The Theory of Natural Selection

Artificial Selection Humans selected (not naturally) traits thought to be advantageous Humans created diversity choosing specific traits to breed – Dogs, Crops, Livestock Only heritable 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 survive 2) Variation: individuals of a population have differences 3) Adaptation: Some variations allow a better chance of survival 4) Descent w/ modification: Those with advantages will reproduce More of the population will have the advantage Not all these wildebeest will survive These 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

New environments are continally created. Life adapts with the changing conditions.

Changing Environments Evidence of a changing Earth – Marine fossils atop mountains – Dry valleys with ocean fossils – Rivers create canyons As environments changed… – Those with adaptation: more likely to survive & reproduce – Those without adaptation: more likely to perish

Sexual selection occurs when certain traits increase mating success. Mating can have an important effect on the evolution of a population. 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

1)What is artificial selection? 2)What is natural selection? 3)How does natural selection lead to population changes? 4)Explain why scientists believe the Earth’s environments have been changing for millions of years. 5)Why don’t populations (such as deer) continue to grow unchecked? REview