16-1 Genes and Variation

• In genetic terms, evolution is any change in the relative frequency of alleles in a population.– Gene pool is the combined genetic information of

all members of a particular population.– Relative frequency is the number of times an

allele occurs in a gene pool compared with the number of times other alleles for the same gene occur.

Sources of Genetic Variation

• A mutation is any change in a sequence of DNA. Mutations increase genetic variation, which is needed for natural selection to bring about evolutionary change.– Mutations do not always affect an organism’s

phenotype or fitness.– Some can affect an organism’s fitness, or its

ability to survive and reproduce in its environment.

Sources of Genetic Variation

• Many heritable differences are due to gene shuffling that occurs during the production of gametes.– The separation of homologous chromosomes during

meiosis is random with a possibility of 8.4 million different combinations of genes.

– Crossing over during meiosis increases the number of different genotypes that can appear in offspring.

– Sexual reproduction produces many different combination of genotypes.

Single-Gene and Polygenic Traits

The number of phenotypes produced for a given trait depends on how many genes control the trait.

- single-gene traits produce only two phenotypes.

- polygenic traits have many possible genotypes and phenotypes.

Fre

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f P

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oty

pe

Phenotype (height)

Generic Bell Curve for Polygenic Trait

Section 16-1

The graph shows the distribution of phenotypes that would be expected for a polygenic trait. The symmetrical bell-like shape of this curve is typical of polygenic traits. This is called a normal distribution.

Fre

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Ph

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)100

80

60

40

20

0 Widow’s peak No widow’s peak

Phenotype

Figure 16–3 Phenotypes for Single-Gene Trait

Section 16-1

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

Figure 16–2 Relative Frequencies of Alleles

Section 16-1

Hardy Weinberg Equilibrium

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Section 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Section 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Section 16-2

Directional Selection

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

Figure 16–6 Graph of Directional Selection

Section 16-2

Key

Per

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f P

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Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Figure 16–7 Graph of Stabilizing Selection

Section 16-2

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Disruptive Selection

Largest and smallest seeds become more common.

Nu

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Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Figure 16–8 Graph of Disruptive Selection

Nu

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onKey

Low mortality, high fitness

High mortality, low fitness

Section 16-2

16-3 The Process of Speciation

• A species is a group of similar organisms that can breed and produce fertile offspring. Individuals in the same species share a common gene pool.– A genetic change that occurs in one individual can

spread through the population as that individual and its offspring reproduce.

– If a genetic change increases fitness, that allele will eventually be found in many individuals of that population.

Isolation Mechanisms

• Speciation is the formation of new species.– As new species evolve, populations become

reproductively isolated from each other.– In reproductive isolation, the members of two

populations cannot interbreed and produce fertile offspring.

– Can be due to behavioral isolation, geographic isolation, and temporal isolation.

Reproductive Isolation

• Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive stratagies.

• Geographic isolation – two populations are separated by geographic barriers (rivers, mountains, or bodies of water).

• Temporal isolation – two or more species reproduce at different times.

Concept Map

Section 16-3

results from

which include

produced by produced byproduced by

which result in

which result in

Reproductive Isolation

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew species

Speciation in Darwin’s Finches

In finches on the Galapagos Islands, a few finches traveled from S. America to one of the islands. They survived and reproduced.

Some birds crossed to other islands, and the different populations no longer shared a gene pool.

Differences in food sources on the islands favored birds with different types of beaks.

This led to the populations becoming reproductively isolated and evolving into separate species.

The reproductive isolation was reinforced by ecological competition between members of the different species whenever they did live in the same environment.

Links from the authors on restricting use of antibiotics

Interactive test

For links on population genetics, go to www.SciLinks.org and enter the Web Code as follows: cbn-5161.

For links on speciation, go to www.SciLinks.org and enter the Web Code as follows: cbn-5163.

Go Online

Interest Grabber Answers

1. Make a list of physical traits that you think are influenced by genes.Then, write next to each trait whether you have the trait or not(e.g., a widow’s peak) or whether there are many variations of thetrait (e.g., hair color).

Students’ answers will include dimples and detached earlobes.

2. Are most of the traits you listed clear-cut or are they mostly traitsthat have many variations? Which traits in your list are difficult tocategorize?

Most traits listed likely have many variations.

3. Compare your list with that of another student. Did he or she think ofany traits that you missed? Why do you think some traits are clear-cut,while others are not?

Some students may suggest that patterns of inheritance for traits with many variations (polygenic) are more complex than for clear-cut (single-gene) ones.

Interest Grabber Answers

1. Choose an animal that you know something about, such as a deer, and write its name at the top of a sheet of paper. Then, divide your paper into two columns, and write the heading Trait in one column and Advantage in the other.

Animal choices should be sufficiently familiar that students can describe several traits.

2. Under Trait, write in several of the animal’s traits.

Students should list traits such as size, color, and specialized behavior.

3. Under Advantage, write in how you think the trait would be helpful to the animal.

Students should indicate that adaptive value is clearer for some traits than for others. For example, white-tailed deer raise their tails upon sensing a predator. This may be an alarm signal for other deer, or it may induce the predator to chase the now-conspicuous deer.

Interest Grabber Answers

1. Make a list of everyday things that this person encounters that you don’t. For example, does he or she eat different kinds of food? Does he or she live in a climate different from yours?

Students’ lists should include several social/environmental factors.

2. All humans are the same species. What might happen if groups of humans were separated for millions of years in very different environments, such as those you have just described?

Students may understand that humans would evolve separately in response to different environmental pressures.

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