In the 1940s, Ernst Mayr coined the term Biological Species Concept that was subsequently widely embraced by the scientific community. The definition stated that “Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups”. Certain mechanisms are in place to prevent species from interbreeding with others and these are referred to as reproductive isolating mechanisms, which are biological incompatibilities. There are many mechanisms acting on natural populations and these are broadly grouped into two categories namely prezygotic and postzygotic mechanisms.
Prezygotic isolating mechanisms are those mechanisms that isolate species before fertilisation i.e. before a zygote is formed. They include geographical, ecological, temporal, ethological, mechanical, morphological and gametic isolation. These mechanisms lesson the possibility of gametes from different species coming into contact and hence forming a zygote.
One prezygotic isolating mechanism is ecological isolation, also referred to as habitat isolation. Ecological isolation prevents different species that live in the same territory but different habitats from interbreeding.
These species are referred to as sympatric species since they occur in the same territories. Individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences. An example of ecological isolation occurs within the Rana genus. R. grylio, the pig frog and R. areolata, the gopher frog both occur around New Orleans, Louisiana. The exceptionally aquatic pig frog lives in deep ponds, lakes and marshes amongst surfacing vegetation and breeds in deep water so has no contact with the gopher frog which lives in burrows during the day, and in the swamp margins at night and breeds in shallow water. This way the possibility of interbreeding between the two species is eliminated.
Temporal isolation, also known as seasonal isolation is a prezygotic mechanism that prevents interbreeding between species. Temporal isolation is the isolation of species by changing the time at which they release gametes. A particularly strong example of temporal isolation occurs in three tropical orchid species of the genus Dendrobium. The species only flower for a single day, opening at dawn and withering by nightfall. The flowering of each species is in response to the same environmental stimuli, such as a sudden storm on a hot day, but the lag time between the occurrence of the stimulus and the flowering is eight, nine, and ten or eleven in the different species.
Since they flower for a single day, inter-species fertilization is made impossible because when one species has flowered the others are either not yet mature or already withered. Other examples of temporal isolation in natural populations are not as pronounced such as in cases were species are isolated due to breeding during different seasons like Sciurus carolinensis, the gray squirrel which mates in July and August; and Sc. niger, the fox squirrel which breeds in May and June.
Another prezygotic isolating mechanism is ethological isolation or behavioural isolation. This mechanism prevents species, mostly animals from interbreeding based on their different behavioural patterns. It affects animals that occur within the same territory and habitat but with different mating behaviours. In most animals, mates are chosen in a species-specific approach often after species-specific mating rituals of some form. Matings follow these rituals which can be ended if at least one of the mating parties decides that the process leading to the mating is not as expected. Ethological isolation can be strong reproductive isolating mechanism in animals also between closely related species.
An effective example of ethological isolation occurs in certain frog species of the Hyla species. H. versicolor, the gray tree frog and the closely related H. femoralis, pine wood tree frog often breed in the same ponds. Both species are physically very similar but their male mating calls, which last about three seconds and sound the same to humans can be differentiated by the female tree frogs and thus insures species-specific mating. This is also the case when considering why dogs and wolves don’t mate as frequently as expected because of their different behavioural patterns.
Mechanical isolation is a mechanism that prevents copulation between different animal species because of incompatible shape and size of the genitalia. This occurs in species are sympatric and live in a common habitat and have overlapping breeding seasons without any ethological isolating mechanisms. In plants, variation in flower structure can inhibit cross species pollination. In California, two sage species, namely Salvia mellifera and S. apiana exhibit this form of prezygotic isolation. Two- lipped S. mellifera has stamens and style in their upper lip, whereas S. apiana has long stamens and style and a specialized floral configuration. Small and medium sized bees that carry pollen on their backs pollinate the two-lipped flowers, and large carpenter and bumble bees that carry pollen on their wings and other body parts pollinate S. apiana. This mechanism thus ensures that pollen cannot be transferred between the species, as only the corresponding pollinators are able to transfer pollen to the style of each flower.
