The Evolution of Sex Flashcards
What is sexual reproduction?
Gametogensis - The production of haploid sperm and eggs via meiosis.
Fertilisation - Fusion of haploid gametes from two different individuals to produce diploid embryos.
Why is sexual reproduction a theoretical problem?
Shouldn’t have evolved, very inefficient compared to asexual reproduction. Sexually reproducing females:
- Share offspring relatedness with the male, so 0.5
- Have to produce offspring who won’t produce any themselves (sons)
Asexual lineages can outcompete sexual lineages in a resource limited environment.
What are the costs of sexual reproduction?
Sharing genetic relatedness of offspring
Production of males
Breakup of coadapted gene complexes through recombination
Requirement to find mates
Sexual conflict
Explain the Fisher-Muller hypothesis
In asexual lineages there is no way to purge mutations, which harm fitness, other than a randomly occuring back-mutation.
Sexual reproduction can counter this - inheritance is not 100% and parents are likely to come from different lineages.
What is the evidence for drift based models like the Fisher-Muller hypothesis
Asexual species do not tend to persist over evolutionary timescales - twigs on the tree of life
Exceptions, such as bdelloid rotifers reproduce asexually have been around with 40 million years with 360 species.
What is the red queen hypothesis?
Over time and space, biotic and abiotic selection pressures vary.
Due to recombination, sexual reproduction increases variation, which increases adaptability, which allows for more rapid evolutionary change.
What is the evidence for selection-based models?
New Zealand mud snails are infected by trematode parasites and can reproduce sexually and asexually.
Asexuals can produce twice as many daughters
Positive relationship between proportion of males and parasite load.
Suggests that sexually reproducing snails (measured by number of males) are more able to cope with parasite load.
What is anisogamy?
Form of sexual reproduction that involves the fusion of two different sized/formed gametes.
Sperm = small
Egg = large
Describe the evolution of anisogamy
Isogamous protogametes fuse together in aquatic environments - there is natural variation in size/number produced.
They must fuse with other protogametes and provision for resulting zygote.
Limited resources means they can prioritise:
- Numerous, small gametes for maximising number of fertilisations.
OR
- Fewer, larger protogametes to produce fitter offspring.
Disruptive selection occurs, producing two different gametes, sperm and eggs.
What is the evidence for anisogamy evolution?
More sperm means more competitive for fusions
Species at higher risk of experiencing sperm competition produce more sperm.
