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Evolution of Sex
I.
Asexual v. Sexual Reproduction
Theories on the evolution of Sex
A.
B.
1)
2)
3)
C.
Unpredictable environment – Red Queen
Deleterious mutation – Muller’s ratchet
Side effect of a cell survival strategy
Evolution of two sexes
Sexual Selection & Dimorphism
II.
A.
B.
C.
Why do females prefer certain phenotypic traits?
Natural Selection v. Sexual Selection
Males more affected
A. Asexual v. Sexual Reproduction
1) Asexual Reproduction
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Binary fission
Parthenogenesis
Adventitious embryony
Vegetative reproduction
2) Sexual Reproduction
Production of haploid gametes
a.


In many species, haploid (n) gametes are
descended from germ cells that are originally
diploid (2n)
 haploid –
Gametes then combine in fertilization to
reconstitute the diploid complement found in
parental cells
Meiosis
Two successive nuclear
divisions, MI & MII,
which produces haploid
gametes that differ
genetically
b. Cost of Sex
Advantage of asexual females
Generation
Sexuals
Asexuals
1
FxM
F
2
FxM
FxM
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3
FxM
FxM
FxM
FxM
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Fraction of
individuals that are
asexual
B. Theories on the evolution of Sex
1)

2)
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3)
4)
Unpredictable environment – Red Queen
sex results in more genetically varied offspring, thus sexual
reproduction can increase fitness by producing an offspring that
will survive in an unpredictable and changing environment.
Deleterious mutations – Muller’s ratchet
In a strain of asexual species the number of deleterious
mutations accumulate with time. The only way to purge the
mutation is by death. When the genotype with the smallest
number of mutations nx dies , the genotype with the smallest
number of mutations in now nx+1 and the "ratchet" has clicked
up one
Sex breaks the ratchet, Sex allows advantageous alleles in
different individuals to be combined together into the same
individual.
Parasite coevolution theory –
Side effect of Cell Survival Strategy
The two alleles are recombined rapidly in (a), a sexual population, but in (b), an
asexual population, the two alleles must independently arise.
Kondrashov argues that the slightly deleterious nature of mutations mean that the
population will tend to be composed of individuals with a small number of mutations.
C. Evolution of two sexes

Essential difference: size of gametes

50/50 sex ratio common in nearly all
obligatory sexually reproducing species.
Different gamete size requires

II. Sexual Dimorphism & Sexual Selection
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Sexual dimorphism =
Sex “complicates life by adding a third major
challenge: finding a member of the opposite
sex and persuading him/her to cooperate”
Sexual Selection

Directional selection that acts on genetically variable phenotypic
traits that affect the reproductive success of the individuals of a
particular sex
Darwin suggested two avenues for the evolution of these traits:
POSSIBLE MECHANISMS:

Pre-mating sexual selection:

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selection for species recognition (females mating w/own species)
could possibly lead to elaborate secondary sexual characteristics
females could detect differences in the number of deleterious alleles
in an male (i.e. athletic ability required for courtship dance confers
fitness level)
Post-mating sexual selection:


sperm competition
polyandrous species have larger sperm, faster swimming
sperm, more aggressive sperm
A. Why do females prefer certain
phenotypic traits?
1)
Direct Benefit: Certain male characters
(nuptial gifts of food or defensive
compounds, care provided to offspring)
2)
Good genes: Male characters are
"indicators" of "good genes", i.e., that males
possess genes that will provide high fitness
for the offspring.
3) Sensory bias or sensory drive: some
aspect of the sensory world biases
females to
B. Natural Selection v. Sexual Selection

In some situations, the
two forces are actually
opposed –

Example:
C. Males more affected by sexual selection
than females
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Females – spend energy on egg production ability
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Males –
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Magnitude of sperm production favors fertilization of
eggs from many females

Therefore traits that increase their ability to acquire many
mates affected, enhance their competitive ability
Males, do they have it easy or what?
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Males do not need to find quality mates,
rather quantity mates…
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Females choose by:
Sex v. Gender
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Sex –
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Gender -