1. The Evolution of Populations
Chapter 23
The Evolution of Populations

2. Overview: The Smallest Unit of Evolution
One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but only populations evolve
Genetic variations in populations contribute to evolution
Microevolution is a change in allele frequencies in a population over generations

3. Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

4. Genetic Variation
Variation in individual genotype leads to variation in individual phenotype
Not all phenotypic variation is heritable
Natural selection can only act on variation with a genetic component

5. Variation Within a Population
Both discrete and quantitative characters contribute to variation within a population
Discrete characters can be classified on an either-or basis
Quantitative characters vary along a continuum within a population

6. Variation Between Populations
Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups

7. Fig. 23-3 1
2.4
3.14
5.18 6
7.15
8.11
9.12
10.16
13.17 19 XX 1
2.19
3.8
4.16
5.14
6.7
Figure 23.3 Geographic variation in isolated mouse populations on Madeira
9.10
11.12
13.17
15.18 XX

8. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

9. Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels
Average heterozygosity measures the average percent of loci that are heterozygous in a population
Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

10. Ldh-B b allele frequency
Fig. 23-4
1.0
0.8
0.6
Ldh-B b allele frequency
0.4
0.2
Figure 23.4 A cline determined by temperature 46 44 42 40 38 36 34 32 30
Latitude (°N)
Maine
Cold (6°C)
Georgia
Warm (21°C)

11. Animation: Genetic Variation from Sexual Recombination
Mutation
Mutations are changes in the nucleotide sequence of DNA
Mutations cause new genes and alleles to arise
Only mutations in cells that produce gametes can be passed to offspring
Animation: Genetic Variation from Sexual Recombination

12. Mutations That Alter Gene Number or Sequence
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Duplication of large chromosome segments is usually harmful
Duplication of small pieces of DNA is sometimes less harmful and increases the genome size
Duplicated genes can take on new functions by further mutation

13. Mutation Rates
Mutation rates are low in animals and plants
The average is about one mutation in every 100,000 genes per generation
Mutations rates are often lower in prokaryotes and higher in viruses

14. Sexual Reproduction
Sexual reproduction can shuffle existing alleles into new combinations
In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

15. Gene Pools and Allele Frequencies
A population is a localized group of individuals capable of interbreeding and producing fertile offspring
A gene pool consists of all the alleles for all loci in a population
A locus is fixed if all individuals in a population are homozygous for the same allele

16. Porcupine herd Porcupine herd range Fortymile herd range
Fig. 23-5
Porcupine herd
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Figure 23.5 One species, two populations
Fortymile
herd range
ALASKA
YUKON
Fortymile herd

17. The frequency of an allele in a population can be calculated
For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2
The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

18. The frequency of all alleles in a population will add up to 1
By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies
The frequency of all alleles in a population will add up to 1
For example, p + q = 1

19. The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

20. Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a population

21. Alleles in the population Frequencies of alleles
Fig. 23-6
Alleles in the population
Frequencies of alleles
Gametes produced
p = frequency of
Each egg:
Each sperm:
CR allele = 0.8
q = frequency of
80%
chance
20%
chance
80%
chance
20%
chance
CW allele = 0.2
Figure 23.6 Selecting alleles at random from a gene pool

22. Hardy-Weinberg equilibrium describes the constant frequency of alleles in a gene pool
If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
p2 + 2pq + q2 = 1
AA + Aa + aa = 1 (you can think of it like this)
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

23. 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) Eggs CW (20%) CR CW
Fig
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
Figure 23.7 The Hardy-Weinberg principle
Eggs
64% ( p2)
CRCR
16% ( pq)
CRCW
4% (q2)
CW CW
16% (qp)
CRCW CW
(20%)

24. Figure 23.7 The Hardy-Weinberg principle
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
Eggs
64% ( p2)
CR CR
16% ( pq)
CR CW
4% (q2)
CW CW CW
(20%)
16% (qp)
CR CW
64% CR CR, 32% CR CW, and 4% CW CW
Figure 23.7 The Hardy-Weinberg principle
Gametes of this generation:
64% CR    +     16% CR    =   80% CR  = 0.8 = p
4% CW      +    16% CW    =  20% CW = 0.2 = q
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants

25. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population
In real populations, allele and genotype frequencies do change over time

26. The five conditions for nonevolving populations are rarely met in nature:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow

27. Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

28. The occurrence of PKU is 1 per 10,000 births
q2 =
q = 0.01
The frequency of normal alleles is
p = 1 – q = 1 – 0.01 = 0.99
The frequency of carriers is
2pq = 2 x 0.99 x 0.01 =
or approximately 2% of the U.S. population