NATURAL SELECTION Pages 300-301

ADAPTATIONS

•A heredity characteristic that helps an organism survive and reproduce in its environment

FITNESS

•Measure of ability to survive and produce more offspring relative to other members of a population

PRACTICE

Example: Duck A has 15 offspring whereas duck B has 6 offspring. Which bird has more fitness?

HOW DID ORGANISMS BECOME SO WELL ADAPTED TO THEIR ENVIRONMENT?

DARWIN’S EXPLANATION

•Darwin proposed that animals evolved to fit their environments

EVOLUTION

•The change in a population of organisms over time

DARWIN’S EXPLANATION

•Darwin realized that animals have many offspring and some don’t survive

•The survivors are better suited to their environment

•He called this Natural Selection

NATURAL SELECTION

•Mechanism by which evolution occurs

•The environment is the selective agent

STEPS OF NATURAL SELECTION

1. OverproductionOrganisms have many offspring, but not all will survive

STEPS OF NATURAL SELECTION

2. Genetic Variation Individuals vary within a population

New traits appear due to genetic mutations

STEPS OF NATURAL SELECTION

3. Struggle to SurviveOrganisms compete for resources (food, water, shelter, and mates)

Organisms have to get away from predators

STEPS OF NATURAL SELECTION

3. Struggle to Survive (continued)Some adaptations allow organisms to survive at a higher rate and individuals are “naturally selected” to survive and produce offspring

STEPS OF NATURAL SELECTION

4. Descent with ModificationOrganisms with the best adaptations will survive and reproduce passing these beneficial traits down to their offspring

IMPORTANT!

•Natural Selection acts on PHENOTYPES not genotypes

•As environments change, different traits will become beneficial

EXAMPLE

Explain how the steps of natural selection will change a population of rabbits over time

1. OVERPRODUCTION

•Many rabbits are born, not all will survive

2. GENETIC VARIATION

•Some rabbits hop slowly, some at a medium, speed and others hop very fast

3. STRUGGLE TO SURVIVE

•There are lynxes in this environment that eat rabbits

•The rabbits that hop slowly are unable to get away from the lynxes

4. DESCENT WITH MODIFICATION

•The rabbits that are able to hop away from the lynx are able to reproduce and pass on the trait for fast hopping