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Introduction

According to the biological species concept, populations are different species if gene flow between them is prevented by biological differences, known as reproductive barriers. If populations exchange genes they are conspecific, i.e. belong to the species, even if they differ greatly in morphology. If they are reproductively isolated, they are different species even if they are indistinguishable phenotypically. Therefore speciation arises from the evolution of biological barriers to gene flow (Futuyma, 1998).

The factors leading to reproductive isolation can be divided into two categories; prezygotic factors, which operate before fertilisation can occur; and postzygotic factors, which operate after fertilisation leading to partial or complete failure of crosses between the two forms. These are summarised below.

A. Prezygotic factors

1. Geographical isolation: forms are separated by land or water barriers that they are unable to cross.
2. Ecological isolation: the forms fail to meet because they live in different places within the same geographic region.
3. Temporal isolation: the forms are active at different seasons or times of day.
4. Behavioural isolation: the forms meet, but do not mate.
5. Mechanical isolation: copulation occurs, but no transfer of male gametes takes place.
6. Gametic incompatibility: gamete transfer occurs, but egg is not fertilised.

B. Postzygotic factors

1. Zygote dies: zygotic mortality soon after fertilisation.
2. F1 hybrids (first generation) have reduced viability (hybrid inviability).
3. F1 hybrids viable but have reduced fertility (hybrid sterility)
4. Hybrid breakdown: reduced viability or fertility in F2 (second generation) or backcross (F1 crossed with parents) generations.

Prezygotic Barriers

One or more of these may be operating within a given population at any time. The evolutionary functions of these mechanisms are the same, to limit or prevent gene flow between species. They may occur only partially; for example, behavioural isolation can be complete or females may only show a slight preference for males of their own species (Dobzhansky et al., 1977).

Geographical Isolation

When populations are kept apart by allopatry, i.e. living in different areas, they are geographically isolated. However, geographic isolation is not a reproductive isolating mechanism brought about by the genetic make-up of a population, geographically isolated populations may be genetically identical (Dobzhansky et al., 1977). Any differences between the populations are likely to be consequences of isolation rather than causes. For example, the species of freshwater fish (Salvelinus spp.) in the lakes of Switzerland, Scandinavia and Great Britain. Almost every lake has a different form (Carter 1954). In this case it is the land between the lakes that acts as a barrier.

There are many ways in which a population can be separated geographically. A change in climate, or a geological change such as an orogenic event or continental drift may, over time, split an originally single population. More likely is the transportation of a small sample of the population to another geographical area; the Galapagos finches are believed to be derived from a single stock that reached the islands in this way (Carter, 1954). This sample of the population would contain only a small proportion of the total genetic variation and differentiation would therefore occur relatively quickly, as any rare alleles will be allowed to become dominant. This is known as the founder effect. Geographical isolation may also be set up by schismogenesis, the principle that two distinct forms within a single population will drive each other towards the parts of their ranges that do not overlap. This will only apply when a new form has arisen that is sterile with the original population. Although theoretically possible, there is no evidence that this process is of any importance in evolution (Carter, 1954).

Once a population has been split by geographical isolation the new populations will evolve independently. This is both from adaptation to environments and non-adaptively by random genetic drift (Carter, 1954).

Ecological Isolation

Differences in the habitats occupied or resources used by different species commonly lower the opportunity for interbreeding and so contribute to reproductive isolation. It is possible that this may be the only barrier to gene flow. For example, two sympatric species of ladybirds in Japan, Epilachna nipponica feed on thistles and E. yasutomii feed on blue cohosh. Each species also mates exclusively on its host plant, which acts as a microhabitat. Hybridisation experiments have yielded no evidence for other barriers to gene flow (Futuyma, 1998).

Temporal Isolation

This can be due to either the timing of breeding cycles (seasonal isolation) of closely related species or the time of day or night that species search for mates. For example, some species of insects seek mates for only a few hours of the day or night, and related species do so at different times. Differences in the timing of breeding cycles is more common, an example being two closely related field crickets, Gryllus pennsylvanicus and G. veletis. These reach reproductive age in the autumn and spring, respectively, in the northeastern United States (Futuyma, 1998). Another good example is that from the pines Pinus radiata and P. muricata, which grow together in California. P. radiata sheds its pollen in early February and P. muricata in April. Hybrids do occur but form only a small fraction of the population and are less vigorous and productive than either parent species (Dobzhansky et al., 1977).

