Natural Selection

The Process of NaturalSelection

In any population of individuals, there is geneticvariation.• Genetic variation is the result of mutation and

recombination.• In a given environment, some variations of

traits are lethal, some make no difference,and some increase or decrease the chancefor survival.

If the environment changes, some individualsmight have one or more variations thatincrease their chance for survival.

Organisms with the variation(s) are morelikely to survive, reproduce, and pass thefavorable variation on to their offspring.

Over time, the population changes as moreindividuals within it have the selected trait.

This is called natural selection because itoccurs through natural processes.

The Factors Involved in the Process of Natural Selection

1. Genetic variation from mutation andrecombination of genes (made up of DNA)during meiosis.

2. Time3. Selection, based on the environment, of

individuals with the trait that allows forincreased survival in the environment.

4. Those with the selected trait reproduce andpass on the trait that allows for survival ofthe offspring.

Darwinian Evolution(Natural Selection)

Giraffes with longer necks tend to reach leavesmore easily. Longer-necked giraffes are morelikely to eat enough to survive and reproduce. Theoffspring inherit their parents’ longer necks.

A Phrase to Summarize

Darwin’s theory of natural selection canbe summarized as:

Common ancestry anddescent with modification.

Is Evolution by NaturalSelection Random?

Overall, no.Evolution requires~Variation due to mutation and recombination

AND~Selection of specific variations.The effect of natural selection on survival oforganisms is NOT random.However, some of the events that may lead tomutation can be random:– Mutations can occur in any part of a gene.– Whether a mutation occurs at all is random.

An Example of Mutation andNatural Selection

View the following segment on sicklecell disease and malaria:

http://www.teachersdomain.org/resource/tdc02.sci.life.gen.mutationstory/

In your science notebook:

Explain why individuals who carry one copy ofthe mutated allele for abnormal hemoglobinare selected for in the natural selection ofsome human populations.

Explain why genetic variation is important inhuman populations that are exposed to newdiseases, which are a type of change in theenvironment.

More on Mutation

Remember from genetics that humans havetwo copies of every gene.Sickle Cell Disease:Two copies of the mutated disease gene(homozygous state) of sickle cell disease canbe fatal.However, people with one copy of themutated disease gene (heterozygous) havean advantage in areas where there is malaria.The heterozygous condition is selected for.