7-9 Flashcards
(59 cards)
5 assumptions of hardy weinburg model
a. infinitely large population size
b. no mutation
c. no selection
d. no gene flow
e. random mating
What is fitness
the average lifetime contribution of individuals of a given genotype to the population after one or more generations (reproductive success)
What is natural selection
A process where indviduals with with favorable traits (phenotypes) are more likely to survive and reproduce than those with unfavourable traits. If the trait is heritable the genotypes/alleles associated with it will increase in frequency over generations
What is an adaption
’a characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states (especially the ancestral condition)’.
What was Lamarck’s theory
Inheritance of acquired characteristics
What is adaptive radiation
the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage
Example of adaptive radiation
Darwins finches - All arose from one ancestral species that colonized the Galapagos islands from S. America. Diverged into a large number of species within relatively short time. The lineages are modified for different ways of life. Differ in morphology of the bill – adaptation to different diets (seeds, insects, nectar etc)
What does natural selection act on
it acts on phenotypes, but selects for genotypes
What is directional selection
favours phenotypes of one extreme
What is diversifying ( or disruptive) selection
Favours the two extremes of a phentoype
What is stabilising selection
favours intermediate types and selects against extreme variants. Often leads to reduced phenotypic variation and status quo.
What is purifying selection
directional selection in favour of the prevalent, advantageous homozygous genotype. Removes mutations that change a phenotype
Example of stabilisng selection
Birth weight in humans (Cavalli-Sforza & Bodmer 1971)
Very heavy or very light babies were most likely to die
Why is selection on rare recessive alleles slow
most copies of the allele is in heterozygotes where the allele is hidden from selection
What is the best chance of removing a rare recessive allele
Best chance is that this goes to zero through drift – not selection!
Why are positive recessive alleles slow to increase initially
because mainly in ‘Aa’ Hz form - hidden from selection
What happens when a positive recessive allele becomes more common
it will increase rapidly once it has reached a certain frequency and eventually will go to fixation once past a certain initial frequency
3 ways variation is maintained
Mutations – (in the wider sense) - not all are deleterious
Gene flow – across different populations with different adaptations
Balancing selection – maintenance of greater variation than expected
2 mechanisms that make it hard to maintain variation
Selection and genetic drift
What 3 mechanisms can cause balancing selection
Heterozygote advantage
negative frequency dependent selection
Fluctuating selection
What is heterozygote advantage (over dominance
When the heterozygote has higher fitness than any of the homozygotes and so both alleles will be kept in the population in successive generations
What is negative frequency dependent selection
When the fitness of a genotype is not constant but depends on the genotype frequency in the population. in this case, the rarer the allele , the greater the advantage
What can negative frequency dependent selection lead too
Leads to oscillations in the phenotype frequency (and underlying alleles) in a population (maintains variation)
What are life history traits
affect the life table of an organism, and can be imagined as various investments in growth, reproduction, and survivorship.
