Lecture 1: Natural Selection

Question 1

Adaption definition:

Answer 1

A characteristic that enhances the survival or reproduction of organisms that bear it, relative to alternative character states

Question 2

to disrupt HW equilibrium you need

Answer 2

4 evolutionary forces:

• mutation
• Genetic Drift
• Migration (gene flow)
• NAtural selection

Question 3

mutation:

Answer 3

• random

- new variation

Question 4

genetic drift:

Answer 4

• random changes in unselected allele frequency
• more in smaller populations
• tends to lower heterozygosity
• can cause isolated populations to diverge

Question 5

migration (gene flow)

Answer 5

-counteracts divergence due to drift (works in opposition to genetic drift)

Question 6

natural selection:

Answer 6

-fitness and adaption focussed

Question 7

Natural selection definition:

Answer 7

differential survival and/or reproduction of classes of entities that differ in one of more characteristics

Question 8

fitness =

Answer 8

probability of survival X average number of offspring for a class of individuals

Question 9

3 points to how we know natural selection exists:

Answer 9

1. Correlations between trait and environment
2. responses to experimental change in the environment
3. correlations between trait & fitness component

Question 10

how do we know natural selection exists? 1. correlations between trait & environment e.g.

Answer 10

• 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

Question 11

how do we know natural selection exists? 2. Responses to experimental change in the environment

Answer 11

• 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)

Question 12

how do we know natural selection exists? 3. Correlation between trait and fitness component

Answer 12

• 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

Question 13

experiment used to justify how we know natural selection exists?

Answer 13

Guppies. Endler 1980

Question 14

Problems with detecting selection

Answer 14

1. Consequence of physical/chemistry not due to selection (our blood is red)
2. Genetic drift can spread traits
3. Ancestral state (useful way back, not adapted anymore, i.e. forming skulls) (preadaptation)
4. Selection might not cause any change
5. selection might not be working at the individual level
6. Linkage

Question 15

linkage:

Answer 15

• Recombination is random process, two parent chromosomes combine to produce F1 generation.
• Results in genetic mash-up in F1 generation

Question 16

linkage disequilibrium:

Answer 16

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

Question 17

linkage equilibrium

Answer 17

complete mash up between genome, random. alleles don’t appear time & time again

Question 18

several reasons for linkage:

Answer 18

• beneficial alleles grouping together
• structural changes e.g. chromosomal inversion, results in twist in DNA so chromosomes are still pairing
• hitchhiking alleles

Question 19

hitchhiking alleles:

Answer 19

selective sweep results in those alleles surrounding beneficial alleles are always often chosen for

Question 20

does linkage disequilibrium mean that selection is working to keep those alleles together?

Answer 20

NO its by chance

Question 21

early example of hitchhiking

Answer 21

• 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

Question 22

why is evidence for hitchhiking in mammals harder to come by?

Answer 22

as we are sexually reproducing, recombination happens more frequently

Question 23

2 examples of hitchhiking in mammals:

Answer 23

• Chan et al (2012) –> Evidence for selective sweeps in mice for body size genes
• Jones et al (2013) -> humans,for the lactose digesting allele

Question 24

What will natural selection not do?

Answer 24

1) ALWAYS lead to adaptation
2) produce perfection
3) ALWAYS progress
4) Produce a balanced, harmonious world
5) Consider ethics

Question 25

adaptations come in all form but not all traits are

Answer 25

adaptations

Question 26

there are _ evolutionary forces, do they always work in the same way

Answer 26

4, and no they don’t

Question 27

is it easy to prove natural selection ?

Answer 27

no. sometimes its hard to even find where it might be working