lecture 20: population genetics

phenotypic variation

observable differences in traits among individuals

genetic variation

differences in DNA sequences or alleles among individuals

either or phenotype

a trait with clear categories where individuals either have the trait or do not

either or phenotype example

mendel's pea plant traits

continuous traits

traits controlled by two or more genes that show a range of variation

Examples of continuous variation

height and skin color

why is variation necessary for evolution

natural selection can only act on existing variation in a population

what is genetic variation

differences among individuals in genes or nucleotide sequences

what are sources of genetic variation

mutation, chromosomal alterations, recombination, and sexual reproduction

mutation

a heritable change in DNA sequence

why are mutations considered random

they occur unpredictably and are not directed by need

how does recombination increase variation

it creates new allele combinations during meiosis meiosis.

why do viruses evolve rapidly

they have high mutation rates and reproduce quickly.

why can bacteria accumulate variation quickly even with low mutation rates?

they reproduce very rapidly, producing many mutation over time

why is sexual reproduction important for variation

meiosis creates unique allele combinations in offspring

what is population genetics

The study of genetic variation within populations and the evolutionary forces acting on them

define a population

a group of organisms of the same species living in the same area at the same time that can interbreed

what is an allele

one of two or more forms of a gene

what is a gene pool

all alleles of all genes in a population at a given time

what is a gene pool

all alleles of all genes in a populations at a given time

what is allele frequency

the proportion of a specific allele in a population

what is a fixed allele

an allele with a frequency of 1 in a population

what equation describes allele frequencies in the Hardy-Weinberg equilibrium

p + q = 1

what does a "q" represent in hardy-weinberg equilibrium

frequency of the recessive allele

what equation describes genotype frequencies in Hardy-Weinberg equilibrium

p² + 2pq+ q^2 = 1

what does p^2 represent

Frequency of homozygous dominant individuals

what does 2pq represent

frequency of heterozygous individuals

what does q^2 represent

frequency of homozygous recessive individuals

what is genetic equilibrium

no change in allele frequencies from generation to generation

what does Hardy-Weinberg equilibrium describe

a non-evolving population

why is Hardy-Weinberg equilibrium useful

ir provides a baseline for comparing evolving populations

Sequence the five mechanisms that cause evolution/disrupt HW equilibrium

1. mutation

2. nonrandom mating

3. natural selection

4. genetic drift

5. gene flow

what five condition are required for Hardy-Weinberg equilibrium

- no mutation

-random mating

-no natural selection

-no genetic drift

-no gene flow

what is the role of mutation in evolution

mutation creates new alleles and provides raw material for natural selection

what is the role mutation in evolution

mutation creates new alleles and provides raw material for natural selection

does mutation usually have a large immediate effect on allele frequencies

no, especially in large populations

random mating

every individual has an equal chance of mating with any individual of the opposite sex

nonrandom mating

some individuals have a greater chance of mating than others

inbreeding

mating between closely related individuals

how does inbreeding affect homozygosity?

It increases homozygosity

define natural selection in population genetics terms

nonrandom change in allele frequency based on fitness

what is fitness in evolution

the ability to survive and reproduce

how does natural selection affect beneficial alleles

Beneficial alleles increase in frequency.

how does natural selection affect harmful alleles

harmful alleles decrease in frequency

does natural selection increase or reduce genetic diversity

it generally reduces genetic diversity

what does natural selection act on directly

phenotypes

what is required for natural selection to occur in a population

at least two phenotypes (polymorphism)

genetic drift

random changes in allele frequency causes by chance events

does genetic drift have a larger effect in large or small populations

small populations

how does genetic drift affect diversity

it reduces genetic diversity.

bottleneck effect

a drastic reduction in population size that changes allele frequencies

founder effect

when a small group colonizes a new environment, producing a gene pool different from the parent population

gene flow

movement of alleles between populations due to migrations

how does gene flow affect a population?

it reduces differences between populations

Why evolution occurs in populations rather than individuals

Evolution is defined as changes in allele frequencies across generations, which only occurs in populations. Individuals only inherit or express traits

why variation is essential for natural selection

without variation, there are no differences in fitness for natural selection to act upon

why natural selection and genetic drift both reduce diversity but differ mechanistically

natural selection changes allele frequencies based on fitness, while genetic drift changes allele frequencies randomly through chance events