1. Chapter 17: Evolution of Populations

2. Evolution of Populations
When Darwin developed his theory of evolution, he did not understand:
how heredity worked.
This left him unable to explain two things:
a. source of variation
b. how inheritable traits pass from one generation to the next

3. Evolution of Populations
In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular population. This evolution of populations is called microevolution.

4. Vocabulary:
Population: group of individuals of the same species living in the same area that breed with each other.

5. gene pool
combined genetic
info. for all members
of a population

6. Allele: one form of a gene

7. 2. relative frequency of an allele: # times an allele occurs in the gene pool compared to other alleles (percent)
Example
Relative Frequency:
70% Allele B
30% Allele b

8. 3. Sources of Variation:
a. mutations: any change in DNA sequence
Can occur because of:
mistakes in replication
environmental chemicals
May or may not affect an organism’s phenotype

9. 3. Sources of Variation
b. Gene Shuffling: recombination of genes that occurs during production of gametes
Cause most inheritable differences between relatives
Occurs during meiosis
As a result, sexual reproduction is a major source of variation in organisms.
Despite gene shuffling, the frequency of alleles does not change in a population. Explain why this is true.
Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) are always there.

10. A) Single gene trait: controlled by single gene with two alleles
4. Gene Traits:
A) Single gene trait: controlled by single gene with two alleles
Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

11. Most human traits are polygenic.
(4. Gene Traits:)
B) Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles
Examples: height, hair color, skin color, eye color
Most human traits are polygenic.

12. why? Only two phenotypes possible type: polygenic
Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain.
type: single gene
why? Only two phenotypes possible
Example: tongue roller or non-tongue roller
type: polygenic
why? Multiple (many) phenotypes possible
Example: height range 4feet to 9 feet all

13. 5. Natural selection acts on phenotypes, not genotypes.
Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white?
Brown, more hidden.

14. 5. If brown is dominant can a predator tell the difference between:
Mouse with highest fitness will have the most alleles passed on to the next generation.
White mouse will have low fitness
BB Bb ?

15. ? BB Bb 5. Which mouse will have the lowest fitness?
White, bb (recessive)
Will the fitness of BB and Bb differ? Why?
No, Both BB and Bb have the same fitness advantage of being brown
BB Bb ?

16. Three ways in which natural selection affects polygenic traits.
Directional Selection
Stabilizing Selection
Disruptive Selection

17. Individuals with highest fitness: those at one end of the curve
Directional Selection: individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait
Individuals with highest fitness: those at one end of the curve
Example: Galapagos finches – beak size
Food becomes scarce.
Key
Low mortality, high fitness
High mortality, low fitness

18. Directional Selection
Food becomes scarce.
Key
Low mortality, high fitness
High mortality, low fitness
Directional Selection

19. Example: human birth weight
Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve
Key
Percentage of Population
Birth Weight
Selection against both extremes keep curve narrow and in same place.
Low mortality, high fitness
High mortality, low fitness
Stabilizing Selection
Individuals with highest fitness: near the center of the curve (average phenotype)
Example: human birth weight

20. Individuals with highest fitness: both ends of curve
Disruptive Selection: individuals at both ends of the curve survive better than the middle of the curve.
Individuals with highest fitness: both ends of curve
Example: birds where seeds are either large or small
Disruptive Selection
Largest and smallest seeds become more common.
Number of Birds in Population
Beak Size
Population splits into two subgroups specializing in different seeds.
Key
Low mortality, high fitness
High mortality, low fitness

21. Stabilizing Selection
Key
Percentage of Population
Birth Weight
Selection against both extremes keep curve narrow and in same place.
Low mortality, high fitness
High mortality, low fitness
Stabilizing Selection

22. Disruptive Selection Disruptive Selection Key
Largest and smallest seeds become more common.
Number of Birds in Population
Beak Size
Population splits into two subgroups specializing in different seeds.
Key
Low mortality, high fitness
High mortality, low fitness

24. However: No examples ever observed in animals
A couple examples that may demonstrate speciation exist in plants and some insects.