Behavioural Isolation

In animals, this is probably the most common and important barrier to gene flow between sympatric species, which frequently encounter each other but simply do not mate. In animals, the courting individual (usually male) often does not display to members of other species, if he does, the courted individual (usually female) does not indicate receptiveness to mating. Both signal and response are usually species-specific, suggesting that these two characteristics have diverged during speciation. An example can be found in crickets, different species have evolved different mating songs and the female will only respond to the song of a male of their own species (Futuyma, 1998). Similarly, species of tarsiers that live in the same geographical region have different mating calls.

More species of animals use chemical communication than any other form. The compounds used are called sex pheromones. Male moths, for example, will respond to very low concentrations of female sex pheromones. When placed in a wind tunnel, male moths will fly to and attempt to mate with an object impregnated with the pheromones of their own species but will not respond to those of other species (Futuyma, 1998).

In many insects, birds and fishes, visual signals are important elements in courtship displays. Among birds, ducks, pheasants and hummingbirds show particular diversity of colour, patterns and ornaments that males display to females. The females of different species are much more similar to each other (Futuyma, 1998).

Mechanical Isolation

Mechanical isolation, resulting from an imperfect structural fit between the sexes, is not often a barrier to gene exchange among animals. One example from animals is that of Drosophila pseudoobscura females and D. melanogaster males, copulation between these can result in injury or even death (Dobzhansky et al., 1977). Analogous situations can be found in the flowering plants, as the structural fit between flowers and pollinators can affect exchange of pollen in some species of plants. For two Swedish orchids Platanthera bifolia is pollinated mostly by hawkmoths, and P. chlorantha mostly by smaller noctuid moths. The moths appear to respond to the different fragrances produced by the two orchids. The pollinia of P. bifolia are situated close to each other and are placed on the base of the proboscis of hawkmoth pollinators, whereas in P. chlorantha they are more widely spaced and stick to the eyes of its pollinators. Hybrid orchids, with intermediate flower morphology, have lower success in pollination and seed production, partly because their pollinia are placed in disadvantageous positions on the moths' faces (Futuyma, 1998).

Gametic Incompatibility

Incompatibility of the gametes of different species is an important reproductive barrier among some externally fertilising animals, such as echinoderms and some molluscs that release eggs and sperm into the water, where they might encounter gametes of other species. In abalones (large gastropods), the sperm carries a lysin protein that dissolves a hole in the egg's envelope, but only in eggs from the same species (Futuyma, 1998).

Postzygotic Barriers

The genotypes of isolated forms will begin to differ as soon as isolation is set up and the differences will increase as long as isolation is maintained. As the forms become progressively dissimilar the hybrids become less and less viable (Carter, 1954).

Hybrid Inviability

Hybrids between species usually have lower survival rates than non-hybrids. Mortality often occurs in the embryonic stages. Sometimes the degree of hybrid inferiority depends on the environment; hybrids often have a higher survival rate in environments intermediate to that of either parent species. Very little is known about the malfunctions in development that cause mortality in hybrids (Futuyma, 1998). Occasionally the opposite occurs and hybrids are extremely vigorous, although sterile. Two end-members of this process are crosses between horses and donkeys, and crosses between sheep and goats. Horse and donkey crosses produce mules, which are vigorous but sterile; while in sheep and goat crosses fertilisation takes place but the hybrid embryos die in the early developmental stages (Dobzhansky et al., 1977).

Hybrid Sterility

The infertility of hybrids ranges from none to complete, depending on the hybrid. There are several causes of hybrid sterility, of which the most common are; 1) the segregation of at least some aneuploid gametes during meiosis, i.e. gametes with an unbalanced complement of chromosomes or genes, due to structural differences between the chromosomes or improper pairing of homologous chromosomes; 2) differences between the genes from the two parents (Futuyma, 1998). Hybrids that are completely sterile may still be otherwise vigorous.

Very frequently only one sex is sterile in the hybrid. This is also true of hybrid inviability. J. B. S. Haldane (1922) noted an important regularity now known as Haldane's rule; when in the F1 offspring of two species or populations, one sex is inviable or sterile, that sex is usually the heterogametic sex. This is the one with two different sex chromosomes or with only one sex chromosome. In mammals the male is heterogametic (XY) and in birds the female is heterogametic. This rule holds generally, regardless of which sex is heterogametic. This is found mostly in species that have recently formed which suggests that the development of sterility or inviability between populations evolves more rapidly in the heterogametic sex than in the homogametic sex (Futuyma, 1998).

