Natural Selection Review Evolution

what is natural selection?

a deterministic [opposite of random] difference in the contribution of different classes of entities to subsequent generation

what is an adaptation?

a mutation that makes an organism more suitable for its environment → increase in fitness

evolution of adaptations occurs via __?

natural selection

what are the requirements for evolution via natural selection?

1. heritability

2. fitness difference [also known as differential reproduction]

3. variation in a population

what is fitness?

the ability to live long enough to reproduce

what is fecundity?

another term for fertility; an ability to produce an abundance of offspring

what unit of biological organization evolves?

population → lots of evolution must occur in a population to form a new species

natural selection acts on the ___ of what unit of biological organization?

the phenotype

natural selection tends to occur when:

1. overproduction [excess fecundity] is present

2. competition [limited resources] is present

what is always required for natural selection?

1. variation in characters

2. differential reproductive success [based on characters]

when does natural selection cause evolution?

relationship between phenotype and fitness + relationship between phenotype and genotype = relationship between fitness and genotype

• ex: if you dye your hair green and everyone wants to mate, natural selection is working, but it is NOT heritable, so evolution is not at work

what are the models of selection?

1. 2 alleles at one locus

   • homozygote advantage [ex: A2 being selected for]

   • heterozygote advantage [ex: A1A2 being selected for]

   • heterozygote disadvantage [ex: one of two alleles will go extinct or population will diverse]

2. quantitative trait

   • directional → increases proportion of genotype with more extreme value of trait

   • stabilizing → does not alter the mean, but may reduce the variance [variation] → standard deviation shortens

   • diversifying → unlikely to be exactly symmetrical, and thus usually shifts the mean → standard deviation lengthens

what prompts directional selection?

advantageous allele and disadvantageous allele

what is overdominance and what prompts it?

heterozygote advantage [stabilizing selection]

what is underdominance and what prompts it?

heterozygote disadvantage [disruptive selection]

• ex: speciation, as 2 phenotypes will change over time as they’re extremes

what is frequency dependent selection?

fitness depends on FREQUENCY OF GENOTYPE

• ex: white bunny survives and has more bunnies than brown, wolves start hunting more white bunnies due to increased availability, which means brown bunnies will then have more bunnies than white, and so on

what is selective sweep?

process where a beneficial mutation rapidly increases in frequency within a population due to positive natural selection, causing a reduction or elimination of genetic variation in the surrounding DNA regions → “sweeps away” other nearby alleles

how can you tell if an area has been recently colonized?

a founder effect takes place, which means genetic diversity is low → this means that there is a high linkage disequilibrium

what is heritability?

fraction of the variation in a population that is due to genetic differences

• if phenotype is not heritable, it is caused by environment

selection occurs ___ generations, while evolution occurs ___ generations

selection occurs within generations, while evolution occurs between generations

what is convergent evolution?

when 2 or more unrelated species independently evolve similar traits or features, often as a result of adapting to similar environmental pressures, creating analogous structures that serve similar functions despite not being inherited from a common ancestor

what is an example of convergent evolution in humans?

lactase persistance [ability to continually digest cow’s milk into adulthood] → at least 6 independent mutations for lactase persistance have occurred in humans in the last 10k years

how would you find the genetic component of lactose persistance?

you would want to look for high linkage disequilibrium → this means that there was low genetic diversity for the LCT and MCM6 genes, leading to a large part of a given population developing lactase persistance

• selective sweep occurred on lactase persistance mutations

how can you determine positive selection?

using omega [dN/dS]

• >1 means that there is more dN than dS → positive selection

• <1 means that there is more dS than dN → no selection, just drift

linkage disequilibrium approaches

• means alleles tend to appear together more often than not

methods based on population differentiation

• calculate F_ST between 2 populations everywhere on the chromosome → where F_ST is highest could be an adaptation