Evolution of sex Flashcards
What is sex?
The process by which an individual is produced from the fusion of gametes produced by two parents.
What is recombination?
The process through which parts of the pair of homologous chromosomes are exchanged - the two copies of the genomes are shuffled together.
What is the other word for sex?
Amphimixis.
Why do we need an evolutionary explanation for sex?
Sex is costly and reduces growth rate relative to asexual reproduction. It reduces the relatedness of parents to offspring and recombination can disrupt good gene combinations.
What is the twofold cost of sex?
The idea that growth rate of a lineage is determined only by the number of females and males are costly since their only role is to be a source of fertilisation.
What is isogamy?
The fusion of two similar-sized gametes.
What is anisogamy?
The fusion of two gametes that are dissimilar in size.
How does anisogamy cause the cost of sex to arise?
As isogamy would not suffer the twofold cost as if males and females had the same gametes, they would be able to interchangably mate.
Why did anisogamy arise if sex arose before?
At some point, the first sexual species must have cast gametes into the environment (water).. Individuals that produced tiny gametes would get the most fertilisations, and individuals that made larger gametes would succeed best if the allowed them to be fertilised by the tiny, common mobile gametes.
What selection caused this anisogamy?
Disruptive selection would have favoured small and numerous sperm and large and less frequent eggs. This resulted in the most successful fertilisation with the largest zygote size.
Why does anisogamy constrain future evolution?
It cannot be reverted back to isogamy so separate sexes are retained.
What are the two basic methods of asexual reproduction?
Apomixis (parthenogenesis) and automixis.
What happens in apomixis?
The gamete does not complete meioisis and the offspring are identical to the mother. There is no loss of genetic variation and no recombination. The species may be obligately or facultatively apomictic or parthenogenetic.
How does automixis work?
Meiosis produces haploid gametes so can include recombination. Haploid nuclei from the same meiosis can fuse. There is little genetic diversity from the parents, but they can be genetically different from the parents.
How common are asexual species in animals?
Rare and lineages are usually short lived.
