In the 19th Century, the naturalist, Charles Darwin proposed that all species have evolved from common ancestors through the use of natural selection. Scientists recognize natural selection as one of the four forces of nature. The other three include genetic drift and gene flow, as well as mutations in DNA.

Evolution can occur on a small or large scale. Microevolution occurs among a species or population. However, macroevolution affects numerous populations, ecosystems, and may even involve mass extinctions. Species refers to one type of organism. It may be a plant or an animal; but all members of the species are similar to each other. Members of a species that live within a certain geographical area are called a population. All humans represent one species. However, everyone who lives in Hawaii is one population. Speciation occurs when a new species develops. Over time, humans evolved from a number of similar species. Our species, Homo sapiens, evolved from a number of ancestors.

This evolution occurs through a number of forces. A gene is a hereditary unit of information located within DNA. DNA may change. When it replicates, errors may occur in the coding of the new DNA strand. Chemicals and other mutagens may also cause changes. These changes are called mutations. A mutation may be a point mutation, affecting only one nucleotide pair. It may also affect an entire area of the DNA. Mutations create diversity within the genetic pool. Diversity is crucial to the gene pool and evolution. Genetic drift allows genes to combine at random chance, further increasing diversity. Parents do not control what genes are passed to their offspring. Chance controls this. The genes may benefit the offspring; they may also lead to the death of the offspring. Parents cannot control this.

Another way to increase diversity in species is through gene flow. It is not always good for populations to completely isolate themselves. An isolated population cannot interbreed with nearby populations. The transfer of genetic material through different populations is called gene flow. When populations are isolated, speciation often occurs. This tends to happen in insular habitats. This may create a different type of diversity in the ecosystem: a new species. However, it does not strengthen the gene pool of the original species. Different species do not interbreed and share their genetic material for a number of reasons. These reasons are called isolating mechanisms. Species are essentially isolated from each other. They don’t share the same environment. They don’t share the same circadian rhythms. They don’t share the same estrous cycles for mammals. They don’t want to reproduce with each other. They prefer to isolate amongst their own species. If they do attempt to breed, it rarely is successfully. Different species may not be able to transfer genetic material between each other. The material may not correctly fuse. Even in the event that it does form a zygote, it may not carry to term. There are cases of hybrids; however hybrids are usually sterile.

Natural selection refers to how species evolve and die over generations due to the effects of these forces. The effects of these forces may be beneficial or harmful to a species. If the effects benefit the species and increase its chances of survival, it flourishes. If it weakens the species over time, the species will die out or become extinct. Natural selection is a nonrandom force of nature; it is the systematic weaning of weaker life forces by the evolutionary process. It also allows stronger forms of life to flourish and advance their genetic material. Through this advancement, new species may further develop to replace the extinct species.

Nature has been often called cruel because of natural selection. Nature is not cruel; it is not kind. Nature does not care what species live and what species die. Nature merely exists and allows those best suited to exist along with it.