Lesson Overview16.3 & 16.4 Darwin Presents

His Case

Conditions under which Natural Selection Occurs

1. Struggle for Existence (After reading Malthus)a. More individuals are produced than can survive, so there

is a “struggle” and they must COMPETE for resources (food, living space, etc.)

2. Variation and Adaptationa. Individual variation occurs among populations and some

are better suited to life in their environment than others.

b. Any “heritable” characteristic that increases an organisms ability to survive is called an “adaptation”. (can be structural or behavioral)

3. Survival of the Fittesta. “Fitness” is how well an organism can survive and

reproduce in its environment.

• Those with a “high fitness” can survive and reproduce

• Those with a “low fitness” are not well suited to their environment and most likely will die without reproducing.

Natural Selection

1. Process by which organisms with variations suited to their local environment survive and leave more offspring.

2. It occurs when:

a. More individuals are born that can survive (struggle for existence)

b. There is “natural heritable variation” (variation & adaption)

c. There is variable “fitness” (survival of the fittest)

Common Descent

Darwin proposed that, over many

generations, adaptation could cause

successful species to evolve into new

species.

He also proposed that living

species are descended, with

modification, from common

ancestors—an idea called

descent with modification.

According to the principle of

common descent, all

species—living and extinct—

are descended from ancient

common ancestors.

A single “tree of life” links all living

things.

Evidence of Evolution

1. Biogeography: where organisms live now and where their ancestors lived in the past.

a. Closely related, but different: closely related species differentiate due to slightly different climates.

b. Distantly related, but similar: distantly related species develop similarities based on similar environments.

Evidence of Evolution

2. The Age of the Earth and Fossils

a. Technology: radioactive dating to accurately determine the age of certain rocks and fossils.

b. The discovery of more intermediate stage fossils.

Evidence of Evolution

3. Comparing Anatomy and Embryologya. Homologous structures: animals with

similar structures evolved from a common ancestor with a “basic” version of that structure. (Descent w/Modification)

i. BUT, don’t confuse common structure with “common function”. These would be considered “analogous structures”, NOThomologous structures.

ii. ALSO, not all homologous structures have important functions. Some are considered “vestigial structures”, where they were inherited, but have lost most if not all of their original function.

Evidence of Evolution

3. Comparing Anatomy and Embryology (Cont.)

b. Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor.

Evidence of Evolution

4. Genetics and Molecular Biology

a. Life’s Common Genetic Code: All living organisms use DNA and RNA to carry information from one generation to the next and to direct protein synthesis.