He recognized that because more individuals are produced than can survive to reproductive age, the members of a population must compete for the resources in an environment. He also concluded that because the more adaptive traits promote survival within a population, they must increase in frequency over generations, and less adaptive traits must decrease in frequency or disappear. He wrapped all of his findings under the umbrella of natural selection. He believed a population can evolve by natural selection. Natural selection is the theory that only the fittest of the population will survive. Darwin's theory of evolution based on natural selection was ultimately attacked by the church but was widely accepted by society when it was discussed openly by him and Wallace. .
Many mechanisms lead to evolutionary change over time. Darwin believed that an individual doesn't evolve; populations do. Microevolution is the changes in organisms to adapt to their habitats and their changing environment. To understand the big picture of evolution, we must begin with the variation of traits in a population's individuals. Variations of traits exist on alleles of an individual. There are many different genes in a population that are available to be passed down to offspring. This gene pool of a population's available genes usually contains genes existing on alleles. Alleles are the alternative molecular forms of the same gene. Offspring from this gene pool inherit different forms and combinations of alleles which contribute to their phenotype and what is passed on to other generations. The mixing of alleles depend on five factors: gene mutation, crossing over, independent assortment, random fertilization, and changes in chromosome structure or number. Of these factors only mutations create alleles; the other four factors create new combinations. The environment around a population also contributes to change of alleles resulting in evolutionary change.
