Lecture 19: Natural Selection and Adaptation

Question 1

What is fitness?

Answer 1

Genetic contribution of individuals to the next generation relative to others as a result of differences in viability and fertility.

Question 2

Who has a selective advantage?

Answer 2

One with better fitness has a selective advantage over the others. This advantage depends on the environment

Question 3

Can evolution occur via natural selection?

Answer 3

Yes, BUT ADAPTATION can only result via Natural Selection

Question 4

What is evolution?

Answer 4

Change in genotype of an organism (allele changes is an evolutionary change)

Question 5

What is artificial selection and 3 examples?

Answer 5

It is humans selecting towards a goal. The examples include:

1. Domesticated plants and animals
2. Selection experiments in genetics
3. Evolution of attenuated vaccines

Question 6

What is natural selection?

Answer 6

Selection by Abiotic and Biotic conditions, it does not have a goal.

It affects ALL ORGANISMS

Question 7

What is positive directional selection?

Answer 7

A single new mutation that gives an individual an advantage.

This is a case in which natural selection is acting on a single allele.

Example of “Natural selection on Alleles”

Question 8

If A is an allele with selective advantage, what about happen to its frequency over generations?

Answer 8

AA: would increase by a lot
Aa: would also increase but with a tiny difference compared to AA

aa: would decrease

-After 50 generations almost 99% is going to be AA. ( this is for discrete selection )

Question 9

What is negative purifying selection?

Answer 9

It reduces mutations that decreases fitness

Question 10

What is variation maintaining selection?

Answer 10

A combination of two alleles should be more beneficial than homozygous alleles.

HH, hh < Hh

Question 11

What are the 3 types of natural selection on alleles?

Answer 11

1. Positive directional selection
2. Negative purifying selection
3. Variation maintaining selection.

Question 12

How are different natural selection of alelles modelled?

Answer 12

1. Y: axis- frequency of the allele
   x-axis : Phenotypic value

If the selection is selective: The graph becomes thinner (favours average traits) (quadratic)

if the selection is direction, the graph shifts to the favored zone (favors one extreme)

if the selection is disruptive: the graph changes its shape.
(favours both extremes)

Question 13

What does disruptive selection result in?

Answer 13

1) Speciation in some cases
2) Leads to TRAIT DIVERGENCE
3) requires spatial heterogeneity or discrete resources

Question 14

What were the results from the graphs, when the colour of the mice were plotted against the probability of survival (on the Y)?

Answer 14

There was a huge directional shift, the light ones survived better on a lighter background, while darker ones survived better on the darker background.

Question 15

Which allele caused resistance against malaria?

Answer 15

G6PD

An increase in A- allele caused a form of anemia but also gave resistance against malaria.

A- was common in tropical region, where malaria was also common,

Question 16

What did genome data on the allele for A- show ?

Answer 16

People with A- allele had very little genetic variation compared to others, it a proof that it is relatively spreading pretty fast and hasn’t had enough time to evolve.

Question 17

How do you know if change in allele is a cause of Natural selection or genetic drift?

Answer 17

If it occurs very quickly then it’s natural selection.

Genetic drift is relatively slow, as it is random genetic frequency in a given population.

Question 18

Why does genetic drift not drive rapid evolution like natural selection?

Answer 18

Because it is a random fluctuation every generation.
While Natural selection leads to really strong selection.

Question 19

Can positive selection act even when you have little as one % or less selective advantage?

Answer 19

Yes it can spread fast with given time.

Question 20

Are many phenotypic traits or polygenic or discrete?

Answer 20

Many are phenotypic traits are polygenic, and polygenic traits are affected by the environment.

Question 21

How does the graph of a continuous polygenic trait look like?

Answer 21

It has the shape of a bell curve. Where even a change of single allele frequency will be favored.

Phenotypic trait value vs Allele frequency

: /——————\
: / \
: /
: /
:/
:…………………………………………

Question 22

What is an example if stabilizing selection?

Answer 22

Human birth weight, beak size in finches in drought conditions

Question 23

How does the graph look for disruptive selection and what is an example?

Answer 23

The graph has peaks to the extremes, and African finches are the examples

Question 24

What happened when HOPI HOEKSTERA changed a single gene(the gene for coat colour) was changed in mice?

Answer 24

They say a rapid change against the frequency of the white allele in the dark background.