Evolution Chapter 16 regents. Copyright Pearson Prentice Hall How Common Is Genetic Variation? Many...

Evolution

Chapter 16regents

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.


In genetic terms, evolution is any change in the occurence 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.


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 occurences and thus to evolution.


Genetic DriftWhat is genetic drift?• A random change in occurrence

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 the occurrence of a trait changes due to migration of a small subgroup of a population it is known as the founder effect.


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.

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.


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 regents. Copyright Pearson Prentice Hall How Common Is Genetic Variation? Many...

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