Population Genetics I - Populations and Hardy-Weinberg

microevolution

change in allele frequencies in a population over generations

// a single trait and how it changes over time

• three mechanisms cause allele frequency change

  • natural selection

  • genetic drift

  • gene flow

• only natural selection causes adaptive evolution

// the other two are random in terms of which allele frequencies may go up or down

hardy- weinberg equation

two alleles for flower color

• C^R= red allele

• C^W= white allele

red and white allele are incompletely dominant

gene pools and allele frequencies

• population: a localized group of individuals capable of interbreeding and producing fertile offspring

  • // individuals that live in the population

• gene pool: consists of the alleles for all loci in a population

  • // one gene out of a hundred

calculating frequencies

genotypic frequency

frequency of a genotype in the population

frequency of homozygous white flowers = 2 white flowers/8 total flowers= .25

allele frequency

frequency of an allele in the gene pool

frequency of white flower allele = 8 white alleles / 16 total alleles= .5

gene pools and allele frequencies

• by convention p and q are used to represent the allele frequencies

  • p = freq o dominant allele

  • q= freq. of recessive allele

• the fre. of all alleles in a population will add up to 1

• we can calculate p and q is if we know the genotypes of the individual in the population

the hardy- weinberg equation

• the hardy-weinberg equation describes the genetic makeup we expect for a population that is not evolving at a particular locus

conditions for hardy- weinberg equilibrium

the five conditions for non evolving populations are arely met in nature

1. no mutations

2. random mating

3. no natural selection

4. extremely large population size

5. no gene flow

considering the genetics of an entire population

p=.9

q=.1

10% chance a gamete will carry a cw allele and 90% chance a gamete will carry a cr allele

hardy- weinberg equation

genetic drift

• describes how allele frequencies fluctuate unpredictably from one generation to the next

• the smaller a sample, the greater the chance of random deviation from a predicted result

the founder effect

• occurs when a few individuals become isolated from a larger population

• allele frequencies in the small founder population can be different from those in the larger parent population

the bottleneck effect

• a sudden reduction in population size due to a change in the environment

• the resulting gene pool may no longer be reflective of the original populations gene pool

if the population remains small, it may be further affected by genetic drift

effects of genetic drift- a summary

1. significant in small populations

2. genetic drift= allele frequencies to change at random

3. can lead to a loss of genetic variation within populations

4. can cause harmful alleles to become fixed

gene flow

• consists of the movement of alleles among population

• alleles can be transferred through the movement of fertile individuals or gametes

• gene flow tends to reduce variation among populations over time