1. The Evolution of Populations
Chapter 23
“Nothing in Biology makes sense except in the light of evolution”
T. Dobzhansky

2. Population Genetics
What was missing from Darwin’s explanation of natural selection?
A way to explain how chance variations can show up in a population, while also accounting for precise transmission from parent to offspring…
Population Genetics:
Emphasizes the variation within populations and recognizes the importance of quantitative characters - those characteristics that vary along a continuum
Modern synthesis:
ties in ideas from paleontology, taxonomy, biogeography, and population genetics
Scientists contributing: Dobzhansky, Wright, Mayr, Simpson, Stebbins (page 446)

3. Quantitative Characters and Discrete Characters
traits that vary along a continuum in a population (like plant height)
Discrete Characters:
traits that can be classified on an either-or basis (like flower color) 3

4. Genes & Variation
While developing his theory of evolution, Darwin worked under a serious disadvantage – he did not know how heredity worked
Without understanding heredity, Darwin was unable to explain 2 important factors:
The source of variation central to his theory
How hereditable traits were passed from one generation to the next
Today, genetics, molecular biology, and evolutionary theory work together to explain how inheritable variation appears and how natural selection operates on that variation (i.e. how evolution takes place)

5. Gene Pools
A gene pool is the combined genetic information of all the members of a particular population
Recall that a population is a collection of individuals of the same species in a given area which share a common group of genes
A gene pool typically contains 2 or more alleles (or forms of certain genes)
Example: mouse populations may have 2 or more alleles for fur color – the gene pool for the trait fur color is the combination of all the alleles in the population
The relative frequency of an allele is the number of times that allele occurs in a gene pool compared to the number of times other alleles occur

6. Relative Frequency of Alleles

7. Sources of Genetic Variation
The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction
A mutation is any change in a sequence of DNA
Mutations that affect an organisms phenotype can lead to an increase in fitness for that organism
Most inheritable differences are the result of gene shuffling that occurs during sexual reproduction
Example: the 23 pairs of chromosomes found in humans can produce 8.4 million different combinations of genes

8. Mutations
Mutation is the ultimate source of new alleles within a population
Mutation is also a source of evolution
A A A
A A
A A A
a a
A a A
A a
T = 0
T = 1

9. Sexual Recombination
Most of the genetic variation in a population results from the unique combination of alleles that each individual receives.
Three mechanisms contribute to the shuffling of alleles during sexual reproduction:
Crossing over
Independent assortment of alleles
Fertilization

10. Population Genetics & Hardy Weinberg
Before we consider the mechanisms that cause a population to evolve, it will be helpful to examine, for comparison, the gene pool of a NONEVOLVING population.
Such a gene pool is described by the Hardy-Weinberg theorum.
The theorum states that the frequencies of alleles and genotypes in a population’s gene pool REMAIN CONSTANT over generations unless acted upon by mechanisms other than Mendelian segregation and recombination of alleles.
Put another way – the shuffling of alleles due to meiosis and random fertilization has no effect on the overall gene pool of a population.

11. Population Genetics
Hardy-Weinberg Principle – states that allele frequencies tend to remain constant in populations unless something happens OTHER THAN Mendelian segregation and sexual recombination.
This situation in which allele frequencies remain constant is called genetic equilibrium
If allele frequencies do not change, the population will not evolve
Hardy-Weinberg is a mathematical model that describes the changes in allele frequencies in a population
Allows us to predict allele and genotype frequencies in subsequent generations (testable) 11

12. Hardy-Weinberg Principle
Model assumptions (conditions required to maintain genetic equilibrium from generation to generation):
random mating population
Large population size – n > 100
No emigration or immigration (no movement into or out of the population)
No mutations
No natural selection (all genotypes have an equal chance of survival and reproduction)
If all 5 conditions are met, there should be NO EVOLUTION – no selection, no gene flow, no genetic drift, no mutation
Describes a NON-EVOLVING POPULATION 12

13. Hardy-Weinburg Principle
The Hardy-Weinburg Principle is neat because it can serve as a null hypothesis for evolution
It can show that evolution IS OCCURING within a population 13

14. Hardy-Weinburg Principle
Let p= frequency of allele A
Let q= frequency of allele a
Let p2= frequency of genotype AA
Let 2pq= frequency of genotype Aa
Let q2= frequency of genotype aa
Law says, given assumptions, that within 1 generation of random mating, the genotype frequencies are found to be in the binomial distribution p2+2pq+q2=1 (genotype frequencies) and p+q=1 (allele frequencies) 14

15. Hardy-Weinberg Example
The allele for the ability to roll one’s tongue is dominant (R) over the allele for the lack of this ability (r).
In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous? 15

