1. Chapter 16: “Population Genetics and Speciation”
Aka: Natural Selection meets Genetics - What Darwin Never Knew

2. 16.1: Genetic Equilibrium Amongst any population, there are always
variations

3. Bell Curve
Graph showing frequencies of occurrences with the majority in the middle and curving out towards the extremes.
Gene frequencies within a population follow this curve.

4. Population Genetics Study of how genetics plays a role in evolution
**New definition of evolution:
A gradual change in the genetic material within a population over time.
**Old definition of evolution
Development of new types of organisms from pre-existing types of organisms over time

5. So what causes these variations in populations?
Variations – caused by:
Environmental factors (food availability, amount of sunlight, etc.)
Mutations – gene and chromosomal
Recombinations (crossing over, independent assortment)
Random fusion of gametes - each brings their own set of genes

6. Gene Pool
A gene pool is the sum of all the individual genes (alleles) in a given population.
A population is the smallest unit of living organisms that can undergo evolution.
Peccaries are small, tough relatives of the modern pig, whose lineage diverged about 40 million years ago. They live in southern Texas, Arizona, and New Mexico.

7. Determining Allele Frequency
If we could take inventory of the gene pool and find out all of the alleles present, then we could apply a simple mathematical formula to make predictions about how the gene pool might change over time – you will see WHY we do this in the next section!!!

8. Suppose… A certain population has two alleles, B and b in 10 gametes.
Let’s say 3 of the gametes carry the B allele, and 7 carry the b allele – what is the allele frequency of B and b?
Answer: 0.3 or 30% and 0.7 or 70%.
So, how do we determine allele frequency?

9. Applications of Allele Frequency
If two parents are each pure breeding for a certain characteristic:
RR x rr
Then their offspring will have these genotypes:
4Rr
Out of 8 total alleles:
R = 4/8 = 0.5 = 50%
r = 4/8 = 0.5 = 50%
Notice that = 1; must be that way because you are dealing with frequencies

10. Applications of Allele Frequency, cont.
If two parents are hybrids for a certain characteristic:
Rr x Rr
Then their offspring will have these genotypes:
RR 2Rr rr
Out of 8 total alleles:
R = 4/8 = 0.5 = 50%
r = 4/8 = 0.5 = 50%
Notice that = 1; must be that way because you are dealing with frequencies

11. So what, big deal, who cares?
Did the phenotype frequencies change from the P to the F1 generation?
Did the genotype frequencies change from the P to the F1 generation?
So, did evolution actually occur, and will it occur with this same population given the same circumstances?
From here, we proudly introduce…

12. Hardy and Weinberg
* A mathematician and a physician who in 1908 developed a mathematical model to predict gene frequencies in future populations in an attempt to explain microevolution (change in genetic material of a population). Macroevolution (chapter 15) is change on a phenotypic level.
This is ONE equation they used: p + q = 1
where p = dominant allele
where q = recessive allele
p equals all of the alleles in individuals who are homozygous dominant (AA) and half of the alleles in people who are heterozygous (Aa) for this trait in a population.
So, if 30% of a population had a certain trait, you could calculate what % must have the other (70%). You would not know, however, what % was heterozygous right-handed with this equation.

13. Applications, cont. We will use the 4 o’clock flower as an example
Remember – this flower has an incomplete dominance inheritance pattern

14. RR RR RR RR Rr Rr Rr Rr Phenotype frequency = 4/8 Red 4/8 Pink
In a population, there are 4 red four o’clock flowers and 4 pink ones (incomplete dominance)
RR RR RR RR Rr Rr Rr Rr
Phenotype frequency =
4/8 Red 4/8 Pink
or 50% 50%
Allele frequency =
R 12/ r 4/16
HW call R = p and r = q and state: p + q = 1

15. Second generation: 5 Red, 2 pink and 1 white
RR RR RR RR RR Rr Rr rr
Phenotype frequency =
5/8 Red 2/8 Pink 1/8 White
Allele frequency =
R 12/16 r 4/16 =1
**So the allele frequencies remain the same from generation to generation in a closed population because you are dipping into the same gene pool!

16. p2 = homozygous dominant allele frequency If q = .25
So what is the likelihood of two “R” genes (RR) being pulled from the gene pool?
If p = .75
then it’s p2 = .75 x .75 = – WHY?
p2 = homozygous dominant allele frequency
If q = .25
- then it’s q2 = .25 x .25 = – WHY?
q2 = homozygous recessive allele frequency
WAIT – why don’t these two values add up to 1? WHAT is missing and how do we find it?

