Maintenance of Variation

Question 1

outline

Answer 1

• maintenance of genetic variation is major q in evolutionary biology
• variation constantly being eroded by drift & selection
• only mutation can create new diversity
• various mechanisms may slow rate of erosion

Question 2

variation contsantly being lost due to chance events

Answer 2

process of genetic drift

Question 3

sometimes alleles go to fixation.

can genetic drift help maintain variation ever?

Answer 3

• fixation isn’t helping maintain variation as…
• 1 allele’s fixation leads to another’s extinction!

Question 4

we have a problem

Answer 4

• evolution requires variation …
• BUT selection & drift continuously eliminate it!!

Question 5

mutation-selection balance

Answer 5

pops in equilibrium between mutation creating variation & selection removing it

equilib. between rate that mutations enter pop & rate they are lost

Question 6

what factors are there that affect rate that new mutation enters pop?

Answer 6

• pop size -> more individs = more chance mutations occuring in 1 of them
• mutation rate -> how often mutations happen

Question 7

factors affecting rate mutations are lost

Answer 7

• strength of selection
• pop size (genetic drift)

Question 8

how is variation so common??

what could be maintaining variation?

Question 9

3 mechanisms that could help maintain genetic variation within pops

Question 10

stabilising selection tends to erode variation slower than directional selection.

why is this?

Question 11

how the env can help maintain genetic variation

Answer 11

• which allele is favoured is heavily dependent on env …
• as fitness is dependent on env
• fittest genotype at 1 time / place may not be fittest at another

Question 12

environmental heterogeneity

Answer 12

• if env varies in time / space, selection will vary in time & space
• scale of this variation relative to gen time / distribution of sp will determine how this works out
• eg. peppered moths

Question 13

multiple niche polymorphism

Answer 13

• diff genotypes better suited to diff niches…
• BUT genotypes also able to select their preferred niche
• eg. Drosophila simulans red & white eye forms
  -> red eyes attracted to light
  -> white eye mutant avoid light

Question 14

Drosophila simulans red & white eye experiment

Answer 14

• cage with 2 halves, lit independently
• when cage lit with white light on both sides -> W allele (responsible for white mutant) freq declines
• when cage lit with red light on both sides -> W allele freq still declines
• when cage lit 1 side red, 1 side white light -> W allele maintained in pop!!
  -> in white side = red eyes prevalent
  -> on red side = white eyes prevalent
• white mutant has lower fitness under all circumstances

summary:

only white light = W mutant declines
only red light = W mutant declines
red & white = W mutant maintained

Question 15

why is W mutant maintained in red & white light cage?

Answer 15

• Drosophila found way to co-exist …
• not because its good for sp but because individs end up in diff parts of env where their fitness is good
• divide habitat up into 2 niches so variation is maintained
• flies show ‘habitat selection’ -> reduces competition, allowing 2 forms to co-exist

Question 16

negative freq dependence can affect…

Answer 16

• predator-prey relationships
  -> eg. blackbirds specialise at finding & eating common snail, so miss other rare snail
• resource use
  -> eg. flowers in shade / sun
• host parasite
  -> eg. asymmetrical-mouthed scale-eating cichlid
• behaviour
  -> eg. in humans - women prefer men shaven over beards

Question 17

general concept of negative freq dependence

Answer 17

• best alleles don’t fix because there isn’t a best allele!
• ‘best’ depends on how common allele is

Question 18

heterozygote adv occurs when…

Answer 18

fitness of AA < fitness Aa > fitness aa

(using w = fitness)

w AA < w Aa > w aa

Question 19

eg. of heterozygote adv

Answer 19

• hybrid vigour
• when crosses between 2 divergent strains of organism => individ who has higher fitness than strains used in cross
• typical eg. maize plants -> the hybrid is bigger but also produces bigger cobs of corn

Question 20

why do we see hybrid vigour?

Answer 20

• because of masking of homozygous deleterious recessive alleles in hybrids
• recessive alleles: not expressed when in heterozygotes
• rare alleles nearly always found in heterozygotes
• alleles with -ve effects on fitness (“deleterious alleles”) can spread by drift …
• they aren’t expressed, so selection doesn’t act against them!
• ^this allows them to persist, making it more likely they will go to fixation by drift

Question 21

hybrid vigour can’t maintain variation for long…

Answer 21

• because pops will merge
• for genetic variation to be maintained, need heterozygotes to have higher fitness within a pop
• selection cannot eliminate homozygotes

Question 22

rare eg. of heterozygous adv within pop

Answer 22

sickle cell anaemia

• kills 100 000 ppl a year
• occurs in homozygotes for haemoglobin allele (S)
• 80% SS individuals die pre-reproduction

Question 23

why does sickle cell disease persist?

Answer 23

• because if heterozygous for condition (AS), you don’t express this sickle phenotypre in RBCs
• but if infected with malaria parasite, if heterozygous, cell sickles and kills parasites -> causes immunity to malaria
• summary: A and S allele both are maintained in pop due to presence of malaria

Question 24

heterozygote adv

overview

Answer 24

• can’t occur in haploid organisms
• haploid organisms (e.g. protists, fungi, some algae show as much polymorphism as diploids
• so heterozygote adv can’t explain all the variation

Question 25

deleterious recessive allele