Sexual Selection (Siva-Jothy) Flashcards
Sexual Selection
Natural selection arising through preference by one sex for certain characteristics in individuals of the other sex.
-Sexual selection works in the opposite direction to natural selection. It is determined when the advantage of one overrides the disadvantage of the other.
E.g. Red colour of stickleback selection in males; selected for by sexual selection (females attracted to it), selected against by natural selection (more likely to be eaten).
Intrasexual selection
Competition within the sexes, e.g. male/male competition, horns
- Fighting
- Courtship display
- Mate guarding
- Sperm competition
Intrasexual selection - Courtship display
Requires a lot of energy. Sometimes it is very extreme, where the loser dies from losing stamina.
Intrasexual selection - Mate guarding
Pre-copulatory: Gamerus sp. Can only reproduce when female has shed their exoskeleton. So they guard her under she sheds so no other male can get access. The ability to find a female close to shedding exoskeleton is selected for.
Post-copulatory: Scatophyga stercoraria Dung beetles. They go to fresh dung, where most reproductive females are. They need to protect the females after mating to ensure eggs aren’t taken by other males. (Best case scenario = 50% of eggs).
Intrasexual selection - Sperm competition
Females can store sperm and other males brush out others’ sperm. They must guard her if they are the one who wants to mate with her. They must disable the sperm of other males.
Intersexual Selection
Competition between the sexes, i.e. traits that make one sex more attractive than the other
Why should females choose one male over another?
- Access to good resources
- Access to good genes
Access to good resources
Courtship feeding (hornbills): Female finds hole in a tree, and builds nest. The female then blocks the female in to protect her from predators. -The female needs to choose a mate who can provide for herself and her offsprings
Access to good genes
The female could be choosing for utilitarian or attractiveness. If choosing for good genes, variation quickly and greatly reduces.
Choice of good genes that will confer high survival on offspring
For example, if she is choosing for resistance to parasites, there will be a different parasite next year and so this offers no real advantage.
Choice of good genes that will make offspring attractive
Differences between the sexes
The only true difference between the sexes is the gametes they produce. Males produce many small sperm, whereas females produce few large eggs.
Males
Males produce lots of sperm. A higher mating rate is favoured by males. Male fitness is not limited to how many gametes they produce. The more mates they have, the better their fitness.
Females
Females produce a few precious eggs. Their fitness is limited by the amount of eggs the produce. The amount of mates they have to reproduce with has no effect on their fitness
Sexual Dimorphism
Distinct difference in size or appearance between the sexes of an animal in addition to the sexual organs themselves.
Sex-role reversal
Male dotterel raises chicks (what is shown to be a more female trait). Female looks for more mates.
- The female is more brightly-coloured than the male so is more able to attract mates.
- Territory is only enough to provide for one adult and one chick, so the male stays in the territory while the female goes out seeking a mate
Runaway sexual selection
Cycle of selection for long tailed males, selection for females choice of long tailed males.
There needs to be two traits for this system to work.
Natural selection for long tail, offers a survival advantage so the female would be selected who favoured males with longer tails. The longer tail + the preference results in runaway sexual selection.
-It only stops when natural selection selects against the trait.
