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A species is an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.Note: sometimes breeding may take place (as it can between a horse and a donkey) but if so, the offspring are not so fertile and/or well adapted as the parents (the mule produced is sterile).
The formation of two or more species often (some workers think always!) requires geographical isolation of subpopulations of the species. Only then can natural selection or perhaps genetic drift produce distinctive gene pools.
|Link to discussion of gene pools and the forces that alter them.|
It is no accident that the various races (or "subspecies") of animals almost never occupy the same territory. Their distribution is allopatric ("other country").
The seven distinct subspecies or races of the yellowthroat Geothlypis trichas (a warbler) in the continental U.S. would soon merge into a single homogeneous population if they occupied the same territory and bred with one another.
Among the animals he studied were 13 species of finches found nowhere else on earth.
Darwin's finches. The finches numbered 1–7 are ground finches. They seek their food on the ground or in low shrubs. Those numbered 8–13 are tree finches. They live primarily on insects.|
1. Large cactus finch (Geospiza conirostris)
2. Large ground finch (Geospiza magnirostris)
3. Medium ground finch (Geospiza fortis)
4. Cactus finch (Geospiza scandens)
5. Sharp-beaked ground finch (Geospiza difficilis)
6. Small ground finch (Geospiza fuliginosa)
7. Woodpecker finch (Cactospiza pallida)
8. Vegetarian tree finch (Platyspiza crassirostris)
9. Medium tree finch (Camarhynchus pauper)
10. Large tree finch (Camarhynchus psittacula)
11. Small tree finch (Camarhynchus parvulus)
12. Warbler finch (Certhidia olivacea)
13. Mangrove finch (Cactospiza heliobates)
(From BSCS, Biological Science: Molecules to Man, Houghton Mifflin Co., 1963)
Several factors have been identified that may contribute to speciation.
The proximity of the various islands has permitted enough migration of Darwin's finches between them to enable distinct island populations to arise. But the distances between the islands is great enough to limit interbreeding between populations on different islands. This has made possible the formation of distinctive subspecies (= races) on the various islands.The importance of geographical isolation is illuminated by a single, fourteenth, species of Darwin's finches that lives on Cocos Island, some 500 miles (800 km) to the northeast of the Galapagos.
The first immigrants there must also have found relaxed selection pressures with few predators or competitors.
How different the outcome, though. Where one immigrant species gave rise to at least 13 on the scattered Galapagos Islands, no such divergence has occurred on the single, isolated Cocos Island.
How much genetic change is needed to create a new species?Perhaps not as much as you might think. For example, changes at one or just a few gene loci might do the trick. For example, a single mutation altering flower color or petal shape could immediately prevent cross-pollination between the new and the parental types (a form of prezygotic isolating mechanism).
If, on the other hand, two subspecies reunite but fail to resume breeding, speciation has occurred and they have become separate species.An example: The medium tree finch Camarhynchus pauper is found only on Floreana Island. Its close relative, the large tree finch, Camarhynchus psittacula, is found on all the central islands including Floreana.
Were it not for its presence on Floreana, both forms would be considered subspecies of the same species. Because they do coexist and maintain their separate identity on Floreana, we know that speciation has occurred.
These act even if fertilization does occur.
Hybridization between related angiosperms is sometimes followed by a doubling of the chromosome number. The resulting polyploids are now fully fertile with each other although unable to breed with either parental type. A new species has been created. This appears to have been a frequent mechanism of speciation in angiosperms.
|Link to a discussion of polyploidy and an example of its role in speciation.|
Even without forming a polyploid, interspecific hybridization can occasionally lead to a new species of angiosperm. Two species of sunflower, the "common sunflower", Helianthus annuus, and the "prairie sunflower", H. petiolaris, grow widely over the western half of the United States. They can interbreed, but only rarely are fertile offspring produced.
However, Rieseberg and colleagues have found evidence that successful hybridization between them has happened naturally in the past. They have shown that three other species of sunflower (each growing in a habitat too harsh for either parental type) are each the product of an ancient hybridization between Helianthus annuus and H. petiolaris. Although each of these species has the same diploid number of chromosomes as the parents (2n = 34), they each have a pattern of chromosome segments that have been derived, by genetic recombination, from both the parental genomes. They demonstrated this in several ways, notably by combining RFLP analysis with the polymerase chain reaction (PCR).
They even went on to produce (at a low frequency) annuus x petiolaris hybrids in the greenhouse that mimicked the phenotypes and genotypes of the natural hybrids. (These monumental studies were described in the 29 August 2003 issue of Science.)Another example. In Pennsylvania, hybrids between
(This study was published in the 28 July 2005 issue of Nature.)
