II. The Evolution of Populations (Chapter 23)

What are the functional units of matter?

Atoms

What are the functional units of living things?

cells

What are the functional units of schools?

Classes

What are the functional units of evolution?

Populations

Important Misconception!

Evolution does not occur in individual organisms—even though natural selection acts on individuals

A. Population genetics

1. Microevolution is evolutionary change on the smallest scale

2. Microevolution refers to the change in the genetic make-up of a population from generation to generation

B. The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance

1. Biologists rapidly accepted Darwin’s theory of evolution, but rejected natural selection as its mechanism because there was no explanation for chance variation and their transmission to offspring

2. Mendel’s work was rediscovered in early 1900’s

Darwin’s ideas required quantitative characteristics that vary along a continuum

Mendel’s ideas were that the factors were “discrete” and “either or” such as purple or white flowers

3. Many geneticists did not believe the continuum of traits Darwin focused on could be explained by Mendel’s theory which seemed to be “either—or” by nature.

5. We know now that Quantitative traits such as:

a. Heightb. Fur lengthc. Weight

Are controlled by several genes and are called:

Polygenic traits

5. Discoveries of the 1930’s led to the development of Population genetics which reconciled Mendelism with Darwinism.

6. Population genetics emphasized quantitative inheritance and genetic variation within a population.

7. 1940 a comprehensive theory known as the modern synthesis was developed that emphasized:

the importance of populations as the units of evolution,

the essential role of natural selection

the gradualness of evolution

C. The genetic structure of a population is defined by its allele and genotype frequencies

1. A localized group of the same species is called a population.

2. A species is a group of organisms whose individuals have the ability to interbreed in nature.

3. Populations may be totally isolated or contiguous.

4. All the genes present in a population at any given time are called the gene pool.

Example 1In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous recessive (bb).

98 BB individuals = 196 B alleles84 Bb individuals = 84 B alleles

and 84 b alleles18 bb individuals = 36 bb alleles200 individuals represents a gene pool

of 400 alleles280/400 alleles are B—70%120/400 alleles are b—30%

0.7 x 0.7 = 0.49 (BB)

2(0.7 X 0.3) = 0.42 (Bb)

0.3 X 0.3 = 0.09 (bb)

D. The Hardy—Weinberg theorem describes a non-evolving Population

1. In the absence of selection pressure and other agents of change, the genetic structure of a population will remain constant from one generation to the next in spite of the shuffling of alleles by meiosis and random mating.

2. When allele frequencies of both alleles and genotypes remain stable in a population that population is said to be in Hardy—Weinberg equilibrium.

3. In a simple case of only two alleles the letters p and q represent the proportion of the two alleles within the population.

4. The combined frequencies of the alleles equals one. (p + q = 1)

The frequency of homozygous dominant individuals (pp) equals p2

The frequency of homozygous recessive individuals (qq) equals q2

The frequency of heterozygous individuals equals pq individuals plus qp individuals, or 2pq.

Everyone must be homozygous dominant, recessive, or heterozygous, therefore the sum of their frequencies equals 1.

p2 + 2pq + q2 = 1

5. Summary:

( p + q) x ( p + q) = p2 +2pq + q2

Allele Allelefrequency frequency For males for females

6. Allele frequencies can be determined from genotype frequencies

7. Always start analysis of allele frequency with the recessive phenotype because the frequency of q is the square root of recessive individuals

Example 3 If the number of individuals with sickle cell

anemia is 4%, then q2 =0.04 the frequency of q is 0.2 (the square root of

0.04 is 0.2) The frequency of p is 0.8 You cannot determine the frequency of p

from the homozygous dominant individuals because:

p2 and 2pq have the same phenotype.

Example 4

Review example 1, which illustrates how Mendelian inheritance preserves genetic variation from one generation to the next. In example 1, the gene frequency for p = 0.5 and q = 0.5. This frequency stays the same for 4 generations and is in equilibrium. Now, assume that p = 0.8 and q = 0.2 and annotate the diagram below:

Example 5

The allele frequency in a population are A = 0.6 and a = 0.4.

Predict the genotype frequencies for the next generation

AA ?

Aa ?

aa ?