17. How about them hybrids?
Frequencies of all types must add up to 1.0
p + q = 1
So, 1.0 – frequency of RR – frequency of rr = frequency of Rr
= (frequency of Rr)

18. Hardy Weinberg Genetic Equilibrium equation
p is defined as the frequency of the dominant allele and q as the frequency of the recessive allele for a trait controlled by a pair of alleles (A and a). In other words, p equals all of the alleles in individuals who are homozygous dominant (AA) and half of the alleles in people who are heterozygous (Aa) for this trait in a population.
p + q = 1
(p + q) x (p + q) = 1
p2 + 2pq + q2 = 1
Hybrid frequency
Pure Dominant frequency
Pure Recessive frequency

19. Let’s Practice! p2 + 2pq + q2 = 1 and p + q = 1
p = frequency of the dominant allele in the population

q = frequency of the recessive allele in the population
p2 = percentage of homozygous dominant individuals

q2 = percentage of homozygous recessive individuals

2pq = percentage of heterozygous individuals

20. Practice, cont. SHOW ALL WORK ON A SEPARATE PIECE OF LOOSE LEAF PAPER
You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: 

  - The frequency of the "aa" genotype.
The frequency of the "a" allele.
The frequency of the "A" allele.
The frequencies of the genotypes "AA" and "Aa."
The frequencies of the two possible phenotypes if "A" is completely dominant over "a."

21. Practice, cont.
Brown hair (B) is dominant to blond hair (b). If there are 168 brown haired individuals in a population of 200, calculate all phenotypic and allele frequencies.
2. If 81% of a population is homozygous recessive for a given trait
What is the predicted frequency of homozygous dominant?
What is the predicted frequency of heterozygotes?

22. Hardy Weinberg Genetic Equilibrium
Hardy and Weinberg showed that allele frequencies in a population remain the same over time unless acted upon by outside influences

23. Assumptions
The allele frequencies of generations within a population will remain the same if:
large population - to insure no sampling error from one generation to the next
random mating - no assortative mating or mating by phenotype
no mutations – allele frequencies don’t show a net change due to mutations
no migration between populations
no natural selection – all genotypes reproduce with equal success

24. Disruption of Genetic Equilibrium (results in evolution!)
1. Mutations – occur spontaneously
Introduce new alleles into population
Harmful mutations are slowly selected out while beneficial mutations drive evolution

25. 2.Migration
Constant movement of animals within a population, both in and out
Immigration – movement into a population
Emigration – movement out of a population
These conditions are referred to as
gene flow – movement of genes from one population to another

26. 3.Genetic Drift Random events or chance Small populations Disease
Natural disasters

27. Genetic Drift occurs when a small population is isolated.
Dipping in to the same gene pool.
Gene frequency changes but not due to natural selection or adaptive advantage. Due to purely chance.
Danger occurs when only 1 allele is left. No variation w/in a population
Simulation

28. 4.Nonrandom Mating
What if mating only occurs with people within one’s own village?
Dipping into the same gene pool could expose disorders due to the repetitive appearance of recessive genes
Assortative mating – organisms tend to mate with others that are similar to them
Sexual selection - attractive phenotypes such as brightly colored feathers (peacock); human pheromones

29. 5.Natural Selection
Most significant factor to disrupt genetic equilibrium
Peppered moths in Industrial England – White trees became soot covered
White on white – > in numbers White on dark - < in numbers Sooty on light - < in #’s

30. Patterns of Natural Selection

31. Stabilizing
Directional
Disruptive

32. 16.3 Formation of a Species – Speciation
a population of organisms that can successfully interbreed and can produce a viable, fertile offspring with others out of their species
Morphologically (internal & external structures) they are all similar; can be confusing because of phenotypic variations

33. What drives speciation?
Isolation – separates breeding populations
1) Geographic isolation – physical separation from original habitat = allopatric speciation
Development of a deep cannon, river changes, land floats, volcanic activity, etc
Isolation stops gene flow between two subpopulations
Populations diverge and can no longer interbreed

34. What drives speciation?
2) Reproductive Isolation- caused by barriers to successful breeding between population groups in the same area such as disruptive selection – two types:
Prezygotic isolation – occurs before
fertilization
* Incompatible behaviors - different mating
seasons and mating calls
* Sympatric speciation – within the same area

35. Prezygotic isolation
Leopard Frog
Wood Frog
Wood frogs mate in late March, Leopard frogs mate in mid April; since both are of the genus Rana, they can interbreed but do not

36. What drives speciation?
Post zygotic isolation
occurs after fertilization
due to a failure of zygote to develop or infertility
wastes gametes

37. Rates of Speciation
Gradualism – evolution is a constant process and occurs at a steady rate
Punctuated equilibrium- Periods of stability separated by accelerated changes
Species arise abruptly then have long periods of little change

38. Chapter Closure
How might the following scientists have responded if they knew about the modern day approaches to evolution as described in this chapter? Why?
Cuvier
Lyell
Lamarck
Mendel
Darwin