Among the evidences that evolutionists use to document the "pattern of evolution" are the fossil record, the distribution patterns of existing species, methods of dating fossils, and comparison of homologous structures.(See evidences of evolution below.) Main articles: Darwinism and Natural selection The second major evolutionary theory is the "theory of modification through natural selection," also known as the "theory of natural selection." This is a dynamic theory that involves mechanisms and causal relationships.The theory of natural selection is one explanation offered for how evolution might have occurred; in other words, the "process" by which evolution took place to arrive at the pattern.

Darwin and Wallace proposed that evolution occurs because a heritable trait that increases an individual's chance of successfully reproducing will become more common, by inheritance, from one generation to the next, and likewise a heritable trait that decreases an individual's chance of reproducing will become rarer.

In the 1930s, scientists combined Darwinian natural selection with the re-discovered theory of Mendelian heredity to create the modern synthesis, which is the prevailing paradigm of evolutionary theory.

As broadly and commonly defined in the scientific community, the term evolution connotes heritable changes in populations of organisms over time, or changes in the frequencies of alleles over time.

A popular definition along these lines is that offered by Douglas J.

Futuyma (1986) in Evolutionary Biology: "Biological evolution…is change in the properties of populations of organisms that transcend the lifetime of a single individual….

The changes in populations that are considered evolutionary are those that are inheritable via the genetic material from one generation to another." In this sense, the term does not specify any overall pattern of change through the ages, nor the process whereby change occurs (although the term is also employed in such a manner).

However, there are two very important and popular evolutionary theories that address the pattern and process of evolution: "theory of descent with modification" and "theory of natural selection," respectively, as well as other concepts in evolutionary theory that deal with speciation and the rate of evolution.

The "theory of descent with modification" is the major kinematic theory that deals with the pattern of evolution—that is, it treats non-causal relations between ancestral and descendant species, orders, phyla, and so forth.

The theory of descent with modification, also called the "theory of common descent," essentially postulates that all organisms have descended from common ancestors by a continuous process of branching.

In other words, narrowly defined, all life evolved from one kind of organism or from a few simple kinds, and each species arose in a single geographic location from another species that preceded it in time. In the broadest sense of the terminology, the theory of descent with modification simply states that more recent forms result from modification of earlier forms.

One of the major contributions of Charles Darwin was to marshal substantial evidence for the theory of descent with modification, particularly in his book, Origin of Species.