Darwin and Natural Selection

•Intro: Why does evolution matter now? http://www.pbs.org/wgbh/evolution/educators/teachstuds/svideos.html

Charles Darwin (1809-1882)

• Ideas supported by fossilEvidence

• On HMS Beagle hestudied and collectedbiological specimens Inthe Galapagos Islands

• Noticed that specieswere unique to theislands they were from,but similar to species onMainland

So what did Darwin say?

1. In nature, organisms produce more offspring than can survive

Example:

Fishes

2. Variation exists in all populations

Example:

fish differ in

color,

size and

speed

3. Individuals with useful variations survive and pass those traits (alleles) on to their offspring

“Survival of the Fittest”• Any useful variation that helps an organism to

survive and reproduce is called an adaptation.

Example: fast fish escape predators, survive and produce more fast fish.

4. Over time, organisms with certain variations become the majority and the population may look very different from the ancestral population

For Example, the Galapagos Finches studied by Darwin

What did he call his theory?

• Natural Selection

• What does it say?

• The environment acts on phenotypes (variations) in a population, causing genetic change over time.

Self Check Quiz

1. What type of evidence did Darwin use to support his Theories of Evolution?

Fossil Evidence

2. Why is variation important to the survival of a population?

Useful variations (adaptaions) survive and the traits (alleles) get passed on to their offspring

3. Natural selection acts on the ___________ of populations.

Phenotypes (variations)

4. What does “Survival of the Fittest” mean?

Organisms with the most useful traits for reproduction and survival are able to be successful and pass their traits on to their offspring.

There are three types of Natural Selection:

1. Stabilizing Selection

2. Directional Selection

3. Disruptive Selection

Stabilizing Selection

Example of Stabilizing Selection: Spiders

• Large spiders are easily seen and eaten by birds• Small spiders can’t compete for food• Medium size spiders are selected for

Directional Selection

Example of Directional Selection: Woodpeckers

• A type of insect livesdeep within the bark oftrees

• Woodpeckers with shortor average-size beakscan’t get to these insects

• Long-beakedwoodpeckers areselected for

Disruptive Selection

Example of Disruptive Selection: Limpets

• Limpets are snails

that live attached to

rocks in the tidepools• Limpets range in

color from white to tan

to dark brown• White colored limpets

have the advantage on

light-colored rocks• Dark brown limpets

have the advantage on

dark-colored rocks

How is Natural Selection related to Evolution?

• It is the mechanisms by which evolution occurs

Do the following:

For each of the following examples, tell me which type of selection it would be analogous to.

• My uncle Ted has a real sweet tooth. He will only eat the sweetest of candies.

• On scantron forms, some types of lead are too soft while others are too hard.

• In a basketball obstacle course game, really large people are good at making baskets and really small people are best at getting through the obstacle course, so teams are usually a mix of very big people with very small people.