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Speciation:
Introduction
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Troy E Wood Indiana University, Bloomington, Indiana, USA
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Loren H Rieseberg Indiana University, Bloomington, Indiana, USA
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Introductory
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doi:10.1038/npg.els.0001709
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Speciation is the
formation of two or more new species from one ancestral species.
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Introduction
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Speciation is defined as the formation of
two or more new species from one ancestral species. The process of
speciation is responsible for the vast amount of biological diversity found
today. Estimates of the number of extant (currently existing) species range
from 5 to 30 million. Despite the seemingly limitless number of examples of
speciation that nature has provided, we know little about the underlying
biological mechanisms of species formation. This lack of knowledge can be
attributed largely to the fact that speciation occurs too quickly to be
recorded in the fossil record and, with few exceptions, too slowly to be
observed in nature or in the laboratory. However, advances in investigative
techniques have allowed us, at least in part, to overcome these inherent
limitations and make inferences about the processes of species
formation. See also: Speciation and the fossil record;
Species and speciation: overview
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Are Species Real
Biological Entities?
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Grouping and naming organisms is a natural
and intuitive process. In fact, it is not hard to imagine that the ability
to distinguish between biological forms was essential to the survival of
our ancestors. For example, being able to tell apart poisonous from
nutritious plants was obviously a critical skill of our forebears. Thus,
our inherent desire to partition and name organisms probably has its roots
in a need for formal classification of the natural world around us.
However, some biologists and philosophers argue that the grouping of
organisms into species is an artificial and subjective enterprise. These
scientists would argue that ‘species’ do not represent discrete groups of
organisms, but rather points along a continuum of diversity of biological
form. In practice, most evolutionary biologists agree that the variation
among organisms is discontinuous; that is, species are not simply
constructs of the human mind, but rather they represent discrete biological
units. For example, consider domestic cats (Felis domestica) and
mountain lions (Felis concolor). Although there is a great deal of
variation within domestic cats and within mountain lions, it is unlikely
that anyone would misidentify a house cat as a cougar. The variation in
house cats does not overlap with the variation in mountain lions. These two
species are easily distinguished because they are not connected by
intermediate forms. See also: History of taxonomy; Philosophy of biological classification
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Species Concepts
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As the example above illustrates, certain
related species can be readily separated, but in many cases the line
between species pairs is blurry. Thus, a precise, operational definition of
species is crucial. Of course, the study of speciation is linked
intrinsically to how we define species, as these are the entities produced
by speciation. Moreover, different definitions of species will lead to
different conclusions about pattern and process in species formation.
Evolutionary biologists have proposed a diverse, almost innumerable list of
species concepts, and the different views expressed in these various
formulations have fuelled an interminable debate. This ongoing debate
emphasizes the fact that species concepts are critical to the study of
evolution and that as yet no one concept is completely satisfactory. Here,
we will focus on the three most instructive concepts and examine the
implications each holds for the study of speciation. See also: Species concepts
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Phylogenetic species concept
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A phylogenetic species is a group of
organisms which share unique morphological or genetic features that make
them diagnosably distinct from other groups. These shared features are
inferred to be the result of common ancestry. This concept has its roots in
the branch of evolutionary biology that focuses on reconstructing the evolutionary
history of organisms. The endpoint of work in this discipline is the
phylogenetic tree, a diagram depicting the hypothetical evolutionary
relationships among related organisms. At the termini of the branches are
phylogenetic species, which cannot be divided further into distinct forms.
In this definition, emphasis is placed on the evolutionary history, or
parental pattern of ancestry, and not on an ability to interbreed with
other groups of organisms. See also: Molecular phylogeny reconstruction
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Recognition species concept
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According to this concept, a species is
the most inclusive population of individual biparental organisms that share
a common fertilization system. By focusing on mate recognition, this
concept suggests that the forces that produce species are associated with
the evolution of mating systems. Consequently, this definition is
especially helpful in understanding patterns of diversification in animal
taxa that have diverged via sexual selection. More generally, this
concept emphasizes characters that are actually subject to natural
selection in a population undergoing speciation rather than those that are
simply a byproduct of divergence.
