Download Evolution Quiz Week 6

“+” = correct answer
1) A. What result is expected from the McDonlad-Kreitman test if a gene is not under selection?
a. Non-synonymous: synonymous (between species) > Non-synonymous: synonymous
(between populations)
b. Non-synonymous: synonymous (between species) = Non-synonymous: synonymous
(between populations) +
c. Non-synonymous: synonymous (between species) < Non-synonymous: synonymous
(between populations)
B. What ratio determines the amount of selective constraint?
d. Replacement (nonsynonymous) mutations : silent (synonymous) +
e. Polymorphic : fixed
f. Polymorphic replacements: fixed replacements
g. Polymorphic silent mutations : fixed silent mutations
2) What is the cost of sex?
a. Producing males
b. Cost of finding mates
c. Cost of mating
d. Cost of recombination
e. All of the above +
3) Why are asexual species typically apomorphic?
a. Because they have high fitness
b. Because they only have high short-term fitness +
c. Because they only have long-term fitness
d. Because mating is a risky business
4) Which is not an example of intrasexual competition?
a. Combat
b. Sensory bias +
c. Sperm competition
d. Alternative mating strategies
e. Infanticide
5) Which two are not a trait to ensure self-fertilization in plants (mark two)?
a. Reduction in flower size, scent, and markings for pollinators
b. Self-incompatibility loci +
c. Cleistogamous flowers
d. Asynchronous female and male reproductive functions +
6) Two questions you had on the paper
Molecular Clock
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Selective constraint determines rate of evolution in proteins
o Globins have less selective constraint so evolve more quickly
o Histones have more constraint and evolve more slowly
McDonald-Kreitman Test
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Used to determine the difference between drift and selection
If gene is not under selection, short term evolution & long term evolution is the same
If under selection, short and long term are not the same
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Fixed between species
o Second position of third codon
polymorphic within species
o third position in first codon
Silent
o Third position
Replacement
o First or second position
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For example, what is the ratio of these?
Linkage disequilibrium and the evolution of sex
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Most expensive part of sex is the males
Hybridogenetic: Female mates with male of another spp, genes of father are not expressed in
daughter or converted to gametes
Gynogenetic: Sperm touches egg but does not transfer material, this stimulates egg to develop
Parthenogenetic: No males necessary, females occasionally go on mating rampages
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Environmental sex determination: Common in reptiles, may be explanation for KT boundary
Meiosis & syngamy distinguish sexual from asexual reproduction
o Syngamy: Fusion of two cells in reproduction
Sexual reproduction entails shift from isogamy to anisogamy
o Isogamy: Gametes are same size
o Anisogamy: Gametes are not same size
The closer two genes are located on the genome, the less likely they are to be separated by
linkage disequilibrium
Women with small eggs or men with large, fewer sperm cannot reproduce, so stable system
Costs of sex
o Finding mates
 Exacerbated by low population density
o Mating
 Risky business, std
o Recombination
 Creates superb combinations of genes but also breaks them up
Why does sexual reproduction persist
o Enhanced adaptive evolution by recombination
o Breaks up bad combinations of genes, or deleterious mutations
o Advantageous mutations are combined across lineages
Asexual spp are typically at the tips of phylogenetic trees, because there is a large short term
benefit, but deleterious long-term consequences
Sex assists in long term with red queen hypothesis
Sexual Selection
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The fitness advantage that some individuals have over other individuals of the same sex, with
respect to reproduction
Fitness in sexual selection
o Viability
o Fecundity
o Longevity
o Mating success
Males are limited by # of females they can mate with, while females are limited by pregnancies
Intrasexual competition: males are competitive
o Combat
o Sperm competition
o Alternative mating strategies
 Sneaker males: look like females but have huge gonads. Not heritable
o infanticide
Intersexual competition: females are choosy
o Direct benefit
o Good genes
