1. 17_Mechanisms of Evolution
By Dr. J. Suris Soto

2. Benchmarks Covered
SC.912.L Describe the conditions required for natural selection including overproduction of species, inherited variations, and the struggle to survive which results in differential reproductive success .
SC.912.L Discuss mechanisms of evolutionary change other than natural selection such as genetic drift and gene flow.
SC.912.L Describe how mutations and genetic recombination increase genetic variations

3. How populations evolve?
Essential Question
How populations evolve?

4. Lesson Objectives By the end of this lesson you should be able to:
Define evolution in genetic terms.
Identify sources of genetic variation.
Explain the four mechanisms of evolution.

5. Chapter Mystery: Epidemic
Read the chapter 17 mystery on page 481
As you read think about the following:
What do you think are some possible causes of the appearance of such a deadly virus?

6. Populations evolve, but individual organisms do not!
What is a population?
A population is an interbreeding group of individuals of one species in a given geographic area at the same time.
A population evolves because the population contains the collection of genes called the gene pool.
As changes in the gene pool occur, a population evolves.
Populations evolve, but individual organisms do not!

7. How populations evolve?
The 4 mechanisms of evolution are:
Mutations
Natural Selection
Genetic Drift
Bottleneck Effect
Founders Effect
Gene Flow

8. Sources of Genetic Diversity
1. Mutations can alter the allelic frequency in a population, but are rare.
2. Most of the genetic diversity in a population comes from sexual reproduction which has the ability to shuffles the alleles in the following ways:
Crossing Over during Prophase I of Meiosis
Independent Assortment
Random Fertilization

9. Sources of Genetic Diversity: Mutations
Are a driving force of evolution. Random change in an organism’s genetic makeup, which influences the population’s gene pool.
It is a change in the nature of the DNA in one or more chromosomes. Mutations give rise to new alleles; therefore, they are a source of genetic variation in a population.
Mutations may be harmful or benign, but they may also be beneficial.

10. Mutations
Only mutations in cell lines that produce gametes (sex cells) can be passed to offspring.
Point Mutations:
Changes in one base in a gene. They can have significant impact on phenotype, as in sickle-cell disease.
Chromosomal Mutations: delete, disrupt, duplicate, or rearrange many loci (places in a chromosome) at once. They are usually harmful.

11. Sources of Genetic Diversity: Sexual Reproduction
a) Crossing Over during Prophase I of Meiosis

12. Sources of Genetic Diversity: Sexual Reproduction
b) Independent Assortment

13. Sources of Genetic Diversity: Sexual Reproduction
c) Random fertilization
A zygote forms by the random joining of two gametes
One egg cell: 1 of 8.4 million possibilities
One sperm cell: 1 of 8.4 million possibilities
223 X 223 = 70 TRILLION DIFFERENT COMBINATIONS
This is not even including variation from crossing over!!

14. Gizmos Activity Evolution: Mutation and Selection
Go to

15. How populations evolve?
The 4 mechanisms of evolution are:
Mutations
Natural Selection
Genetic Drift
Bottleneck Effect
Founders Effect
Gene Flow

16. Natural Selection
1. There is variation in traits. Some beetles are green and some are brown.
2. There is differential reproduction. Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
3. There is heredity. The surviving brown beetles have brown baby beetles because this trait has a genetic basis
4. End result: The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.
Natural selection increases the frequencies of alleles that enhance survival and reproduction.

17. Structural/ functional
Adaptations
Variations that provide a competitive advantage to an organism to survive, thus increasing the reproductive success .
The physical or functional features of an organisms that helps them survive.
Structural/ functional
The things organisms do to survive
Behavioral
Cactus have long roots to get more water in the desert
Birds migrate to find food.

18. Types of Selection Three modes of selection:
Directional selection favors individuals at one end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes

19. (a) Directional selection
Figure 23.13a
Original
population
Evolved
population
Figure Modes of selection.
(a) Directional selection

20. (b) Disruptive selection
Figure 23.13b
Original
population
Evolved
population
Figure Modes of selection.
(b) Disruptive selection

21. (c) Stabilizing selection
Figure 23.13c
Original
population
Evolved
population
Figure Modes of selection.
(c) Stabilizing selection

22. Frequency of individuals
Original population
Frequency of
individuals
Phenotypes (fur color)
Original
population
Evolved
population
Figure Modes of selection.
(a) Directional selection
(b) Disruptive selection
(c) Stabilizing selection

23. Sexual Selection
Sexual selection is natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

24. Sexual Selection
Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

25. How do female preferences evolve?
The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should increase in frequency

26. Which of the following is a correct statement about the relationship between natural selection and evolution?
a. Natural selection results from evolution. b. Natural selection includes evolution as a part of it. c. Natural selection is one mechanism of evolution. d. Natural selection and evolution are the same thing.

27. Which of the following best illustrates natural selection?
A. An organism with favorable genetic variations will tend to survive and breed successfully.
B. A population monopolizes all of the resources in its habitat, forcing other species to migrate.
C. A community whose members work together utilizing all existing resources and migratory routes.
D. The largest organisms in a species receive the only breeding opportunities.

