Download Natural Selection

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. Overproduction
Organisms 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 Survive
Organisms 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 Modification
Organisms 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