1. Evolution of Populations
Chapter 16

2. Gene and Variation
Although Mendel and Darwin both worked in the 1800’s, they were not able to share information as scientists do today. By the 1930’s evolutionary biologists started to focus on genes as a way to understand evolutionary change.

3. 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

4. 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.

5. 16-1 Genes and Variation
How common is genetic variation?

6. How Common Is Genetic Variation?
Many genes have at least 2 forms or alleles
Animals such as horses, dogs, mice, and humans often have several alleles for traits such as body size or coat color

7. 2. allele: one form of a gene

8. 16-1 Genes and Variation Variation and gene pools
Gene pool – all genes present in a population
Relative Frequency – the number of times an allele occurs in the gene pool, compared to other alleles

9. In genetic terms, evolution is any change in the relative frequency of alleles in a population.

10. Genetic Variation
Genetic variation is studied in populations. A gene pool consists of all the genes, including the different alleles that are present in a population.

13. 16-1 Genes and Variation
Scientists now recognize two main sources of genetic variation
Mutations
Gene shuffling

14. 16-1 Genes and Variation
Mutations
Any change in a sequence of DNA

15. Mutations
Can occur because of:
mistakes in replication
environmental chemicals
May or may not affect an organism’s phenotype

16. 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.

17. 16-1 Genes and Variation Sexual Reproduction Think of a deck of cards
Remember…an organism is successful if it reproduces
Can produce many phenotypes, BUT the relative frequency will stay the same
Think of a deck of cards
52 different cards
Chance of an Ace off the top – 1/13 (4/52)
Many combinations, BUT the frequency stays the same

18. 16-1 Genes and Variation Single-gene and polygenic traits
Widows peak is a single gene trait – a single gene with two alleles
Polygenic Traits are controlled by two or more genes
This means that a polygenic trait can have many possible genotypes ant thus phenotypes.

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

20. Most human traits are polygenic.
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.

21. 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

22. Review Which of the following statements describes what
all members of a population share?
a.They are temporally isolated from one another.
b.They are geographically isolated from one another.
c.They are members of the same species.
d.They have identical genes.

23. Review The combined genetic information of all members
of a particular population is the population’s
a.relative frequency.
c.genotype.
b.phenotype.
d.gene pool.

24. Review
The two maun sources of genetic variation are ___________ and _________________________
Mutations
Genetic shuffling from sexual reproduction

25. Population Genetics Study of how genetics plays a role in evolution
**New definition of evolution:
it’s a gradual change in the genetic material within a population over time.

26. Among any population, there are always variations
Line up in the front of the class from the shortest person to the tallest person. Let’s graph the frequencies of the heights in this class.

27. 5 4 3 2 1
Frequency
4’ 5” 6” 7” 8” 9” 10” 11” 5” 1” 2” 3” 4’ 5” 6” 7”
Height

29. Gene frequencies within a population follow this curve.
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.

30. So what causes these variations in height among your classmates?
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

31. All the members of the same population contribute their genes into a gene pool.
If the people on the right side of the classroom during the height lineup were to all throw their genes into a gene pool, what would be the expected outcome of their offspring?
The tall trait would show up more often thus increasing the tall gene (allele) frequency.

32. Amount of times a certain gene (allele) appears in a population.
Allele frequency
Amount of times a certain gene (allele) appears in a population.
If two parents are hybrids for hand preference, it’s 0.5: 0.5
or 50:50
If one is a hybrid and the other is pure recessive, it’s 25:75

33. Review!!! Section 16-1 Genes and Variation
1. Is the Following sentence true or false? Mendel’s work on inheritance was publish after Darwin’s lifetime.
False
2. Which two important factors was Darwin unable to explain without an understanding of heredity?
The source of variation and how genes were passed from generation to generation
3. List the three fields that collaborate today to explain evolution
a. Genetics b. Molecular Biology c. Theory
Gregor Mendel

34. Gene Pools
4. A collection of individuals of the same species in a given area is a
population
5. The combined genetic information of all members of a particular population is a
gene pool
6. Is the following statement true or false? A gene pool typically contains just one allele for each inheritable trait
False
7. The number of times that an allele occurs in a gene pool compared to the number of times other alleles occur is called
the relative frequency

35. Source of Genetic variation
Sources of Genetic Variation
8. Complete the concept map
Source of Genetic variation
Include
Mutations
Gene Shuffling
9. What is a mutation?
Any Change in the sequence of DNA
10. Why do mutations occur?
As a result of mistakes in the replication of the DNA or as a result of radiation or chemicals in the environment

36. 11. Circle the letter that is true about mutations
They can be limited to a single base of DNA
They always affect lengthy segments of a chromosome
They always affect an organism’s phenotype
They always affect an organism’s fitness
12. Is the following statement true of false? Most inheritable differences are due to gene shuffling that occurs during the production of gametes
13. Circle the letter of each choice that is true of sexual reproduction
It is a major source of variation in many populations
It can produce many different phenotypes
It can produce many different gene combinations
It can change the relative frequencies of a population

