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Genetic variation among a population of a species naturally occurs. It can be caused by: - recombination - crossing over - mutations REMEMBER: only mutations in gamete cells can be passed on to offspring Variation- difference in physical, behavioral, or chemical feature of an organism within a population. Some mutations will help organisms survive and reproduce more offspring. This will help them survive as a species. A population changes as a result of differential reproduction. This means some individuals will produce more offspring then others. Over many generations, species develop adaptations that help the species survive. As the environment changes, those organisms better adapted to the environment will survive and reproduce. - they had a trait that helped them survive -because they were able to reproduce they pass those traits on to their offspring Adaptation- is a feature that is common in a population because it has improved their ability to survive and reproduce For example, a tiger's striped fur gives it camouflage in its shadowy forest habitat. Being hard to see helps make the tiger a successful hunter. Darwin proposed that natural selection changes in species over time. can lead to What is natural selection? The process by which individuals with favorable variations survive and reproduce more successfully than those with less favorable traits They are naturally selected to survive. Populations change over time. Evolution - change in the characteristics of a populations over time (over many generations) Evolution will happen if: -their is potential for a population to increase in numbers (grow) -there is genetic variation - there is a finite amount of resources required for life (there is competition for resources) - selection by the environment of the offspring that are better able to survive These are things that make it so that some individuals are more likely to survive then others. What is an anatomical similarity? Alligators and crocodiles are related. If they are related they share a common ancestor. The more related they are the more similar features they will have. A phylogenetic tree shows the relationships among organisms. The closer organisms are on a branch, the more closely related they are. All organisms on the tree share a common ancestor. By using the tree, we can see that mammal are more closely related to birds and reptiles than they are to mollusks. Homologous structuressimilar structures that originated in a shared ancestor, but may be used for a different function. Example: Bat wing and human arm. Analogous Structures When organisms have similar structures because they have a similar function. - they have originated somewhere different on the evolutionary chain - they are the result of adapting to the environment in the same way Example: honeybee and hummingbird wing Gel electrophoresis can be used to compare DNA.