natural selection 1

Question 1

Natural selection definition

Answer 1

the differential survival of phenotypes that vary in fitness. Fitness is a measure of the relevant gene’s representation in future generations.

Question 2

Malthus essay on population growth inspired Darwin

Answer 2

In 1798, an English minister economist named Thomas Robert Malthus published
‘An Essay on the Principle of Population, As It Affects the Future Improvement of Society.’

He reasoned that while populations grew geometrically, the resource needed to sustain them would grow arithmetically
— leading to some gloomy predictions…

Darwin was inspired by Malthus’ paper which showed populations grew geometrically whereas their resources grow arithmetically – so some will survive and others wont.

Question 3

Charles Darwin

Answer 3

After his return from his time as naturalist on HMS Beagle (1831-1836), Darwin spent decades writing about the expedition including his popular journal account, and writing four monographs on barnacles.

From his notebooks, Darwin explored some of the ideas he would develop further later on, especially in the ‘Origin of Species’

Darwin had been working on a major work to be in two parts

1) the first on domestication, originally planned as a short ‘abstract’ the piece on Domestication was published in 1859 as a 502 page book. The first part of his larger book was eventually published in 1868 as Variation of Animals and Plants Under Domestication.

2) the second on natural selection(~1856-1858), which he abandoned to work on the Origin of Species, started a month after he received the letter from Wallace.

Question 4

Alfred Russell Wallace

Answer 4

Wallace was an explorer and collector of natural history specimens.
It is said that he was delirious with fever in Indonesia when the idea of evolution by natural selection came to him.

He wrote a manuscript on the subject and sent it to Darwin, leading to the joint publication by both in 1858

Question 5

How does natural selection work?

Answer 5

Natural selection works with the available genetic variation in natural populations that has accumulated by mutation over time – such variation will be going to fixation or loss due to genetic drift, and the rate depends on the size of the population

Question 6

Fundamental model of natural selection

Answer 6

i) the frequency, p’, of an allele after selection is proportional
to:
the frequency of that allele among newborns x their probability of survival.
ii) Their probability of survival is typically referred to as their ‘viability’ and denoted ‘w’.
iii) For example, the frequency of A1A1 adults in the population is expected to be proportional to p^2 w11

Question 7

There are many components to fitness, including:

Answer 7

• Fertility
• developmental time
• Mating success
• And more -
  all aspects of phenotype that affect the contribution of that individual’s genotype in the next generation.

For the sake of this analysis, we’ll lump all these aspects together into the viability

Question 8

Fundamental model of natural selection, based on a
single locus in a diploid organism with two alleles —
Al and A2

Answer 8

See diagrams and equations in notes

Frequency after selection =
(initial freq2iinitial viability + initial freq x allele 1) / 1

1= initial freq^2i initial viability+2initial freq x allele 1 viability + allele 2 x viability of allele 2

Question 9

Allele frequencies in the next generation

Answer 9

Selection biases the probability of sampling the two alleles in the next generation

Genotype frequencies remain in H-W equilibrium after selection

at the frequencies defined by p’ (A1)and q’ (A2)

see notes for equations

Question 10

change due to selection (random mating) and reaching equilibrium

Answer 10

see notes for equations

If A1A1 has the highest fitness, then at equilibrium there will only be A1 alleles ( so equal to 1)

If the heterozygote has the highest fitness, then the frequency of A1 will remain intermediate (between 0 and 1)

It is simpler to consider not the absolute, but the relative
fitness, for example relative to the fitness of the A1Al
genotype (which would then be given a fitness of l).

s= selection fitness
If s > 0, then A2A2 is less fit than A1A1, etc.

s varies between 0 and 1, and even 10% (0.1) is considered a relatively strong effect.

Question 11

Heterozygous effect - measuring dominance

Answer 11

Genotype:
Relative fitness:
A1A1 AlA2 A2A2
1 1-hs 1-s

Heterozygous effect (h) is the fitness of the heterozygote relative to the selective difference between the two homozygotes - essentially a measure of dominance:

h=0 Al dominant, A2 recessive
h= 1 A2 dominant, Al recessive
0<h<1 = incomplete dominance
h<0 = overdominance (heterozygote favoured)
h>1 = underdominance (either homozygote favored)

Incomplete dominance = codominance ( this is v. common)

see notes for graphs and equations

Question 12

Selection & Mutation

Answer 12

Change through the selection of p taking into account mutation. Value is small as mutation is rare.

See derivations in Gillespie, Chapter 3

Question 13

Selection and drift summary

Answer 13

1) Selection is a weak force when alleles are rare.

e.g. for a favorable new mutation, the frequency of the mutation is 1/2Ne, so the strength of selection is roughly s(1/2Ne), which is likely to be less than 1/2Ne.So in finite populations, a favourable new mutation is usually lost due to genetic drift.

2) selection dominates over drift when the allele becomes common

Once established, selection will be likely to influence the fate of an allele when the selection coefficient for that allele is greater than the effects of drift, which depends on Ne:

s>1/2Ne

For example, if Ne = 1000, s would need to be greater than 0.0005 for Ne = 100, s > 0.005, for Ne = 10, s > 0.05

Note that when Ne is small, even strong selection is
overpowered by genetic drift, and the relative strength of
selection is small compared to drift.

Question 14

Summary so far:

Answer 14

1) Natural selection is the differential survival of phenotypes that vary in fitness. Fitness is a measure of the relevant gene’s representation in future generations.

2) Selection only works on available variability – doesn’t provide a species with whatever it needs.

3) The frequency p’ of an allele after selection is proportional to the frequency of that allele among newborns, times their probability of survival (which we designate as the viability).

4) Directional selection is selection in favour of a particular allele through the homozygous genotype (with codominance); Balancing selection includes selection for heterozygote and frequency dependence, and Disruptive selection is against the heterozygote