Natural Selection

“SURVIVAL OF THE FITTEST?”

Who is the fittest Cricket?

What is it?

• Natural selection is the process by which favorable traits that are heritable become more common in successive generations of a population of reproducing organisms, and unfavorable traits that are heritable become less common.

• http://en.wikipedia.org/wiki/Natural_selection

Certain Conditions Must Exist:1. Variation in the population. (Multiple alleles for one gene, Caused

by mutation.)2. Traits must be heritable (caused by genetics not acquired traits, 3. Limited Resources (Not all individuals born will survive)4. Selective pressure:

a. Predationb. Natural disasterc. Diseases d. Food availabilitye. Competitionf. Sexual selection

5. Differential survival and reproduction. a. Individuals with certain alleles will leave more offspring than

others.• http://evolution.berkeley.edu/evosite/evo101/IIIMechanisms.shtml

Example

1. Variation in the population.

2. Selective pressure

a. Predation

3. Reduced Fewer green than Brown Beetles

4. Only Brown beetles left after

many generations.

Survival of the Fittest

• The individuals who are the best fit for the environment at the time will survive.

• If the Environment changes then the traits that are the “Most Fit” will change

• Fitness does not mean always mean the biggest or strongest:

• It can mean fewest parasites, fastest, smallest, best able to avoid predators or attract a mate.

Limits to Fitness

• Cheetahs with longer legs will run faster

• But will also have more fragile bones…

• The leg length and speed are constrained by the limits to the physical strength of the bone.

Darwin’s Finches

• Natural Selection can eventually lead to new Species.

• A Species is a group of organisms that can interbreed and produce fertile offspring under natural conditions.

Hybrids

• Example = Mule

• Horse (64 Chromosomes)

• Donkeys (62 Chromosomes)

• Mules (63 Chromosomes and can not divided evenly, Mules are sterile)

Different Beaks for Different Foods

• http://www.biology-online.org/2/11_natural_selection.htm