Lecture 5 Flashcards
What happened between the 1920s and the 1950s?
- the creation of the mathematical evolutionary theory for population genetic change
- initiated by Fisher, Haldane, and Wright
- provided the foundations for Neo Darwinism and the New Synthesis
- showed that continuous variation and Darwinian natural selection are entirely consistent with Mendel’s Laws
- demonstrated the evolutionary significance of genetic variation
state the 5 forces that influence patterns of genetic diversity and evolution and what effect they tend to have
- mutation - increases diversity
- recombination - increases diversity
- genetic drift - decreases diversity
- natural selection - increases/decreases diversity
- migration - increases diversity
how does mutation influence patterns of genetic diversity?
- increases genetic variation in populations
- caused by errors during replication
how does recombination influence patterns of genetic diversity?
- increases genetic variation in populations
- creates new combinations of alleles and mutations
how does genetic drift influence patterns of genetic diversity?
defined as the change in the frequency of an existing gene variant (allele) due to random chance
- acts to decrease genetic variation in populations
- random sampling affects every generation
- more important for populations that are smaller
three types of natural selection
- negative (purifying) selection
- positive (directional) selection
- selection favouring diversity
negative (purifying) selection
- decreases genetic variation in populations
- mutations that reduce fitness are removed by natural selection
positive (directional) selection
- decreases genetic variation in populations
- mutations that increase fitness will eventually become fixed in a population
selection favouring diversity
- increases or retains genetic variation in populations
- natural selection can act to maintain diversity over the long term (eg heterozygote advantage)
how does migration (gene flow) influence patterns of genetic diversity?
- increases genetic diversity in populations
- migration influences the structuring of diversity over a large spatial scale
describe the two metrics of genetic variation
Heterozygosity (H)
- fraction of individuals that are heterozygous, averaged across gene loci
Polymorphism (P)
- proportion of gene loci that have 2 or more alleles in the population
- a locus can be polymorphic without being heterozygous
define fixation
occurs when a polymorphic locus becomes monomorphic due to the loss of all but one allele (can occur due to natural selection or genetic drift)
decreases differences between populations.
what maintains genetic variation?
- mutation-selection balance
- selection maintaining variation
mutation-selection balance
- less fit types reintroduced by mutation
- followed by selection acting to remove them
selection maintaining variation
- the maintenance of genetic variants at intermediate frequencies over long periods of time
what are the two models of population genetic variation?
- the “classical” theory asserted that most genetic variation within species is maintained by mutation-selection balance
- the “balance” theory proposed that genetic variation is maintained primarily by overdominant selection or some other type of balancing selection.
classical model
- Morgan, muller
- low heterozygosity
- low polymorphism
- wild type is ‘normal’ genotype
- selection typically negative
balance model
- Dobzhansky, Ford
- heterozygote advantage
- high heterozygosity
- high polymorphism
- selection favours diversity
define gene flow
movement of genetic material from one population to another
how was genetic diversity studied before 1966?
morphological
- eg snail colour polymorphism
cytological
- eg chromosome inversions
what was the early quantitative evidence for the existence of genetic variation?
- rather than focussing on Mendelian discrete traits, there was a focus on continuous polygenic traits
- selection experiments were done on different groups of organisms
- these involved controlled breeding of individuals with particular traits for many generations
= artificial selection
evolutionary responses of continuous traits
- demonstrates existence of heritable variation in fitness-related phenotypes
- due to many underlying genes
give a summary of the artificial selection experiments that were done on quantitative traits
- selection responses demonstrate that abundant genetic variation exists for polygenic quantitative traits
- but often no information on P & H as key population genetic parameters
- also comparative studies difficult as traits studied often are group specific
still no solution to the question: what maintains genetic variation?
describe Richard Lewontin and the Electrophoresis Revolution
- allozyme gel electrophoresis provided a way to ask
- what proportion of genes are variable (p&h)?
- addresses fundamental dispute between classical and balance schools
advantages of studies of enzyme polymorphism
- many loci can be examined
- can be used in nearly any organism
- loci co-dominant, heterozygotes can be identified
- variation examined close to DNA level
- provides genetic marker loci for other studies
describe the neutral theory by Motoo Kimura
- negative selection rapidly eliminates detrimental mutations
- positive selection rapidly fixes beneficial mutations
- the only mutations left to create genetic variation are selectively neutral
describe DNA variation in maize vs teosinte
- corn has reduced genetic diversity compared to its wild ancestor teosinte
- a consequence of population bottlenecks during domestication
describe human genetic variation
- humans show a loss of genetic variation with increasing distance from East Africa
- reflects founder events as humans migrated from source population
describe comparisons of polymorphism in Arabidopsis lyrata
- regions that were recently glaciated have lower DNA diversity
- genetic drift following recolonisation
