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20 Cards in this Set

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  • Back
In the 1960’s several techniques were developed for screening genetic variability in
natural populations (e.g., allozyme electrophoresis and DNA sequencing). Evolutionary
biologists were surprised by the relatively high levels of diversity observed. Why were
they surprised? What evolutionary processes might explain the patterns of diversity
observed.
Phenotype isn’t genotype so animals who looks similar and have similar traits could seem similar based on convergent evolution whereas their genomes are much less similar than you would think. Genetic information can carry a lot of different things, such as transposons and old viruses.
What effect does genetic drift have on the distribution of genetic variation within and
among populations? What effect does gene flow have on genetic variation within and
among populations?
Genetic drift decrease genetic variation, but maintains certain levels of variation. During genetic drift, certain alleles are either fixed or lost, often by chance. Gene flow, on the other hand, increases various in one population, but decreases variation as a whole among several populations.
What determines the strength of genetic drift in nature?
Population size. A smaller population will experience much strong genetic drift. You can also measure the effective population size, Ne. Ne is the number of individuals that have the same intensitiy of random genetic drift in an idealized population. In an idealized population, the population size is constant and all individuals have the same chance of leaving offspring. Small Ne = big genetic drift. The actual size (N) can be very different from Ne and factors that affect Ne are changes in N through time, population subdivision, and unequal numbers of males and females. Ne is usually much less than N. An example of changes in N would be the founding effect and the bottleneck effect. Population subdivision would be like when all parts of the population get separated. An example would be the prairies of Illinois.
What is the probability that a new autosomal mutation in a diploid population of size N
will be lost due to drift?
The probability of fixation is 1/2N. So if you have a large N, the allele can fix pretty fast. However, if you have a small N, it could take a long time. The probability of loss, in comparison, is 1-1/2N. So in a big population the probability is going towards 1 and in a small population, the probability is going toward negative infinity.
Describe a scenario depicting how gene flow can prevent local adaptation. Similarly,
describe a scenario depicting how gene flow can promote adaptive evolution.
If alleles are being introduced into the population that are not advantageous, these alleles might make the population a little less fit. If a species is already genetically well-adapted for their environment, genetic pollution can sometimes cause individuals to lose these adaptive traits due to sex and recombination. A big population giving alleles to a small population is going to give more of a variety to the small population, which may not be advantageous. Small populations tend to have less variation. However, if an advantageous alleles is given to the small population, it could make the population more fit and help them adapt to their environment.
Define effective population size.
The effective population size is used to measure the strength of drift. Ne is the effective population size and it is the number of individuals in an idealized population that would have the same intensity of random genetic drift. A small Ne means that there is strong drift in the population.
What are two main types of sexual selection? How do they differ?
The two types of sexual selection include intra-sexual selection and inter-sexual selection. Intra-sexual selection is male-male competition for mates. The reason for this kind of selection is because the male has a lower parental investment and shows greater mating effort. Because the male only has to donate sperm, his fitness relies on his availability and access to mates. Inter-sexual selection is choosiness on the part of females when picking a mate. The 2-fold cost of sex is the idea that females put in all the resources, but only half of the genetic material when reproducing. For this reason, females have a higher parental investment and will often be choosy in mate selection.
Name two different mechanisms by which male-male sexual selection works. Give an
example of each.
There are actually three different mechanisms of male-male sexual selection.
Combat: Males will fight each other for mates. Examples include Elephant seals who fight for the right to breed with the females. The winning male will have a harem of females and the losing males essentially doesn’t get to breed with anybody. Only a ⅓ of elephant seal males get mates. Elk (red deer) fight each other with their antlers to get mates. 23% of mature males get injured in fights, 6% end up with permanent injuries, but if some winning males get 10x the number of matings.
Sperm competition: Some species compete for insemination and have special techniques to make sure that their sperm inseminates the female. Dragonflies have a penis that scoops out all other sperm that might have been inside the female previously. Other species have types of penile structures that break off and block the canal so that no other sperm can enter. There is a lot of diversity in sperm competition, particularly in insects.
Infanticide: Some male species will kill another male’s offspring. Male lions kill cubs. This is probably due to the fact that lionesses don’t ovulate while they’re taking care of their cubs so killing another lion’s cubs allows the male to mate with the female.
Verbally describe the process of Fisher’s “runaway” evolution of male display and
female preference. What genetic considerations are critical for the runaway to occur?
The idea is that two traits can become linked - a male trait and a female preference for the male trait. This will in turn create a female offspring which has an even bigger preference for a certain male trait and the connection between the two traits will grow progressively stronger. The increase in female preference causes sexual selection to favor the more extreme male trait and will cause the male trait to increase which can cause an explosive change in female preference and the male trait.
What is Muller’s ratchet? What evolutionary forces play a role in the ratchet?
Muller’s ratchet is the idea that in asexual reproduction mutations accumulate overtime. Generations will go from one to two to three mutations without being able to decrease their number of mutations. It’s like the “click” of a ratchet. In asexual reproduction, deleterious mutations can accumulate faster on a chromosome than selection can remove them. Sex, in contrast, can be a way to purge deleterious mutations. Due to recombination, sex can reset and remove deleterious mutations on a chromosome.
