Lecture 1: Natural Selection Flashcards
Adaption definition:
A characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states
to disrupt HW equilibrium you need
4 evolutionary forces:
- mutation
- Genetic Drift
- Migration (gene flow)
- NAtural selection
mutation:
- random
- new variation
genetic drift:
- random changes in unselected allele frequency
- more in smaller populations
- tends to lower heterozygosity
- can cause isolated populations to diverge
migration (gene flow)
-counteracts divergence due to drift (works in opposition to genetic drift)
natural selection:
-fitness and adaption focussed
Natural selection definition:
differential survival and/or reproduction of classes of entities that differ in one of more characteristics
fitness =
probability of survival X average number of offspring for a class of individuals
3 points to how we know natural selection exists:
- Correlations between trait and environment
- responses to experimental change in the environment
- correlations between trait & fitness component
how do we know natural selection exists? 1. correlations between trait & environment e.g.
- Endler (1980)
- Trinidad
- Guppy (Poecillia reticulata)
- 2 predators:
- -Rivulus hartii (eat juveniles)
- -Crenicichla alta (eat all guppies)
- 2 different predation pressures
- colouration differs between predation rate
- probably due to trade-off between attracting females but not getting eaten
how do we know natural selection exists? 2. Responses to experimental change in the environment
- Guppies: Endler 1980
- add predators after 6 months
- Black, yellow + red spots different vary due to predation, blue & iridescent colours spots did drop as predation increased
- spot size matched background when predation is high (larger gravel, larger spots)
how do we know natural selection exists? 3. Correlation between trait and fitness component
- Guppies: Endler 1980
- when there was no predators the spots were doing an opposite to the background - they were trying to stand out! female attraction
- colouration is costly, rarer pattern more attractive to females
- more carotenoids (organic colours) = faster and more viable sperm
experiment used to justify how we know natural selection exists?
Guppies. Endler 1980
Problems with detecting selection
- Consequence of physical/chemistry not due to selection (our blood is red)
- Genetic drift can spread traits
- Ancestral state (useful way back, not adapted anymore, i.e. forming skulls) (preadaptation)
- Selection might not cause any change
- selection might not be working at the individual level
- Linkage
linkage:
- Recombination is random process, two parent chromosomes combine to produce F1 generation.
- Results in genetic mash-up in F1 generation
linkage disequilibrium:
linkage resulting in the same alleles across the generations that keep popping up together time and time again
-alleles appearing together more often than u would expect
linkage equilibrium
complete mash up between genome, random. alleles don’t appear time & time again
several reasons for linkage:
- beneficial alleles grouping together
- structural changes e.g. chromosomal inversion, results in twist in DNA so chromosomes are still pairing
- hitchhiking alleles
hitchhiking alleles:
selective sweep results in those alleles surrounding beneficial alleles are always often chosen for
does linkage disequilibrium mean that selection is working to keep those alleles together?
NO its by chance
early example of hitchhiking
- Atwood et al (1951)
- his allele
- his is hitchhiking due to association with an advantageous mutation
- his alleles RESPOND to selection but are not UNDER selection
why is evidence for hitchhiking in mammals harder to come by?
as we are sexually reproducing, recombination happens more frequently
2 examples of hitchhiking in mammals:
- Chan et al (2012) –> Evidence for selective sweeps in mice for body size genes
- Jones et al (2013) -> humans,for the lactose digesting allele
What will natural selection not do?
1) ALWAYS lead to adaptation
2) produce perfection
3) ALWAYS progress
4) Produce a balanced, harmonious world
5) Consider ethics
adaptations come in all form but not all traits are
adaptations
there are _ evolutionary forces, do they always work in the same way
4, and no they don’t
is it easy to prove natural selection ?
no. sometimes its hard to even find where it might be working
