EVOLUTIONBy Natural Selection

What is evolution?

• changes in living

organisms over

time

• explains how • explains how

modern organisms

have descended

from ancient

organisms

Fitness and Adaptation (page 380-82)

• Organisms compete for limited resources and

some organisms are more “fit” than others

• Fitness – the ability of an individual to survive • Fitness – the ability of an individual to survive

and reproduce in its specific environment

• Adaptation – any inherited characteristic that

increases an organism’s chance of survival,

including reproduction

What determines survival?

• Traits that help individuals survive

• survive predators

• survive disease

• compete for food

compete for territory • compete for territory

• Traits that help individuals reproduce

• attracting a mate

• compete for nesting sites

• successfully raise young

Natural Selection (page 380-82)

1. There is variation in traits.For example, some beetles are green and some are brown.

2. There is differential reproduction.Since the environment can’t support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.

Natural Selection

3. There is heredity.The surviving brown beetles have brown baby beetles because this trait has a genetic basis.

4. End result: Natural SelectionThe more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.

Natural Selection• Over time, natural selection results in changes in the

inherited characteristics of a population.

• These changes increase a species’ fitness in its

environment. environment.

Sources of Genetic Variation (page 394)

• Gene flow – movement of genes from one population to another (migration)

• Mutation – any change in a sequence of DNA

• Some mutations can affect an organism’s fitness while others • Some mutations can affect an organism’s fitness while others have no effect on fitness.

• Gene Shuffling • Independent assortment

• Crossing over

• Sexual reproduction

• Sexual reproduction produces many different combinations of genes, but in itself it does not alter the relative frequencies of each type of allele in a population