1. Population and Community Dynamics
Chapter 21 – PART 1
Population and Community Dynamics

2. Populations
Successful individuals of a species are able to reproduce and adapt to their environments. What role might their genetic makeup play in meeting these two demands?

3. Populations
The population size of marine green turtles shows little change over time, while the locust population in the Canadian prairies varies widely from year to year.
What might this suggest about the environments in which they live?
Which environment is most stable?

4. Populations Population:
A group of organisms of the same species that live in the same habitat or ecosystem at the same time.
Each individual has a different genotype.
Differences in genotypes and environmental influences account for differences among the phenotypes of individuals of the same species.
All of the genes that occur in a population are referred to as the gene pool.
Maintains continuity of traits from generation to generation.

5. Hardy- Weinberg Principle
1908 – G.H. Hardy & W. Weinberg, independently derived the basic principle of population genetics, the Hardy-Weinberg principle

6. Hardy-Weinberg Terminology
Allele frequency:
The proportion of allele copies in a population of a given gene.
Changes in populations can be measured in part by looking for changes in allele frequencies.
Note: not all genes exhibit variation
Fixed frequency:
The frequency of an allele within a population when only a single allele is present for a particular gene.
The allele’s frequency is always 100% (only option)

7. The conditions under which no change will occur are:
1) The populations must be closed (no immigration/emigration)
2) Random mating takes place (no mating preferences with respect to genotype)
3) No selection pressure. A single gene must not affect the survival of the offspring
4) No mutations
5) Population must be large

8. Hardy-Weinberg Terminology
Genotypic ratio:
The ratio of offspring with each possible allele combination from a particular cross
Ex. 250 AA : 500 Aa : 250 aa
Phenotypic ratio:
The ratio of offspring with a dominant trait to the alternative, recessive trait.
Ex. 750 Squid : 250 not Squid

9. Hardy-Weinberg Principle
The idea behind Hardy-Weinberg is that scientists studying inheritance of traits in populations wanted an equation to calculate the number of alleles in a population.
They did this by measuring the allele’s frequency.

10. Solving a Hardy-Weinberg Problem
What am I being asked to find?
Create Dominance Hierarchy.
Identify what info. has been given to you.
If allele frequency is given, you already have either p or q (easy to solve).
If no allele frequency is given, use info. you have been given to find q2, then take √q2 to get q and solve.
Be sure you are solving what the question is asking (know your terminology)

11. Remember…
An allele is one of several forms of the same gene. Ex. The gene for wing colour in moths has 2 alleles: Brown and white
Brown is dominant to white
Let’s say that A= brown and a= white
Write down the 3 possible genotypes:

12. Hardy-Weinberg Principle
In our example population there are 500 moths:
320 homozygous brown
160 heterozygous brown
20 white
What are the genotype frequencies?
Type of moth/total number of moths
What are the allele frequencies?
Remember: each individual contributes 2 alleles to the gene pool

13. Hardy-Weinberg Principle
320 AA = 640 A (from AA genotype)
160 Aa = 160 A a (from Aa genotype)
20 aa = 40 a (from aa genotype)
TOTAL: 1000  the number of alleles in the gene pool
Allele frequency:
A =
a =

14. p + q = 1 p2 + 2pq + q2 = 1 Hardy-Weinberg Equation
For a gene with only 2 alleles
p + q = 1
p= frequency of allele A (dominant)
q= frequency of allele a (recessive)
p2 + 2pq + q2 = 1
p2= frequency of homozygous dominant
2pq= frequency of heterozygous
q2= frequency of homozygous recessive

15. Example:
Suppose a certain allele A has a frequency of 0.6 in a population. The frequency of allele a must be 0.4 because A + a must equal 1. (1 – 0.6 = 0.4). Let’s see what happens during reproduction. We can arrange the alleles and their frequencies in a Punnett square.
A (0.6) a (0.4)
A (0.6) AA (0.36) Aa (0.24)
a (0.4) Aa (0.24) aa (0.16)
Genotype ratio: 36% AA; 48% Aa: 16% aa
Unlike the genetic Punnett square used to determine individual traits, the eggs & sperm of this Punnett square represent the genes for the entire population.
Or use Hardy Weinberg:
Frequency of AA = p2 = (0.6)2 = 0.36
Frequency of Aa= 2pq = 2(0.6x0.4)= 0.48
Frequency aa = q2 =( 0.4)2 = 0.16

