Neutral & Adaptive Evolution, Pop Genetics, Heritability

Question 1

other causes of evolution

Answer 1

• mutation
• gene flow
• genetic drift

Question 2

what causes changes in gene freq?

Answer 2

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

Question 3

1.mutations

Answer 3

• 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)

Question 4

different effects of mutations

Answer 4

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

Question 5

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

Answer 5

only 2% of mammalian genome encodes for proteins

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

Question 6

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

explain why

Answer 6

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

Question 7

2.genetic drift

Answer 7

• 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

Question 8

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

Answer 8

• 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)

Question 9

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

Answer 9

• 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!

Question 10

eg. of genetic drift

Answer 10

• 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

Question 11

when you have genetic drift…

Answer 11

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

Question 12

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

Answer 12

• 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

Question 13

“founder effect”

Answer 13

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

Question 14

founder effects in man

Answer 14

• 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

Question 15

3.gene flow

Answer 15

• 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

Question 16

bottlenecked wolves in Scandanavia eg. of gene flow

Answer 16

• 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

Question 17

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

give general eg. of this

Answer 17

• 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

Question 18

gene flow important at margins of sp

eg. pine trees

Answer 18

• 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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

• (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

Question 21

population genetics

Answer 21

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

Question 22

dominance

Answer 22

• 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

Question 23

hardy-weinberg equilibrium

Answer 23

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)

Question 24

using Hardy-Weinberg…

Answer 24

• 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!

Question 25

suggest how assumptions of hardy-weinberg equilib could be wrong

Answer 25

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

Question 26

hardy-weiberg used to determine if selection is going on, but how **strong** is that selection??

Answer 26

- 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**

Question 27

selection coefficient

Answer 27

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

Question 28

3 forms selection can take

Answer 28

- directional - stabilizing - disruptive

Question 29

directional selection

Answer 29

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

Question 30

pink salmon in Pacific NW

Answer 30

- 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 **↓**

Question 31

stabilising selection

Answer 31

- most common in nature - **av. members** of pop are **favoured** over extremes - eg. birth weight in humans

Question 32

disruptive selection

Answer 32

- 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**

Question 33

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?

Answer 33

- important cause only heritable traits can evolve -

Question 34

mid-parent trait

Answer 34

**mean trait** of 2 parents

Question 35

mid parent & 1 parent difference

Answer 35

- 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

Question 36

total variation in the pop =

Answer 36

**genetic** variation + **environmental** variation

Question 37

can we predict how a pop will evolve?

Answer 37

using R : response to selection

Question 38

heritability

Answer 38

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