Another prezygotic isolation mechanism is morphological isolation, which in some cases overlaps with mechanical isolation mechanisms. This isolating mechanism prevents mating due to differences in size and shape between species. Morphological isolation prevents the spread of genes between the oak toad, Bufo quercicus and the Gulf Coast toad, Bufo vallicpes due to the size variation between the two species. The female oak toad has a maximum length of approximately 3 centimetres whereas the smallest Gulf Coast males are about twice as long. This way the size differences between the spaces make copulation extremely unlikely as male oak toads are too small to grasp the female Gulf Coast toad and male Gulf Coast toads are large enough to, and generally do, eat the female oak toads.
The last prezygotic isolating mechanism is gametic isolation also known as gametic mortality. Gametic isolation is the mechanism in which fertilization cannot occur between species generally due to chemical incompatibilities between their gametes. In animals that practice internal fertilization, sperm may not be able to survive in the female’s sperm receptacles. Some plant species, pollen grains of one species usually cannot germinate on the stigma of another species thus preventing fertilization between species. Since many aquatic animals disperse their ova and sperm into the water, gametesof different species do not have affinities for each other. This was demonstrated between the sea urchins Stronglocentrotus purpuratus and S. franciscanus when after induction of simultaneous realease of eggs and sperm, all resultant fertilizations were between eggs and sperms of the same species.
Postzygotic isolation mechanisms are those reproductive isolating mechanisms, which are effective after the union of gametes of different species i.e. after fertilisation. Postzygotic isolating mechanisms include hybrid inviability, hybrid sterility and hybrid breakdown. Postzygotic isolating mechanisms reduce the viability or fertility of hybrids or their progeny. These mechanisms come into play when hybridzygotes are formed either naturally or unnaturally.
Hybrid inviability is the postzygotic isolating mechanism that occurs between fertilisation and birth. This mechanism prevents the full development of zygotes and thus leads to their subsequent death. An example in animals is the death of artificially created sheep-goat hybrid embryos in early developmental stages before birth. Hybrid inviability also occurs in plants, most commonly resulting in hybrid seeds either failing to germinate or dying shortly after germination.
Hybrid sterility or hybrid infertility is a postzygotic reproductive isolating mechanism which affects hybrids that survive to adulthood. This mechanism creates sterile hybrids i.e. hybrids that cannot produce offspring. The most well known example is the case of the mule. When a horse and a donkey mate they are capable of producing offspring called mules. But mules are sterile, leaving them ‘out’ of the gene pool thus maintaining species integrity.
Hybrid breakdown is the last postzygotic isolating mechanism in place to maintain species differentiation. It occurs in cases of crossbreeding were hybrids of inter species matings are viable and fertile. Hybrid breakdown occurs commonly and results in the hybrid’s offspring or the F2 generation being unviable or significantly weaker and unable to contribute genes to a next generation. Certain cotton plants show this form of reproductive isolation such as Gossypium barbadense, G. hirsutum and G. tomentosum which produce hybrids that seem viable and fertile, but their offspring die either as seeds, early during development or develop into weaker plants unable to compete and reproduce.
Reproductive isolation combines traits that reduce gene flow, such as mate choice or fertilization barriers, with traits that select against genes that have flowed, such as hybrid incompatibility. Reproductive isolating mechanisms are ultimately mechanisms evolved in species to prevent interbreeding with other species and thus allow for scientists to differentiate between species using the biological species concept.
Futuyma, DJ. 1998. Evolutionary biology, 3rd ed., Sinauer Assoc. Inc., USA
Griffiths, A.J.F., Miller, J.H. Suzuki, D.T., Lewontin, R.C. and Gelbart, W.M. 1999. Introduction to Genetic Analysis. W.H Freeman & Co. New York.
Hale, W. and Margham, J. 1988. Collins Dictionary of Biology. Collins, UK.
Mayr, E. 1942 Systematics and the Origin of Species. Columbia University Press, New York, USA.
Mayr, E. (1970) Populations, Species, and Evolution. Harvard University Press, Cambridge, Massachusetts, USA.
Starr, C. and Taggart, R. (2001). Biology, The Unity and Diversity of Life, 9th ed. Brookes/ Cole, USA.