Scientist typically look at multiple source to see if evolution in a population has occurred. These sources include:
2. Comparative anatomy - Look at homologous structures, vestigial and analogous structures
3. Comparative embryology
4. Biogeography (species distribution)
5. Molecular biology - DNA
6. Direct observation
Homologous structures are parts of different organisms that are similar because they come from a common ancestor but have evolved differently (e.g. arms of a human and cat). Vestigial structures are body parts that do not have a function but at one time most likely did (e.g. human intestines). Analogous structures are are parts of different organisms that are similar but did not come from a common ancestor (e.g. wings of the bird and butterfly).
Jean-Baptise Lamarck was one of the first people to investigate the idea of evolution. However, he reasoned that evolution occurred through use and disuse and the inheritance of acquired characteristics. He claimed that a body part could develop if it was used more and that these developed parts would be passed on. While it is true that is a body part it used more it can become bigger (muscle) this is not passed onto offspring because it is not part of the genetic material.
Darwin said that natural selection is the cause of evolution. There is variation in all populations (usually the result of mutations); some traits increase ones fitness (ability to survive and reproduce) and are considered an adaptation. Organisms with these traits will survive in the environment better and be able to reproduce and pass on these traits; therefore, they are selected for from the environment.
Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print. PACK, P. (2013). AP BIOLOGY. (4TH ED., PP. 73-81). BOSTON: HOUGHTON MIFFLIN HARCOURT.
Stabilizing selection is when the individual with the most common trait is selected for and the frequency of the ones with the extreme traits decreases (e.g. birth weight of humans). Directional selection is when one of the extremes is selected for and the opposite extreme is selected against (e.g. peppered moth). Disruptive selection is when extremes are selected for the common is not.
Sexual selection occurs typically when a male has a trait that increases their chance of mating. This can be a trait or behavior that the female selects for (e.g. size and color of a peacock tail or courting rituals) or through male competition (e.g. elks fighting with antlers). This type of selection often results in sexual dimorphism which is when there are distinct difference between males and females. This is a type of disruptive selection.
Artificial selection is when humans choice to breed for specific traits. This is a type of directional selection.
Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print. PACK, P. (2013). AP BIOLOGY. (4TH ED., PP. 73-81). BOSTON: HOUGHTON MIFFLIN HARCOURT., N.d. Photograph. Biology NotebookWeb. 22 Feb 2014. .