7-9 Flashcards
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
Example of variation in number size of offspring
Blue tits can lay 14 eggs whilst kiwis lay 1 which is 25% mothers weight
Example of variation in the
Fruit flies live 26 days, Ming the icelandic clam llived 507 years
In terms of r what is fitness
the per capita rate of increase of a genotype = r
What will increase r (per capita rate of birth - per capita rate of death)
Higher survival up to and through the reproductive ages
Earlier age of first reproduction
Higher fecundity at each reproductive age
Higher fecundity early in life
Longer reproductive lifespan
What don’t all organisms evolve traits that max out their fitness/r
Phylogenetic constraints Genetic constraints Trade offs (e.g antagonistic pleiotropy)
How can antagonistic pleiotropy affect fitness maximisation
due to allocation tradeoffs
- genotypes differ in investment to reproduction or maintenance/growth
- e.g increase fecundity but decreasein growth/survival
What is senescence
- physiological degeneration with age - increased likelihood of death
What are theories of senescence based on
Based on the principle that
– due to extrinsic mortality less individuals survive / breed at older ages
– selective advantage of factors that enhance survival decreases with age
Who are the people who gave the 2 main theories on senescence
Williams (1957)
Medewar (1952)
What was Medewar (1952) theory about senescence
He argued that it was the accumulation of deleterious mutations that caused senescence - ie Medewar (1952) – accumulation of deleterious mutations
that only affect later age classes accumulate in populations
because selection against them is weak
What was Williams (1957) theory on senescence
Because of greater contribution of earlier age classes to fitness, alleles that
provide an advantage in early life have a selective advantage… even if
deleterious later in life. ie antagonistic pleiotropy
When is it an advantage ti be iteroparous
- If early age reproduction causes disproportionately high mortality - by investing more in early growth mortality may greatly reduce and lead to higher later fecundity
- Extrinsic mortality is low so enough time to repeatedly reproduce
- Population growth is low
in terms of offspring number , what is the best fitness strategy
All else being equal, genotype with higher fecundity will have
higher fitness than one with lower fecundity.
When are r-selected organisms better
In unstable or unpredictable environments with low density dependence, - the ability to reproduce quickly is crucial.
When are k selected species better
In stable or predictable environments subject to strong density dependence, - ability to compete for limited resources is crucial
Why are r and k selected strategies concept criticised
It’s polarised and does not
cover all the important elements of life-history theory
What is anisogamy
difference in investment in sperm and eggs
What is protogyny
– sequential hermaphroditism from female to male
What is protandry
– sequential hermaphroditism from male to female
What is sequential hermaphroditism associated with
Associated with changes in the relative reproductive success of the two sexes as an individual gets bigger
What is heritability
The extent to which the variation of a trait within a population can be attributed to genetic factors
What do you need to calculate heritability
You need phenotypic variance or you can’t calculate heritability , can’t divide by 0
How do heritable affect the rate of change on a trait
Low heritabilities combine with strong selection
to produce change, but much more slowly
High heritabilities combine with strong
selection to produce rapid change
Why is heriitabiity not the same as genetic determination
Heritability is a measure of what is associated with variation in a trait, not what causes a trait.
Example of a trait with low heritability
Humans have 10 finger and toes because of genetics determine our body plant. Whilst some variation ca be explained genetically, most variation is explained by the environment
Why is heritability not fixed
Heritability estimates are calculated for a given population, at a given time in certain environmental conditions.
Why are studies on heritability in human behaviour difficult to conduct
People that share genetics also usually share environments
We can’t put clones of humans into controlled environmental conditions
We can’t separate children from their parents and do a regression analysis
What is used to study heritability in human behaviour
Twin studies - is monozygotic twins resemble each other more on a certain trait than the dizygotic twins do, that trait is genetically
influenced
What does heritability help us explain
Heritability helps us explain how much variation in a trait within a population can be attributed to genetic variation