37. Frequency of Phenotype
Single- Gene and Polygenetic traits
14. Is the following sentence true or false. ?The number of phenotypes produced for a given trait depends upon how many genes control the trait
15. Is the following sentence true or false? Most traits are controlled by a single gene
16. Label the two graphs to show which one represents a single gene trait and which one represents a polygenetic trait
Single gene trait
Polygenetic Trait
Frequency of Phenotype
Frequency of Phenotype
(%)
Phenotype (height)
Widow’s peak
No widow’s peak
Phenotype

38. 16-2 Evolution as Genetic Change
How do we view an individual’s fitness?
How do we view an evolutionary adaptation?
By an organisms success in passing genes on
to the next generation
As any genetically controlled physiological,
anatomical, or behavioral trait that increase
an individual’s ability to pass on its genes

39. Evolution as Genetic Change
Natural Selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color may be more visible to predators and therefore less likely to survive and reproduce than organisms of another color.

40. 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.

41. ? If brown is dominant can the 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

42. ? 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

43. Will the fitness of BB and Bb differ? Why?
No, Both BB and Bb have the same fitness advantage of being brown

44. 16-2 Evolution as Genetic Change
A. Natural Selection on Single-Gene Traits can lead to changes in allele frequencies and evolution
How does the color affect the fitness of the lizard?
What do you predict the lizard population will look like by the 50th generation? Explain.

45. Pg 397. With your desk mate; answer:
How does the color affect the fitness of the lizard?
What do you predict the lizard population will look like by the 50th generation? Explain.

46. Natural Selection of Polygenic Traits
The effects of natural selection
are more complex when traits are controlled
by more than one gene.

47. Three ways in which natural selection affects polygenic traits.

48. Individuals with highest fitness: those at one end of the curve
a. 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

49. Directional Selection (page 398)
Food becomes scarce.
Key
Low mortality, high fitness
High mortality, low fitness
Directional Selection

50. Example: human birth weight
b. 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

51. 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

52. Individuals with highest fitness: both ends of curve
c. 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

53. Disruptive Selection (pg 399)
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

54. Which type is it?

56. 16-2 Evolution as Genetic Change
Genetic Drift – Random change in allele frequency
Founder effect – when allele frequencies change because of migration

57. Genetic Drift
In small populations, individuals that carry a particular allele may leave more descendants than other individuals BY CHANCE.
Over time, these alleles become more common

58. Video

59. 16-2 Evolution as Genetic Change
Evolution vs. Genetic Equilibrium
Hardy-Weinberg principle – allele frequencies will stay constant unless some factor changes the frequency
Genetic Equilibrium – when allele frequencies remain constant

60. 16-2 Evolution as Genetic Change
Five Conditions to Maintain Equilibrium
Random Mating
Large Population
No Movement into or out of the population
No Mutations
No Natural Selection

61. 16-3 The Process of Speciation
Recall, a species is a group of organisms that breed with one another and produce fertile offspring

62. Isolating Mechanisms
If the gene pools of two populations are isolated; they can not breed
This can lead to the formation of new species.
The populations were REPRODUCTIVELY ISOLATED

63. Reproductive isolation
At this point, the populations have separate gene pools

64. 16-3 The Process of Speciation
Behavioral Isolation
Two populations will not breed because of differences in courtship

65. 16-3 The Process of Speciation
Geographical Isolation
Rivers, mountains, or bodies of water separate two populations

66. 16-3 The Process of Speciation
Temporal Isolation
Different species mate at different times
Times of day
Times of year

67. 16-3 The Process of Speciation
Testing Natural Selection in Nature
Variation
Differences in population
Natural Selection
Which beak is best in a drought?
Rapid Evolution
Natural selection takes place frequently and sometimes rapidly

68. 16-3 The Process of Speciation
Speciation in Darwin’s Finches
Founders Arrive
How did the finches get to the Galapagos Islands?
Separation of Populations
How did they become separated?

69. 16-3 The Process of Speciation
Changes in the Gene Pool
How did they become individual species?

70. 16-3 The Process of Speciation
Reproductive Isolation
If a small beak bird moves back to the first island will it mate with a big beaked bird?

71. 16-3 The Process of Speciation
Ecological Competition
What happens to the birds that are different that don’t have to compete for food?
Continued Evolution
How did we get all of the 13 species?

72. 16-3 The Process of Speciation
Studying Evolution Since Darwin
Limitations of Research
Unanswered Questions

73. Review The Galapagos finches are an excellent example of A) speciation
B)genetic equilibrium
C) stabilizing selection
D) selection on single-gene traits

74. Review
The genetic equilibrium of a population can be disturbed by all of the following EXCEPT
A) Nonrandom mating
B) movement into and out of the population
C) a large population size
D) mutations

75. Review Natural selection acts directly on A) alleles B) genes
C) phenotypes
D) mutations