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evolution > Neutral & Adaptive Evolution, Pop Genetics, Heritability > Flashcards

Neutral & Adaptive Evolution, Pop Genetics, Heritability Flashcards

w5

1
Q

other causes of evolution

A
  • mutation
  • gene flow
  • genetic drift
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2
Q

what causes changes in gene freq?

A
  • selection (natural, sexual or artificial)
  • mutations
  • genetic drift
  • gene flow
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3
Q

1.mutations

A
  • heritable changes in nucleotide seq
  • sometimes resulting in alteration in products coded for by gene
  • types of mutations…
    -> substitution
    -> deletion
    -> insertion
    -> inversion
  • low mutation rates (1x10⁻⁹/ base pair / gen)
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4
Q

different effects of mutations

A
  • harmful
  • neutral
  • beneficial -> rare as random changes unlikely to result in improvement
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5
Q

as beneficial mutations are rare, it is thought that most mutations are harmful or beneficial.

suggest why there are more neutral mutations than harmful ones

A

only 2% of mammalian genome encodes for proteins

rest is junk DNA -> if mutation occurs here => no effect!

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6
Q

what is av. no. of mutations in each human zygote?

explain why

A

6 new mutations

  • low mutation rates (1x10⁻⁹ / base pair / gen)
  • haploid human genome ~3x10⁹ base pairs long
  • ^also human has 2 copies of each gene, so double this
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7
Q

2.genetic drift

A
  • change in gene freq in a pop…
  • due to random events
  • is a sampling issue -> so v important in small pops, but omnipresent in big pops too (just can’t see its effect as much)
  • rare alleles can drift to fixation by chance! (fixation = when 1 allele becomes only allele in that pop (no genetic variation at that locus))
  • alleles can also be lost by chance
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8
Q

random events are v important in evolution and history of life on earth.

A
  • if there is no selection, genes in each gen are random sample of prev. gen
  • in small pops, gene freqs can change by chance ( = genetic drift)
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9
Q

why are conservationists worried about little genetic diversity in a sp?

A
  • if eg. new disease, if it affected 1, there is danger to them all
  • limits robustness to new forms of selection, eg. new viruses

-> genetic variation (diversity) is essential for adaptive evolution!

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10
Q

eg. of genetic drift

A
  • cheetahs had extreme bottleneck 1000s of years ago
  • as result, when we look at modern cheetah pops (which are larger than they were 1000s ya), they have extremely limited genetic diversity
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11
Q

when you have genetic drift…

A
  • probability allele drifts to fixation = freq. of allele in pop
  • rate of drift depends on pop size
  • can experiment this with Drosophila (Dobzhansky & Pavlovsky)
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12
Q

describe Dobzhansky & Pavlovsky’s experiments with Drosophila in genetic drift

A
  • cages with either 20 or 4000 flies…
  • large pop saw stable decline in PP allele
  • small pop had range of freqs that PP allele was found at

conclusion: drift is v important in small pops

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13
Q

“founder effect”

A

bottleneck that occurs when a new pop is founded

  • small founder pops may have non-representative sample of genes
  • probability of over-representation of allele / lack of allele depends on…
    -> no. of founders
    -> gene freqs
  • can use as way of mapping colonisation events
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14
Q

founder effects in man

A
  • isolated pops usually have high freqs of normally v rare alleles
  • Afrikaners -> ~30 000 afrikaners carry gene for rare porphyria variegata (causes reaction in barbiturates)
    -> all descended from 1 couple
  • Mauritius -> all cases of Huntingdon’s disease traced to single french nobleman
  • Tristan de Cunha islanders -> asthma
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15
Q

3.gene flow

A
  • intro (or loss) of new alleles into a pop through immigration (or emigration)
  • causes rise in genetic diversity
  • eg. bottlenecked wolves in Scandanavia
  • gene flow = migration -> requires actual individs to move
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16
Q

bottlenecked wolves in Scandanavia eg. of gene flow

A
  • heterozygosity was declining in 70s-80s
  • because basically 1 pack of wolves left, so have to breed with each other every gen as no other wolves …
  • so becoming inbred & ↓ heterozygosity (usually bad thing for pops)
  • BUT in 90s, 1 male wolf came to Scandinavia & brought bunch of genetic diversity!
  • he mated with alpha female => ↑ heterozygosity after this in pop (this is genetic rescue)
  • pop size also grew after this
17
Q

gene flow can limit local adaptation (not positive like in Scandinavian wolves).

give general eg. of this

A
  • pop A has parasite absent in B
  • pop A under selection to resist parasite BUT B ISN’T
  • so if B individs enter pop A, they bring parasite-susceptible genes
  • ↓ rate of fixationof anti-parasite gene as susceptible genes are continually arriving into pop
18
Q

gene flow important at margins of sp

eg. pine trees

A
  • range margin in terms of altitude…
    -> pine trees cannot grow in mountains
    -> pine trees cannot grow lower as out-competed by another plant
  • range not completely homogenous
    -> at top: env arid & cold
    -> at bottom: env moist & warm
  • so pine trees have diff adaptations throughout their range
19
Q

why are pops unable to adapt to variation in its range?