AA 0.36

Aa ?

aa 0.16

0.36 + 0.16 = 0.52

1.0- 0.52 = 0.48

Aa = 0.48

Or, Aa = 2pq

Aa = 2 x 0.6 X 0.4 = 0.48

Example 6

Suppose that one gene locus determines stripe pattern in skunks. SS skunks have two broad stripes; Ss have two narrow stripes; and ss skunks have white speckles down their backs. A sampling of a population of skunks found 65 broad-striped skunks , 14 narrow-striped skunks, and 1 speckled. Determine the gene frequencies.

Total number of skunks in population = 80

65 SS skunks represent 130 A alleles 14 Ss skunks represent 14 S alleles

and 14 s alleles 1 s skunk represent 2 s alleles

S alleles 130 + 14 = 144s alleles 14 + 2 = 16

S alleles = 144s alleles = 16Total Population of skunks = 8080 skunks, each with 2 alleles means a

gene pool of 160 alleles

144/160 = 0.9 frequency of S16/160 = 0.1 frequency of s

You can also solve using Hardy- Weinberg equation:

1 skunk out of 80 is homozygous recessive

1/80 = 0.0125 = q2

Therefore, q = 0.1118 or 0.1 and p = 0.9

E. Conditions for Hardy—Weinberg Equilibrium

1. Extremely large population sizeThe smaller the population, the greater the role played by chance fluctuations in allele frequencies

2. No gene flow

Gene flow is the transfer of alleles between populations and can alter allele frequencies

3. No mutations

By introducing or removing genes from chromosomes or by changing one allele into another, mutations modify the gene pool.

4. Random mating

If individuals preferentially choose mates with certain genotypes, including close relatives, (inbreeding), random mixing of gametes does not occur

5. No natural selection

Differential survival and reproductive success of individuals carrying different genotypes will alter allele frequencies.

Concept Check

What did Mendel's findings about genetics add to Darwin’s theory of evolution by natural selection?

Mendel showed that the inheritance is particulate, and subsequently it was shown that this type of inheritance can preserve the variation on which natural selection acts

Concept Check

Which parts of the Hardy-Weinberg equation correspond to the frequency of individuals that have at least one PKU allele?

2pq + q2

F. Primary Causes of microevolution

1. Mutations

New genes and alleles originate by mutations, such as a point mutation which is the cause of Sickle Cell Anemia. Some point mutations may be harmless, such as those found in the wobble position and those found in non-coding DNA.

“On rare occasions however, a mutant allele may actually make its bearer better suited to the environment, enhancing reproductive success.” (Campbell 459)

2. Gene flow

Genetic additions or subtractions resulting form the movement of individuals or gametes

3.Genetic drift

The smaller the sample size the greater the chance of deviation from the predicted result. 2 causes of genetic drift are: Bottleneck effect (p.461) Founder effect (p. 462)

Bottleneck effect—sudden changes in the environment

Elephant seals in California

Have no genetic variation at 24 gene loci—only one allele

4.Sexual Recombination

Produces much more recombination than mutation from generation to generation.

5. Natural selection

Individuals in a population exhibit variation in their heritable traits, and those with variations that are better suited to the environment tend to produce more offspring than those that are less well suited

G. Genetic Variation occurs within and between populations

1. Polymorphism occurs when two or more forms are evident in a population (freckles and absence of freckles are different “morphs” and examples of polymorphism)

2. Genetic variation can be measured by gel electrophoresis

a. can identify the variation in protein products of specific loci.

b. Proteins produced by different alleles may vary in amino acid composition and still have the same overall charge and be indistinguishable by electrophoresis.

c. Meiosis and random mating generate genetic variation

most mutations are not viable Mutation can produce genetic variation in

bacteria and other microorganisms which have short generation times

Variation in plants and animals is usually due to sexual recombination because mutations are deleterious and usually result in defective protein products.

d. Sexual selection may lead to pronounced secondary differences between the sexes called sexual dimorphism. (example: size, plumage, lion manes, deer antlers or other adornments in males)

e. Natural selection cannot fashion perfect organisms

Selection can only edit variations that already exist

New genes are not formed by mutation on demand.