Species are created as a result of reproductive isolation
Speciation is defined reproductive isolation, but isolation can take many forms. In general, isolating mechanism are one of two types: prezygotic or postzygotic. That means that either mating (zygote formation) is blocked (prezygotic) or that the fertile zygotes don't form (postzygotic).
(Why is it important to say that "fertile" zygotes don't form? If horses and donkeys regularly mated in the wild to produce infertile mules would that make them a single species?)
There are eight categories of isolating mechanisms:
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Prezygotic Isolation: |
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Temporal Isolation |
Populations overlap geographically, but mate at different times |
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Behavioral Isolation |
Mating behavior is different between populations that they can not mate |
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Mechanical Isolation |
Essentially a plumbing problem; sperm can not be productively deposited |
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Gametic Mortality |
Sperm are deposited, but can not fertilize, and eventually die |
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Ecological Isolation |
Potential mates occupy different habitats |
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Postzygotic Isolation: |
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Zygotic Mortality |
The egg is fertilized, but the zygote or embryo dies |
| Hybrid Inviability | F1 hybrids form, but have very low fitness |
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Hybrid Offspring |
Hybrids are sterile |
The biological species concept works very well on live populations of sexually reproducing individuals.
It is obviously not a viable definition when we look at the fossil record. There is no way to be certain that certain fossils were or were not able to cross fertilize. For fossil species the operative definition must be morphological. This has hazards, but works well for the majority of cases. This means that we must be conservative in our designation of distinct species in the fossil record.
It is also not useful when speaking about various microbial species which do not have a true sexual cycle, or which only rarely reproduce sexually. There are, however, mechanisms which operate with these species which assure lateral movement of alleles between and within populations.
