An alternative definition is the Ecological Species Concept, which attempts to define species ecologically. An ecological species is that which exploits a single niche, and consequently members of a species are similar to each other because they share a common ecological niche, and such groups of organisms are therefore reproductively isolated.
Probably the most widely accepted species concept amongst zoologists is the Biological Species Concept (BSC). This concept defines a species as a reproductive community, who can interbreed and produce fertile offspring under natural conditions. Mayr (1942) defined species as ‘…groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.’ The BSC is popular amongst population geneticists in particular as it takes into consideration genetics, as reproductive isolation has obvious implications for the gene pool, as genes cannot mix between species. Members of species are similar to each other because they share a common gene pool. The BSC has problems when dealing with plants, or asexual populations. A number of land plants undergo interspecific hybridisation between clearly different species, and the definition also has limited application to land plants that primarily self-pollinate. It is also not always possible to know whether two groups of organisms can successfully interbreed, as they may not have the opportunity to do so.
Speciation is the splitting of one gene pool into two gene pools, as a result of reproductive isolation (no mixing of genes can occur). In order for speciation to occur there must be a factor causing two populations to diverge, but there must also be factors preventing the two populations from combining back into a single gene pool.
There are 3 modes of speciation. These are:-
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Allopatric speciation – the result of physical isolation of populations, for example a bridge connecting the population breaks, or some of the population get transported to an island and therefore are isolated from the rest of the population. Allopatric speciation is the most common cause of speciation as there is a physical barrier preventing gene flow. Once the two populations are physically separated, they start to genetically diverge, as a result of the Founder effect and adaptive radiation. The founder effect is caused because a founding population has only a proportion of the ancestral population genes, causing gene frequencies to diverge rapidly by random drift. It is likely that the founder population will have a gene pool which is not representative of the ancestral population, as it is likely that there is something distinct in that population which caused them to found a new population. Random chance will also cause bias in the selected gene pool. Adaptive radiation causes genetic divergence as the population isolated in the novel environment will be subjected to different selection pressures and therefore undergo rapid adaptation. Genetic divergence eventually causes reproductive isolation, as the two populations have diverged so much that they can no longer successfully interbreed.
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Sympatric speciation – this occurs without geographical isolation. Sympatric speciation is far less common than allopatric speciation, as gene flow is still possible between the two populations. There are two modes of sympatric speciation, the first of which is ecological speciation, whereby the population is not separated by a geographic barrier, but by an ecological one which prevents gene flow between the groups. For example, different forms of parasites of the same species may live and reproduce in different hosts, and there will therefore be no gene flow between the forms reproducing in different hosts. There is therefore assortive, rather than random mating. Due to the lack of gene flow, natural selection and genetic drift will eventually produce two reproductively isolated species. The second mode of sympatric speciation is strong disruptive selection. If the two extremes of a trait are strongly selected for, and the hybrids have reduced fitness, then successful reproduction between the two extremes will be reduced and gene flow will be minimal. This type of speciation in similar to Parapatric speciation, accept that parapatric speciation occurs across a geographical boundary, where as Sympatric speciation occurs in the same place.
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Parapatric speciation– occurs when the two populations are geographic neighbours, and are free to mate and move between the two areas. Speciation is unlikely without some form of geographical isolation, so in order for parapatric speciation to occur there must be strong selection counteracting the effect of gene flow. It is thoughy that parapatric speciation may occur when there is an abrupt change in the environment over a geographical border. Some the population will have a higher fitness on one side of the border, and others will have a higher fitness on the opposite side of the border. Hybrids between the two forms do not have high fitness in either area, forming a hybrid zone. For example, there are areas where there is a sudden change in soil type from one place to the next. Plants that have high fitness in one soil type have low fitness in the other. There will be strong natural selection for the particular type in each area, and this will counteract the effect of potential gene flow. This isolation by a hybrid zone will cause the two different forms to develop into different species.
The reproductive isolation causing speciation is of two types, prezygotic or postzygotic. Prezygotic isolation means that a zygote is never formed, either because the mates do not meet (seasonal or habitat isolation), the mates do meet but do not mate (ecological or behavioural isolation), or the mates meet but no sperm transfer occurs (mechanical isolation). Postzygotic isolation means that a zygote is formed but the zygote either dies, produces and F1 adult that has reduced viability, or the hybrid is viable, but sterile. It is also possible that the F1 is fertile and viable, but the F2 may be deficient and unable to reproduce, thereby preventing sustainable gene flow.
The theory of Punctuated Equilibrium was pioneered by Niles Eldredge and Stephen Jay Gould in 1972. By studying the fossil record, they inferred that there were long periods of stasis in which virtually no evolution occurred, and that these periods of stasis are ‘punctuated’ by relatively very short periods of development of new forms. The fossils of organisms found in subsequent geological layers show long periods of equilibrium in which nothing changes, and then sudden transitions in which species became extinct and replaced by new forms. These jumps in the fossil record were originally thought to be due to the fact that the fossil record was incomplete, however it now appears that there are genuine gaps in the fossil record, and the presence of transitional forms is very rare. The theory claims that the speciation event is not often present in the fossil record due to the fact that it probably occurs in a small population occupying a small geographical range, and because it occurs over a relatively short period of time, perhaps 1,000 to 10,000 years. Theses factors make the probability of finding a fossil preserved during a speciation event very unlikely compared to the probability of finding a fossil of the expanded, established population of the new species over the millions of years of its existence. Punctuated Equilibrium is more of an observation of evolution than a mechanism. It has been noted that adaptive radiations tend to occur following mass extinction events. For example, the extinction of Dinosaurs allowed the adaptive radiation of mammals. It is thought that the extinction freed ecological niches. Allowing mammals to diversify and fill these niches.
The theory of punctuated equilibrium is an alternative to Darwin’s (1959) belief that evolution was a slow, continuous and gradual process without sudden jumps. This theory is known as phyletic gradualism, and has been the widely accepted theory of evolution throughout the late 19th century and most of the 20th century. However, it was gradually realised that there was overwhelming evidence from the fossil record and theoretical studies that punctuated equilibrium is the foremost pattern in evolution. There is however, some fossil evidence supporting phyletic gradualism. One group of fossils which provides support for phyletic gradualism is the foraminifera fossils. By examining marine plankton fossils which appear in the Paleocene and extending 66 million years, Hunter et al. (1988) demonstrated a nearly continuous morphological change through time. However such evidence is scarce, and seems to be apparent in only a few taxa.
Punctuated equilibrium seems to explain the jumps in morphological forms observed in the fossil record, and studies have provided evidence that evolutionary processes can and do incite this pattern. Although phyletic gradualism is evident, it is seen to only be important in a few taxa. The evolution of life has been shaped by both punctuated equilibrium and phyletic gradualism, but current data suggests that punctuated equilibrium plays a more significant role.
