Natural selection works on populations of organisms over time to produce changes in genetic composition that make successive generations better adapted to the environment, thereby producing evolutionary change. Variation within a species is another mechanism affecting evolution. The variation in individuals promotes adaptive evolutionary changes. For example, within a particular plant species, those plants that are better able to tolerate less rainfall will survive and continue to reproduce if drought conditions occur, propagating drought-resistant qualities in forthcoming generations.
Because all living things pass on genetic material, the understanding of genetics is an important quantitative aspect of biology. The study of population genetics depends on understanding and interpreting the Hardy-Weinberg Equilibrium, which scientists use to determine if gene frequencies have changed in the population. The Hardy-Weinberg theory dispels the misconception that the dominant allele, or expression of a gene, will always have the highest population frequency, and the recessive allele the lowest. Gene frequencies often change based on the environment. Population genetics shows whether gene frequencies have changed, and thus if evolution has occurred, according to Judith Stanhope of the Woodrow Wilson Biology Institute.
Cells are common to all living things and are classified as either prokaryotes or eukaryotes. Prokaryotes are unicellular organisms that consist of a cell wall, cell membrane, DNA, cytoplasm and ribosomes. The cytoplasm is a liquid substance enclosed by the cell membrane; the DNA (deoxynucleic acid) contains the genetic material; ribosomes contain RNA (ribonucleic acid) and are important for protein synthesis; and the cell wall, which is absent in animal cells, maintains the structural integrity of the cell. Eukaryotes have membrane-bound organelles such as the nucleus, containing the genetic material, and the mitochondria, which are important for metabolism. Prokaryotes include bacteria and blue-green algae, while eukaryotes include fungi, some types of plants and all animals.
Evidence that biology dictates behavior is seen in behaviors that are species-specific. According to the Oak Ridge National Laboratory, the chickadee exhibits different feeding characteristics than do finches, with chickadees carrying one sunflower seed at a time to eat at a branch, while finches eat large amounts of seeds at the feeder. Specific behaviors can also be reproduced in successive generations; for example, the herding instinct of border collies or the speed of racehorses. Behaviors also change in response to biological alterations such as brain injuries or drug abuse.