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Speciation
M.Tevfik
Dorak, M.D., Ph.D.
Species: A population of organisms interbreeding only with
each other. Subspecies are genetically diverged groups of a species. In taxonomy,
the term race is used interchangeably with subspecies. Species ranks below
family and genus. In practice, taxonomists identify species on the basis of
morphological characteristics with the help of more-detailed genetic and
chromosomal analyses if necessary.
Different species concepts
Biological species concept (Dobzhansky,
1937; Mayr, 1940): Species are groups of actually or
potentially interbreeding natural populations which are reproductively isolated
from other such groups. Speciation is thus seen in terms of the evolution of
isolating mechanisms and is said to be complete when reproductive barriers are
sufficient to prevent gene flow between the two new species. The problem is
that capacity to interbreed cannot always be tested nor can the potential for
interbreeding. For asexually reproducing organisms and fossils, this concept
does not apply.
Recognition species concept (Paterson, 1985): A species is
defined as an inclusive population of individual biparental
organisms which share a common fertilization system. Speciation thus occurs
when a different fertilization system evolves. This definition can sometimes be
applied to fossil species. In a single habitat in USA, up to 40 cricket species
live together. Each one of them, however, has a different song sung by the male
and recognized by the female which altogether make up the mate recognition
system. The crucial event in the formation of a new species is the evolution of
a new mate recognition system. The advantage of this concept is that mate
recognition systems can be observed directly whereas interbreeding (the
biological species concept) may have to be inferred indirectly.
Biological
and recognition species concepts only apply to species that reproduce sexually
and neither allows for the existence of hybrids between species.
Evolutionary species concept (Simpson, 1951):
A species consists of all individuals that share a common evolutionary history.
It is not always clear what constitutes a common evolutionary history, and chronospecies (species occurred with gradual evolution
along the same line) are part of the same lineage but still different species.
The predecessor of a chronospecies is said to have
gone pseudo extinction. It is applicable to both living and fossil species and
also to both sexual and asexual species. It fails to say anything about how
speciation occurs.
Description of species
Allopatric: If they occupy
different ranges
Sympatric:
If they coexist in the same habitat
Parapatric: If their ranges
are adjacent and if they have a zone of contact
Speciation
The
formation of new species may involve transformation of one species into another
(anagenesis) or splitting up of one species into a
number of others (cladogenesis). Speciation is
generally described as multiplication of species by the division of one species
into two or more separate species, thus leaves out the anagenetic
speciation. Speciation is the direct result of changes in the gene pool.
Isolation (with subsequent reduction in gene flow) and disruptive or diverging
selection result in speciation. The common factor in all mechanisms of
speciation is a reduction in gene flow between two populations. This starts the
divergence and speciation eventually occurs. Speciation is not necessarily
adaptive. Gene flow in allopatric models is reduced
by geographical separation, whereas gene flow in sympatric or parapatric models is reduced by other means.
The
relative importance of adaptation in speciation is controversial. Chance,
however, plays some part in speciation as in emergence of allopatry.
When divergences (eventually leading to speciation) between populations are not
adaptive, they are attributed to genetic drift (some Hawaiian Drosophila
species and the North American flowering plant Clark lingual).
Allopatric speciation: This
is the most common pattern and it takes place when populations become
geographically separated. Progressive divergence as a result of physical
separation leads to speciation. The debate is whether adaptation or chance
(genetic drift) plays a major role in divergence in allopatry
(in small population it is more likely to be drift).
Speciation of Drosophila in Hawaii following volcanic eruptions is an example.
When two species get together after a period of allopatric
divergence (secondary contact): (1) they have already speciated
and cannot interbreed; (2) their hybrids have lowered fitness and natural
selection rapidly acts to develop reproductive isolation mechanisms; (3) they
interbreed successfully and mix again as single species. If one of the split
populations is very small a major role will be played by the founder population
in the development of divergence (drift will be fast, inbreeding will lead to
increased homozygosity, ecological shift will occur rapidly, intraspecific competition will be low and the population
will have a flush phase). In such small population faster
divergence results in a rapid budding kind of speciation (peripatric
speciation or founder effect speciation). However, in small
populations likelihood of extinction is very high before any allele is fixed
and many alleles may be lost as a result of inbreeding.
