Chapter 16: Darwin’s Theory of Evolution Section 16-3: Darwin Presents His Case

Darwin’s Ideas Darwin wanted to gather evidence before making his ideas public Waited 20 years after returning Published On the Origin of Species in 1858

The Struggle for Existence More individuals are produced than survive Members of a population compete for food, living space, other limited necessities Malthus

Variation and Adaptation Individuals have natural variation in heritable traits, some better suited for the environment Any heritable characteristic that increases an organism’s ability to survive and reproduce in its environment is an adaptation Adaptations can be morphological, physiological, behavioral

Survival of the Fittest An organism’s fitness as how well it can survive and reproduce in its environment Adaptations increase fitness Differences in fitness (survival rate and reproduction) called survival of the fittest Survival = passing adaptations to next generation

Natural Selection The process by which organisms with variations most suited to their local environment survive and leave more offspring Mechanism of evolution Environment influences fitness Populations continue to change as they become better adapted and their environment changes Acts only on inherited traits

Natural Selection

Natural Selection

Natural Selection

Natural Selection

Natural Selection Does not make organisms “better” Does not move in a fixed direction Extinction can occur if conditions change faster than species can adapt

Common Descent Over many generations adaptation caused successful species to evolve into new species Descent with modification from common ancestors Life has been on Earth a long time Evidence in fossil record Explains diversity of life