Week Two Flashcards
What is the Ruby in the Rubbish genetic hypothesis?
Background selection means that newly arisen good genes get stuck together with bad ones in asexual populations.
- Get pulled down by their bad neighbors.
- May be lost due to selection upon the net fitness of the genome they arose in.
- Only beneficial mutations that occur in very high fitness classes have a chance.
Review Why does fitness decline in asexuals?
Directional effect of mutation on fitness.
Lines are also lost due to genetic drift (by chance).
Why does sex allow faster adaptation of organisms?
In asexuals, only beneficial mutations in the FITTEST class (very unlikely) are propagated.
In sexuals, beneficial mutations can move in ‘both directions’ (high fitness or lower fitness); so they can arise anywhere in the distribution and still be selected if they do not go extinct early on.
What was the experiment that tested the effect of sexual recombination on the rate of accumulation of beneficial mutations?
They had two groups; sexual reproducer with competitor, and asexual reproducer with competitor.
-Used special ‘clone generators’ to stop recombination in Asex Treatment.
Design: Genotypes were drawn randomly. Half experiments were with sex, and half with asex.
+10% advantage given to asex and sex flies for beneficial effect.
Results: There was more variation when mutation arises in an asexual genetic background (lineages due to limited sharing of genes), than a sexual one.
- Asexuals had greater fitness over shorter time!! Many asex lineages died out.
- Faster average rate of increase in sexuals. (good gene accumulated faster).
- Biological data is often noisy: Many, many experiments are needed in biology.
What is the fate of a gene that promotes recombination (in an unchanging environment)?
Unchanging environment - no selection, sex loses. Very unrealistic.
Genetic mixing did NOT evolve, increasing variation is detrimental.
What were the mathematical models in the 60s + 70s based on?
The early mathmatical models were developed around hypothetical genes coding for increased recombination.
These models failed because there was an unrealistic assumption of populations adapting to an unchanging environment.
What were the mathematical models in the 80s + 90s based on?
Kondrashov, Barton, Charlesworth used mathematical treatments, assumed populations are under selection, and that genes interact with one another.
- Mutations can combine non-additively. Ex. each additional mutation makes its bearer disproportionately sicker than it would if it was the only mutation carried.
-Genetic mixing can evolve…..sometimes! under a very restrictive range of conditions, sex can evolve.
What is the Mutational deterministic genetic theory of sexual reproduction? (Kondrashov)
Like Muller, Kondrashov envisions elimination of mutations as the key advantage to sex.
Assumptions: Mutation rates affecting Darwinian fitness are high: > 1/ genome / generation
- Mutations have a synergistic, negative impact on fitness.
What is synergistic epistasis?
Sex helps create genotypes with more mutations, which are eliminated by selection.
-Slow decrease in fitness with number of mutations…..then hits a threshold and plummets.
What is antagonistic epistasis?
Individuals with few mutations are rapidly eliminated without the help of sexual recombination.
-Sharp decrease in fitness with number of mutations, then it levels off.
Why does the MD theory come up short?
The range of conditions was not overly realistic.
- Fitness (reproductive success x survival) must be curvilinearly-related to number of mutations in a specific way.
- Data from experiments looking for synergistic effects of mutation are few and far from convincing.
ALL THREE PATTERNS HAVE BEEN OBSERVED!
Mutation rates have to be higher than what has been measured in most short-lived organisms.
Which species and methods did Goddard, Godfray and Burt use to show how Sex facilitates adaptation?
Used yeast; Saccharomyces cerevesiae, which naturally undergoes both sexual and asexual reproduction.
Knocked our two genes responsible for recombinaiton, creating an obligately asexual, diploid strain.
What were the results of Goddard, Godfray and Burt’s experiment?
Under unchanging control conditions, sexual and asexual strains were indistinguishable.
Under environmental challenge, sexual strains adapted more rapidly.
Weismann (1889) suggested that the significance of sex lies in the generation of variation to fuel selection.
What are the molecular signatures of hitch hiking?
How do asexual and sexual reproduction affect this?
Bad genes get dragged to high frequencies due to linkage with good genes.
Sexual reproduction will eventually break the linkage through new combinations of alleles. Bad allele will increase, then decrease.
For asexual reproduction, the allele will go to fixation and not reduce.
What is the main idea behind the Diversity/Uncertainty hypotheses?
Reduction via meiosis and fusion via gametes creates diversity.
What is the Williams’ Elm-Oster model (diversity)
Environmental heterogeneity and specificity (spatial or temporal) may favour diverse offspring.
A gap in the forest may only support one new tree and have very particular prevailing conditions.
Sending many siblings (1000s of seeds) into the gap increases the odds that the one that survives (the best genotype) is your offspring)
What is the Tangled Bank model? (diversity)
Michael Ghiselin emphasized that genetically diverse offspring can differ in resource use.
- Having different niches would allow more siblings to survive in the same environment than if reproduction were clonal and all offspring do the same thing.
- Sexual species more successful because of higher coverage of micro-environment and reduced sibling competition.
-predicts higher carrying capacity for the same environment if occupied by sexual species than by asexual species.
What are some criticisms of the Tangled bank?
1) Overlooks the fact that populations of clones may be genetically diverse and occupy a variety of niches.
2) Some experimental studies show that the carrying capacities of cultures of genetically populations are no greater than those of individual component genotypes in monoculture.
3) Presupposes siblings are in competition with one another locally. Critics have questioned how widespread the conditions for this kind of sibling competition would be, and note that we rarely see sexual and asexual forms of a species in immediate competition.
What is the Red Queen Hypothesis?
Leigh Van Valen likened evolution to an arms race.
-Because other species are constantly improving as competitors, or predators, a species has to constantly evolve just to ‘stay in one place’ (survive in its niche).
- Van Valen’s formulation was directed at macroevolutionary patterns such as species extinctions in the fossil record.
- A microevolutionary interpretation has become synonymous with the RQ hypothesis.
According to Van Valen, when do species go extinct?
He showed that species go to extinction whether or not disturbances occur. A species was as likely to go extinct if it had existed for a million or a hundred million years.
How is the Red Queen linked to parasitism?
Parasites are typically smaller and undergo more generations per unit time than their hosts.
- A Host’s offspring may therefore face a very different suite of parasite genotypes
- Sex allows the host to produce an array of offspring genotypes
- If the parasites have cracked the parents’ defenses, their offspring may have a better chance if they express recombined, novel genotypes.
Challenge with biotic versus abiotic factors
Unlike abiotic challenges, an organism’s enemies evolve against it
What would the Red Queen hypothesis predict with Trematode parasitism and Snails?
1) Sexual snails should be more common in lakes where the probability of infection is highest.
2) Parasites from one lake should be LESS successful infecting snails from another.
3) Parasites should ‘track’ most common snail genotypes in a given lake, so that rare genotypes have some degree of resistance.
4) Within a lake, sexual snails will be found where ducks dabble.
If Red Queen arms race is present, predict incident of sex and parasitism.
The frequency of sexuals should correlate with the probability of infection.