A

gene flow inhibits adaptation & prevents pops at becoming really good at dealing with conditions at edges of its margin

20
Q

why are small pops more strongly affected by genetic drift than large pops?

A
  • (probability allele drifts to fixation is same as freq. of allele in pop)
  • (founder effect: in small pops founded by a few individs -> initial allele freqs may not accurately represent o.g pop => fixation of certain alleles through genetic drift)
  • fixation much more random in small pops, regardless of if allele is advantageous or not
  • chances of allele being lost / fixed in pop are much higher as each individ in small pop represents a larger proportion of the entire pop
  • smaller populations have less variation & ∴ lower ability to adapt to changing conditions
21
Q

population genetics

A
  • study of processes affecting gene freq
  • pop genetics & genetic models allow us to…
    -> predict evolution
    -> identify processes eg. selection
22
Q

dominance

A
  • most eukaryotes are diploid
    -> paired chromosomes
    -> 2 copies of every gene
    -> ^1 from mother, 1 from father
  • generally, both alleles are expressed
    -> co-dominant eg. AB blood group
  • sometimes 1 allele (dom) masks expression of another (recessive) THIS IS DOMINANCE
  • dom doesn’t mean advantageous - doesn’t affect fitness
23
Q

hardy-weinberg equilibrium

A

p² : 2pq : q²

  • achieved after 1 gen of random mating
  • ratio of parental genotypes does’t affect result…
  • so even if eg. loads of genetic drift & all heterozygotes have gone extinct… within 1 gen, freq of alleles will dictate future genotype freq (goes back to equilibrium)
24
Q

using Hardy-Weinberg…

A
  • assume…
    -> large pop
    -> random mating
    -> no selection
  • allows calc of genotype freqs from gene freqs
  • deviations from Hardy-Weinberg equilib. eg. more heterozygotes than you’d expect: shows 1 of assumptions is wrong!
25
Q

suggest how assumptions of hardy-weinberg equilib could be wrong

A
  • smaller pop
  • mating not random -> individs choosing to mate according to own genotype
  • most common: selection removing / favouring 1 genotype than others
26
Q

hardy-weiberg used to determine if selection is going on,

but how strong is that selection??

A
  • genotypes may vary in their selection coefficient (chance of survival to reproduction)
  • alleles may vary in effect on phenotype (dom. & recessive)…
  • which also affects rate of evolution - being exposed to selection

conclusion: use idea of selection coefficient to determien how strong selection is

27
Q

selection coefficient

A

how much more / less likely are those genotypes to survive & reproduce

28
Q

3 forms selection can take

A
  • directional
  • stabilizing
  • disruptive
29
Q

directional selection

A
  • individs on 1 side of distribution are favoured (eg. small body size)
  • pop evolves in this direction
  • eg. pink salmon in Pacific NW
30
Q

pink salmon in Pacific NW

A
  • 1945: fisherman started being paid by the pound not no. of fish -> so only wanted large fish caught…
  • => selection against large size
  • over time, mean body mass of salmon ↓
31
Q

stabilising selection

A
  • most common in nature
  • av. members of pop are favoured over extremes
  • eg. birth weight in humans
32
Q

disruptive selection

A
  • happens when drastic change in env
  • intermediates are disfavoured
  • eg. pop diversifying into 2 habitats
  • can immitate selection against intermediates in lab with bristle numbers in Drosophila
    -> do not allow intermed. bristle no. individs to breed
33
Q

how much of the variation in traits is heritable?

(heritable: passed from parents -> offspring)

OR

how much of pop variation in a trait is due to genes?

A
  • ## important cause only heritable traits can evolve
34
Q

mid-parent trait

A

mean trait of 2 parents

35
Q

mid parent & 1 parent difference

A
  • with mid-parent data, heritability is the slope

BUT

  • with 1 parent data (a.k.a if trait is measured in 1 parent eg. mother’s milk), 2x the slope as 2 parents contribute genes to offspring
36
Q

total variation in the pop =

A

genetic variation + environmental variation

37
Q

can we predict how a pop will evolve?

A

using R : response to selection

38
Q

heritability

A
  • measures how much of variation in pop is genetic
  • rate of evolution depends on strength of selection & heritability of trait

evolution (8 decks)

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