28. How populations evolve?
The 4 mechanisms of evolution are:
Mutations
Natural Selection
Genetic Drift
Bottleneck Effect
Founders Effect
Gene Flow

29. Genetic Drift
Genetic drift—along with natural selection, mutation, and gene flow (migration)—is one of the basic mechanisms of evolution.
In each generation, some individuals may, just by chance, leave behind a few more descendants (and genes, of course!) than other individuals.
The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals.
That, in a nutshell, is genetic drift. It happens to ALL populations—there’s no avoiding the whims of chance

30. Genetic Drift
Genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process.
So although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.

31. Effects of Genetic Drift
Through sampling error, genetic drift can cause populations to lose genetic variation
Imagine that our random draws from the marble bag produced the following pattern: 5:5, 6:4, 7:3, 4:6, 8:2, 10:0, 10:0, 10:0, 10:0, 10:0...
Why did we keep drawing 10:0?
Because if the green marbles fail to be represented in just one draw, we can’t get them back—we are “stuck” with only brown marbles.

32. Effects of Genetic Drift
The impact on small populations: The marble-drawing scenario also illustrates why drift affects small populations more.
Imagine that your bag is only big enough for 20 marbles (a tiny bag!) and that you can only draw four marbles to represent gene frequencies in the next generation.
Something like this might happen:

33. Effects of Genetic Drift
The impact on small populations
Notice how quickly and drastically the marble ratio changed: 1:1, 1:3, 0:1.
The same process operates in small populations.
All populations experience drift, but the smaller the population is, the sooner drift will have a drastic effect.
This may be a big problem for endangered species that have low population sizes.

34. Bottleneck Effect
Population bottlenecks occur when a population’s size is reduced for at least one generation.
Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster.
By chance, blue and gold marbles are over-represented in the new population and red and green marbles are absent.

35. Bottleneck Effect In the bottleneck effect
A sudden change in the environment may drastically reduce the size of a population
The gene pool may no longer be reflective of the original population’s gene pool
Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources, because the genetic variation that selection would act on may have already drifted out of the population.

36. Example of bottleneck effect
Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s.
Hunting reduced their population size to as few as 20 individuals at the end of the 19th century.
Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
Elephant seal image courtesy of David Smith, UCMP

37. Founder effect
A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have:
reduced genetic variation from the original population.
a non-random sample of the genes in the original population.

38. Example of Founder effect
The Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists.
Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency.
This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.

39. A small portion of the population that is geographically isolated from the rest of the population experiences genetic drift. This population runs the risk of decreased
Genetic drift causes reduced genetic variation.
Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources, because the genetic variation that selection would act on may have already drifted out of the population.
A. Directional selection
B. mutation rate
C. natural selection
D. genetic variation

40. How populations evolve?
The 4 mechanisms of evolution are:
Mutations
Natural Selection
Genetic Drift
Bottleneck Effect
Founders Effect
Gene Flow

41. Gene Flow Gene flow, also called migration
is any movement of individuals, and/or the genetic material they carry, from one population to another.
Causes a population to gain alleles (immigration)
or lose alleles (emigration)
Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If gene versions are carried to a population where those gene versions previously did not exist, gene flow can be a very important source of genetic variation.

42. Misconceptions about evolution
Understanding misconceptions about evolution:

43. What Is Theoretical about the Darwinian View of Life?
In science, a theory
Accounts for many observations and data and attempts to explain and integrate a great variety of phenomena

44. How populations evolve?
Essential Question
How populations evolve?

45. There are four mechanisms that shape the process of evolution
There are four mechanisms that shape the process of evolution. They are:
Mutations
Natural Selection
Genetic Drift
Bottleneck Effect
Founders Effect
Gene Flow

47. Let’s Practice

48. Which of the following statements correctly compares a scientific theory and a scientific law?
A. A law is a fact and a theory is an opinion.
B. A law is a theory that has been proven to be true.
C. A law is a description and a theory is an explanation.
D. A law is always true and a theory is sometimes true

49. In his trips to the Galapagos Islands, Charles Darwin observed that 4 of the 13 species of the islands' finches have beaks adapted to eating specific foods. Which best explains how these facts provide evidence for divergent evolution?
A. The finches were different species but resemble each other because of how they evolved in a similar environment. B. The finches descended from similar ancestors and have evolved adaptations in response to each other's influences. C. The finches descended from the same ancestor but evolved along their own lines in isolation from each other. D. The finches descended from a common ancestor but evolved differently in response to their environment.

50. B. limited food resources C. limited beak variations
The climate of an island became drier over a long period of time. This caused changes to the populations of various island finch species. Finch populations with a certain beak shape thrived, while those not having that beak shape decreased.
Which of the following describes a necessary condition for these changes in the finch populations to occur?
A. fewer mutations
B. limited food resources
C. limited beak variations
D. overproduction of offspring
B. Limited resources are a major driving factor for natural selection. Since there was not enough food for all of the finches, the finches that had beaks which allowed them to get food more easily were more likely to survive and reproduce.

51. Mutations within a DNA sequence are
A. natural processes that produce genetic diversity
B. natural processes that always affect the phenotype
C. unnatural processes that always affect the phenotype
D. unnatural processes that are harmful to genetic diversity
A. natural processes that produce genetic diversity
Random mutations in DNA provide genetic variation in a population of species and allow for natural selection to occur.