25. Genetic Drift
random change in allele frequency that occurs in small populations

26. Two phenomena that result in small populations and cause genetic drift
Founder Effect
Bottleneck Effect

27. Genetic Drift
The results of genetic crosses can usually be predicted using the laws of probability.
In small populations, however, these predictions are not always accurate.

28. Founder effect
Allele frequencies change due to migration of a small subgroup of a population

29. Founder Effect
Two groups from a large, diverse population could produce new populations that differ from the original group.

30. 2. Bottleneck effect
Major change in allele frequencies when population decreases dramatically due to catastrophe
Example: northern elephant seals
decreased to 20 individuals in 1800’s, now 30,000
no genetic variation in 24 genes

31. Bottleneck Effect: Northern Elephant Seal Population
Hunted to near extintion
Population decreased to 20 individuals in 1800’s, those 20 repopulated so today’s population is ~30,000
No genetic variation in
24 genes

33. Bottleneck Effect
Original population

34. Bottleneck Effect
Catastrophe
Original population

35. Bottleneck Effect
Catastrophe
Surviving population
Original population

36. Evolution Versus Genetic Equilibrium
What conditions are required to maintain genetic equilibrium?
According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium:
The population must be very large
there can be no mutations
there must be random mating
there can be no movement into or out of the population
(5) no natural selection can occur

37. Genetic equilibrium = no evolution
A population is in genetic equilibrium if allele frequencies in the population
remain the same. If allele frequencies don’t change, the population will not evolve.
The Hardy-Weinberg principle describes the
conditions under which evolution does not occur.
The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.

38. Hardy-Weinberg principle
1. Large Population
Genetic drift can cause changes in allele frequencies in small populations.
Genetic drift has less effect on large populations.  
Large population size helps maintain genetic equilibrium
2. No Mutations
If mutations occur, new alleles may be introduced into the gene pool, and allele frequencies will change.

39. Hardy-Weinberg principle
3. Random Mating
All members of the population must have an equal opportunity to produce offspring.
Individuals must mate with other members of the population at random.
In natural populations, however, mating is not random.
Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers.
Such non-random mating means that alleles for those traits are under selection pressure.

40. Hardy-Weinberg principle
4. No Movement Into or Out of the Population
Individuals who join a population may introduce new alleles into the gene pool. (Immigration)
Individuals who leave may remove alleles from the gene pool. (emigration)
Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population.

41. Hardy-Weinberg principle
5. No Natural Selection
All genotypes in the population must have equal probabilities of surviving and reproducing. No phenotype can have a selective advantage over another.

42. Sexual Reproduction and Allele Frequency
Meiosis and fertilization do not change the relative frequency of alleles in a population.
The shuffling of genes during sexual reproduction produces many different gene combinations but does not alter the relative frequencies of alleles in a population.

43. The Process of Speciation
The formation of new biological species, usually by the division of a single species into two or more genetically distinct one.

44. Three Isolating Mechanisms: Isolate species forming subspecies and perhaps causing speciation.
Geographic Isolation
Behavioral Isolation
Temporal Isolation

45. Example: Eastern and Western Meadowlark
Male birds sing a matting song that females like, East and West have different songs. Females only respond to their subspecies song.

46. 1. Geographic Isolation
Two populations separated by a geographic barrier; river, lake, canyon, mountain range.

47. Example: 10,000 years ago the Colorado River separated two squirrel populations.
Kaibab Squirrel Abert Squirrel

48. This resulted in a subspecies, but did not result in speciation because the two can still mate if brought together.
Kaibab Squirrel Abert Squirrel

49. Example: Eastern and Western Meadowlark
Eastern and Western Meadowlark populations overlap in the middle of the US

50. 2. Behavioral Isolation
Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.

51. 3. Temporal Isolation Populations reproduce at different times January

52. Example: Northern Leopard Frog & North American Bullfrog
Mates in: Mates in:
April July

53. Conclusion: speciation. Geographic, Behavioral and
Temporal Isolation are all
believed to lead to
speciation.