Lecture 5

Question 1

What happened between the 1920s and the 1950s?

Answer 1

• 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

Question 2

state the 5 forces that influence patterns of genetic diversity and evolution and what effect they tend to have

Answer 2

1. mutation - increases diversity
2. recombination - increases diversity
3. genetic drift - decreases diversity
4. natural selection - increases/decreases diversity
5. migration - increases diversity

Question 3

how does mutation influence patterns of genetic diversity?

Answer 3

• increases genetic variation in populations
• caused by errors during replication

Question 4

how does recombination influence patterns of genetic diversity?

Answer 4

• increases genetic variation in populations
• creates new combinations of alleles and mutations

Question 5

how does genetic drift influence patterns of genetic diversity?

Answer 5

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

Question 6

three types of natural selection

Answer 6

• negative (purifying) selection
• positive (directional) selection
• selection favouring diversity

Question 7

negative (purifying) selection

Answer 7

• decreases genetic variation in populations
• mutations that reduce fitness are removed by natural selection

Question 8

positive (directional) selection

Answer 8

• decreases genetic variation in populations
• mutations that increase fitness will eventually become fixed in a population

Question 9

selection favouring diversity

Answer 9

• increases or retains genetic variation in populations
• natural selection can act to maintain diversity over the long term (eg heterozygote advantage)

Question 10

how does migration (gene flow) influence patterns of genetic diversity?

Answer 10

• increases genetic diversity in populations
• migration influences the structuring of diversity over a large spatial scale

Question 11

describe the two metrics of genetic variation

Answer 11

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

Question 12

define fixation

Answer 12

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.

Question 13

what maintains genetic variation?

Answer 13

1. mutation-selection balance
2. selection maintaining variation

Question 14

mutation-selection balance

Answer 14

• less fit types reintroduced by mutation
• followed by selection acting to remove them

Question 15

selection maintaining variation

Answer 15

• the maintenance of genetic variants at intermediate frequencies over long periods of time

Question 16

what are the two models of population genetic variation?

Answer 16

• 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.

Question 17

classical model

Answer 17

• Morgan, muller
• low heterozygosity
• low polymorphism
• wild type is ‘normal’ genotype
• selection typically negative

Question 18

balance model

Answer 18

• Dobzhansky, Ford
• heterozygote advantage
• high heterozygosity
• high polymorphism
• selection favours diversity

Question 19

define gene flow

Answer 19

movement of genetic material from one population to another

Question 20

how was genetic diversity studied before 1966?

Answer 20

morphological
- eg snail colour polymorphism
cytological
- eg chromosome inversions

Question 21

what was the early quantitative evidence for the existence of genetic variation?

Answer 21

• 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

Question 22

evolutionary responses of continuous traits

Answer 22

• demonstrates existence of heritable variation in fitness-related phenotypes
• due to many underlying genes

Question 23

give a summary of the artificial selection experiments that were done on quantitative traits

Answer 23

• 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?

Question 24

describe Richard Lewontin and the Electrophoresis Revolution

Answer 24

• allozyme gel electrophoresis provided a way to ask
• what proportion of genes are variable (p&h)?
• addresses fundamental dispute between classical and balance schools

Question 25

advantages of studies of enzyme polymorphism

Answer 25

- 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

Question 26

describe the neutral theory by Motoo Kimura

Answer 26

- negative selection rapidly eliminates detrimental mutations - positive selection rapidly fixes beneficial mutations - the only mutations left to create genetic variation are selectively neutral

Question 27

describe DNA variation in maize vs teosinte

Answer 27

- corn has reduced genetic diversity compared to its wild ancestor teosinte - a consequence of population bottlenecks during domestication

Question 28

describe human genetic variation

Answer 28

- humans show a loss of genetic variation with increasing distance from East Africa - reflects founder events as humans migrated from source population

Question 29

describe comparisons of polymorphism in Arabidopsis lyrata

Answer 29

- regions that were recently glaciated have lower DNA diversity - genetic drift following recolonisation