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BIO120 > Lecture 5 > Flashcards

Lecture 5 Flashcards

1
Q

What happened between the 1920s and the 1950s?

A
  • 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
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2
Q

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

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

how does mutation influence patterns of genetic diversity?

A
  • increases genetic variation in populations
  • caused by errors during replication
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4
Q

how does recombination influence patterns of genetic diversity?

A
  • increases genetic variation in populations
  • creates new combinations of alleles and mutations
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5
Q

how does genetic drift influence patterns of genetic diversity?

A

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

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6
Q

three types of natural selection

A
  • negative (purifying) selection
  • positive (directional) selection
  • selection favouring diversity
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7
Q

negative (purifying) selection

A
  • decreases genetic variation in populations
  • mutations that reduce fitness are removed by natural selection
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8
Q

positive (directional) selection

A
  • decreases genetic variation in populations
  • mutations that increase fitness will eventually become fixed in a population
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9
Q

selection favouring diversity

A
  • increases or retains genetic variation in populations
  • natural selection can act to maintain diversity over the long term (eg heterozygote advantage)
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10
Q

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

A
  • increases genetic diversity in populations
  • migration influences the structuring of diversity over a large spatial scale
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11
Q

describe the two metrics of genetic variation

A

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

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12
Q

define fixation

A

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.

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13
Q

what maintains genetic variation?

A
  1. mutation-selection balance
  2. selection maintaining variation
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14
Q

mutation-selection balance

A
  • less fit types reintroduced by mutation
  • followed by selection acting to remove them
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15
Q

selection maintaining variation

A
  • the maintenance of genetic variants at intermediate frequencies over long periods of time
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16
Q

what are the two models of population genetic variation?

A
  • 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.
17
Q

classical model

A
  • Morgan, muller
  • low heterozygosity
  • low polymorphism
  • wild type is ‘normal’ genotype
  • selection typically negative
18
Q

balance model

A
  • Dobzhansky, Ford
  • heterozygote advantage
  • high heterozygosity
  • high polymorphism
  • selection favours diversity
19
Q

define gene flow

A

movement of genetic material from one population to another

20
Q

how was genetic diversity studied before 1966?

A

morphological
- eg snail colour polymorphism
cytological
- eg chromosome inversions

21
Q

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

A
  • 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

22
Q

evolutionary responses of continuous traits

A
  • demonstrates existence of heritable variation in fitness-related phenotypes
  • due to many underlying genes
23
Q

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

A
  • 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?

24
Q

describe Richard Lewontin and the Electrophoresis Revolution

A
  • allozyme gel electrophoresis provided a way to ask
  • what proportion of genes are variable (p&h)?
  • addresses fundamental dispute between classical and balance schools
25
Q

advantages of studies of enzyme polymorphism

A
  • 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
26
Q

describe the neutral theory by Motoo Kimura

A
  • negative selection rapidly eliminates detrimental mutations
  • positive selection rapidly fixes beneficial mutations
  • the only mutations left to create genetic variation are selectively neutral
27
Q

describe DNA variation in maize vs teosinte

A
  • corn has reduced genetic diversity compared to its wild ancestor teosinte
  • a consequence of population bottlenecks during domestication
28
Q

describe human genetic variation

A
  • humans show a loss of genetic variation with increasing distance from East Africa
  • reflects founder events as humans migrated from source population
29
Q

describe comparisons of polymorphism in Arabidopsis lyrata

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

BIO120 (32 decks)

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