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Biological species concept
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Biological species are groups of actually
or potentially interbreeding natural populations that are reproductively
isolated from other such groups. This concept was developed to explain the
absence of intermediates between closely related, yet discontinuous species
that co-occur in nature. If two species are unable to interbreed, then the
lack of intermediates is no longer puzzling. One of the most useful
features of this definition is that it suggests a diagnostic test to
distinguish between true species and variants within a species. If two
populations cannot interbreed to form viable and fertile progeny, then they
represent two distinct species. One weakness of this definition is that
many populations never interbreed in nature because they occupy different
geographical ranges, hence the ad hoc ‘potentially’. A more serious
problem is the frequent occurrence of morphologically and ecologically
distinct species that do interbreed in areas of contact. Furthermore, like
the recognition concept, this definition is not applicable to organisms
that reproduce asexually.
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If we accept the premise of the biological
species concept, that species are delimited in terms of reproductive
isolation, we can then view speciation as the evolution of mechanisms that
prevent gene flow between divergent populations. Thus, the biological
species concept provides a logical framework for the study of speciation,
and consequently, most speciation theory is based on this concept.
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Isolating
Mechanisms
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Because species are defined as biological
entities that are reproductively isolated, it is important to focus on the
evolution of barriers to the exchange of genes. Barriers that are based on
biological attributes of species are called isolating mechanisms. Isolating
mechanisms result from the evolution of morphological, behavioural or
chromosomal differences between closely related species. Isolating
mechanisms can be divided into premating and postmating barriers, depending
on when they act to prevent gene flow (Table 1). Most speciation
events are thought to result from a combination of two or more types of
barriers, with physical barriers (geological or climatic changes that
separate formerly contiguous populations) setting the stage for the evolution
of isolating mechanisms. See also: Isolating mechanisms
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Premating barriers
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Temporal isolation
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Two closely related species that mate at
different times or during different seasons are said to be temporally
isolated. Often, temporal isolation prevents gene flow between populations
that are fully interfertile. Pink salmon, Oncorhynchus gorbuscha,
migrate from salt water to fresh water to spawn and represent an
interesting example of how temporal isolation may result in speciation.
Because of the timing of reproductive maturation, pink salmon spawn in
two-year cycles. Those individuals that result from a spawn in an even year
return to spawn two years later, in an even year, while individuals that
are produced in odd years return to spawn in an odd year. ‘Odd-year’ and
‘even-year’ salmon are thus genetically isolated in time. While odd-year
and even-year pink salmon are still considered the same species, recent
studies suggest that the two types are diverging. Forced crosses between
odd- and even-year individuals yield F1s and F2s
with lowered fitness. Temporal isolation is also an effective barrier to
gene flow between two species of pine, Pinus muricata and P.
radiata, which are highly interfertile, but do not produce hybrids in
nature because the anthers and stigmas mature at different times. See also: Fitness
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Ecological isolation
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Ecological isolation, also called habitat
or resource isolation, can occur when two species are adapted to different
niches. Two species that utilize different resources or occupy different
habitats are less likely to encounter each other and, consequently,
opportunities for interspecific matings are rare. One of the most
convincing examples of ecological isolation at work in speciation involves
the threespine stickleback, Gasterosteus aculeatus. Different
forms of this species are adapted to feeding at different lake depths; one
form feeds in open water and the other is a bottom feeder. Though the
species status of these forms is unclear, they are morphologically distinct
and females prefer to mate with males of the same morphology.
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Behavioural isolation
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Patterns of behaviour during courtship and
mating are important in mate recognition systems. If two closely related
species have distinct courtship and mating rituals they will not recognize
each other as potential mates or mating attempts generally will not result
in successful copulations. There is evidence that behavioural isolation has
played an important role in the diversification of Hawaiian crickets.
Species of this group almost always have different calling songs and
females of many species can identify conspecific mates, even when
heterospecific songs are only slightly different. Although behavioural
isolation is thought to be more common in animals than plants, the latter
can be isolated by the behaviour of their pollinators. In some cases,
pollinators display remarkable flower constancy – they preferentially visit
one species of flower based on its floral morphology, colour or scent.
Because the pollinators move only between flowers of the same species, no
gene flow between different species occurs.
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Mechanical isolation
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Certain species are reproductively
isolated because of morphological differences that prevent successful
interbreeding. Mechanical isolation is common in plant species that rely on
animal pollinators to transport gametes between individuals. For example,
two species in the snapdragon family, Pedicularis groenlandica and P.
attollens, are both pollinated by bumblebees. Owing to structural
differences between the flowers of these two species, the plants deposit
and remove pollen from different parts of the bee. Consequently, little or
no pollen is exchanged between the two species.