Sympatric speciation: Results from disruptive selection for
alternative adaptive models (disruptive selection= selection for the extreme
phenotypes instead of the intermediate ones). Changes in host, food or habitat
preference, resource partitioning may start sympatric speciation. Habitat
isolation within a sympatric species would stop interbreeding and may lead to
speciation. It is interesting that closely related sympatric insect species
usually use different host plants while closely related allopatric
species use identical or similar plants. A common example of sympatric
speciation from the same species would involve their colonizing different trees
to lay their eggs. The European mosquito Anopheles group consists of six
morphologically indistinguishable species. They are isolated reproductively as
they breed in different habitats. Some breed in brackish water, some in running
fresh water and some in stagnant fresh water. Therefore, they never meet to
breed. If this happens for subpopulations of a species, speciation may follow.
In
North American species of lacewings (genus Chrysopa),
speciation may have been initiated by disruptive selection on genetic variation
in color, favoring homozygotes in their respective habitats, where they are
protected against predators. The intermediate ones do not have a cryptic color
and are eliminated quickly (in fact, C.downesi and C.carnea have different breeding times). In this example of
sympatric model, speciation is initiated by disruptive selection operating on a
single freely interbreeding population which is followed by physical separation
(only after the genetic divergence has already begun). In allopatric
model, geographical separation always occurs before genetic divergence (and
initiates it).
Another
sympatric model in which speciation does not require habitat isolation is
competitive speciation which results from slight differences in resource
utilization. This intraspecific competition can lead
to the establishment of a stable polymorphism in sympatry
even in a homogeneous environment. The end result is the division of a single
gene pool into two or more adaptive types.
Parapatric speciation: In
this model, geographical separation is not complete and two diverging
populations share a boundary with no barrier to dispersal across it. This is
also a result of disruptive selection. An ancestral species spreads over a
spatially variable area and this leads to geographical separation by primary
contact area. Geographical differentiation leads to the formation of a cline,
which acts as a barrier to gene flow so that further divergence can take place
in the populations on each side of the cline. Hybrids between two parapatric populations are less fit and assortative
mating is favored by natural selection. The reduction of gene flow through a
hybrid zone will depend on the dispersal distances, selection against hybrids
and selection against pure types either in the hybrid zone or on the wrong side
of the hybrid zone. Indeed, in nature, when a species covers a large
geographical area, individuals at the extreme ends of its distribution can be
very different.
Reproductive isolation
After
subsequent sympatry (secondary contact), initially
slight differences in mate recognition traits are exaggerated by selection in
favor of pre-zygotic isolation -through assortative
mating- (reinforcement theory). Pre-zygotic isolating mechanisms due to mate
recognition may evolve during allopatry. This is
called recognition in allopatry hypothesis by Maynard
Smith. For sympatric species, pre-zygotic isolation -through natural selection-
evolves more rapidly between species who produce unfit
hybrids.
Isolating
mechanisms prevent gene flow between sympatric species. These may be
pre-zygotic preventing the formation of hybrids or post-zygotic preventing the
reproduction of hybrids. Pre-zygotic isolating mechanisms are the result of
natural selection favoring isolation to prevent the waste of reproductive
effort. Genetic drift may also play a role in the development of reproductive
isolation (see below).
Some
pre-zygotic (post-mating) isolating mechanisms:
1.
Allopatric separation
2.
Ecological or habitat isolation (sympatric) (Anopheles group)
3.
Different flowering, pollination (in plants) or mating season (temporal
isolation) (Pinus radiata
and P. muricata are sympatric but shed their pollens
at different times. Hybrids are rare and less vigorous. The American toads Bufo americanus and B. fowleri have different breeding times and do not mate.
Their habitat preferences are different too.
4.
Ethological (or sexual) isolation: The sexual attraction between males and
females is reduced or absent. Differences in courtship patterns in Drosophila
species in Hawaii is an example. Sexual selection and assortative
mating result in ethological isolation. In general, when strong, positive assortative mating maintains the polymorphism, the
different types may become genetically distinct, and these may eventually
become true species.
Slight
changes caused by genetic drift could lead to rapid divergence in reproductive
morphology and sexual behavior between two populations as a result of runaway
selection. Changes in only a few genes controlling a male sexual trait and
female preference for that trait could lead to pre-mating reproductive
isolation between two populations and hence to speciation without any adaptive
value.
5.
