1. The Evolution of Populations
Chapter 11

2. Genetic Variation in Populations
Microevolution - the observable change in the allele frequencies of a population over time.
Occurs on a small scale
In populations there are a wide range of phenotypes
Ex: short/fat penguins and taller/slim penguins

4. Genetic Variation in Populations
In order to have a wide range of phenotypes, there must be genetic variation
The greater the variation in phenotypes, the more likely it is that at least some individuals can survive in a changing environment

5. Genetic Variation in Populations
For example, a short/fat penguin might be better able to stay warm in an especially cold winter. A tall/slim penguin might be able to dive better, thus catching more fish in a period of food shortage

6. Genetic Variation in Populations
Gene pool - All of the alleles of every individual in a population

7. Genetic Variation in Populations
The allele frequency is a measure of how common a certain allele was in the population

8. Genetic Variation in Populations
To calculate allele frequency for “b”:
7 b
7 B
Total alleles = 14
7b/14 TOTAL = 50%

9. Genetic Variation - 2 Main Sources
Mutations
A random change in DNA
Can form a new allele
Mutations that occur in sex cells can be passed on to offspring
Recombination
Occurs during meiosis
Parents’ alleles can be arranged in new ways during crossing over

12. How do we analyze genetic variation in a population?
With graphs!

13. Natural Selection in Populations
“mean” = average
“distribution” = range of data; a set of data and their frequency of occurrence

14. Natural Selection in Populations
Normal distribution/bell shaped curve
frequency is highest near the mean and decreases toward each extreme end of the range

15. Natural Selection in Populations
Phenotypes in the middle of the range are most common
Ex: people of average height (5-6ft) would fall in the shaded region
Anyone shorter or taller (the extremes) would fall outside the shaded region

16. Directional Selection
Directional selection - Favors a phenotype at one extreme of a trait’s range.
Causes a shift in the population’s phenotypic distribution
Extreme/rare phenotype becomes more common

17. Directional Selection
Ex: Peppered moth example
After the tree trunks became dark again, the trait for white bodies was selected against because it was no longer beneficial. This caused a shift in the distribution. The mean range now included mostly black moths.

18. Stabilizing Selection
Stabilizing Selection- The intermediate phenotype is favored and becomes most common in the population.
Distribution becomes stable in the middle range/decreases genetic variation

19. Stabilizing Selection
Ex: Human birth weight
Babies with either a very low or very high weight at birth are more likely to experience complications than babies of average weight

20. Disruptive Selection
Disruptive selection- Both extreme phenotypes are favored
Intermediate is selected against

21. Disruptive Selection
Ex: The African swallowtail butterfly has traits to be either orange, black (the extremes), or tan (the intermediate)
Predators don’t eat the orange or black ones because orange and black butterflies of different species are poisonous. Thus, the extreme trait is favored.

22. Other Mechanisms of Natural Selection
Gene flow: Organisms move to new population & reproduce --> its alleles are added to new population’s gene pool and removed from original one.
Increases genetic variation of receiving population
Lack of gene flow can lead to speciation
Speciation - the rise of two or more new species from a previously existing species

23. Other Mechanisms of Natural Selection
Genetic Drift
Small populations = more affected by chance
Elimination/sudden increase of alleles in a small population = drift
Drift = loss of genetic diversity

24. Other Mechanisms of Natural Selection
Bottleneck Effect
Drift that occurs after an event reduces population size
Reduced population size = reduced genetic variation

25. Other Mechanisms of Natural Selection
The Founder Effect
Small number of individuals colonize a new area
Gene pool of new colony is usually very different (less diverse) from original population

26. Other Mechanisms of Natural Selection
Effects of Drift
Loss of genetic variation
Less likely to have a large amount of individuals that can survive a changing environment
Higher chance of lethal genes being passed on instead of eliminated

27. Other Mechanisms of Natural Selection
Sexual Selection
Males continually produce sperm, small investment, can have many mates
Females have limited number of eggs, long gestational periods, invest more time/resources
Makes females choosy about mates, don’t want to have loser babies

28. Other Mechanisms of Natural Selection
Sexual Selection
Certain traits increase mating success
Intra- : competition between males
Inter- : showy males to attract mates
Some showy traits can put animals in danger, but suggest good health/fertility/ability to care/defend to potential mates. GOOD QUALITY.