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Postmating barriers
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Gametic incompatibility
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Experimental crosses between related
species frequently reveal that mating systems are incompatible due to
interactions between egg and sperm or interactions between the female
reproductive tract and sperm. For example, heterospecific sperm (or
pollen in plants) often suffer a competitive disadvantage. In
plants, heterospecific pollen is sometimes unable to germinate on
the stigmas of different species and when it does it may be unable to
fertilize ovules. In insects, studies of females serially mated to males of
the same and different species show that intraspecific sperm is more likely
to fertilize eggs, regardless of mating order. In some fruitfly species,
interspecific crosses initiate an immunological response by females that
targets and kills sperm of other species. See also: Sperm–egg interactions: sperm–egg binding
in invertebrates; Sperm–egg interactions: sperm–egg binding
in mammals
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Hybrid inviability
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Progeny of crosses between species
frequently show greatly reduced vigour. Because these progeny are unlikely
to reproduce, gene flow between the parental species is limited.
Developmental abnormalities that prevent gestation or seed production are
also a frequent outcome of interspecific crosses. See also: Hybrid speciation
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Hybrid sterility
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Sterility barriers are important in
maintaining the genetic integrity of very closely related species that have
not yet evolved other effective isolating mechanisms. Breeding between
recently diverged taxa can result in viable offspring that reach
reproductive maturity. However, these offspring are commonly sterile due to
chromosomal or genic differences between the species that disrupt gamete
formation. A classic example of this is seen in crosses between female
horses and male donkeys. Mules that result from these crosses are
completely sterile.
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Geography of
Speciation
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Allopatric and peripatric
speciation
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Populations of the same or related species
that occupy nonoverlapping ranges are referred to as allopatric
populations. Because allopatric populations are separated by physical
barriers (e.g. mountains, water, large expanses of unoccupied land, etc.),
they evolve independently, free of the homogenizing effects of gene flow.
Under these conditions, populations diverge because they are subject to
different selection pressures and undergo unique changes due to genetic
drift. Barriers to gene flow can arise easily as a byproduct of this
divergence, which would be unlikely to occur if the populations were in
contact. See also: Speciation: allopatric; Drift: introduction; Drift: theoretical aspects
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Two major types of allopatric speciation
are recognized by evolutionary biologists. The first type, called vicariant
speciation, is based on geological changes that split formerly connected
populations. For example, the uplift that created the spine of mountains
that runs down the centre of the Baja peninsula also resulted in the
formation of the Gulf of California. Since at least
the Late Pliocene this sea has isolated populations of many species that
now exist on both the peninsula and mainland Mexico. Some of these
populations have differentiated enough to be distinguished as separate
species. In this example, the same geological processes that split
populations also resulted in ecological and climatic shifts that created
new selection regimes. The combination of these processes has produced a
number of intriguing organisms adapted to environmental conditions unique
to Baja California.
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The second model of allopatric divergence,
peripatric speciation, involves the colonization of habitats that lie along
the periphery of a species range. This model is based on the idea that
small, founding populations may be genetically distinct because they
represent only a small, random sample of the larger source population. The
model predicts that subsequent evolutionary changes within these small
colonies will result in rapid differentiation or ‘genetic revolutions’ as
selection establishes novel, adaptive gene complexes. Reproductive
isolation could result as a secondary effect of these changes. The
geographical pattern of morphological differentiation within the New Guinea kingfisher, Tanysiptera
galeata, is consistent with the predictions of the model. Kingfishers
that live on coastal islands are strikingly different from mainland
kingfishers, and the variation of plumage phenotypes across these small
islands is much greater than the variation seen within the much larger
mainland population. However, there is little direct evidence that small
population size has played a critical role in speciation in this or other
examples. See also: Darwin′s finches
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Sympatric speciation
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Sympatric speciation occurs when
populations become reproductively isolated in spite of significant amounts
of gene flow during the initial stages of divergence. While allopatric
speciation inherently allows for differentiation without the constant
production of intermediate forms, models of sympatric speciation must meet
this challenge head on. These models rely on strong disruptive selection to
drive divergence in the presence of gene flow. For example, consider a
population that occupies a habitat in which two different resources are
distributed in discrete patches. If individuals begin to specialize on one
of the two resource types, two distinct forms, which are adapted to utilize
the two different resource types, may evolve. In situations where
disruptive selection is strong enough to eliminate hybrids between the two
morphs, the evolution of reproductive isolation may be possible. Many
examples of sympatric speciation through ecological isolation have been
reported. For example, many closely related insect species occupy the same
range but feed and reproduce on different host plants. In most cases, it is
hard to prove that speciation did in fact occur while the incipient species
were in sympatry. Purported examples of sympatric speciation can be
explained by the alternative view that reproductive isolation evolved in
allopatry, and the distribution patterns we see today came about through
secondary contact. Some theoretical issues related to sympatric speciation
are discussed below in the context of reinforcement. See also: Speciation: sympatric and parapatric
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Genetic Basis of
Speciation
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If speciation is equated with the
evolution of reproductive isolation, the goal of speciation genetics then
is to reconstruct the sequence of genetic changes necessary for the
development of isolating mechanisms.
