Lecture 2: Variation and polymorphism Flashcards
Types of selection:
- Directional
- Divergent (–> disruptive)
- stabilising
Directional selection:
shifts the overall population by avoiding an extreme e.g. Drug resistance in Plasmodium falciparum Nwakanma et al 2014
Divergent selection:
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
what is divergent selection among same population known as
disruptive selection (v specific type of divergent)
divergent selection in 2 different environments between populations is the base for
ecological speciation
Stabilising selection
acts against extreme phenotypes
-e.g. Sztepanacz & Rundle 2012
Drosophila serrata Genetic variance was greater among low-fitness individuals compared to high fitness individuals
is selection always clear?
no, hard to tell which type of selection is taking place. e.g. Martin and Pfennig 2009 Spadefoot toads, speak multiplicata (divergent, directional, stabilising?)
Variation is necessary for selection but selection often
reduces variation
how is genetic variation and polymorphism maintained?
1) Diploidy
2) Gene flow
3) Mutation
4) Balancing selection
a) Heterozygous advantage
b) Frequency dependent selection
Diploidy
- 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
Gene Flow / migration e.g.
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
Mutation
- introduce a lot of genetic variation
- can get somatic (not in evolutionary bio) / gremline –> reproducing cells
types of germline mutations:
-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
mutation example:
-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
Balancing selection: 2 proposed mechanisms
- Heterozygote advantage
- Frequency dependent selection
balancing selection is when:
selections acts to maintain the different morphs in the population
heterozygote advantage:
-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
reasons for Heterozygote advantage not being that common: associative overdominance
- 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
selection coefficient
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
associative overdominance:
increase in fitness of heterozygotes at a neutral locus because it is in linkage disequilibrium at a locus that is under selection
reasons for Heterozygote advantage not being that common: Heterozygote advantage imposes a ‘load’ on the population
‘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
reasons for Heterozygote advantage not being that common: Heterozygote advantage is unstable
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
reasons for Heterozygote advantage not being that common:
- associative over dominance
- Heterozygote advantage imposes a ‘load’ on the population
- Heterozygote advantage is unstable
HA
- maintains variation in population
- shows how balancing selection can work
- theories against it
Balancing selection: Frequency -dependent selection
- Positive
- -fitness increases with more individuals that have it
- Negative
- advantage of the rare
e.g. of positive F-d selection
- Mullarian mimicry,
- cytotype exclusion
- – doesn’t help with maintaining polymorphism
e.g. of negative F-d selection
- Batesian mimicry
- host-parasite cycles
- DOES maintain polymorphism
negative frequency -dependent selection is a big driver of
BALANCING SELECTION
specific negative frequency dependant selection examples to maintain many morphs (also left handed people - Faurie & Raymond 2005)
Lake Tanganyika cichlid Perissodus microlepis
- right / left mouthed, bite chunks out other fish
- 2 different morphs increase & decrease continuously
- Hori 1993
