What is ecological isolation and how does it prevent the occurrence of inter-species hybrids? In this BiologyWise article, we will answer these questions, and at the same time provide examples of this isolation mechanism, to make it easier for you to understand the same.
Ecological Speciation Definition
When ecologically-based divergent selection creates reproductive barriers between populations, it is referred to as ecological speciation.
Ecological isolation is one of the five pre-zygotic isolation mechanisms that prevent interbreeding between species. Interbreeding does little or no good to the population of the species in question, as most hybrids are sterile. On the contrary, it expends energy of these species on a process that is far from fruitful. Mechanisms of reproductive isolation are not just important because they prevent the birth of sterile offspring, but also because they play a crucial role in speciation.
What is Ecological Isolation?
Also known as habitat isolation, ecological isolation is a form of reproductive isolation, wherein habitat preferences of species lowers the probability of mating.
In ecological isolation, mating is prevented because of the separation of species resulting from the difference in their habitat. This usually occurs when the species are used to different habitat types, or different parts of the same habitat. Even if the geographic range of two species overlaps, the difference in their ecological needs and breeding requirements is enough to isolate them from each other. The slightest difference in their habitat can ensure that they never come together for mating or even come across each other, despite the fact that they live in the same territory.
In birds, for instance, if two similar species of birds live in a specific area, they are most likely to choose a different location. If the location is the same, the type of tree chosen by either species will be different. If both choose the same tree, they will choose different parts of that tree to make their nests and lay eggs.
Premating and postmating prezygotic isolation: Like behavioral isolation, wherein two distinct species do not mate because of the discrepancies in their mating ritual, even ecological isolation is a type of premating prezygotic isolation. In contrast, mechanical isolation, which prevents mating because of genital incompatibility, is a postmating prezygotic isolation.
Ecological Isolation Examples
❒ One of the most-cited examples of ecological isolation is the case of two closely-related birds of the Turdus genus, the blackbird (Turdus merula) and ring ouzel (Turdus torquatus). The population of the blackbird, which is a woodland species, is ecologically isolated from that of the ring ouzel, a moorland-breeding species, as a result of which the chances of these species producing a hybrid is as good as none.
❒ Ecological reproductive isolation is also seen in different species of frogs. Of the many examples of the same, two prime examples are found in North America and Australia. In North America, the population of northern red-legged frogs (Rana aurora) is isolated from the population of American bullfrogs (Rana catesbeiana), as the former breed in fast-moving, ephemeral streams, while the latter breed in permanent ponds.
❒ In Australia, the crucifix toad (Notaden bennettii) and desert tree frog (Litoria rubella) are both found in desert environment with an overlapping geographic range. However, they are unlikely to end up mating, as the crucifix toad is a fossorial or ground-dwelling species, which only comes to the surface when it rains, while the desert tree frog is a species of tree frog, thus largely arboreal.
❒ Yet another example of this type of reproductive isolation is seen in stickleback species (species of the family Gasterosteidae). While the freshwater species of sticklebacks live in freshwater streams all year round, marine species which are found in the sea in winter migrate to river estuaries for breeding in spring and summer. It is their adaptation to distinct salt concentrations that acts as a reproductive barrier, and prevents the two groups from mating.
❒ Then there are the six species of mosquitoes, which together form the Anopheles maculipennis group. Despite their similarities―they are morphologically indistinguishable―these sibling species seldom produce hybrids as they breed in different habitats. While some species breed in brackish water, others prefer freshwater. Within the freshwater breeding species, there are some that prefer running water, while some prefer stagnant water.
❒ In plants, an apt example of ecological isolation can be seen in the case of two species of spiderworts (genus Tradescantia): the Ohio spiderwort (Tradescantia ohiensis) and the zigzag spiderwort (Tradescantia subaspera). Despite their overlapping geographic range and ability to produce hybrids, these species are unable to mate because of the difference in their habitats. The Ohio spiderwort grows in sunny areas, while the zigzag spiderwort prefers shade.
Trivia: Besides ecological isolation, the Ohio spiderwort and zigzag spiderwort also demonstrate an apt example of temporal isolation, as they flower at different times of the year.
As with other mechanisms of reproductive isolation, even ecological isolation leads to speciation, as the lack of opportunity to reproduce restricts gene transfer, and splits the population of a species into two distinct groups.