1. Chapter 16: Evolution of Populations

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

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

6. 2. 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 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 ?

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

17. 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

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

19. 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

20. 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

21. 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

22. 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

24. Evolution review ½ sheet. (yes some questions are missing)
Quiz Monday!!
Evolution review ½ sheet. (yes some questions are missing)
Thursday and Friday’s concepts will be on the quiz:
Directional, Stabilizing and Disruptive selection.
Geographic, Behavioral, Temporal Isolation
Small populations caused by bottleneck and founder effect

25. 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.

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

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

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

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

30. 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.

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

32. 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.

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

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

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

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

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

39. The results of genetic crosses can usually be predicted using the laws of probability. In small populations, however, these predictions are not always accurate.
a. Founder effect: allele frequencies change due to migration of a small subgroup of a population
Example: fruit flies on Hawaiian islands

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

41. Founder effect
allele frequencies change due to migration of a small subgroup of a population

42. Founder Effect: : Fruit Flies on Hawaiian islands
Sample of
Original Population
Founding Population A
Founding Population B
Descendants

43. 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

44. 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

45. Bottleneck Effect
Original population

46. Bottleneck Effect
Catastrophe
Original population

47. Bottleneck Effect
Catastrophe
Surviving population
Original population

48. Another picture to illustrate bottleneck effect