17. H.W. Equation Contd..... p2 + 2pq + q2 = 1
p= frequency of A = 0.8 or 80%
q= frequency of a = 0.2 or 20%
p2 + 2pq + q2 = 1
AA genotype =
Aa genotype =

19. Practice Pg. 720 #1-3 Pg. 722 #5-8 Hardy-Weinberg Worksheets
Discuss Tomorrow

20. Chapter 21 – PART 2
Changes in Gene Pools

21. Review of Hardy-Weinberg…
Conditions for H.W. Equilibrium include: 1. 2. 3. 4. 5.
In a H.W. Equation:
p = q =
p2= q2= 2pq=

22. When is a Gene Pool predicted to change?
When a population is small, chance fluctuations in numbers will cause changes in allele frequencies
When individuals migrate
Mutations: new alleles will arise or existing ones will change
Natural Selection occurs
When mating is not random
In other words.....gene pools are always changing because the environment is in a constant state of change. Hardy-Weinberg only exists as a model, not in real life.

23. 5 Agents of Evolutionary Change
Mutation
Gene Flow
Non-random mating
Genetic Drift
Selection

24. Genetic Drift Is a change in the genetic makeup of a
population resulting from chance (random
events).
Ex. Small populations - can lead to fixation of alleles.
Increases the % of homozygous individuals within a population and reduces its genetic diversity
Watch to 3:50

25. Genetic Drift Amplified in low populations

26. Founder Effect Genetic drift that results when a small number of
individuals separate from their original population
and find a new population.
Allele frequencies likely to not be the same as those of original population

27. Bottleneck Effect A dramatic, often temporary, reduction in population
size, usually resulting in significant genetic drift.
Frequency of alleles in the survivors is very different from that in the original population.
Narrows gene pool
Ex. Elephant seals (Fig 4 pg. 724)

28. Bottleneck Effect

29. Gene Flow (Migration) The movement of alleles from one
population to another through the
movement of individuals or gametes.
Alters both populations
Occurs in wild populations
Ex #1 Seed & pollen distribution by wind & insects
Ex #2 Migration of animals
sub-populations may have different allele frequencies
causes genetic mixing across regions
reduce differences between populations

30. Mutations Can be beneficial or harmful Mutation creates variation
new mutations are
constantly appearing
Mutation changes the DNA sequence
changes in protein may change phenotype & therefore change fitness

31. Natural Selection Differential survival & reproduction due to changing
environmental conditions
climate change
food source availability
predators, parasites, diseases
toxins
Combinations of alleles that provide “fitness” increase in the population
adaptive evolutionary change
Ex. The Peppered Moth & Malaria/Sickle Cell Anemia

32. Natural Selection

33. Non-Random Mating Sexual Selection
Differential reproductive success that results from variation in the ability to obtain mates
When combined with evolutionary pressures can create Sexual Dimorphism within a species:
Striking differences in the physical appearance of males and females not usually applied to behavioral differences between sexes.

34. Antibiotic-Resistant Bacteria
Case Study on page

35. Speciation Speciation refers to the formation of a new species.
There is an enormous diversification between species that evolution alone cannot explain.
A group of similar organisms that can interbreed and produce fertile offspring in the natural environment.

36. How does speciation occur???
It is important to note that speciation and evolution are NOT necessarily the same.
Natural selection does not always cause speciation! (Ex. The evolution of the peppered moth did not lead to a new species).
How does speciation occur???

37. a. Instantaneous Speciation
Occurring in one generation because of major changes to the chromosomes
Usually a result of nondisjunction
Polyploids can mate with each other, but not with members of parent generations, because of different chromosome numbers.

38. b. Gradual Speciation
Most species arose slowly and gradually evolved differences through time.
i.e. Galapagos finches

39. c. Geographic Speciation
Speciation occurs if a population is divided into 2 or more smaller populations, that are physically separated from one another.
i.e. mountains or bodies of water from floods establish physical barriers. Over time the species cannot reproduce within the original group

40. d. Punctuated Equilibrium
Periodic rapid evolution (within generations ) followed by little change over a long time
Suggests that population remain stable and unchanging for very long periods of time

41. e. Phyletic Gradualism
Evolution occurs at a constant rate over time

42. Practice
For each type of change in a gene pool that we have discussed, write down how each affects the alleles in a population.
Genetic Drift
Gene Flow
Mutations
Natural Selection
Non-Random Mating
Pg. 730 #2-12