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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.


What are the disadvantages of sec

Halves the potential reproductive rate
Means you share your genetic reproduction
Break up co-adapted gene complexes
Cost of finding mates


What are the advantages of sex

Stops the accumulation of deleterious mutations
Fisher- Muller hypothesis
Stops the accumulation of deleterious mutations
Fisher- Muller hypothesis


What are the advantages of dispersal

Colonisation of new empty habitat
Inbreeding avoidance in sexual organisms
Find a ‘better’ environment


What is dispersal

movement of organisms from location at birth to other locations where they breed


What does natural selection select against alleles that promote dispersal

. Within a population there is automatic selection against dispersal alleles – they remove themselves from the population straight away!
Local adaptation – selection means that individuals are best suited to local conditions.
Offspring that disperse (and the dispersal allele) will end up in environments to which they are less well adapted – lower fitness
Dispersal is dangerous  higher mortality


Examples of where organisms vary in life history/demographic traits

Number and size of offspring
– Age distribution of reproduction
– Life span
– Alternative mating strategies
– Dispersal
– Mode of reproduction