Speciation

Speciation is the evolutionary process by which new biological species arise. There are four modes of natural speciation, based on the extent to which speciating populations are geographically isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry or laboratory experiments. Observed examples of each kind of speciation are provided throughout.

Natural speciation

All forms of natural speciation have taken place over the course of evolution, though it still remains a subject of debate as to the relative importance of each mechanism in driving biodiversity. available online

There is debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant over time, some palaeontologists such as Niles Eldredge and Stephen Jay Gould have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as punctuated equilibrium.

Allopatric (geographic)

During allopatric speciation, a population splits into two geographically isolated allopatric populations (for example, by habitat fragmentation due to geographical change such as mountain building or social change such as emigration). The isolated populations then undergo genotypic and/or phenotypic divergence as they (a) become subjected to dissimilar selective pressures or (b) they independently undergo genetic drift. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes.

Observed instances

Different sub-species of Juniperus communis (an alpine plant) from the White Mountains in California are observable in at the Cambridge University Botanic Garden. All have stemmed from an original plant but have altered phenotypically and perhaps genetically, as can be seen by their differing heights and shape. See Van der Merwe et al, 2000.

Another example is that of certain species of sub-saharan cattle, like the Cape Buffalo, which has undergone such genetic drift that it can no longer successfully breed with other breeds of cattle.

Island genetics, the tendency of small, isolated genetic pools to produce unusual traits, has been observed in many circumstances, including insular dwarfism and the radical changes among certain famous island chains, like Komodo and Galapagos, the latter having given rise to the modern expression of evolutionary theory, after being observed by Charles Darwin. Perhaps the most famous example of allopatric speciation is Darwin's Galápagos Finches.

Peripatric (mostly geographic)

In peripatric speciation, new species are formed in isolated peripheral populations which are prevented from exchanging genes. It is related to the concept of a founder effect, since small populations often undergo bottlenecks. Genetic drift is often proposed to play a significant role in peripatric speciation.

Parapatric (somewhat geographic)

In parapatric speciation, the zones of two diverging populations are separate but do overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms which prevent breeding between the two species.

Ecologists refer to parapatric and peripatric speciation in terms of ecological niches. A niche must be available in order for a new species to be successful.

Observed instances

Ring species
- The Larus gulls form a ring species around the North Pole.
- The Ensatina salamanders, which form a ring round the Central Valley in California.
- The Greenish Warbler (Phylloscopus trochiloides), around the Himalayas.
the grass Anthoxanthum has been known to undergo parapatric speciation in such cases as mine contamination of an area.

Sympatric (non-geographic)

In sympatric speciation, species diverge while inhabiting the same place. Examples of sympatric speciation are found in insects which become dependent on different host plants in the same area.

Polyploidy is a mechanism often attributed to causing some speciation events in sympatry. It should be noted that not all polyploids are reproductively isolated from their parental plants, so an increase in chromosome number may not result in the complete cessation of gene flow between the incipient polyploids and their parental diploids.

Observed instances

Polyploidy is observed in many plant species:
- wheat
- Salsify or goatsbeard
Cichlids of Lake Victoria, Lake Tanganyika and Lake Malawi

Reinforcement

Reinforcement is the process by which natural selection increases reproductive isolation.Ridley, M. (2003) "Speciation - What is the role of reinforcement in speciation?" adapted from Evolution 3rd edition (Boston: Blackwell Science) tutorial online

Reinforcement may occur after two populations of the same species are seperated and then come back into contact. If their reproductive isolation was complete, then they will have already developed into two seperate incompatible species. If their reproductive isolation is incomplete, then further mating between the populations will produce hybrids, which may or may not be fertile. If the hybrids are infertile, or fertile but less fit than their ancestors, then there will be no further reproductive isolation and speciation has essentially occured (e.g., as in horses and donkeys.) If the hybrid ofspring are more fit than their ancestors, then the populations will merge back into the same species within the area they are in contact.

Reinforcement is required for both parapatric and sympatric speciation. Without reinforcement, the geographic area of contact between different forms of the same species, called their "hybrid zone," will not develop into a boundary between the different species.

Reinforcement may be induced in artificial selection experiments as described below.

Artificial speciation

New species have been created by domesticated animal husbandry, but the initial dates and methods of the initiation of such species are not clear. For example, domestic sheep do not breed with Ovis orientalis, from which they are descended, and attempts at artificial insemination between domestic sheep and their ancestor species almost always fail.Nowak, R. (1999) Walker's Mammals of the World 6th ed. (Baltimore: Johns Hopkins University Press) Although domestic sheep share a clear heritage from Ovis orientalis supported by their mitochondrial DNA, they have a acquired additional chromosomes in the past 10,000 years.Hiendleder S., et al. (2002) "Molecular analysis of wild and domestic sheep questions current nomenclature and provides evidence for domestication from two different subspecies" Proceedings of the Royal Society B: Biological Sciences 269:893-904 (Cattle on the other hand, can be considered the same species as several varieties of wild ox, bison, yak, etc., as they willingly and readily reproduce, producing fertle offspring, with their ancestor and several related "other" species.)

The best-documented creations of new species in the laboratory were performed in the late 1980s. Rice and Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies which came out of two of the eight exits were set apart to breed with each other in their respective groups. After thrity-five generations, the two groups and their offspring would not breed with each other even when doing so was their only opportunity to reproduce.

Diane Dodd was able to show speciation in Drosophila pseudoobscura fruit flies after only eight generations using different food types (starch and maltose) and random separation (instead of allowing the flies to choose their separation in a maze.)Dodd, D.M.B. (1989) "Reproductive isolation as a consequence of adaptive divergence in Drosophila pseudoobscura." Evolution 43:1308–1311. Dodd's experiment has been easy for many others to replicate, including with other kinds of fruit flies and foods.Kirkpatrick, M. and V. Ravigné (2002) "Speciation by Natural and Sexual Selection: Models and Experiments" The American Naturalist 159:S22–S35 DOI

The history of such attempts is described in Rice and Hostert (1993).

References

External links

Observed Instances of Speciation from the Talk.Origins Frequently Asked Questions
Speciation, and
Evidence for Speciation from Understanding Evolution by the University of California Museum of Paleontology
Speciation from John Hawks' Anthropology Weblog - paleoanthropology, genetics, and evolution

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