|Reproductive Isolating Mechanisms|
Reproductive isolating mechanisms prevent interbreeding between species. The term, which was first used by Theodosius Dobzhansky in 1937 in his landmark book Genetics and the Origin of Species, refers to mechanisms that are genetically influenced and built-in.
Geographic isolation can prevent interbreeding among populations, but it is an external factor. The standard model for speciation requires that populations be geographically isolated long enough to diverge genetically.
Later, if the geographic barriers break down, built-in isolating mechanisms maintain reproductive isolation between the divergent populations. As these mechanisms continue to prevent hybridization, continued divergence leads to new species.
Reproductive isolating mechanisms function only between sexually reproducing species. They have no applicability to forms that reproduce only by asexual means.
Hermaphrodites, organisms with both male and female reproductive organs that reproduce only by self-fertilization (rare in animals, more common in plants), represent a distortion of the sexual process that produces essentially the same results as asexual reproduction.
Many lower animals, many plants, and protists regularly employ both asexual and sexual means of reproduction, and the significance of isolating mechanisms in such forms is essentially the same as in normal sexual species.
Reproductive isolating mechanisms are usually classified into two main groups. Premating, or prezygotic, mechanisms operate prior to mating, or the release of gametes, and therefore do not result in awaste of the reproductive potential of the individual.
By contrast, postmating,or postzygotic, mechanisms come into play aftermating, or the release of gametes, and could result in a loss of the genetic contribution of the individual to the next generation. This distinction is important in the theoretical sense, in that natural selection should favor genes that promote premating isolation.
Genes that do not promote premating isolation presumably would be lost more often through mating with an individual from another species, which often leads to no offspring or infertile offspring, in turn leading to a reinforcement of premating isolation.
Ecological isolation (habitat isolation) often plays an important role in both animals and plants. Different forms may be adapted to different habitats in the same general area and may meet only infrequently at the time of reproduction.
Different plant species may occur on different soils, on different drainage profiles or exposures, or at different altitudes. This type of isolation, although frequent and widespread, is often incomplete, as the different forms may come together in transitional habitats.
The importance of ecological isolation, however, is attested by the fact that when hybrid swarms (groups of organisms that show signs of extensive hybridization) are produced between forms that normally remain distinct, they have often been found to result from disruption of the environment, usually by humans.
Mechanical isolation is a less important type of premating isolation in plants, though it does occur in some. Complex floral structures in certain plants (such as orchids) may favor one species of animal pollinator over others. Finally, temporal differences often contribute to premating isolation.
The most common type of temporal isolation is seasonal isolation: Species may reproduce at different times of the year. One type of western pine normally sheds its pollen in February, while another does not shed its pollen until April. Differences can also involve the time of day.
In one species of desert plant, the flowers open in the early morning, while in another species of the same genus the flowers open in the late afternoon. Such differences, as in the case of ecological isolation, are often incomplete but may be an important component of premating isolation.
If premating mechanisms fail, postmating mechanisms can come into play. If gametes are released, there still may be a failure of fertilization (intersterility).
In plants, the pollen may not germinate on the foreign stigma or the pollen tube may fail to develop. Fertilization failure is almost universal between remotely related species and occasionally occurs even between closely related species.
If fertilization does take place, other postmating mechanisms may operate. The hybrid may be inviable (zygotic inviability). In other cases, development may be essentially normal, but the hybrid may be ill-adapted to survive in any available habitat (hybrid adaptive inferiority).
Even if hybrids are produced, they may be partially or totally sterile (hybrid sterility). Hybrids between closely related forms are more likely to be fertile than those between more distantly related species, but the correlation is an inexact one.
The causes for hybrid sterility are complex and can involve genetic factors, differences in gene arrangements on the chromosomes that disrupt normal chromosomal pairing and segregation at meiosis, and incompatibilities between cytoplasmic factors and the chromosomes.
If the hybrids are fertile and interbreed or backcross to one of the parental forms, a subtler phenomenon known as hybrid breakdown sometimes occurs.
It takes the form of reduced fertility or reduced viability in the offspring. The basis for hybrid breakdown is poorly understood but may result from an imbalance of gene complexes contributed by the two species.
A Fail-Safe System
Inmost cases of reproductive isolation that have been carefully studied, more than one kind of isolating mechanism has been found. Even though one type is clearly of paramount importance, it is usually supplemented by others. Should the predominant type fail, others may come into play. In this sense, reproductive isolation can be viewed as a fail-safe system.
A striking difference in the overall pattern of reproductive isolation between animals and plants is the much greater importance of premating isolation in animals and the emphasis on postmating mechanisms in plants.
Behavioral isolation, together with other premating mechanisms, is highly effective in animals, and postmating factors usually function only as a last resort. In contrast, behavioral factors contribute little to premating isolation in plants.
Pollen in many forms is widely distributed either by the wind or by unselective animal pollinators, and post mating factors consequently are much more likely to come into play. This difference is reflected in a much higher incidence of natural hybridization in plants as compared with animals.