Trivers-Willard model

The Trivers-Willard model describes the behavior of an organism in a situation where the external conditions permit determination in advance of whether sons or daughters will have greater fitness (Trivers & Willard 1973). Under these conditions, it is advantageous for an individual if it is capable of adapting to the momentary conditions and can produce individuals of a single particular sex.

            The situation amongst parasitic hymenopterous insects can serve as an example (King 1994). A simple mechanism exists in hymenopterous, haplodiploid insects, which can determine in a female whether an egg should develop into a male or female. Males are haploid and are hatched from unfertilized eggs, while females are diploid and hatch from fertilized eggs. Because, following copulation, females store the male sperm in their spermatheca, sometimes for the rest of their lives, rotation of the egg in the oviduct can determine whether it is fertilized or not and whether it will produce a female or a male. For example, the larvae of parasitic wasps of the genus Nasonia parasitize on the larvae of grain-eating insects, so that the amount of nutrients that they have available during their development and thus the size to which they grow and will also have as adults can be estimated in advance according to the size of the grain into which the female lays its eggs. If the grain is small, the grain-eating larvae inside will also be small and the future larvae and adults of the parasitic wasp will not grow to the required size. This is a handicap for females, as the number of eggs produced during its lifetime is directly proportional to its size. In contrast, even a small male is capable of producing more than enough sperm to fertilize every female that it encounters during its lifetime. Thus, it is advantageous for a female to lay unfertilized eggs in small grains and fertilized eggs in large grains.

            In diploid organisms with heterogametic male sex, the sex of the embryo is determined by the sex chromosome brought into the zygote by the male gamete. Thus, it might seem that the female has rather limited means of influencing the sex of her progeny. However, even here, appropriate mechanisms can exist, consisting, for example, in separation of sperm with chromosome X or Y, or possibly the possibility of selectively killing zygotes, embryos or, in the extreme case, young of a certain sex (Komdeur et al. 1997; Mendl et al. 1995). The effectiveness of such mechanisms in humans is reflected, for example, by a secondary sex ratio close to a value of 5 (in favor of males) in some areas of India and Pakistan (Judson 1994).

            Shifting of the sex ratio in humans in dependence on the physiological condition of the mother apparently also occurs by natural means. For example, it has been observed that, if a mother gives birth to a girl with low birth weight (under 3.2 kg) in her first pregnancy, it is more probable that the next child will also be a girl (64%). If the first-born girl was heavier than 3.2 kg, the probability that another girl will be born is lower (33%) (Pawlowski & Cieplak 2002). Similarly, it has been observed that women with latent toxoplasmosis (approximately one quarter of women of fertile age in the population under study) have several dozen percent greater chance of giving birth to a boy than a girl (Fig. XIV.7) {12801, 13715}. It is possible that the partial immunosuppression induced by the parasite means that a greater percentage of male zygotes survive in the female organism (in general, male zygotes induce a greater immune response than female zygotes), as a consequence of which the secondary sex ratio in women infected with toxoplasmosis is much closer to the primary sex ratio (ratio of male and female zygotes immediately after fertilization of the egg).

            In some animals, specifically in a number of species of birds and mammals including humans, the female can react, not only to the external factors in the environment, for example the amount of food available, but also, for example, to her social position in the flock or herd. Because the social position of the parents can substantially affect, either genetically or nongenetically, the social position of the progeny, it is advantageous for the parents if they are capable of adapting the sex of their progeny to their social position. In most societies, if they hold a low social position, it is more advantageous for them to produce daughters, as even females with a low social position are usually able to reproduce. On the other hand, if the parents hold a high social position, it is evolutionarily more advantageous for them to produce sons. In a great many species, males with higher social position become the fathers of most of the young in the population. This phenomenon has, of course, also been studied in humans but the results are so far not entirely unambiguous (Chahnazarin 2003). In any case, the substantially greater number of sons in the descendants of the European aristocrats, American presidents or American generals (Mueller & Mazur 1998), or in women who were in the care of more expensive gynecologists {12801} is a fact for which it is rather difficult to find an alternative explanation.  The results of an extensive study performed on data derived from pre-industrial Scandinavia suggest a possibility. In contrast to the number of daughters, the number of sons is negatively correlated with the length of the life of the mother (every son meant shortening of the mother’s life by 34 weeks). This suggests that “bringing up” sons is more demanding on the maternal organism than “bringing up” daughters and only women in good condition with sufficient material security can allow themselves to invest their reproduction potential into sons {10869}.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more