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Why Sex?
Two features that distinguish sexual
from asexual reproduction: meiosis
and syngamy
Meiosis: process by which
diploid organisms produce
haploid gametes
Syngamy is the fusion of two haploid
gametes to produce a diploid zygote.
Sex typically associated with anisogamy
Ansiogamy = differences in size between two gametes
Sex = Recombination
Recombination
Species with separate sexes in separate
individuals are dioecious. (includes most
animals)
Animals in which the same individual can
produce both eggs and sperm are either
simultaneous hermaphrodites or sequential
hermaphrodites.
Hermaphrodites
Simultaneous: many snails, annelid worms
Sequential: echinoderms, some fish (e.g.,
gobies, wrasses, parrot fish)
Asexual Reproduction
Common in plants, less common in animals.
Found in all groups of vertebrates, except
mammals
Many organisms capable of both sexual and
asexual reproduction
hydra, Aphids, Daphnia, nematodes
Sexual reproduction is costly
Gonads are expensive organs to produce and
maintain.
Mating is risky and costly, often involving
elaborate structures and behaviors, risk of
injury and disease.
Recombination breaks up beneficial
combinations of alleles at different loci.
So why does sexual reproduction exist at all?
Asexual reproduction is
evolutionarily advantageous
Assume:
Reproductive mode does not affect how
many offspring a female can produce.
Reproductive mode does not affect the
survival or reproductive success of the
offspring.
Is asexual reproduction is
evolutionarily advantageous?
Does reproductive mode affect how many
offspring a female can produce?
Does reproductive mode affect the
survival or reproductive success of the
offspring?
(Dunbrack et al. 1995)
Evolutionary advantage to sex:
Deleterious mutation and drift --> “Muller’s ratchet”
Faster adaptive evolution
Muller’s
Ratchet Model
In asexual populations,
mutations will accumulate in
lineages because there is no
recombination.
Faster adaptive evolution
If a different beneficial
mutation occurs in two
different families in a
population. If the
population is sexual, the
two different mutations can
end up in the same
organism through meiosis
and syngamy.
Why did ansiogamy evolve?
Why did ansiogamy evolve?
Once we have sexual reproduction, two selection
pressures on gametes:
Bigger gametes (increase zygote size and survival)
More gametes (increase potential number of offspring)
Both strategies increase reproductive potential, just differently
Why did ansiogamy evolve?
Can lead to conflict among sexes (which will invest more).
Bigger gametes (more parental investment - be choosy
when selecting mates)
More gametes (less parental investment - mate as many
times as possible)
Darwin (1871), The Descent
of Man, and Selection in
Relation to Sex
Sexual Selection: “the
advantage which certain
individuals have over others
of the same sex and
species, in exclusive relation
to reproduction.”
Sexual Selection occurs in two forms
Intrasexual: Male-male competition
Intersexual: Female choice