Lecture 2: Variation and polymorphism

Question 1

Types of selection:

Answer 1

• Directional
• Divergent (–> disruptive)
• stabilising

Question 2

Directional selection:

Answer 2

shifts the overall population by avoiding an extreme e.g. Drug resistance in Plasmodium falciparum Nwakanma et al 2014

Question 3

Divergent selection:

Answer 3

favours variants of opposite extremes

• -normally in 2 different environments, between populations, when in the same population it is disruptive selection
• e.g. Senior et al 2014, great tits, stripe made from melanin (costly), thicker stripe = better quality male but variation in urban&rural
• • urban: smaller stripe = fitter
• • rural opposite

Question 4

what is divergent selection among same population known as

Answer 4

disruptive selection (v specific type of divergent)

Question 5

divergent selection in 2 different environments between populations is the base for

Answer 5

ecological speciation

Question 6

Stabilising selection

Answer 6

acts against extreme phenotypes
-e.g. Sztepanacz & Rundle 2012
Drosophila serrata Genetic variance was greater among low-fitness individuals compared to high fitness individuals

Question 7

is selection always clear?

Answer 7

no, hard to tell which type of selection is taking place. e.g. Martin and Pfennig 2009 Spadefoot toads, speak multiplicata (divergent, directional, stabilising?)

Question 8

Variation is necessary for selection but selection often

Answer 8

reduces variation

Question 9

how is genetic variation and polymorphism maintained?

Answer 9

1) Diploidy
2) Gene flow
3) Mutation
4) Balancing selection
a) Heterozygous advantage
b) Frequency dependent selection

Question 10

Diploidy

Answer 10

• 2 copies of alleles, back up! (i.e 2 parents, one iffy, use other) &
• dominant & recessive- recessive always hidden, phenotype resulting from dominant shown.
• Selection cannot effect recessive allele, can’t be seen.
• if environment changes may be useful

Question 11

Gene Flow / migration e.g.

Answer 11

e. g. Spea multiplicata (Speadefoot toads) movement
- once tadpoles grow up they can move from one pond to another, taking all genetic information and background neutral alleles with it , use in new pond
- genetic information moving between populations

Question 12

Mutation

Answer 12

• introduce a lot of genetic variation

- can get somatic (not in evolutionary bio) / gremline –> reproducing cells

Question 13

types of germline mutations:

Answer 13

-Point mutations
–Substitution
–insertion
–deletion
–inversion
can be silent/synonymous or non-synonymous –> resulting in: missense (changes aa), nonsense (doesnt make anything anymore) , frame shift
-block mutations (whole chunk of DNA changed)
–deletion
–insertion
–translocation
–inversion
–duplication

ALL ADD VARIATION

Question 14

mutation example:

Answer 14

-Neurofibromatosis type 1
1 in 3000-4000
effects myelin sheath –
-highst mutation rates described for any human disorder
-it spans ~350 kb of genomic DNA
-codes for a protein of 2818 aa
so far 255 different mutation shave been reported

Question 15

Balancing selection: 2 proposed mechanisms

Answer 15

• Heterozygote advantage

- Frequency dependent selection

Question 16

balancing selection is when:

Answer 16

selections acts to maintain the different morphs in the population

Question 17

heterozygote advantage:

Answer 17

-initially doesn’t make sense, e.g. sickle cell anaemia gene in Africa
- 0 copies
- 1 copy resistant to malaria
-2 copies have sickle cell
SELLIS ET AL 2011
–> says make sense as has to be fit in heterozygous form as this is how it first arises through mutation

Question 18

reasons for Heterozygote advantage not being that common: associative overdominance

Answer 18

• what appears to be HA might not actually be HA (linkage disequilibrium, might not realise theres a second locus also affecting it –> selection coefficient)
• -> ASSOCIATIVE OVERDOMINANCE

Question 19

selection coefficient

Answer 19

the difference between the mean relative fitness of individuals of a given genotype and that of a reference genotype
-amount the mutation affects the fitness of an individual

Question 20

associative overdominance:

Answer 20

increase in fitness of heterozygotes at a neutral locus because it is in linkage disequilibrium at a locus that is under selection

Question 21

reasons for Heterozygote advantage not being that common: Heterozygote advantage imposes a ‘load’ on the population

Answer 21

‘any genotypes that aren’t ‘the best’

• Genetic load = difference between maximum fitness and mean fitness
• -> homozygous being less fit and this isn’t how selection works as it normally works towards 100% fitness

Question 22

reasons for Heterozygote advantage not being that common: Heterozygote advantage is unstable

Answer 22

due to duplication! (mutation)

• advantageous allele duplicated and becomes fixed at a different location
• after recombination most members of population will have the gene –> high fitness, low load
• • evidence of duplication can be seen in globin family of genes e.g. Wheeler et al 2001

Question 23

reasons for Heterozygote advantage not being that common:

Answer 23

• associative over dominance
• Heterozygote advantage imposes a ‘load’ on the population
• Heterozygote advantage is unstable

Question 24

HA

Answer 24

• maintains variation in population
• shows how balancing selection can work
• theories against it

Question 25

Balancing selection: Frequency -dependent selection

Answer 25

• Positive
• -fitness increases with more individuals that have it
• Negative
• • advantage of the rare

Question 26

e.g. of positive F-d selection

Answer 26

• Mullarian mimicry,
• cytotype exclusion
• – doesn’t help with maintaining polymorphism

Question 27

e.g. of negative F-d selection

Answer 27

• Batesian mimicry
• host-parasite cycles
• • DOES maintain polymorphism

Question 28

negative frequency -dependent selection is a big driver of

Answer 28

BALANCING SELECTION

Question 29

specific negative frequency dependant selection examples to maintain many morphs (also left handed people - Faurie & Raymond 2005)

Answer 29

Lake Tanganyika cichlid Perissodus microlepis

• right / left mouthed, bite chunks out other fish
• 2 different morphs increase & decrease continuously
• Hori 1993