Speciation is a term which aims to describe the manner in which different species come about. This is very important because it explains how the original common ancestor has diverged into the multitude of species that are living today.
Major Theories on Speciation
Biological Species Concept
The Biological Species concept aims to explain speciation by looking at the likelihood of species to interbreed and produce viable and fertile offspring. In other words, if species are able to mate and bear healthy, fertile offspring, this will not lead to speciation, but if there is anything that barriers this interbreeding, a new species could arise.
Reproductive isolation is an example of a case where due to the lack of interbreeding within a species, a new species could arise. Factors which lead to reproductive isolation could be mechanical differences meaning the two species could not physically mate successfully. Behavioral differences could also lead to a lack of mating between species.
The Morphospecies concept tries to explain speciation by looking at similarities in structures from species to next.
One example of this is mammalian forelimbs. A human, a cat, a whale and a bat all have similar forelimbs.
It is through these similarities in morphological structure that scientists can deduce some sort of common ancestor which contained these structures, and from there hypothesize on where the speciation occurred. Of course, it is much harder to deduce this for something such as a bat and a human, but there are many cases which this concept can be more directly applied. For example, the different species of finches due to there different beak sizes and shapes.
Phylogenic Species Concept
The Phylogenic Species concept aims to explain speciation by looking at genetic history of species. In this method physical characteristics and molecular sequences of organisms are compared to find the set of organisms that share a unique genetic history. As a result, the groups of the individuals that are sufficiently different are separate species. This analysis has some advantage; for instance, it can be applied to all types of organisms - asexual organisms, such as bacteria. In addition, Sibling Species - Species that are very similar that they cannot be distinguished by morphological analysis - can be recognized. Phylogenic Species analysis also has some disadvantages. For example, good phylogenies are not available for most organisms and it is not clear that how much genetic divergence represents long-term isolation and thus speciation.
Different types of Speciation
To recognize speciation, it is significant to have a definition of a species. A species is a group of populations through which genes can flow and whose offspring have a fitness equal to the parents. As evolution progresses, gene flow is ultimately stopped between one population and the species as a whole. As a population undergoes changes in gene frequencies it could diverge entirely until it is reproductively distinct from other populations of the species from which it diverged. This is called phyletic evolution or anagenesis. Alternatively the population could evolve into two distinct species that exist simultaneously. This is called true speciation or cladogenesis.
Cladogenesis occurs because reproductive-isolating mechanisms prevents two sub-populations from interbreeding. This reproductive barrier could be the result of
1) the isolation of one portion of the population by some physical barrier
2) a sub-population becoming established in a new ecological niche not previously occupied by the species
3) a polymorphism becoming established in a population before it moves to a new ecological niche
Speciation involves the genetic transform in a subgroup of a population that renders the new population unavailable of reproducing offspring with the original population. If a reproductive barrier occurs because of isolation by a physical barrier, the isolated population can evolve and develop into a new species. This process is termed allopatric speciation. This has been considered for a long time to be the primary mode of speciation. Speciation can also occur when a subpopulation migrates into a new niche. This is termed parapatric speciation, and this process seems to have been used by some annual plants.
The final form of speciation is called sympatric speciation. This type of speciation occurs when a subpopulation that occupies the same niche as the remainder of the species develops a unique mutation that prevents it from mating with the original population. That new species may have an ecological advantage which permit its establishment as a species in the same niche. A nice example of this method of speciation is the development of the new saltmarsh species Spartina townsendii, that was derived from S. alterniflora and S. maritima, but is reproductively incompatible with either parent. This new species is better adapted to the coastal regions of Holland than either of the parental species and was able to better establish itself in that niche.
Theories behind Evolutionary Speciation
At the current time there are two accepted theories on ways in which the evolution of a species, leading to speciation, can occur. The two theories are gradualism and punctuated equilibrium. The difference between these two theories primarily lie in the time scale between when speciation may occur in a particular organism.
Gradualism is the theory that selection and variation happens in a more gradual state. In other words, over a short period of time, it is difficult to recognize the change that is happening, either phenotype or genetically, and small variations that fit an organism become gradually better to its environment. Over a long peroid of time, one could recognize a change and can lead to subsequent speciation.
Punctuated Equilibrium speaks of change that comes in spurts. There is a period of very little change and then one or few huge changes occur. This usually occurs through mutations in the genome of an individual organism. Mutations are random changes int he DNA sequence that are not inherited from the previous parental generation. Although some mutations are harmful, some mutations prove to be beneficial to an organism surroundings and are naturally selected. Once selected, speciation follows suit to forever change the organisms to its better adaption to its environment.
Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. Biochemistry. 6th ed. New York: W. H. Freeman and, 2006. Print.