16. How Hardy-Weinberg Works
The equation: p2 + 2pq + q2 = 1
Therefore, p + q = 1
500 organisms, 25% are rr; thus q2 = .25
so 125 organisms are rr
If q2 = .25, then q=.5
Thus, p + .5 = 1, leaves p = .5
So, p2 = .25, so 125 organisms are RR
2pq leaves the heterozygotes, so
2(.5)(.5) = .5 or 50%, so 250 organisms are Rr 16

17. Causes of Microevolution
Natural selection, genetic drift, and gene flow can alter allele frequencies in a population and cause MOST evolutionary changes.
Microevolution:
generation to generation change in a population’s allele frequencies
Three main causes:
natural selection
genetic drift
gene flow

18. Natural Selection on Polygenic Traits
Natural selection: differential survival and reproduction among members of a population
Natural selection is NOT random – it leads to adaptive evolution – evolution that results in a better match between organisms and their environment.
Can affect the distribution of genotypes in any of three ways:
Stabilizing selection
Directional selection
Disruptive selection

19. Genetic Drift
Natural selection is not the only force that can lead to evolution:
In addition to natural selection, genetic drift is a way by which allele frequencies can change
In the real world, population sizes fluctuate
Because populations fluctuate in size, sometimes there can be changes in allele frequencies due to random chance
These changes are called random genetic drift 19

20. Genetic Drift
In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by change
Over time, a series of chance occurrences of this type can cause an allele to become common in a population 20

21. The Power of Genetic Drift
Genetic drift is a powerful force when a population size is very small
Can and does lead to allele fixation
Allele fixation means that a population changes (evolves) from many alleles represented to only 1 allele represented
Depends on starting frequency (which allele becomes fixed) 21

22. Consequences of Genetic Drift
Can and does lead to fixation of alleles
Effect of chance is different from population to population
Small populations are effected by genetic drift more often than larger ones
Given enough time, even in large populations genetic drift can have an effect
Genetic drift reduces variability in populations by reducing heterozygosity
REAL WORLD EXAMPLES OF GENETIC DRIFT:
The Bottleneck Effect
The Founder Effect 22

23. Real World Examples of Genetic Drift
The Bottleneck Effect
Occurs when only a few individuals survive a random event, resulting in a shift in allele frequencies within the population
Small population sizes facilitate inbreeding and genetic drift, both of which decrease genetic variation
Reduces genetically variability because at least some alleles are likely to be lost from gene pool
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. 23

24. Figure 23.5 The bottleneck effect: an analogy24

25. Real World Examples of Genetic Drift
The Founder Effect
Occurs when individuals from a source population move to a new area and start a new population
This new population is often started by relatively few individuals that do not represent the population well in terms of all alleles being represented 25

26. The Founder Effect http://bcs. whfreeman
What determines which variants survive the event or get to the new location?
Random chance
Genetic drift has the larges effect on small populations ( individuals) 26

27. The Founder Effect Sample of Original Population Descendants
Founding Population A
Founding Population B 27

28. The Founder Effect Sample of Original Population Descendants
Founding Population A
Founding Population B 28

29. The Founder Effect Sample of Original Population Descendants
Founding Population A
Founding Population B 29

30. Gene Flow Gene flow can also change allele frequencies
Gene flow is the physical flow of alleles into or out of a population.
Immigration – alleles coming in (added)
Emigration – alleles moving out (lost)
Gene flow counteracts differences that arise through mutation, natural selection, and genetic drift.
Gene flow helps keep separated populations genetically similar – reduces differences between populations 30

31. The Power of Natural Selection
Natural selection is the ONLY mechanism that consistently causes adaptive evolution.
Evolution by natural selection is a blend of chance and “sorting” – chance is the creation of new genetic variations and sorting as natural selection favors some alleles over others.
Because of this sorting effect – ONLY natural selection consistently increases the frequencies of alleles that provide reproductive advantage and thus leads to adaptive evolution.
Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.
Relative fitness conferred by a particular allele depends on the entire genetic and environmental context in which it is expressed.

32. Three Modes of Selection
The effect of selection on a varying characteristic can be:
Stabilizing
Directional
Disruptive (Diversifying)
This effect on allele frequency depends on which phenotypes in a varying population are being favored.

33. Stabilizing Selection
selection is against phenotype with arrows
selection is against both extreme phenotypes
intermediate survives and reproduces at a higher rate than others
phenotypic extremes are eliminated, variance has decreased
population has stabilized around mean
But…remember that mutation and gene flow can increase variance by counteracting selection

34. Stabilizing Selection
When individuals near the center of the curve have higher fitness than individuals at either end of the curve
Intermediate forms of a trait are favored and alleles that specify extreme forms are eliminated from a population
Counteracts the effects of mutation, gene flow, and genetic drift – preserves the most common phenotypes.

