The Theory ofNatural Selection

Artificial Selection

• Humans created diversity choosing specific traits to breed– Dogs, Crops, Livestock

• Humans selected (not naturally) traits thought to be advantageous

• Only inheritable traits can be passed down• Importance: Shows life can change over a period of time

Dog traits have been selected by breeders for centuries to produce various breeds.

Plants have been bred to produce certain traits for human use also!

• Four factors:1) Overpopulation: more offspring are born than can survive2) Variation: individuals of a population have differences3) Adaptation: Some variations allow a better chance of

survival4) Descent w/ modification: Those with advantages will

reproduce more of the population will have the advantage

Not all these wildebeest will

surviveThese kittens

have variations

The Myth

• Natural selection can act only on traits that already exist.

• Structures take on new functions in addition to their original function.

wrist bone

five digits

Which rabbit is best adapted to this environment?

Over time, those not adapted most likely die…that’s natural selection!

Which rabbit is best adapted?

Do the dark rabbits turn white? No! They get eaten. (survival of the fittest)

Struggle for Survival

• Populations do not grow unchecked– Limiting Factors: food, water, shelter, disease, predators

• Fitness: measure of the ability to survive & produce more offspring

Natural and artificial selection are not the only factors in evolution.

• We’ve learned about natural selection and artificial selection, but what about sexual selection?– Mating can have an important effect on the evolution of a

population.

Sexual selection occurs when certain traits increase mating success.

• Sexual selection occurs due to higher cost of reproduction for females.– males produce many

sperm continuously– females are more limited

in potential offspring each cycle

• There are two types of sexual selection.

– intrasexual selection: competition among males– intersexual selection: males display certain traits

to females

Natural Selection in Action:

The Peppered Moth Analysis

Before the industrial revolution, the trees in the forest around Manchester, England were a light grey/green due to the color of lichens (fungus) on their trunks. Peppered moths, or Biston betularia, which lived in the area, were mostly lightly colored with dark spots. However, there was some color variation between the moths. The most common moth color was light with dark spots and the least common were dark with light spots. Because there were not enough resources to support the entire population, the members of this species had to compete for survival. Their coloring, when against a similar background, served as camouflage against predators such as birds and lizards, thus increasing chances of survival.

Natural Selection in Action:

The Peppered Moth Analysis

As the Industrial Revolution progressed, the trees became covered with sulfur dioxide, turning the trunks dark.