Darwin and human reproduction: a long way to gynaecology

the following 9 chapters summarize the most essential steps that through evolution led to the reproductive behaviour of humans. Hence, the text is not necessarily confined to our species. it is a brief personal view, written as background information for reproductive specialists and gynaecologists. the objective is to give a rough sketch of the evolutionary framework in which some current reproductive phenomena may fit. 1. The long way from isogamous reproduction to gender differences Most sexual eukaryotic species are anisogamous, which means that male and female genotypes are contained in gametes of different size and characteristics. Anisogamy has evolved by disruptive selection from ancestors with isogamous reproduction (bulmer et al., 2002), and is believed to be the origin of all further sexual dimorphism, based on two assumptions. First, the amount of resources that an individual allocates to reproduction is constant, necessitating a trade off between gamete size and the number of gametes produced. the second assumption is that the survival of a zygote depends on its size, which is determined by the size of the two gametes that fuse to form the zygote. selection favoured offspring of gametes provided with reserve substances (yolk), leading to the production of macrogametes (egg cells). Other gametes of the same species didn't invest in the ballast of yolk, allowing them to move faster to the macrogametes for fertilization than the competing gametes. they became the mobile microgametes (sperm). two opposing forces therefore impinge on the evolution of gamete size: production of a large number of small gametes increases the number of fusions, whereas a smaller number of larger gametes increases the survival of the resulting zygotes. Moreover , the relative importance of these forces is frequency-dependent: the more large gametes are present, the more advantageous it is to produce many small gametes. Parker et al. (1972) showed that this frequency-dependence can lead to disruptive selection on gamete size. Alleles encoding for large and small gametes are favoured over alleles encoding for gametes of intermediate size if the range of gamete size is large enough. For the same amount of energy and raw materials many more sperm than eggs are produced, resulting in a permanent excess of sperm available for a relative shortage of eggs. by extension , the competition between sperm for fertilization of ova is the basis of male competition and behaviour in search for fertile females, rather than vice versa. 2. Parental …