35. Stabilizing Selection
Section 16-2
Stabilizing Selection

36. Directional Selection
selection against 1 extreme in favor of the other extreme
after time we see a shift in the direction of the population toward 1 of the 2 homozygous extremes
Variation is reduced here and alleles can be lost from the population

37. Directional Selection

38. Disruptive/Diversifying Selection
selection is against phenotype with arrows
selection is against intermediate phenotype in favor of BOTH extremes
number of intermediates after a few generations is low, but variation is maintained here
in the real world, this can lead to speciation
if this occurs long enough and there is barrier to gene flow, speciation can occur

39. Disruptive or Diversifying Selection
When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
Forms at both ends of the range of variation are favored and intermediate forms are selected against – selection creates two, distinct phenotypes
Ex. Bird beak size – no middle sized seeds, only large seeds and small seeds; thus, small and large beaks are favored

40. Disruptive Selection

41. Selection Graphs

42. Figure 23. 12 Modes of selection http://bcs. whfreeman

43. Key Role of Natural Selection in Adaptive Evolution
Natural selection increases the frequencies of alleles that enhance survival and reproduction, thus improving the match between organisms and their environment.
The physical and biological components of an organism’s environment may change over time.
As a result, what constitutes a “good match” between an organism and its environment can be a moving target – making adaptive evolution a continuous, dynamic process!

44. Sexual Selection
Sexual selection may lead to pronounced secondary differences between the sexes:
Sexual selection is a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates
Maintained by natural selection
May lead to pronounced differences between sexes – sexual dimorphism
a marked difference between the two sexes in secondary sexual characteristics

45. Figure 23.16x1 Sexual selection and the evolution of male appearance

46. Types of Sexual Selection
Intrasexual Selection means selction within the same sex – typically males
Individuals of one sex compete directly for mates of the opposite sex
Often it is based on rituals and displays that don’t risk injury
Intersexual Selection is also called “mate choice” – typically females
Female choice is typically based on showiness of the male’s appearance and/or behavior
Males will often weight the attraction of predators versus the attraction of mates

47. The Preservation of Genetic Variation
Tendency for directional and stabilizing selection to reduce variation is countered by mechanisms that preserve or restore it:
Diploidy
Balanced Polymorphism
Neutral Variations

48. Diploidy Diploidy refers to organisms carrying genes in pairs:
Recessive traits can be preserved in heterozygotes – this maintains a large pool of genes that may not be useful today, but could be in the future.

49. Balanced Polymorphism
Balancing selection maintains two or more forms in a population:
Heterozygous Advantage: sometimes a heterozygote has an advantage to homozygotes and survives
Frequency Dependent Selection: the fitness of a phenotype declines if it becomes too common in a population

50. Figure Shells 50

51. Figure 23x2 Polymorphism 51

52. Neutral Variation
Changes in the DNA (typically non-coding) that provide no selective advantage or disadvantage.
However, these variations MAY influence survival and reproduction in ways that are difficult to measure.
The variation may also be neutral in ONE environment but beneficial in ANOTHER environment.
The point is that this variation is an enormous reservoir of raw material for natural selection!

53. Figure 23.7 A nonheritable difference within a population
Genetic Variation is the raw material for natural selection
Genetic variation occurs within and between populations
many are at molecular level and cannot be seen…not all are heritable – some are environmentally induced
(Ex. Map butterflies – figure 23.7)

54. Measuring Genetic Variation
Population Geneticists use whole gene measurements and molecular measurements – gene diversity and nucleotide diversity
Ex. Fruit flies – gene diversity using loci, nucleotide diversity using DNA fingerprinting
Note: humans have little genetic variation compared to other species – same nucleotide sequence at 999 out of every 1000 nucleotide sites in your DNA

55. Geographic Variation
Differences in gene pools between populations or subgroups of populations
Due to fact that at least SOME environmental factors are likely to differ from one place to another; thus, natural selection can contribute to this….
Ex. In population, one type of geographic variation is a Cline - graded change in trait along a geographic axis

56. Figure 23.8 Clinal variation in a plant

57. What Can’t Natural Selection Do?
NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS!!!
Limited by historical constraints
Ex. Body structure for erect posture
Adaptations are often compromises
Ex. Seal on land vs. water
Not all evolution is adaptive
Ex. Storm blows ALL orgs to new place, not just best suited
Selection can only edit existing variations
See page 461