29. Video Bell Ringer!
What kind of sexual selection is feautured in this video? What traits might the beetle have to make him successful?
Sexual Selection of Darwin Beetles
What kind of sexual selection is featured in this video? Why are females so choosy about mates?
Sexual Selection Birds of Paradise

30. Hardy-Weinberg Equilibrium
Describes populations that are not evolving
“Equilibrium” - Genotype frequencies in a population stay the SAME over time as long as 5 conditions are met.

31. 5 Conditions Very large population - no drift
No emigration/immigration - no gene flow
No mutations - no new alleles can be added to gene pool
Random mating - no sexual selection
No natural selection - all traits aid in survival equally

32. Hardy-Weinberg Equilibrium
Equilibrium is only met if ALL 5 conditions are met!!!!
Real populations rarely meet all 5

33. Why study Hardy-Weinberg?
Biologists can compare real data to data predicted by the model.
Can learn more about the ways populations evolve and about how to test factors that can lead to evolution.

34. Review on what leads to evolution
Genetic drift
Gene flow
Mutation
Sexual selection
Natural selection

35. Speciation Through Isolation
Isolated populations - when gene flow stops
Isolated populations adapt to their environments differently, which can lead to differences in the gene pools

36. Speciation Through Isolation
Reproductive isolation - members of different populations can not mate successfully
Reproductive organs not compatible
Producing barren offspring
Final step of speciation

37. What causes isolation?
Behavioral isolation - differences in courtship or mating behaviors
Bird dances
Frog songs
Firefly flash patterns

38. What causes isolation?
Geographic isolation - Physical barriers that can separate populations
Mountain ranges
Rivers
Valleys

39. What causes isolation? Geographic
Isthmus of Panama separated a large population of wrasse fish
Gene pools changed over time
Now 2 separate species

40. What causes isolation?
Temporal isolation - mating seasons do not line up
High competition for mates at one time

41. Evolution Through Natural Selection is Not Random
“Chance” and “random” relate to how easily an outcome can be predicted
Genetic drift
Mutations

42. Evolution Through Natural Selection is Not Random
The only “random” thing is a change in the environment
Natural selection pushes a population’s traits in an advantageous direction
The response of species to environmental challenges is not random

43. Patterns In Evolution
Convergent Evolution - evolution toward similar characteristics in unrelated species
Adaptations to similar environments

44. Patterns In Evolution Shark and dolphin tails
Separated by 300 million years of evolution

45. Patterns In Evolution
Divergent Evolution - Closely related species evolve in different directions
Different appearances due to adapting to different environments

46. Patterns In Evolution Red fox and Kit fox
Red coat, temperate region vs. Sandy coat/big ears, desert regions

47. Patterns In Evolution
Species interactions can lead to connected evolutionary pathways
Coevolution - process in which two or more species evolve in response to changes in each other

48. Patterns In Evolution Acacia plant and stinging ant
Ant protects plant from predators and plant provides nectar for the ant

49. Patterns In Evolution
“Evolutionary Arms Race” - coevolution in competitive relationships

50. Patterns In Evolution Garter snake and rough skinned newt
Newt produces a neurotoxin that concentrates on their skin
Garter snakes have evolved a resistance to the toxin
Driven newts to have extreme levels of toxins

51. Patterns In Evolution
Extinction - The elimination of a species from Earth
Most often occurs when a species cannot adapt to an environmental change

52. Patterns In Evolution
Background Extinction - occur continuously at a low rate
Usually affect one or a few species in a small area

53. Patterns In Evolution
Mass Extinction - Very rare, but very intense, often on a global level
Can destroy many species
There have been at least 5 mass extinctions in the last 600 million years

54. Patterns In Evolution
Punctuated Equilibrium - episodes of speciation occur suddenly in geologic time and are followed by long periods of stability

55. Patterns In Evolution
Adaptive Radiation - Diversification of one ancestral species into many descendant species
Descendant species usually adapted to a wide variety of environments