∞ generated and posted on 2016.12.08 ∞
Reproductive isolation mechanismthat acts prior to conception
|Prezygotic Barriers block reproduction prior to conception, that is, prior to formation of hybrid progeny and as such prior to use and/or loss of one or both gametes, male (sperm) and female (ovum, i.e., egg). Formation of such reproductive barriers is key to formation of separate biological species from a single species, i.e., completion of a speciation event.|
Prezygotic reproductive barriers, contrasting postzygotic barriers, are important particularly because they do not waste gametes, particularly the oh so valuable eggs, nor is energy wasted in gestation or subsequent raising of offspring. That is, parental investment is minimal and even nonexistent if conception – production of the zygote – is avoided in combination with a sparing of gametes.
Prezygotic isolation mechanisms include mechanisms that…
Note that all of these mechanisms have a genetic basis except for geographical isolation.
These barriers, as listed, represent increasing potential costs to the participants. Thus, mating itself can be costly both energetically and because it can be distracting, to animals, so is best avoided unless it has some potential to produce successful progeny, that is, assuming that post-mating, including postzygotic barriers are not present (hence the utility of behavioral barriers to mating).
Furthermore, if mating is attempted, then it may be preferable, even for males, to avoid wasting gametes, an issue that is particularly relevant for plants in terms of wasting pollen, and particularly so if that pollen is insect transferred (thus the utility of mechanical barriers associated with flowers that can bar all but a preferred pollinator).
Lastly, even if sperm are able to reach egg, such as may be seen especially among those many species that release sperm or pollen en mass into the water or air, then it can be in the egg's best interest to have mechanisms that serve to prevent sperm from different species from successfully fertilizing.
All of these mechanisms, though resulting in increasing costs, nonetheless as prezygotic barriers are preferable to allowing zygotes that are destined to be unsuccessful from forming in the first place (with "unsuccessful" here representing various postzygotic barriers).
The evolution of prezygotic reproductive barriers is a key step in speciation events since these barriers serve to block hybridization and therefore the movement of alleles between two populations. If movement of alleles between populations is minimal, and opportunities for such movement are not minimal, then by the biological species concept the two populations can reasonably be described as both separate and distinct species.