So speciation can occur as a result of hybridization between two related species, if the hybrid
Thus the reuniting of the two groups may create an intense selection pressure leading to one of two possible outcomes.
The range of beak sizes of Camarhynchus pauper on Floreana and Camarhynchus psittacula on Isabela is roughly the same. This reflects the fact that the two species feed on the same type and size of insect.On Floreana, however, character displacement has occurred: C. psittacula has a substantially larger beak than C. pauper does, and thus differs enough in its food requirements for the two species to coexist there.
The histogram shows the data. The larger beak of C. psittacula on Floreana reduces the competition with C. pauper (which is not found on Isabela). (Redrawn with permission from David Lack, Darwin's Finches, Cambridge Univ. Press, 1947. The number of collected specimens (16) of C. psittacula on Floreana was too small to compute a reliable percentage but all fell within the range indicated.)
The character displacement described above was inferred from the data shown. However, over the past several years, the research teams led by Peter and Rosemary Grant have been able to observe character displacement actually occurring. This is their story.
The formation of a number of diverse species from a single ancestral one is called an adaptive radiation.
Some evolutionary biologists don't believe that it ever occurs. They feel that interbreeding would soon eliminate any genetic differences that might appear.But there is some compelling (albeit indirect) evidence that sympatric speciation can occur.
The three-spined sticklebacks, freshwater fishes that have been studied by Dolph Schluter (who received his Ph.D. for his work on Darwin's finches with Peter Grant) and his current colleagues in British Columbia, provide an intriguing example that is best explained by sympatric speciation.They have found:
Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's Lake Baikal.
Perhaps not very many.
The European corn borer, Ostrinia nubilalis, (which despite its common name is a major pest in the U.S. as well) exists as two distinct races designated Z and E. Both can be found in the same area; that is, they are sympatric.
But in the field, they practice assortative mating — only breeding with mates of their own race.
The females of both races synthesize and release a pheromone that is a sex attractant for the males. Both races use the same substance but different isomers of it. Which isomer is produced is under the control of a single enzyme-encoding gene locus. The ability of the males to respond to one isomer or the other is controlled by 2 loci.
So here is a prezygotic isolating mechanism where only 3 gene loci could begin the process of sympatric speciation. (This work is described in Lassance, J-M., et al., Nature, 22 July 2010.)
There is another possible way for new species to arise in the absence of geographical barriers.
The occurrence of gradual phenotypic (and genotypic) differences in a population across a large geographical area is called a cline. Successful interbreeding occurs freely at every point along the cline, but individuals at the ends of the cline may not be able to interbreed. This can be tested in the laboratory.
And, on occasions, it is tested in nature. If a cline bends around so that the ends meet, and the populations reunited at the junction cannot interbreed, then the definition of separate species has been met. Such species are called ring species and this type of speciation is called parapatric speciation.
1. The Caribbean slipper spurge Euphorbia tithymaloides.
Genetic analysis shows that this wildflower originated in Central America where Mexico and Guatemala share a common boundary. From there it spread in two directions
Reunited in the Virgin Islands, the two populations have diverged sufficiently that they retain their distinctive genotypic and phenotypic traits. Ongoing studies will determine to what degree they may be reproductively isolated.2. The large-blotched salamander Ensatina eschscholtzii.
This animal is found in California where it occurs in a number of different subspecies or races. A single subspecies is found in Northern California, and it is thought to be the founder of all the others. Over time that original population spread southward in two directions:
South of the valley, the eastern group has moved west and now meets the western group, closing the ring. Here the two populations fail to interbreed successfully, maintaining their distinct identities. But each subspecies interbreeds in an unbroken chain up the two paths their ancestors took.
Ring species present a difficult problem in assigning species designations. It is easy to say that the populations at the ends of the cline represent separate species, but where did one give rise to the other? At every point along the cline, interbreeding goes on successfully.
The same problem faces paleontologists examining the gradual phenotypic changes seen in an unbroken line of ever-younger fossils from what one presumes to be a single line of descent. If one could resurrect the ancestral species (A) and the descendant species (B) and they could not interbreed, then they meet the definition of separate species. But there was no moment in time when one could say that A became B. So the clines of today are a model in space of Darwin's descent with modification occurring over time.
Although clines present a problem for classifiers, they are a beautiful demonstration of Darwin's conviction that the accumulation of small inherited differences can lead to the formation of new species.