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Evolutionary Biology
Selection and adaptation
| Question | Answer |
|---|---|
| Natural selection | Individuals with favorable traits (phenotypes) are more likely to survive and reproduce than those with unfavorable traits. Heritable trait - genotypes/alleles associated with it will increase in frequency over generations – adaptive evolution! |
| Natural selection | Fitness – average lifetime contribution of individuals of a given genotype to the population after one or more generations |
| Natural selection | Components of natural selection that may affect the fitness of a sexually reproducing organism Not random, but no plan |
| Adaptations | ’a characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states (especially the ancestral condition)’ |
| The giraffe neck | Selection of quantitative genetic variation in a trait |
| Darwin's finches | Adaptive radiation: the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage |
| Modes of selection | Natural selection - acts on phenotypes! - selects for genotypes! |
| Directional selection | Favours phenotype of one extreme |
| Diversifying selection | = disruptive selection Favours different extreme phenotypes |
| Stabilising selection | Selects against extreme variants - leads to reduced phenotypic variation |
| The effects of selection and the HW equilibrium | • Natural selection will change the genotype frequencies and (consequently) the allele frequencies (over a generation) in a population • E.g. For co-dominant alleles |
| Selection against recessive alleles | Selection against recessive ‘a’ allele (in system with two alleles A and a) e.g. 50% of ‘aa’ die before reproducing but Aa not affected (because a is recessive) |
| Selection against recessive alleles | • When recessive is common = many ‘aa’ exist – therefore rapidly selected against • But when rare - most copies of ‘a’ are in Hz form ‘Aa’ - hidden from selection! – slow rate of selection against |
| Selection against recessive alleles | • Hard to eliminate recessive alleles and get maximum fitness in a population! |
| Selection for recessive alleles | • New beneficial recessive allele increases in frequency slowly at first because mainly in ‘Aa’ Hz form - hidden from selection • Increases rapidly once it has reached a certain frequency (because more aa’s) |
| Selection for recessive alleles | • Recessive alleles selected for will go to fixation once past a certain initial frequency |
| Selection for recessive alleles | at low frequency positive recessives = very susceptible to loss through genetic drift! |
| Selection operates on variation to produce adaptation | But there is a problem here Selection eliminates variation! Genetic drift also removes variation Paradox - how is variation maintained? |
| At least three (potentially interacting mechanisms) cause balancing selection | - Heterozygote advantage - Negative frequency dependent selection - Fluctuating selection |
| Heterozygote advantage (overdominance) | Both alleles will be kept in the population in successive generations Example. Sickle cell trait Beta hemoglobin gene A = normal; S = sickle allele (codominant) |
| Negative frequency-dependent selection | The fitness of a genotype is not constant but depends on the genotype frequency in the population Negative Frequency Dependence = rarer form has an advantage (rare allele advantage) |
| Negative frequency-dependent selection | Why would a rare allele be at an advantage ? Leads to oscillations in the phenotype frequency (and underlying alleles) in a population (maintains variation) |
| Competition for food (eating chunks of prey!) in two morphs of a cichlid species | less competition/prey awareness for the right form. So these ‘right’ predator morphs feed/breed well/ increase in frequency (so freq of the left form alleles decreases) Right form of predator becomes common and selection then favours the rarer left form! |
| Fluctuating selection | Gene conferring resistance to powdery mildew in Arabidopsis thaliana (RPW8) is polymorphic in natural populations. Why? In the absence of pathogen infection, weight and seed yield are higher in plants without the resistance gene |
| Fluctuating selection | A cost of having the resistance gene! Resistant plants do better when pathogen is abundant Temporal and spatial variation in pathogen presence – determined by external factors (e.g rainfall) - maintains the polymorphism |
| How to infer an adaptation | Factors that may explain the occurrence of a trait: • genetic drift • correlated evolution (pleiotropy or linkage) • an ancestral character – a result of phylogenetic history • natural selection – adaptation |
| Methods used to infer adaptation | • Complexity / apparent design • Experiments • The comparative method |
| Experiments for inferring adaptation | • Assess the effect a change in a single, well-defined factor has on a phenotypic trait • Test how it affects fitness |
| The comparative method | Adaptations inferred from patterns observed across species, correlations among traits, or correlations between traits and the environment |
| The comparative method | Rationale: if a certain relationship between trait and environment has evolved repeatedly (independently), then it is likely to be adaptive |
| The comparative method | A scatter plot may provide only weak evidence that two traits evolve in tandem ! Problem - a lack of independence of the data points 2 groups that share common ancestry rather than independent adaptive evolution |
| Independent contrasts | Felstein’s method for evaluating phylogenetically independent contrasts |
Created by:
rose.coo
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