1. The Evolution of Populations: Population Genetics

2. There are two ingredients for evolution:
Genetics
Natural Selection

3. There are also two types of evolution:
Macroevolution:
Evolution that takes place over LONG periods of time
Is change on a grand scale
Includes mass extinctions.
Microevolution:
Change that occurs within a species
This is a change in gene frequencies.
This is the focus of Darwin’s theory of evolution via natural selection.

4. Microevolution and Natural Selection
end result of natural selection is development of features which increase an organism’s likelihood for survival and reproduction.
adaptations.
Darwin’s explanation
Adaptation + natural selection  change within species.

5. How can we study microevolution?
The study of population genetics helps us to understand changes that occur within a species.

6. What is a population?
All individuals of a species that live together in one place at one time.

7. What do we already know about populations?
We know that variety exists among organisms and among species:
Blood groups in humans
Enzymes among some vertebrate species have striking similarities.

8. But why does variety persist?

9. Will dominant alleles eventually replace recessive ones?
Things that made them go “Hmmmm…”

10. Hardy-Weinberg Principle
Godfrey Hardy, a mathematician.
Wilhelm Weinberg, MD

11. They pointed out that:
In a large population that has random mating members, AND in the absence of evolutionary forces…
The proportions of genotypes for a given trait will remain constant generation to generation.
Dominant alleles will NOT replace recessive ones.
Their proportions do NOT change.
This is known as the Hardy-Weinberg Equilibrium.

12. There is a mathematical equation that represents this principle:
It is known as a binomial expansion.

13. Let’s say we have a gene for fur color in fuzzy bunnies.
There are only two possible kinds of fuzzy bunnies:
B: brown
b: white

14. Using the Hardy-Weinberg Principle…

15. p2 + 2pq + q2 = 1
Where:
p2 represents individuals homozygous for alleles B, so BB
2pq represents individuals heterozygous for alleles B and b, so Bb
q2 represents individuals homozygous for alleles b, so bb.

16. A few things you need to know about H-W
P always represents the more common allele. It is NOT always dominant!
Q always represents the least common allele. It is NOT always recessive!

17. Other things you need to know
The equation is derived from:
(p + q)2 = 1
p + q must always equal 1!
Think: what are the chances an organism has of inheriting a gene if there are only two alleles for a specified trait?

18. Applying the Principle

19. Fuzzy Bunnies
In the FooFoo Forest, a population of cute fuzzy bunnies lives. Brown fur is the dominant trait in this population. In a population of 100 cute fuzzy bunnies, 36 of them have white fur. What is the frequency of the white fur gene? b

20. What do we already know? We know white fur is recessive.
We know that to have a recessive trait, an organism must have two recessive alleles.
Therefore, we can set the problem up as follows…

21. p2 + 2pq + q2 = 1 Which part of the equation will you use?
THINK: Which part deals with the recessive allele?
Now, set up your problem.

22. Solving for Q q2 = 36/100 bunnies q2 = .36 √ .36 = .6
Therefore, q = .6 b = .6

23. Could you solve for the frequencies of the heterozygotes in that same population?Bb

24. What do you already know?
You know the frequency of the recessive allele (represented by q).
You know that p + q must equal 1.
Therefore…you can figure out what p is.
Now figure out what part of the equation you need to use in order to find the frequency of the heterozygote.

25. Solving for P
q = .6
p + q = 1
p = .4 B = .4

26. Now, find the frequency of the heterozygote.
p2 + 2pq + q2 = 1
Which part of the equation will you use?

27. Solving for 2pq
q = .6
p = .4
2pq = 2(.6)(.4)
2pq = .48 Bb =
.48

28. Does it add up? BB bb Bb + +
.16
.48
.36
= 1

29. What things can change a H-W equilibrium?
There are a few things.

30. Mutations
Mutations introduce new alleles that can be selectively advantageous, but most are harmful.
Polydactyly, or extra digits.

31. Gene Flow
This is the introduction or removal of alleles from a population that occurs when individuals enter (immigrate) or leave (emigrate) that population. bb BB BB BB BB BB

32. Natural Selection
This is a change in gene frequencies due to changes in the environment.

33. Genetic Drift This is a random increase or decrease in alleles.
This has a greater effect if the population is very small.
There are two kinds:
Founder effect: when allele frequencies in a group of migrating individuals are not the same as that of their population of origin.
Bottleneck: the population experiences a huge decrease in size. This results in a severe reduction in diversity of the original gene pool. This is a problem with many endangered species.

34. Nonrandom Mating
When individuals choose mates based on their particular traits. Nonrandom mating can also occur when mates choose only nearby individuals. Two kinds:
Inbreeding: individuals mate with relatives
Sexual selection: females choose males based on appearance or behavior.

35. In other words, for a H-W equilibrium to hold true…
There can be NO mutations.
There can be NO gene flow.
There can be NO genetic drift. In other words, the population is BIG.
Mating must be TOTALLY random.
There must be NO natural selection of any kind.

36. Next time…
Fishy Frequencies…