpanzees, and gorillas. Charles Darwin originally proposed
that chimpanzees and gorillas were closely related to
humans. However, subsequent study placed humans in the
family Hominidae and the great apes (chimpanzees, gorilla,
orangutan, and gibbon) in the family Pongidae. There are
three possible phylogenetic trees for human, chimpanzees,

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