What is linkage disequilibrium? Give a specific example of how it can play an
important evolutionary role.
Linkage disequilibrium is the linkage of two alleles more often or less often than would be expected in reproduction. Linkage disequilibrium is affected by factors like genetic drift, non-random mating, and rate of mutation. Mating is not random and so certain traits evolve together in a population. However, if random mating were to occur, we would get back to linkage equilibrium immediately. Fisher’s runaway model is related, but applies more to specific phenotypic traits than to allelic linkages. Fisher’s runaway model applies specifically to the coevolution of a male trait and the female preference for that trait.
What are the key pieces of data required to test the sensory bias hypothesis for the
evolution of female preference?
Sensory bias hypothesis is the idea that female preference may simply be a side effect of a kind of sense that has evolved. If the female has evolved a certain sense for predation purposes, she may seek out males with traits that use that sense and so the male trait evolves. The key point is that the sensory bias comes before the evolution of the male trait. Then intra-sexual selection in male-male competition will begin exploiting this bias to compete for mating. The key experiment is the testing of female preference in swordtail males. The males have this tail that is sexually selected and so the researchers attached these long tails to a clade species of fish that didn’t have the tail. They found out that the attached tail made males more sexually successful, hence proving that preference predates the male trait. They also test female preference for frog sounds which denotes the idea that females evolved the sensory bias and preference before the male trait. What exactly is the experiment?
Describe three possible costs of sexual reproduction.
The two-fold cost of sex: Females donate all of the nutrients, but only have of the genes while males donate half of the genes and none of the nutrients. If females were asexual, they could donate twice as many genes. Sexual females have to produce males as well so they aren’t able to donate as much genetic material. Asexual populations would double in size each generation whereas the sexual population might just stay the same size because it takes two individuals to make one offspring.
Finding a partner can be a problem.
STDs which occur in many different species: 40 million people were infected with HIV in 2001 and it’s the 4th leading cause of mortality.
Sex breaks up good combinations of genes: If you have a good thing going, you could lose a good adaptive genome by having to only donate half your genetic material.
How can sexual reproduction speed up adaptive evolution?
This is the Fisher-Muller Theory. Essentially, new mutations are most likely to arise in different individuals. We are all heterozygous for some nasty diseases, but if there is a favorable mutation that occurs, sexuals can recombine and bring together favorable mutations. Asexuals, on the other hand, have to wait for consecutive advantageous mutations to occur in a single lineage. Advantageous mutations can be trapped by bad ones, but recombination allows good mutations to escape and be favored in the environment.
Why might sex be valuable in a variable environment?
Because of recombination, a good mutation can more quickly fix and doesn’t have to be trapped by a deleterious mutation. If the environment is often changing, being able to segregate and recombine can allow for different allele combinations on the chromosome which will could be beneficial. Asexuality can block good mutations if they don’t occur in the right place and time in the genome.
Describe one scenario where selection may be in conflict across levels of biological
organization.
Sexual selection can be in conflict with natural selection. Female preference may be for a bullfrog with a very loud call, but if the bullfrog is also more likely to be heard by predators, sometimes the selection for his mating traits can conflict with his survival.
Why do organisms die? Provide at least two plausible hypotheses based on
evolutionary theory.
Accumulation of damage theories (wear and tear)
Cell division limit (telomeres)
Programmed cell death
Rate-of-life
Waster accumulation/free-radical damage
Evolutionary theories (result of/lack of natural seleciton)
Mutation accumulation
Tradeoff hypothesis (you pay later)
Selection is expected to drive deleterious mutations to extinction in natural
populations. Nonetheless, deleterious mutations exist (sometimes in high frequency)
in natural populations. Give two explanations for why this might occur.
We all are heterozygous for some pretty nasty diseases and mutations, but because we’re heterozygous, we’re phenotypically normal and that trait is not selected against in the environment. Also, sometimes the heterozygous trait is selected for in the population and the mutation is kept around. For example, heterozygous individuals for Sickle Cell are more likely to survive in high-malaria climates and are selected against where malaria isn’t a big deal.
Heterozygote advantage: The heterozygous phenotype is more advantageous than the homozygous traits, like in sickle cell anemia.
Trade-off hypothesis: Mutations may occur that involve a pleiotropic trade-off between reproduction early and survival late in life. The mutation may not really affect your ability to reproduce, like Huntington’s where you start to see the effects when you’re like 40.
Describe Hamilton’s kin selection model for the evolution of altruistic behavior.
rB - c > 0. r is the relatedness of the actor and recipient. B is the benefit to the actor. c is the cost on the actor of being altruistic. If the combination of rB is more than c, then the organism is more likely to be altruistic.
What effect will random genetic drift have on populations that initially are genetically
similar?
Genetic drift is the change in allele frequencies due to random sampling. If two populations start out similar and genetic drift is acting on both of the populations, certain alleles can be fixed or lost in one population and not the other. Genetic drift maintains a base level of variation while losing certain alleles based on random events. A period of time afterward, the two population can end up looking very different, but not necessarily due to specific natural selection. It could just be due to random allele fixation and loss from genetic drift.