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Postmating barriers
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Much of the theoretical work on speciation
genetics is founded on the standard model, which was formulated soon after
the synthesis of the theories of natural selection and genetics. This model
is based on the evolution of genetic divergence during allopatry. If two
populations are separated in space, natural selection causes the
populations to evolve along different paths as they adapt to their
different environments. Because these populations are isolated and
experience different selective regimes, favourable mutations that arise and
become fixed are very likely to be different. When the populations come
back into contact, the mutated genes may interact unfavourably, thereby
causing reproductive isolation. The reason that this model is so powerful
is that the mutated genes that contribute to isolation actually confer a
fitness advantage to individuals within the population in which they arise.
Only when they interact with mutated genes that arose in another isolated
population do they cause hybrid incompatibilities. Thus, according to this
model, the genetic changes that cause reproductive isolation are a side
effect of natural selection. This model has recently been confirmed in a
pair of fruitfly species. In this example, hybrid male sterility appears to
be a direct result of strong positive selection on an important developmental
gene.
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Premating barriers
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Unlike postmating barriers, which appear
to be a side effect of adaptive divergence, characters affecting premating
isolation often are selected on directly. Thus, the genetics of premating
barriers can be viewed as being synonymous with the genetics of adaptation.
In the classical view, very small mutations are thought to drive the
evolution of adaptations because mutations of large effect are unlikely to
be favourable. However, others have argued, based on results from classical
crossing studies, that mutations with large
morphological effects often contribute to adaptive evolution. This latter
viewpoint has received increased support recently because of the
recognition that larger mutations, while less likely to be favourable, are
more likely to be fixed in the population when advantageous and because
mutations with large morphological effects are often detected in genetic
mapping studies. For example, genetic analyses of floral isolation between
two species of monkeyflowers indicate that nine of the twelve floral
differences studied involve major mutations. See also: Adaptation: genetics
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Reinforcement
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Reinforcement is one of the most
interesting and intensely debated ideas in speciation theory. Reinforcement
occurs when premating barriers evolve in sympatry to prevent the production
of hybrid offspring. For example, if two incipient species, A and B, come
into secondary contact after a period of allopatry, genetic differences
that arose during separation may cause hybrid offspring between A and B to
have reduced fitness. If a mutation arises that
increases within-species mating, individuals that carry this mutation will
have increased fitness relative to individuals that mate indiscriminately.
Thus, natural selection operates to isolate the incipient species by
increasing assortative, or within-species, mating. The term ‘reinforcement’
is based on the idea that this process ‘reinforces’ reproductive barriers
that evolved in allopatry, and reinforcement is often viewed as a mechanism
that serves to complete the process of speciation. See also: Reinforcement
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Reinforcement is a popular concept because
it describes a scenario in which natural selection plays a direct role in
speciation. Some evolutionary biologists would argue that the isolating
mechanisms discussed above are not really ‘mechanisms’ of speciation,
because they have not evolved directly to prevent hybridization between
divergent populations. Rather they are thought to be a byproduct of
allopatric divergence. In contrast, reinforcement is a mechanism of
speciation because it evolves directly to prevent gene flow between
divergent populations.
