XIV.6 An individual’s sex is mostly genetically determined
Theoretically, it is possible to imagine a number of mechanisms that would control differentiation of individuals into two types, male and female.However, basically, there are only two alternatives.The sex of an individual can be determined by his genotype or by factors of the external environment.Evolution chose the first variant in the vast majority of species.The sex of the adult individual is determined at the moment of joining of the two gametes and formation of the zygote.There are endless specific mechanisms through which this state can be achieved.Two sex chromosomes exist in the chromosomal set of the species in the most of important taxa.These chromosomes have two variants, which we will call A and B.If the zygote contains both sex chromosomes of the same type, for example A, then a member of the homogametic sex will develop from it; if it has one sex chromosome A and the other B, then a member of the heterogametic sex will develop from it.In some taxa, the homogametic sex is the male sex and the heterogametic sex is the female sex, in other taxa it is the opposite.A member of the homogametic sex produces gametes (microgametes or macrogametes) only with one type of sex chromosome, let’s say A, while a member of the heterogametic sex produces half of its gametes with sex chromosome A and half with sex chromosome B.As a consequence, under normal circumstances, only zygotes of two kinds can be formed, with pairs of sex chromosomes AA and AB.Zygotes with the BB combination can never be formed.Many groups of organisms have substantially more complicated systems of determining the sex of individuals.Haplodiploid organisms constitute an extreme case, where the members of one sex are diploid and the zygotes that develop from them are formed by normal fusion of two gametes, while the members of the other sex are haploid and derived from unfertilized macrogametes (Hymenoptera, Thysanoptera, and another approx. 15 independently formed groups of metazoa) (Koivisto & Braig 2003).
Another way of determining sex consists in determination of the direction of ontogenesis by factors in the external environment.The green spoonworm(Bonellia viridis) is a well-known example.Larvae that develop solitarily form macroscopic females, while larvae that are settled on the surface of the body of the female develop into microscopic males.In a great many reptiles, the temperature and humidity at the time of incubation of the eggs affect the sex of the young (Janzen 1994)and a similar situation is known for some crustaceans (Dufresne & Hebert 1994).
At first glance, it may seem that, as is demonstrated by the Trivers-Willard model, the latter means of determining sex is more advantageous than the former.More males could be born under conditions more favorable for the development of males and more females under conditions more favorable for the development of females.However, in actual fact, this mechanism exposes the population of the particular species and thus the species itself to greater risk of extinction.If a random fluctuation of external factors creates permanent conditions favorable for the development of individuals of one sex for a prolonged period of time, the population of this species could become extinct because of the absence of individuals of the other sex.
The degree to which the risk of extinction as a consequence of a long-term fluctuation of the external conditions (and thus the action of species selection) is actually a reason why evolution chose the genetic means of determining sex in the vast majority of cases is a matter of conjecture. Opponents point out the fact that, in a population exposed to conditions preferring the birth, for example, of females, a mutant capable of producing males would have such a great selection advantage that its progeny would very rapidly shift the entire structure back to equilibrium.However, at the very least, it is doubtful whether a similar shift could occur in the opposite direction, i.e. under conditions preferring the birth of males and whether the density of the population in the transition period would not decrease below the critical limit for survival of the particular species.s