Isolation by different pollinators: The two species attract different kinds of
insects etc. as pollinators and their gametes never get together.
6.
Mechanical isolation: The reproductive organs of the sexes are not anatomically
identical and this impedes reproduction. Lack of pollen tube growth down style
of a different plant species is an example.
7.
Gametic isolation: Gamete transfer takes place but fertilization does not
occur. Many species of Drosophila show an insemination reaction as a result of
which sperm is killed in the vagina.
8.
A genetic change in some members of the population (like chromosomal
reorganizations such as polyploidy)
Some
post-zygotic (post-mating) isolating mechanisms:
1.
F1 hybrids inviable: Fertilization occurs but
embryonic development does not. In crosses between sheep and goats, the embryos
die early in their development.
2.
F1 hybrids infertile: The hybrids occur but do not produce functioning gametes.
The classic example is the cross between female horse and male ass, the mule.
3.
Hybrid breakdown: F1 hybrids are viable and fertile, but F2, backcross or
later-generation hybrids are inviable or infertile.
Reproductive
character displacement is a post-mating mechanism imposed by natural selection
to prevent hybrid formation when they are inviable or
infertile. It involves increased differentiation between the reproductive
systems of two species living in sympatry. This
phenomenon has nothing to do with speciation because it involves already
differentiated species.
Endemic species
Speciation
on isolated islands is an example of allopatric speciation.
These areas provide a unique combination of empty habitats, novel environments,
geographical isolation and sometimes lack of predators. In these new habitats,
selection pressures will be strong and invading groups will evolve quickly,
producing new species as a by-product. The ancestors of the faunas and floras
of oceanic islands are believed to have arrived by long-distance dispersal
mechanisms. Once the organisms have arrived in these isolated places, in the
absence of competitors, they would diversify and speciate
as they occupy specialized niches. The only constraint would be intraspecific competition. This kind of rapid speciation is
driven by natural selection rather than genetic drift and is called adaptive
radiation. In these small founder populations, genetic drift would also play a
role at neutral loci in addition to strong selective forces acting on other
loci.
1.
Darwin’s Finches of Galapagos Islands: Of the 14 species of finches
living on the islands today, 13 are endemic. Finches are poor fliers and
colonization from the mainland could not have been frequent. On the other hand
the marine birds are strong fliers and only two of the 13 marine birds are
endemic. The differences in beak shape and size of finches correlate with their
feeding habits.
2.
Cichlid fishes of lake Victoria: There are 170 species
of the cichlid genus Haplochromis alone in Lake
Victoria believed to have originated from a single ancestral species or a group
of species. None but one lives elsewhere. The difference in
dental patterns correlate with their feeding habits. Reproductive
isolation between species is very marked, and male coloration may be a major
factor in species recognition during mating. It is believed that they diverged
each time water levels fell in the lake isolating peripheral population from
the main body of the lake. Sexual selection seems to have involved in the
speciation process.
3.
Hawaiian Drosophilids: More than a quarter of the
known species of the family Drosophila live in the Hawaii archipelago. Out of
probably more than 800 species in Hawaii, 95% are endemic. All of the endemic
species may be descendants of a single gravid female arrived at the oldest
island Kauai about six million years ago. The habitat on
these volcanic islands is very patchy and have been repeatedly split up
by lava flows. It is clear that founder events have occurred frequently in
these patches leading to new species. Despite the huge morphological diversity,
there is little genetic diversity among the endemic species. It appears that
courtship behavior has driven speciation in Hawaiian islands.
Even closely related sympatric species have differences in their reproductive
behavior. This is an example of sexual selection causing speciation probably
due to the effects of genetic drift in small founder population.
(Hawaiian
Honey Birds)
4.
Silversword alliance: In Hawaii, 95% of the native
plant species are endemic.
5.
Tristania (endemic snails of the Tristan de Cunha
archipelago): All six species of snails from the genus Tristania
are different from the rest of the snails on other Atlantic islands.
6.
Lemurs in Madagascar and Comoro islands.
Suggested reading
Coyne JA: Genetics and Speciation (1992)
Coyne JA & Orr HA: The Evolutionary Genetics of Speciation (1998)
Vias
S: The Ecological Genetics of Speciation (2002)
M.Tevfik Dorak, MD, PhD
Last updated 9 January 2007
Evolution Genetics Population
Genetics HLA MHC Inf & Imm Genetic
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