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Though recent theoretical and empirical
work suggests that reinforcement is plausible, unequivocal evidence of
reinforcement is lacking. The scarcity of natural examples is not
surprising considering the following theoretical constraints on the
operation of reinforcement. First, a gene that increases mate
discrimination in zones of contact may not be favoured in allopatric
populations that serve as a source of migrants. Thus, gene flow between the
zone of contact and allopatric populations will dampen the effects of
selection on the gene that increases mate discrimination. Second, the
intensity of selection will decrease as crossmating decreases because
inviable hybrids become increasingly rare. Hence, reinforcement is unlikely
to result in complete reproductive isolation. Finally, for reinforcement to
work effectively, it must generate an association between the genes that
cause reduced hybrid fitness and the gene for mate-discrimination. Because
reduced fitness in hybrids is usually based on the effects of many genes,
this association is unlikely to be formed. Even if reduced hybrid fitness
is caused by only one gene, recombination between this gene and the
mate-discrimination gene will limit the effects of natural selection. In fact,
selection to increase hybrid fitness may operate more effectively than
selection against crossmating.
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The majority of evidence for reinforcement
comes from studies that compare levels of premating isolation between
sympatric and allopatric populations of divergent species or subspecies.
These studies test the hypothesis that premating isolation should be more
pronounced in areas where the ranges of related species overlap. Many
studies have revealed this pattern. However, this type of comparison does
not provide direct evidence for reinforcement. In most of these studies,
the species pairs examined are not able to hybridize. In these cases,
increased premating isolation in sympatry may simply be a result of
selection to avoid wasting energy on incompatible mating, and not on
selection to avoid producing unfit hybrid offspring. In cases where species
pairs cannot hybridize, increased premating isolation in sympatry is called
reproductive character displacement. The difference between reproductive character
displacement and reinforcement is subtle, but the distinction is important.
Reproductive character displacement, because it operates between species,
is not a mechanism of speciation. Reinforcement, which operates within
species, selects for a reduction in gene flow, and therefore is a direct
mechanism of speciation. Demonstration of reinforcement requires evidence
that gene flow occurs between the populations under comparison, and that in
spite of this gene flow, selection is operating to reduce interbreeding.
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One recent study meets all of the
stringent requirements imposed by opponents of the reinforcement
hypothesis. The pied (Ficedula hypoleuca) and collared (F.
albicollis) flycatchers are primarily distributed allopatrically, but
come into contact in parts of Central and Eastern Europe. In allopatry,
males of the two species have very similar black and white markings, but in
sympatric zones, males of the pied flycatcher are brown and collared males
have more extreme white markings. Researchers demonstrated that this
phenotypic difference plays a role in female mate choice. Females from
sympatric areas show a strong preference for males of their own species,
with pied females even preferring the brown males over conspecific black
and white males. Less than 3% of matings are heterospecific, and the hybrid
progeny of these matings are one-third as fit as pure progeny. Therefore,
females that choose conspecific mates accrue a significant reproductive
advantage. The conclusion that selection for assortative mating is
responsible for the phenotypic differences of sympatric and allopatric
males is firmly supported by the data. This study, coupled with recent
theoretical work, has reinvigorated enthusiasm for the reinforcement
hypothesis.
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Speciation via
Hybridization
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Often geological or climatic causes of
isolation are impermanent. When two populations come back into contact
after a period of separation, the evolution of barriers to gene flow may be
incomplete. In these cases, hybridization between the two populations will
occur. This mixing of genomes can result in the following four outcomes: (1)
the populations may merge and evolve as a single species; (2) reinforcement
may complete the process of speciation initiated in allopatry; (3) the two
populations may retain their genetic identity, with hybrids restricted to
zones of contact; (4) new species can arise from the recombination of the
differentiated genomes (Figure 1). Natural examples of the third scenario are
often a focus of study, because they provide windows on the processes of
speciation. In this section, we will discuss the fourth outcome: speciation
via hybridization. See also: Hybrid zones
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Figure 1
Possible outcomes of contact between populations that were previously
isolated by a physical barrier. Broad red arrows represent evolutionary
lineages (species). Narrow horizontal arrows indicate gene flow between
otherwise independently evolving lineages. The colouration of the region
between diverging lineages depicts the evolution of intrinsic isolating
barriers (white, no intrinsic barri ...
