Week 4 Flashcards
What is natural selection?
Individuals with favorable traits (phenotypes) are more likely to survive and reproduce than those with unfavorable traits
What is adaptive evolution?
If the trait is heritable the genotypes/alleles associated with it will increase in frequency over generations
What does natural selection forget?
Components of natural selection that may affect the fitness of a sexually reproducing organism
What are the chances of natural selection?
Natural selection is not random there is no plan. There is no design in natural selection
What are the stages of organism development and the selection placed on at that stage?
Zygote to adut –> viability selection
Adult to parents –> sexual selection
Parents to gametes –> Fecundity selection and gametic selection
Gametes to zygotes –> Compatibilty selection
What is adaptation?
Characteristic that enhances the survival or reproduction of
organisms that bear it, relative to alternative states
(especially the ancestral condition)
What was the idea of Jean-Baptiste Lamarck in the 1800’s?
Inheritance of acquired characteristics ie giraffe had long neck because the stretch for food or if you bodybuild all the time then your baby would be muscular
What was the correct idea of evolution?
Wallace and Darwin were correct
Selection of quantitative genetic variation in a trait
What is adaptive radiation?
The evolution of ecological and phenotypic diversity within a
rapidly multiplying lineage
What is a classic example of adaptive radiation?
Darwins finches –> descended from one finch species and form the natural variation in the population have become 14 different species each with their specialised niches
What does natural selection act on?
Acts on phenotypes but selects for genotypes
What causes quantitative variation?
Quantitative variation underpinned by genetic component
What is directional selection?
Favours phenotype of one extreme
What is diversifying selection?
Same asdisruptive selection
Favours different extreme phenotypes
What is stabilising selection?
Selects against extreme variants.
Leads to reduced phenotypic variation
What is an example of stabilising selection?
Body size in cliff swallows
Measured after a cold year population was shown to bigger than previous year
Why cant the directional selection of cliff swallows constantly select for being larger?
Trade offs with growing bigger eg more weight when flying so more energetically costly
If year is warm then the population will shrink in size creating a stabalising effect
What is an example of diversifying selection?
Bill morphology in black-bellied seedcracker, Pyrenestes ostrinus
Diversifying selection arising from the superior fitness of different phenotypes on different resources
What is an example of natural selection?
Birth weight in humans
Very small and very large babies are most likely to die, leaving a narrower distrubution of birthweights
What are the effects of natural selection?
Natural selection will change the genotype frequencies and
(consequently) the allele frequencies in a population
What will the frequency of genotype of zygotes relative to frequency of each genotype?
They will be in Hardy-Weinburg equilibrium
What happens to the frequency of the different alleles in the genotype of the zygote population?
Viability selection occurs killing off members of a genotype
What happens to the genotype frequency after the zygote stage?
As the populaiton demographic has changed this removes the alleles from HWE. Then when the current population goes to reproduce this will create a new ratio for the alleles so the next generation zygotes will be in a new HWE
What is the overview of selection against recessive alleles?
When recessive is common = many ‘aa’ – therefore rapidly selected against
But when rare - most copies of ‘a’ are in Hz form ‘Aa’ - hidden from selection so slow rate of selection against
Hard to eliminate recessive alleles and get maximum fitness in a population
What could eliminate recessive alleles at low frequency?
Genetic drift, the recessive allele is more at risk due to its prescence in a low number of genotypes
What is the overview of selection for recessive alleles?
New 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)
Recessive alleles selected for will go to fixation once past a certain initial frequency
What can be an issue with selection for recessive alleles?
At low frequency positive recessives = very susceptible to loss through genetic drift
What does selection operate on?
Variation to produce adaptation
What is a problem with variation operating on adaptation?
Selection eliminates variation!
Genetic drift also removes variation
How is variation maintained if its a paradox with selection and drift removing selection?
Heterozygote advantage
Negative frequency dependent selection
Fluctuating selection
What is heterozygote advantage?
Overdominance - both alleles will be kept in the population
in successive generations
What is an example of heterozygote advantage?
Sickle cell trait - resitant to malaria
Beta hemoglobin gene
A = normal; S = sickle allele (codominant)
What is the trade off with sickle cell trait?
Two sickle cell traits cause sickle cell disease which can result anemia and frequent pain needing blood transfusion
What is negative frequency-dependant selection?
The fitness of a genotype is not constant but depends on the genotype frequency in the population
Negative Frequency Dependence = rarer form at advantage
(rare allele advantage)
Why would a rare allele be at an advantage?
Leads to oscillations in the phenotype frequency (and underlying alleles) in a population (maintains variation)
What is an example of rare allele advantage?
Competition for food (eating chunks of prey!) in two morphs of a cichlid species
What was the overview of the two morphs of a cichlid species?
Left genotype of predator is common to begin with (mapped over time)
Less competition/prey awareness for the right form. So these predator morphs feed/breed well and 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
What is an example of fluctuating selection?
Gene for resistance to powdery mildew in Arabidopsis thaliana (RPW8) is polymorphic in natural populations
Why is the RPW8 gene polymorphic?
In the absence of pathogen infection, weight and seed yield are higher in plants without the resistance gene.
A cost of having the resistance gene!
Resistant plants do better when pathogen is abundant… but poorly when it is rare.
Temporal and spatial variation in pathogen presence – determined by external factors (e.g rainfall) - maintains the polymorphism
What are factors that can explain the occurance of a trait?
Genetic drift
Correlated evolution (pleiotropy or linkage)
An ancestral character – a result of phylogenetic history
Natural selection – adaptation
What are methods used to infer adaptation?
Complexity / apparent design
Experiments
The comparative method
What is the use of experiments inferring adaptation?
Assess the effect a change in a single, well-defined factor
has on a phenotypic trait
Test how it affects fitness
What is an example of an experiment inferring adaptation?
The adaptive value of the RPW8 resistance gene
Produce transgenic plants
Individuals differing only with respect to the presence or absence of the resistance gene.
Assess fitness
What is the comparative method?
Adaptations inferred from patterns observed across species,
correlations among traits, or correlations between traits and the environment.