Neutral & Adaptive Evolution, Pop Genetics, Heritability Flashcards
w5
other causes of evolution
- mutation
- gene flow
- genetic drift
what causes changes in gene freq?
- selection (natural, sexual or artificial)
- mutations
- genetic drift
- gene flow
1.mutations
- 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)
different effects of mutations
- harmful
- neutral
- beneficial -> rare as random changes unlikely to result in improvement
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
only 2% of mammalian genome encodes for proteins
rest is junk DNA -> if mutation occurs here => no effect!
what is av. no. of mutations in each human zygote?
explain why
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
2.genetic drift
- 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
random events are v important in evolution and history of life on earth.
- 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)
why are conservationists worried about little genetic diversity in a sp?
- 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!
eg. of genetic drift
- 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
when you have genetic drift…
- probability allele drifts to fixation = freq. of allele in pop
- rate of drift depends on pop size
- can experiment this with Drosophila (Dobzhansky & Pavlovsky)
describe Dobzhansky & Pavlovsky’s experiments with Drosophila in genetic drift
- 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
“founder effect”
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
founder effects in man
- 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
3.gene flow
- 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
bottlenecked wolves in Scandanavia eg. of gene flow
- 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
gene flow can limit local adaptation (not positive like in Scandinavian wolves).
give general eg. of this
- 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
gene flow important at margins of sp
eg. pine trees
-
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
why are pops unable to adapt to variation in its range?
gene flow inhibits adaptation & prevents pops at becoming really good at dealing with conditions at edges of its margin
why are small pops more strongly affected by genetic drift than large pops?
- (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
population genetics
- study of processes affecting gene freq
- pop genetics & genetic models allow us to…
-> predict evolution
-> identify processes eg. selection
dominance
- 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
hardy-weinberg equilibrium
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)
using Hardy-Weinberg…
- 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!
suggest how assumptions of hardy-weinberg equilib could be wrong
- smaller pop
- mating not random -> individs choosing to mate according to own genotype
- most common: selection removing / favouring 1 genotype than others
hardy-weiberg used to determine if selection is going on,
but how strong is that selection??
- 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
selection coefficient
how much more / less likely are those genotypes to survive & reproduce
3 forms selection can take
- directional
- stabilizing
- disruptive
directional selection
- individs on 1 side of distribution are favoured (eg. small body size)
- pop evolves in this direction
- eg. pink salmon in Pacific NW
pink salmon in Pacific NW
- 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 ↓
stabilising selection
- most common in nature
- av. members of pop are favoured over extremes
- eg. birth weight in humans
disruptive selection
- 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
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?
- ## important cause only heritable traits can evolve
mid-parent trait
mean trait of 2 parents
mid parent & 1 parent difference
- 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
total variation in the pop =
genetic variation + environmental variation
can we predict how a pop will evolve?
using R : response to selection
heritability
- measures how much of variation in pop is genetic
- rate of evolution depends on strength of selection & heritability of trait