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In some cases, crosses between divergent
genomes can result in instant, or ‘abrupt’
speciation. If two divergent populations come into secondary contact,
hybrids may be sterile due to chromosomal factors that disrupt meiosis. One
mechanism of avoiding such sterility is polyploidy. Polyploid individuals
result from genome duplication and often are of hybrid origin. Because
polyploids have two identical copies of each chromosome, pairing during
meiosis is not affected. However, backcrosses to either parent yield
sterile, triploid offspring. Thus polyploid lineages that originate after
hybridization are instantly isolated from their parental species and will
evolve as distinct entities. Polyploidy, which is often associated with
extreme habitats, is very common in ferns and their allies, and in
flowering plants, suggesting that hybridization has played an important role
in the diversification of vascular plants. The fern genus Ophioglossum
provides an extreme yet interesting example. Members of this genus have
chromosome counts ranging from 120 to 630. These elevated chromosome counts
are apparently a result of polyploid speciation through hybridization.
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Hybridization can also lead to new species
that retain the chromosome number of their progenitors. This mode, referred
to as homoploid hybrid speciation or recombinational speciation, is
believed to occur less frequently than polyploid speciation, because the
possibility of backcrosses makes the establishment of reproductive
isolation less likely. Like polyploid species, homoploid hybrid derivatives
usually possess extreme phenotypes and are adapted to extreme habitats.
While the genetic basis of these extreme phenotypes is not fully
understood, experimental evidence suggests that homoploid hybrid species
combine favourable genes from each parental species to form novel, adaptive
genotypes. If these novel genotypes become associated with a distinct
mating type, or if individuals with these genotypes become separated
spatially or ecologically, the homoploid derivative can establish itself as
a distinct species. Though this mode of speciation is believed to be rare, putative
examples exist for all major animal and plant groups.
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Because hybrid speciation can occur
rapidly, investigators have been able to study this type of speciation in
the laboratory. Forced crosses between Gilia malior and G.
modocensis yielded highly sterile, homoploid hybrids. This sterility
was due to chromosomal differences that disrupt meiosis. Hybrid sterility
was ameliorated by artificial selection and continued crossing of the most
fertile hybrids. After nine generations, hybrid lines recovered full
fertility and were reproductively isolated from the parental species. The
hybrid plants were morphologically and chromosomally distinct from the
parent species. In a more recent study that employed modern molecular
techniques to identify chromosomal blocks, researchers were able to compare
the genomic composition of synthetic hybrids of two sunflower species, Helianthus
annuus and H. petiolaris, to that of H. anomalus, the
supposed natural hybrid derivative of the two species. The molecular
data indicated that the genomes of the synthetic and natural hybrids were
highly similar, suggesting that hybrid speciation is a deterministic
process. In addition, this study reveals that hybridization can result in
rapid genomic change. See also: Hybrid speciation
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Summary
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Progress in speciation research has been
constrained by debates on how to define species. If we assume that
reproductive isolation is the most important aspect of species boundaries,
we can avoid the semantic quagmire created by definitional debates.
Students of speciation who have adopted the biological species concept have
made considerable progress in elucidating the evolutionary mechanisms of
species formation. Reproductive isolation has been shown to result from a
combination of isolating mechanisms that act in concert to preserve the
genetic integrity of species. Theoretical models of speciation indicate
that isolating mechanisms arise while populations are geographically
isolated, although sometimes populations come back into contact before
isolation is complete. This can lead to the merger of species through
hybridization, the origin of new hybrid derivatives, or the completion of
speciation via the reinforcement of preexisting isolating mechanisms.
Analyses of the genetic basis of these isolating mechanisms suggest that
postmating isolating barriers typically arise as a byproduct of adaptive
divergence, whereas premating barriers are more likely to be selected on
directly. Further study of the genetic basis of isolation will allow us to
estimate the relative frequency and tempo of the different modes of species
formation. See also: Ring species and speciation
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Originally published: July 1999
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Further Reading
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Avise JC and
Wollenberg K (1997) Phylogenetics and the origin of species. Proceedings
of the National Academy of Sciences of the
USA 94:
7748–7755.
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Bush GL (1975)
Modes of animal speciation. Annual Review of Ecology and Systematics
6: 339–364.
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Coyne JA and Orr HA
(1998) The evolutionary genetics of speciation. Philosophical
Transactions of the Royal Society B 353: 287–305.
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Dobzhansky T (1937)
Genetics and the Origin of Species. New York: Columbia University Press.
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