Transcript Linkage, sex, and natural selection

Origins and maintenance of sex
Sexual conflict
Sex ratios
Dr. Sally Otto, UBC
Asexuality vs. self-fertilization
• Self-fertilization:
• Asexuality:
Costs of sex
Advantages of asexuality: which
sex limits population growth?
Asexual lineage
Sexual lineage
Advantages of asexuality: fitness
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of individuals
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SEX
ASEX
So, is sex good?
99.9% of species
can’t be wrong
Cnemidophorus
Sexual
Asexual
Dandelions
Daphnia
(but a few asexual lineages persist)
Bdelloid rotifers
freshwater filter
feeders
Hypotheses to explain the
maintenance of sex
Harmful mutations: Mller’s ratchet
Speed of adaptation and the Red Queen
Disadvantages of asexuality:
Müller’s ratchet
Mutations happen
and accumulate
mutation-free
chromosome
Müller’s ratchet
“CLICK”
frequency
frequency
LLC: “least-loaded
class”
# of deleterious
mutations
# of deleterious
mutations
Müller’s ratchet in sexuals? No.
frequency
frequency
“CLICK”
# of deleterious
mutations
frequency
# of deleterious
mutations
# of deleterious
mutations
Müller’s ratchet : an experiment
• Set up 444 cultures of Salmonella
• Transfer one individual every 24 hours
• 1700 generations
• Prediction: lower fitness
• Test: generation time:
– parental:
– Experimental populations:
Evidence for Müller’s ratchet : the
human Y chromosome
• In XX females, recombination
• Y chromosome does not pair with X
Müller’s ratchet overall
Reason’s for sex: adaptation
A1 B1
A1 B2
A2 B1
A2 B2
A1 B1
A1 B2
A1 B1
meiosis
A2 B2
A2 B1
A2 B2
Low fitness
High fitness
High fitness
Low fitness
parental
recombinant
recombinant
parental
Does recombination increase
genetic variance for fitness?
D = 0.25
D = -0.25
D=0
If parents have higher than average fitness, what effect of
recombination?
The results of sex: cost of recombination
A1 B1
A1 B2
A1 B1
meiosis
A2 B2
A2 B1
A2 B2
parental
recombinant
recombinant
parental
Experimental test of recombination
and adaptation
Adaptation may require new combinations of
alleles
Asexuality does not allow this
Experiment: flour beetles (Tribolium)
• Have stock population
• Allow one population to evolve
• The other is restocked from original population, as if
asexual.
• Asexual has 3x reproductive advantage
• Asexuals start 0.5 of population
• Selection: pesticide Malathion
• What proportion are sexual?
Advantage of sex: adaptation
Proportion sexual
Malathion
concentration
30
Generations
figure 7.18
Do organisms need to adapt? Red
queen hypothesis
Red Queen to Alice:
“Now, here, you see, it takes all the running
you can do, to keep in the same place.”
--Lewis Carrol,
Through the Looking Glass (1872)
Concept: constant adaptation needed. Why?
Parasites and hosts
Imagine four parasite genotypes, four host
defense genotypes
Parasite
Host
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Parasites and host: Red queen
Parasite
start
Host
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Parasites and host: Red queen
Parasite
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Host
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Evolution over time: cycling
genotypes
Red queen, evidence: topminnows
of Mexico
• Interspecific hybrids from asexual triploids
• or: sexual diploid fish
• Infected by parasites that cause spots
Data I: sexuals vs. asexuals:
which has more parasites?
Parasites
Asexuals have more
parasites
Fish length (mm)
What if there is no variation in
sexual population?
• Heart pool: dried up in 1976
• Recolonized by just a few sexual minnows
and a few asexual minnows.
• Which will have the higher fitness?
Why sex, summary
Why two sexes?
Most species have two sexes
Some have multiple sexes (mating types):
mating type 1 can mate with anyone
except mating type 1, etc.
Advantage: higher proportion of population
available for mating
Why only two mating types??
Implication of two sexes
• Sexual selection
• Sexual conflict
Intrasexual competition: sperm
competition
• When females mate with multiple males,
sperm can compete
Example: yellow dung fly
Scathophaga stercoraria
Yellow dung flies
• Females mate with multiple males
• Last male to mate fertilizes most ovules
• Selection experiment: compare wild to 10
generations monogamous, 10 generations
polyandrous
• Expected effect on sperm competition?
Monogamous
♀+♂
Polygamous
10 gen.
♀ + 3♂
10 gen.
Yellow dung flies: polyandrous
males vs. monogamous males
• Mate each female with two males: one
from polyandrous line, one from
monogamous line
Hosken & Ward
2001. % offspring
sired by second
male to mate
Sexual selection and intersexual
conflict evolution
• Traits that favour male success may harm
female success
• Example: sperm competition
faster sperm win: higher
fitness for male
too many sperm:
polyspermy – egg is lost
human spontaneous
abortions: 3% of
conceptions end due to
triploid embryos (1 egg, 2
sperm)
Female evolutionary response:
slow the sperm
• Yellow dung fly experiment
Evolution of sex ratios
• Which sex should be more common?
• Why aren’t there many females per male?
– imagine monogamous species
– females become more common
– which would be better to have has offspring:
male or female?
Sex bias? If females could choose:
What if polygamous species?
some males have many mates
most males do not mate
If in excellent condition, should a female
produce male or female offspring?
If in poor condition, should a female produce
male or female offspring?
Sex bias: data
Case 1: Kakapo (NZ parrot)
Captive breeding, plenty of food.
When well fed: 70% sons
Adequately fed: 50% sons
Sex bias in humans: data
Mormon second (or higher) wives
Sons
Daughters
One co-wife
4,217
3,994
More than one 712
548
Ratio
105.6
129.9
References
Hoskins, Garner, and Ward. 2001. Sexual conflict selects for male
and female reproductive characters. Current Biology 11:489493.
Hoskins and Ward. 2001. Experimental evidence for testes size
evolution via sperm competition. Ecology Letters 4:10-13.
Lively, Craddock, & Vrijonhoek. 1990. Red queen hypothesis
supported by parasitism in sexual and clonal fish. Nature
344:864-866.
Ridley, M. 1993. The red queen: sex and the evolution of human
nature. Harper. Nice job reviewing Muller’s ratchet and other
hypotheses for the maintenance of sex. Goes boldly into
explaining human nature, far beyond the evidence.
Robertson et al. 2006. Sex allocation theory aids species
conservation. Biology Letters 2:229-231.
Readings and questions
1. In the beetle evolution experiment (figure 8.18) Dunbrack et al
did not actually asexual beetles, as there aren't any. Instead
they used two different lines of beetles that differed in color,
treating one line as if it were asexual by replacing individuals
with individuals from a stock population. The researcher's
simulated asexual population was not allowed to evolve at all
in response to competition and the presence of the
insecticide. Is this realistic?
2. In general, would you expect asexual lineages to persist longer
with small population sizes or large population sizes? Why?
3. If the offspring of sexual and asexual individuals have
equivalent fitness, why would asexuals take over a
population?
4. Explain how Muller's ratchet affects sexual populations
differently from asexual populations.
5.
Explain how the Red Queen hypothesis relates to the
maintenance of sex. Why might sex be advantageous in the
face of parasites or disease?
6.
In most species, the sex ratio is 50 / 50 male: female. Why
wouldn’t evolution favor a higher proportion of females,
since one male could mate with many females? Discuss
using the idea of frequency dependent selection.