Evolution Genetics Population Genetics HLA MHC Inf & Imm Genetic Epidemiology Epidemiology Glossary Homepage
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
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.
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.
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.
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
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