evolution chapter 16 honors. copyright pearson prentice hall how common is genetic variation? many...

Evolution

Chapter 16honors

Copyright Pearson Prentice Hall

How Common Is Genetic Variation?Many genes have at least two forms, or alleles.All organisms have genetic variation that is “invisible”

because it involves small differences in biochemical processes.

An individual organism is heterozygous for many genes.


Variation and Gene PoolsA population is a group of individuals of the same species

that interbreed. A gene pool consists of all genes, including all the different

alleles, that are present in a population.

The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage, and it is not related to whether an allele is dominant or recessive.


Gene Pool for Fur Color in MiceSample Population Frequency of Alleles

allele forbrown fur

allele forblack fur


In genetic terms, evolution is any change in the relative frequency of alleles in a population.

Sources of Genetic VariationThe two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.


MutationsA mutation is any change in a sequence of DNA.Mutations occur because of mistakes in DNA replication

or as a result of radiation or chemicals in the environment.

Mutations do not always affect an organism’s phenotype.


Gene ShufflingMost heritable differences are due to gene shuffling.Crossing-over increases the number of genotypes that

can appear in offspring.Sexual reproduction produces different phenotypes, but

it does not change the relative frequency of alleles in a population.

Single-Gene and Polygenic TraitsThe number of phenotypes produced for a given trait depends on how many genes control the trait.


A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.


Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes.Height in humans is a polygenic trait.


• A bell-shaped curve is typical of polygenic traits.• A bell-shaped curve is also called normal distribution.


16-2 Evolution as Genetic Change

Natural selection affects which individuals survive and reproduce and which do not.

Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, can evolve over time.


Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.


Natural Selection on Polygenic TraitsHow does natural selection affect polygenic traits?

Natural selection can affect the distributions of phenotypes in any of three ways:

directional selectionstabilizing selectiondisruptive selection


Directional Selection When individuals at one end of the curve have higher

fitness than individuals in the middle or at the other end, directional selection takes place.


Stabilizing Selection When individuals near the center of the curve have higher

fitness than individuals at either end of the curve, stabilizing selection takes place.


Disruptive Selection When individuals at the upper and lower ends of the

curve have higher fitness than individuals near the middle, disruptive selection takes place.


Genetic DriftWhat is genetic drift?• A random change in allele frequency

Genetic drift may occur when a small group of individuals colonizes a new habitat. Individuals may carry alleles in different relative frequencies than did the larger population from which they came.


Genetic Drift


Genetic DriftDescendants

Population A Population BWhen allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect.


Evolution Versus Genetic EquilibriumThe Hardy-Weinberg principle states that allele

frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

When allele frequencies remain constant it is called genetic equilibrium.


Evolution Versus Genetic Equilibrium

Five conditions are required to maintain genetic equilibrium from generation to generation:• there must be random mating,• the population must be very large,• there can be no movement into or out of the population,• there can be no mutations, and • there can be no natural selection.


16-3 The Process of SpeciationThe Process of Speciation


16-3 The Process of Speciation

Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation.Speciation is the formation of new species.A species is a group of organisms that breed with one another and produce fertile offspring.


What factors are involved in the formation of new species?The gene pools of two populations must become separated for them to become new species.


Isolating MechanismsAs new species evolve, populations become reproductively isolated from each other.When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.


Behavioral Isolation Behavioral isolation occurs when two populations are

capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.


Geographic Isolation Geographic isolation occurs when two populations are

separated by geographic barriers such as rivers or mountains.


Temporal Isolation Temporal isolation occurs when two or more species

reproduce at different times.

Generation 1: 1.00 not resistant0.00 resistant

Resistance to antibacterial soap

How natural selection works


mutation!





Testing Natural Selection in NatureStudies showing natural selection in action involve

descendants of the finches that Darwin observed in the Galápagos Islands.

The finches Darwin saw were different, but he hypothesized that they had descended from a common ancestor.


Peter and Rosemary Grant tested Darwin’s hypothesis, which relied on two testable assumptions:

• For beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection.• Differences in beak size and shape must produce

differences in fitness, causing natural selection to occur.


When food was scarce, individuals with large beaks were more likely to survive.


Speciation in Darwin's FinchesSpeciation in the Galápagos finches occurred by:• founding of a new population• geographic isolation• changes in new population's gene pool• reproductive isolation• ecological competition


Founders Arrive A few finches—species A—travel from South America to one of the Galápagos Islands.

There, they survive and reproduce.


Geographic IsolationSome birds from species A cross to a second island.

The two populations no longer share a gene pool.


Changes in the Gene Pool

Seed sizes on the second island favor birds with large beaks.

The population on the second island evolves into population B, with larger beaks.


Reproductive IsolationIf population B birds cross back to the first island, they

will not mate with birds from population A.Populations A and B are separate species.

evolution chapter 16 honors. copyright pearson prentice hall how common is genetic variation? many...

Documents

gene poolsa population

singlegene traits

relative frequency of

genetic shuffling

different alleles

typical of polygenic

possible phenotypes

given trait