Evolution of Populations

Chapter 16

Genetic Variation

• Heterozygotes make up between 4-8% in mammals and 15% in insects.

• The gene pool is total of all the alleles in a population. All the genes.

• Relative Frequency is the number of times a particular allele appears in a gene pool.

• In genetic terms evolution is any change in relative frequency in a gene pool.

 Relative Frequencies of Alleles

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

Brown is the recessive trait but more common in frequency.

Sources of Variation

• The two main sources for genetic variation are Mutation and Gene Shuffling.

• Mutations occur as DNA is changed by mistakes in replication, radiation or chemicals in the environment.

• Gene Shuffling occurs as meiosis creates gametes. Crossing Over and Independent Assortment creates new combinations.

Single Gene & Polygenic Traits

• Single-gene traits are controlled by one gene with two phenotypes.

• The variation leads to only two phenotypes.

• Polygenic traits are controlled by two or more genes and have variable phenotypes.

• The phenotypes usually fall in a bell curve.

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Phenotype (height)

Generic Bell Curve for Polygenic Trait

Natural Selection on Single-gene Traits

• Evolutionary fitness can be measured by the ability to reproduce and pass genes.

• Natural selection works only on the organism and its phenotype to survive.

• Natural selection on a single-gene trait can change gene frequencies and cause evolution. Fig 16-5

• Red, Brown, & Black Lizards

Natural Selection on Polygenic Traits

• Natural Selection on Polygenic traits is more complex.

• Directional Selection: occurs when members on one end of the curve have greater fitness and create a shift in that direction

• Stabilizing Selection: the middle of the curve is most fit and the ends shrink.

• Disruptive Selection: the lower and upper ends of the curve have greater fitness and the middle declines.

Directional Selection

Food becomes scarce.

KeyLow mortality, high fitness

High mortality, low fitness

 Graph of Directional Selection

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Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

 Graph of Stabilizing Selection

Low mortality, high fitnessHigh mortality, low fitness

Stabilizing Selection

Disruptive Selection

Largest and smallest seeds become more common.

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Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

 Graph of Disruptive Selection

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Low mortality, high fitness

High mortality, low fitness

Genetic Drift• In small populations random mating leads

to a change in gene frequency called Genetic Drift.

• By chance in a small population some organisms might leave more offspring thus increasing their gene frequency.

• The founder effect occurs when a new population is created from an existing one and the founding gene frequency is different than the original population due to chance.

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift

Evolution vs. Genetic Equilibrium

• Hardy-Weinberg Principle states that allele frequencies will remain in genetic equilibrium and unchanged unless acted upon by evolutionary forces.

• There are five conditions for Hardy-Weinberg to work:– Random mating– Very large population– No movement in or out of a population– No mutations– No natural selection

Speciation

• Speciation is the process of a species evolving into a new species.

• Separation of the gene pool is necessary for speciation to occur.– Behavioral Isolation: differences in mating

rituals lead to isolation– Geographic Isolation: a population is divided

by a geographic boundary. River, mountains– Temporal Isolation: different breeding times

separate a population.

Concept Map

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

Testing Natural Selection

• Peter & Rosemary Grant tested Natural Selection on the Galapagos Islands.

• Variation in the Finches followed the bell curve and was genetic.

• During drought bigger beaked birds were naturally selected, also a behavior for big beaked birds mating with like birds led to directional selection.

• The finches’ beaks became bigger in only a few decades.

Speciation in Darwin’s

• The species of Galapagos finches evolved by:– Founding population from South America– Separation by geographic boundaries– Changes in gene pool by directional selection– Reproductive Isolation keeps birds separate– Ecological Competition may change the

original population– Evolution continues on the islands– Discuss Limits and Questions to finch

evolution