1. The Evolution of Populations
Chapter 23
BCOR 012
February 05 and 07, 2007

2. Outline: The Evolution of Populations (Chapter 23)
February 5
Introduction
Evolution is a population-level phenomenon
Linking Darwinian evolution and mendelian inheritance
The Modern Synthesis
Population Genetics
A population’s gene pool is defined by its allele frequencies
Hardy-Weinberg theorem
Manipulating the H-W equation
Example from health science – Cystic fibrosis
Assumptions of H-W
Microevolution
Causes
Drift
Bottleneck
Founder Effect
Natural Selection - the English Peppered Moth

3. Darwin’s arguments that life has evolved were accepted
more readily than his contention that natural selection was
the mechanism. This was partly because it was not known
how characteristics were passed from generation to generation.

4. The smallest unit that can evolve is the population ...
So what is a population?
A population is a set of individuals of the same species
that live close enough together to interbreed.

5. The study of genetic change in populations.
Population Genetics
The study of genetic change in
populations.

6. Gene pool: the total collection of alleles present in a popu-lation is that population’s gene pool.
A population’s gene pool is defined by its allele frequencies.

7. Example:
Flower color in Hypothetica notrealia is determined by alternative alleles at the color locus.
R is dominant to r, and results in red color. The rr genotype yields white color.
In one population, the frequency of R has been determined to be 0.8, whereas the frequency of r is (Note that allele frequencies sum to 1.0)

8. Hardy-Weinberg Theorem
The Hardy-Weinberg theorem states that, in a non-evolving population, allele and genotype frequencies remain constant through time.

9. The Hardy-Weinberg equation
A mathematical expression that connects allele frequencies and genotype frequencies.
Let p = frequency of dominant allele.
Let q = frequency of recessive allele.
p + q = 1
p2 + 2pq + q2 = 1

10. So, if our population of Hypothetica notrealia is in Hardy-
Weinberg equilibrium, what are the expected genotype
frequencies?
RR = .64
Rr = .32
rr = .04
In a population of 500 plants, how many will have the white
flowered phenotype?
A.: 20 plants (0.04 X 500)

13. A Real Example:
Frequency of cystic fibrosis phenotype in the caucasian population = 1/ What proportion of the population are carriers? (The CF allele is recessive.)
q = √.0004 = 0.02
p = 0.98
q2 = 1/2500 =
2pq =
or about 1 in 25 people

14. Assumptions of Hardy-Weinberg:
No mutations
Large population size
No migration
No natural selection (i.e., all members survive and reproduce)
Random mating
These conditions are almost never met in nature. Thus HW is
an ideal case.

15. Q. So how do populations evolve?
A. Through genetic drift and natural selection

16. Genetic drift can alter population allele frequencies.

17. Two situations can shrink a population to a size small enough for genetic drift to operate:
The bottleneck effect
The founder effect

18. The bottleneck effect

19. The African cheetah population experienced one (perhaps two) bottlenecks.

20. Founder effect is the establishment of a new population by a few original founders which carry only a small fraction of the total genetic variation of the source population.

22. The Juan Fernandez Islands, located some 700 km west of Santiago in Chile. There and nowhere else in the world exists Thyrsopteris elegans, a genus of tree ferns endemic to the islands.

23. A. Genetic change in small populations as a
Q. What permits novel life forms like Thyrsopteris elegans and Darwin’s finches to evolve on islands?
A. Genetic change in small populations as a
result of founder effect, and adaptation to the
new environmental conditions.