Evolution involves both macroevolution and microevolution. Macroevolution refers to evolution that occurs on a large scale. It is also referred to as vertical evolution. Microevolution refers to the small changes that occur within a small pool of species. This is also called horizontal evolution. Species refers to a group of animals that have the ability to share genes with each other. The offspring of the animals must also be fertile. Some species have been cross-bred; however, their offspring are sterile and unable to reproduce. If the species all live within the same area, they are called a population. A population may cover a large space, or a relatively small one. Larger populations have greater genetic diversity. Smaller populations tend to have a limited gene pool, which often leads to genetic problems. This is seen in humans as a species. Groups that tend to isolate themselves and have a limited genetic pool often have significant genetic problems as a result. One example of this is the Amish, a religious group that is isolated from other groups. The lack of new genetic material into their population allows harmful mutations to appear multiple times. The Amish are known for several genetic conditions. They are, of course, not the only group that is isolated.

Evolution tends to occur by four different forces. If it is a random chance by which an offspring gets a gene, it is referred to as genetic drift. It is entirely up to chance as to whether an offspring will inherit gene x or gene y. This does make a longterm difference in the chances of survival. All genetic changes are not random though. However, gene flow refers to new genes being introduced into a population due to migration. If an animal enters a new population, it brings new genes with it. This tends to strengthen the genetic diversity of a population. This also changes the possible genetic combinations that may develop. Regardless of migration, though, there are some changes in the genetic makeup of populations. Another way by which the combinations are changes is through mutation. This is a spontaneous change of genetic material. All of these forces work together to cause natural selection. Natural selection allows some species to die off while others live (Berkeley University, n.d.).

As a result of these mechanisms, there is variation between species within a population and between different populations. If enough variation occurs between the populations, a new species may develop. This is called speciation. This normally develops as a result of isolating mechanisms between species. An isolating mechanism refers to something the keeps members of a species from reproducing and sharing genetic material. Because of isolating mechanisms, a wide variety of changes may result in the genetic makeup of a population. A common isolating mechanism is geography. Species from one population tend to be isolated from species in another population. A population which started as one species may be isolated from the rest of the species. This may occur after the first population migrated to an area where they then became isolated. As a result, they cannot share genetic material with the population where they originated. Any mutations or genetic changes in each population are no longer shared with the other species. If this happens over a long enough period of time, a new species may develop. This commonly occurs on islands, which accounts for the wide range of different species on an island, such as where Darwin did his research. As islands broke away from main lands with changes in the continents, they often isolated a number of populations that then changed over the centuries. This is one reason why islands are known for their distinct plant and animal life. (University of Wisconsin, 2003).