1. The Evolution of Populations21
The Evolution of Populations 1

2. Overview: The Smallest Unit of Evolution
One common misconception is that organisms evolve during their lifetimes
Natural selection acts on individuals, but only populations evolve
Consider, for example, a population of medium ground finches on Daphne Major Island
During a drought, large-beaked birds were more likely to crack large seeds and survive
The finch population evolved by natural selection

3. Figure 21.1

4. 1976 (similar to the prior 3 years) 1978 (after drought)
Figure 21.2 10 9
Average beak depth (mm) 8
1976
(similar to the
prior 3 years)
1978
(after
drought)

5. Microevolution is a change in allele frequencies in a population over generations
Three mechanisms cause allele frequency change
Natural selection
Genetic drift
Gene flow
Only natural selection causes adaptive evolution

6. Concept 21.1: Genetic variation makes evolution possible
Variation in heritable traits is a prerequisite for evolution
Mendel’s work on pea plants provided evidence of discrete heritable units (genes) that provided an understanding of genetic differences on which evolution is based

7. Genetic Variation
Genetic variation among individuals is caused by differences in genes or other DNA sequences
Phenotypic variation often reflects genetic variation
Some phenotypic differences are due to differences in a single gene and can be classified on an “either-or” basis
Other phenotypic differences are due to the influence of many genes and vary in gradations along a continuum

8. Single gene
Many genes

9. Nucleotide variation rarely results in phenotypic variation
Genetic variation can be measured at the whole gene level as gene variability
Gene variability can be quantified as the average percent of loci that are heterozygous
Genetic variation can be measured at the molecular level of DNA as nucleotide variability
Nucleotide variation rarely results in phenotypic variation
Most differences occur in noncoding regions (introns)
Variations that occur in coding regions (exons) rarely change the amino acid sequence of the encoded protein

10. Phenotype is the product of inherited genotype and environmental influences
Natural selection can only act on phenotypic variation that has a genetic component
Same Species!
(a) Caterpillars raised on a diet of
oak flowers
(b) Caterpillars raised on a diet of
oak leaves

11. Sources of Genetic Variation
New genes and alleles can arise by mutation, gene duplication, gene alteration, rapid reproduction and sexual reproduction

12. Formation of New Alleles
A mutation is a change in the nucleotide sequence of DNA
Only mutations in cells that produce gametes can be passed to offspring

13. A “point mutation” is a change in one base in a gene
The effects of point mutations can vary
Mutations in noncoding regions (introns) of DNA are often harmless
Mutations to genes can be neutral because of redundancy in the genetic code
Mutations that alter the phenotype are often harmful
Mutations that result in a change in protein production can sometimes be beneficial

14. Altering Gene Number or Position
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Duplication of small pieces of DNA increases genome size and is usually less harmful
Duplicated genes can take on new functions by further mutation
An ancestral odor-detecting gene has been duplicated many times: Humans have 350 functional copies of the gene; mice have 1,000

15. Rapid Reproduction Mutation rates are low in animals and plants
The average is about one mutation in every 100,000 genes per generation
Mutation rates are often lower in prokaryotes and higher in viruses
Short generation times allow mutations to accumulate rapidly in prokaryotes and viruses

16. Sexual Reproduction
In organisms that reproduce sexually, most genetic variation results from recombination of alleles
Sexual reproduction can shuffle existing alleles into new combinations through three mechanisms: crossing over, independent assortment, and fertilization

17. Concept 21.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

18. 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
An allele for a particular locus is fixed if all individuals in a population are homozygous for the same allele

19. Gene Pools and Allele Frequencies
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 times 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

20. By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies (usually p for the dominant allele and q for the recessive allele)
The frequency of all alleles in a population will add up to 1
For example, p + q = 1

21. Figure 21.UN01
CRCR
CWCW
CRCW

22. Example
Consider a population of wildflowers that is incompletely dominant for color
320 red flowers (CRCR)
160 pink flowers (CRCW)
20 white flowers (CWCW)
Calculate the number of copies of each allele
CR = (320 × 2) = 800
CW = (20 × 2) = 200

23. To calculate the frequency of each allele
p = freq CR = 800 / ( ) = 0.8 (80%)
q = 1 − p = 0.2 (20%)
The sum of alleles is always 1
= 1

24. 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

25. 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

26. Allele Frequencies (p and q)
Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
Consider, for example, the same population of 500 wildflowers and 1,000 alleles where
p = freq CR = 0.8
q = freq CW = 0.2

27. 80% chance 20% chance 80% chance 20% chance
Figure 21.7
Frequencies of alleles
p = frequency of CR allele
= 0.8
q = frequency of CW allele
= 0.2
Alleles in the population
Gametes produced
Each egg:
Each sperm:
80%
chance
20%
chance
80%
chance
20%
chance

28. Genotype Frequencies (p2, 2pq and q2)
The frequency of genotypes can be calculated
CRCR = p2 = (0.8)2 = 0.64
CRCW = 2pq = 2(0.8)(0.2) = 0.32
CWCW = q2 = (0.2)2 = 0.04
The frequency of genotypes can be confirmed using a Punnett square

29. CR CW CR CW + + Figure 21.8 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm
CRCR
0.16 (pq)
CRCW
Eggs CW
0.16 (qp)
CRCW
0.04 (q2)
CWCW
q = 0.2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants) +
16% CR
(from CRCW plants) =
80% CR = 0.8 = p
4% CW
(from CWCW plants) +
16% CW
(from CRCW plants) =
20% CW = 0.2 = q
With random mating, these gametes will result in the same
mix of genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants

30. 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
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

31. Figure 21.UN02

32. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population that is not evolving
In real populations, allele and genotype frequencies do change over time 32

33. The five conditions for nonevolving populations are rarely met in nature; they are:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow (migration)

34. Natural populations can evolve at some loci while being in Hardy-Weinberg equilibrium at other loci
Some populations evolve slowly enough that evolution cannot be detected

35. Applying the Hardy-Weinberg Principle
Phenylketonuria (PKU) is a recessive genetic disorder
We can assume the locus that causes (PKU) is in Hardy-Weinberg equilibrium given that:
The PKU gene mutation rate is low
Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele
Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
The population is large
Migration has no effect, as many other populations have similar allele frequencies 35

36. The occurrence of PKU is 1 per 10,000 births q2 = 0.0001 q = 0.01
The frequency of normal alleles is
p = 1 – q = 1 – 0.01 = 0.99
The frequency of carriers is
2pq = 2 × 0.99 × 0.01 =
or approximately 2% of the U.S. population 36