Hybrid breakdown may occur in varying degrees in F2 offspring and backcrosses with the parent populations. The cause of this is the formation of genotypes with low fitness, due to the recombination of the gene complements of the parental species. This has been observed in crosses both between species and between different geographic populations of the same species. For example, survival of F2 larvae in a cross between Drosophila psuedoobscura from California and from Utah was lower than in either pure population (Futuyma, 1998).

Evolutionary Origin of Reproductive Isolation

There are two theories that try to explain the origin and development of reproductive isolation between genetically diverging populations. The first of these considers isolation to be due to genetic divergence. As populations grow genetically more dissimilar it becomes less likely that hybrids will be viable. The second theory considers reproductive isolation to be a product of natural selection. If hybrids are less fit than either of the parental species and offspring from intraspecific (same species) matings are more fit, then genetic characteristics that enhance the likelihood of intraspecific matings will be promoted and those permitting interspecific (different species) matings will be discriminated against (Dobzhansky et al., 1977). Any of the prezygotic mechanisms discussed in the previous section may contribute to this.

Prezygotic isolating mechanisms are probably caused by natural selection and postzygotic factors are likely to arise due to genetic divergence. Once postzygotic factors have arisen it becomes advantageous to the species concerned to minimise the occurrence of interspecific matings. This leads to a process known as reproductive character displacement. This is the evolutionary process by which closely related species become more different, when they are sympatric than when they are allopatric, in characters used in species recognition. This means that closely related sympatric populations will evolve strongly due to evolutionary selection as a consequence of postzygotic problems. Selection is strongest on females. This is because females make a greater "investment" in the offspring. If the offspring is inviable then this is wasted as none of her genes will be moved forward. Males do not have this problem as they can mate many more times and usually put much less "investment" into the offspring, so they can afford to make a few "mistakes". Therefore females in particular must be discriminating when finding a mate.

In a zone of geographic overlap, where hybridisation occurs, the evolution of prezygotic isolating mechanisms will be intense. Once reproductive isolation is complete the populations will become biological species and will undertake independent evolutionary pathways. In some cases populations may fuse instead of diverging. This is the case with the toads Bufo americanus and B. fowleri, this occurs in regions where they are sympatric and hybridise to form a "hybrid swarm" (Dobzhansky et al., 1977).

Conclusions

Biological species are reproductively isolated from other such populations. Mechanisms leading to reproductive isolation are:

A. Prezygotic factors:

1. Geographical isolation
2. Ecological isolation
3. Temporal isolation
4. Behavioural isolation
5. Mechanical isolation
6. Gamete incompatibility

B. Postzygotic factors:

1. Hybrid inviability
2. Hybrid sterility

These factors are not mutually exclusive and one or more may be operating in any given population at any time. They may occur completely or be only partially developed. This is because speciation is a dynamic process occurring all the time and as a result very closely related species may not have fully evolved mechanisms to avoid cross-breeding and the hybrids may still be viable although less fit than either parent species. As a result of this it can be difficult to distinguish between sub-species and true biological species. Geographical isolation differs to the other forms in that the differences between species occur after separation due to random genetic drift. The other isolating factors are brought about by the slight differences that already exist in closely related species. Prezygotic isolating mechanisms are probably caused by natural selection and postzygotic factors are likely to arise due to genetic divergence. When the populations evolve mechanisms to aid in species recognition (prezygotic mechanisms), to avoid postzygotic problems, the process is referred to as reproductive character displacement. Selection will always be strongest on females as there is a much greater genetic "investment" made by females in the offspring.

When looking at the fossil record it is only really possible to see speciation as a result of allopatric speciation. This is because it is possible to document cases through geological history in relation to, for example, tectonic events. Other mechanisms leading to speciation are not likely to be recognised as they involve characters that cannot be found in the fossil record; such as elaborate colouring and ornamentation, and the differences in mating calls. In cases such as these all that can be seen in the fossil record is the end result, i.e. a speciation event at some point in geological time.

References

Carter, G. S., 1954. Animal Evolution. Sidgwick and Jackson Limited, London.

Dobzhansky, Ayala, Stebbins and Valentine, 1977. Evolution. W. H. Freeman and Company, San Francisco.

Futuyma, D. J., 1998. Evolutionary Biology. Sinauer Associates, Inc.