 So fit that it can produce outrageous characteristics
o Sensory bias: Gene where female prefers a particular trait
 They want huge tails on the sword-tail
Genetics of sexual selection
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Runaway selection
 One gene of sensory bias, one gene for desirable trait
 Two genes become close on genome through linkage disequilibrium
 Co-segregate: Daughters have sensory bias, sons have desirable trait
Inter-locus conflict
 2 separate genes that increase fitness or male or female, therefore genes are in
conflict
 Female must choose male that gives high fitness to her daughters & sons
equally
Intra-locus conflict: Same gene provides fitness for female & male
 Female wants to take male with high fitness, but that would result in daughters
of low fitness
 Only way to get out is multiple mating’s with males of low fitness
EVOLUTION OF MATING SYSTEMS
Evolution of sex ratio
 Population sex ratio: Males to females in population
 Individual sex ratio: ratio of progeny from female
 Homogametic: Females in humans are XX, males in birds are ZZ
 Heterogametic: Males in humans are XY, females in birds are WZ
 Sex chromosomes do not guarantee a 1:1 sex ratio
 R.A. Fisher provided genetic explanation for evolution of stable sex ratio of 1:1
o Males & females have same average fitness
o Each individual has 1 mother & 1 father
o However, variance in fitness in males can be much larger than in females (northern
elephant seal)
o Frequency-dependent selection results in stable equilibrium sex ratio of 1:1
o Fitness must be considered across 2 generations
 Experiment confirming Fisher’s theory
o Platys showed that, even with 3 genes controlling sex, sex ratio stabilizes around 1:1
 Exceptions to Fishers theory
o Local mate competition (ie Parasitoid wasps)
 High inbreeding, no need for males
o Condition-dependent sex allocation
 Lots of food, females can produce strongest offspring possible
 Best offspring to make is a male, as sexy sons produce more offspring
 Occurs in polygyonous spp where females invest heavily in young
 Polygyonous: multiple mates
o Bi-directional sex change: goby in Japan: sex ratio biased towards females when food,
males when little food
 Why change sex?
o If no change sex, reproductive success increases equally with body size in both sexes
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Protogynous hermaphroditism affects reproductive success
 Protogynous is first female then male
Protandrous species: reproductive success of males does not increase as rapidly with size, as
it does for females
 Protandrous: First male, then female
The evolution of inbreeding and outbreeding
 Inbreeding is not great, as deleterious mutations cannot be replaced
 Many spp evolved traits to avoid inbreeding
o Asynchronous female and male functions with plants of both
o Monoecious: same plant has male and female parts separated
o Dieocious: One plant is one sex
o Self-incompatibility loci: prevents selfing or breeding with close relatives
o Heterostyly: Results in pollen transferred more effectively between plants than within
plant
 Primrose has two different kinds of flowers
 However, some plants evolved characteristics to ensure self-fertilization
o Reduction in size of flower, scent & markings for pollinators, pollen production
o Flowers may be cleistogamous (not open at all)
 Peas, used by Gregor Mendel, are cleistogamous
o Partial selfers have fitness advantage because genes can still be transmitted enough to
avoid ratchet
 Contain benefit if pollinators do not pull through (reproductive assurance)
 Paper used selfing population & sexual population of snail, then subjected both
to selection by removing largest snails. Selfing population did poor
Basolo, A.L., 1995. Phylogenetic evidence for the role of a pre-existing bias in sexual
selection. Proceedings of the Royal Society of London B: Biological Sciences, 259(1356), pp.307311.
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This paper attempts to build a phylogeny & determine whether whether swordtails arose from
platys or vice versa
Genus xiphophorus includes sworded swordtails & unsworded platyfish, which both prefer long
tails
o Molecular phylogeny proposes that platys arose within swordfish
o Maximum parsimony suggests the same, as both spp prefer swords, but males in platys
do not have swords (decoupling of co-segregated genes in swordtails?)
Researchers exposed females to males with taped on tails for 10 minutes to determine
preference
The longer his sword, the longer the female spent in his presence