Lecture 19: Natural Selection and Adaptation Flashcards
What is fitness?
Genetic contribution of individuals to the next generation relative to others as a result of differences in viability and fertility.
Who has a selective advantage?
One with better fitness has a selective advantage over the others. This advantage depends on the environment
Can evolution occur via natural selection?
Yes, BUT ADAPTATION can only result via Natural Selection
What is evolution?
Change in genotype of an organism (allele changes is an evolutionary change)
What is artificial selection and 3 examples?
It is humans selecting towards a goal. The examples include:
- Domesticated plants and animals
- Selection experiments in genetics
- Evolution of attenuated vaccines
What is natural selection?
Selection by Abiotic and Biotic conditions, it does not have a goal.
It affects ALL ORGANISMS
What is positive directional selection?
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”
If A is an allele with selective advantage, what about happen to its frequency over generations?
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 )
What is negative purifying selection?
It reduces mutations that decreases fitness
What is variation maintaining selection?
A combination of two alleles should be more beneficial than homozygous alleles.
HH, hh < Hh
What are the 3 types of natural selection on alleles?
- Positive directional selection
- Negative purifying selection
- Variation maintaining selection.
How are different natural selection of alelles modelled?
- 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)
What does disruptive selection result in?
1) Speciation in some cases
2) Leads to TRAIT DIVERGENCE
3) requires spatial heterogeneity or discrete resources
What were the results from the graphs, when the colour of the mice were plotted against the probability of survival (on the Y)?
There was a huge directional shift, the light ones survived better on a lighter background, while darker ones survived better on the darker background.
Which allele caused resistance against malaria?
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,
What did genome data on the allele for A- show ?
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.
How do you know if change in allele is a cause of Natural selection or genetic drift?
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.
Why does genetic drift not drive rapid evolution like natural selection?
Because it is a random fluctuation every generation.
While Natural selection leads to really strong selection.
Can positive selection act even when you have little as one % or less selective advantage?
Yes it can spread fast with given time.
Are many phenotypic traits or polygenic or discrete?
Many are phenotypic traits are polygenic, and polygenic traits are affected by the environment.
How does the graph of a continuous polygenic trait look like?
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
: /——————\
: / \
: /
: /
:/
:…………………………………………
What is an example if stabilizing selection?
Human birth weight, beak size in finches in drought conditions
How does the graph look for disruptive selection and what is an example?
The graph has peaks to the extremes, and African finches are the examples
What happened when HOPI HOEKSTERA changed a single gene(the gene for coat colour) was changed in mice?
They say a rapid change against the frequency of the white allele in the dark background.
