genetic diversity

Change over time:

• Within a species (change in gene pool)

=Microevolution

• Type & number of

species

=Macroevolution

microevolution= evolution within a species

differences between species

darwins major problems: Natural selection requires variation – But Blending inheritance gets rid of it

– As does the process of natural selection

adding in mutation and heredity:

mutation generates variation, permits novelty

diploidy, segregation maintains variation in absences of selection

mutation + mendel solve the genetic variation problem

types of mutation

-no effect on the protein as amino acid is the same

-might change protein

-stops protein from being produced

-effects when and where the protein is produced

other mutational effects

• gene duplication

• novel genes

• endosymbiosis

• changes in karyotype

• polyploidy (=genome duplication)

most selection on mutations of small effect → gradual, slow changes

macromutations important in microbes

analysis of microbial genomes → widespread transfer of genetic material between microbes

influence of population size on mutation

occurs 1 in one billion individuals/generation:

n=1000: takes 1x10^6 generations

n=10^6: takes 1000 generations

n=10^9: takes 1 generation

so, if population small, slows adaptation

use in HIV therapy:

-first drug AZT (1987) → supressed virus for 22 weeks before HIV evolved resistance

triple therapy (2006) → knocked viral titres very low, resistance takes a long time to evolve

for simple genetic traits:

-can measure frequency of alleles from data, and observe changes over time

-direct genotyping

-inferred from phenotype if know dominance

3 types of variant:

deleterious :selected against

neutral: do not effect fitness

advantageous: selected for

on average, each person carries 4 lethal/ deleterious mutations

vast majority are recessive (little phenotypic effect unless homozygous

mostly, individuals have different lethal mutations → each genetic disease is rare

cystic fibrosis is one of the most common diseases in the uk - 1/2500 births

summary

Alleles causing genetic disease present because mutation is recurrent

Selection removes them, but does so slowly when rare (recessive)

• Thus there are many different lethal mutations, all rare

Founder effects make particular genetic diseases more common

neutral mutations: don’t affect phenotype

-outside of genic regions

-in introns

-in third base pair positions, no aa change =affect phenotype in insignificant ways

-amino acid with same properties

neutral mutations fix or are lost rapidly in small populations, but can hang around for a long time in large populations

Large populations have large numbers of neutral mutations/generation (as bigger)

• Neutral mutation frequency is affected only by random sampling

=> Neutral mutations retained for longer periods in large populations

Thus.... Large populations have more genetic diversity

SUMMARY

Genetic diversity is high:

• This is mostly associated with ‘neutral mutations’ whose frequency undergoes a random walk (genetic drift)

• Deleterious mutations persist across multiple loci at low frequency: mutation- selection equilibrium

• Natural selection can also maintain diversity