natural selection 2-12

Darwin’s Theory of Natural Selection:A Mechanism For Evolution

Be careful not to suggest…

Organisms can acquire/change characteristics during their lifetime based on NEED

These characteristics can only be passed on genetically.

Example: giraffes needed longer necks so they stretched them!

Charles Darwin

1809 -1882

“ I have called this principle, by which

each slight variation, if useful, is preserved,

by the term Natural Selection.”

Important Features of Darwin’s

Theory of Natural Selection:A Mechanism for Evolution

1. Organisms produce more OFFSPRING than can possibly SURVIVE.

Thomas Malthus1766-1834

What "struck" Darwin in Essay on the Principle of Population (1798) was Malthus's observation that in

nature plants and animals produce far more offspring than can survive, and that Man too is capable of overproducing if left unchecked. Malthus concluded

that unless family size was regulated, man's misery of famine would become globally epidemic and eventually consume Man.

REMEMBER…

Overproduction (of Offspring)

2. As populations grow in size,

become limited. This forces organisms to

NATURAL RESOURCES

COMPETE…

What do lions & hyenas compete for? Food Water Shelter – bear & raise young Space – hunt, travel

What else to lions compete for with other lions? Mates Do all lions mate?

REMEMBER…

Competition

3. Because organisms must COMPETE for limited resources, they STRUGGLE to EXIST. If they do not obtain needed resources, they ____.

VARIATIONS exist in all populations of organisms.

The 2 causes of these differences can be attributed to:–Mutation–Meiosis (crossing-over, unique

combination of chromosomes in egg/sperm)

Because organisms are built somewhat differently from others in the same population, some have an ADVANTAGE

What determines which variety of finch has a beak that will help it survive more successfully???

Adaptations make an organism FIT for the environment.

Fitness can be described as the ability to survive (out compete) and reproduce.

Why not just survive?

REMEMBER…

Survival Of TheFittest

Environment creates challenges & opportunities Selective Pressures are applied to

each population of organisms making it difficult for them to survive.

REMEMBER…

The environment applies a

Selective pressure

5. Organisms that survive can pass on their traits to their offspring. They are able to

REPRODUCE.

WHY?

Remember:

Reproductive Advantage

How much change?

Populations of organisms may change but not evolve into a new species.

Evolution does not mean that new species must arise, just that they can if enough changes take place.

REMEMBER…

Darwin referred to this as:

Descent with

Modification

6. Over time,hundreds or thousands of

generations, the characteristics (VARIATIONS)

of a population change.

If members of the same species can no longer reproduce successfully

(their offspring can reproduce), they are considered to be two

distinct species.

REMEMBER…

Speciation -Over time, members of a species may become SO different that they are now 2 different species

How does evolution really work?

Overview

1. Overproduction of offspring2. Competition for limited resources3. Variations exist which are

inherited from their parents.4. Those more fit for the

environment have a selective advantage

5. Reproductive advantage6. Population characteristics may

change