Kønsidentitet og seksuel orientering er uafhængige dele af en persons seksuelle identitet, som normalt er i harmoni med genitalia, men ikke altid. Denne gennemgang diskuterer, hvordan prænatale faktorer, især hormonelle påvirkninger, former hjernens udvikling og seksuel adfærd. Der er dog undtagelser, og genetiske og immunologiske faktorer spiller også en rolle. Samlet set indikerer beviser, at biologiske faktorer væsentligt bidrager til udviklingen af seksuel identitet og orientering.
Titel på undersøgelse:
Gamete Competition, Gamete Limitation, and the Evolution of Two Sexes.
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Hele abstrakt på originalsprog:
Males and females are a fundamental aspect of human reproduction, yet procreation is perfectly possible without this division into two sexes. Biologically, males are defined as the sex that produces the smaller gametes (e.g. sperm), implying that the male and female sexes only exist in species with gamete dimorphism (anisogamy). Our ancestors were isogamous, meaning that only one gamete size was produced. The question of the evolutionary origin of males and females is then synonymous to asking what evolutionary pressures caused gamete sizes to diverge. Studying the ancestral evolutionary divergence of males and females relies largely on mathematical modelling. Here, we review two classes of models explaining the evolutionary origin of males and females: gamete competition and gamete limitation. These seemingly alternative explanations are not mutually exclusive, but two aspects of a single evolutionary process. Once evolved, anisogamy and the two sexes are evolutionarily very stable. This explains the maintenance of anisogamy in organisms with internal fertilization, which can cause large decreases in both gamete competition and gamete limitation. The ancestral divergence and maintenance of gamete sizes subsequently led to many other differences we now observe between the two sexes, sowing the seeds for what we have become.