Lecture 5 - again Flashcards
What defines males and females?
Gamete size and anisogomy.
Gametes are the reproductive genes, e.g. sperm and eggs. Males have microgametes and females have macrogametes.
Anisogamy is the sexual reproduction by the fusion of dissimilar gametes. It is the fundamental sex difference.
What is sexual selection?
It is natural selection arising through preference by one sex for certain characteristics in individuals of the other sex.
Males often have traits that have been selected through sexual selection, that females do not have. They serve only to give one male an advantage over another.
What is the difference between natural selection and sexual selection
Natural selection is the survival of the fittest.
Sexual selection is the survival of the sexiest.
Fundamentally, both work in the same way, but sexual selection focuses its attention on the selective consquences of sexual interactions within a species. Natural selection operates in the context of other environmental factors (such as disease, climate, adaptations for food finding etc).
What is batemans principle (1948)
He suggested that in most species, variability in reproductive success (or reproductive variance) is greater in males than in females. This can be seen as the result of anisogamy.
He studied the fertility of male and female fruit flies as a function of the number of mates. Males had many more mates, whereas females had fewer. The more mates the males had the more offspring they had, but women plateu at 1 mate, sleeping with more mates does not increase their chances of offspring.
The main findings were that variance in male reproductive success than female, and males reproductive success depends on number of mates.
The sex which invests more in the young becomes a resource for which other members of the less parental sex compete.
What are the two mechanisms of sexual selection
Male-male competition. Whereby, males compete for access to females, and females are the choosier sex. Males compete for access to females in the majority of species - females are choosy, therefore males are competitive.
Female choice, where females in certain species use physical or chemical mechanisms to control a male’s success of inseminating them; namely by selecting whether sperm are successful in fertilizing their eggs or not.
Both of these lead to sex differences in both morphology and behaviour. As a generalisation, the bigger the difference in parental investment between sexes, the more different the sexes are. For example, size dimorphism. Increasing polygyny reflects stronger male-male competition. Stronger competition leads to increase sexual dimorphism, males are much larger and often brighter in most mammal and bird species.
Discuss sperm competition
Perhaps the greatest constraint on male promiscuity is post-copulatory competition. Sperm competition. This occurs post copulation where there is male-male competition. Mate guarding is a way an individual can prevent sperm competition, making sure their mate is not inseminated by another.
Another way is sperm displacement, blocking the females reproductive tract.
Lastly, crytpic female choice. The complexitiy of the female tract can be used to select the best sperm. There is a positive correlation between frequency of remating and complexity.
Discuss sperm competition and body size dimorphism in primates
Testes size and body size dimorphism are linked to mating systems in primates. Multi-male/multi-female groups provide a lot of opportunity for sperm competition.
In humans, spending more time apart from partner increases mens attraction to partner, interest in sex and belief that their partner wants to have sex.
Discuss female choice
Females may prefer males with secondary sexual traits - the power to charm the female.
Female preference for elaborate feature is seen in the African widowbird (Andersson, 1982). Small birds with very long tails employed during courtship. They found that females prefered males with longer tailed, even though they were fake.
There is three models for this, the runaway selection, good genes and sensory exploitation
What is the runaway selection model by Fisher?
There is some variation in females preference for a male trait. The sons of these females will inherit the trait, and daughters will inherit the preferences (genetic covariance). There is then positive feedback in selection - females prefer males with the trait because they enjoy an advantage.
Fisher originally envisioned a functional advantage to male traits which can set the ball rolling.
What is the good genes hypothesis
Females can gain directly from mating, e.g. nutrition. But it many cases they don’t but are still choosy.
Sometimes females choose even though they don’t seem to gain anything other than sperm. Therefore the handicap principle was suggested, stating that secondary sexual characteristics may be attractive because they are detrimental to survival.
For example, testosterone-dependent traits demonstrate the immunocompetence handicap hypothesis is things such as bird song and morphology.
What is the sensory exploitation theory
Pre-exisiting female choice.
Female preferences and male traits can coevolve rapidly, especially when females are penalized reproductively (for example, through reduced survival or mating success of females or their offspring) for mating with males exhibiting an “exploiting” stimulus.
For example in humans, high heels.
