Can someone defend the theory that natural selection causes species to evolve?
Natural selection is one of the mechanisms
The main point of the argument is that populations evolve, not individuals. Evolution is a change in the gene pool of the population (change in allele proportion for example)
There are four mechanisms that contribute to evolution:
Mutation: It is a random change in a gene. Consequently, it leads to the apparition of new alleles and change the allele frequency of a population. A mutation will contribute to evolution if it brings an advantage (ex: the individuals who have it are less likely to perish or are more successful to reproduce). Over time, it will spread in the population, changing the gene pool.
Gene flow: Gene flow occurs when an individual from another population breeds with individuals from another population. This mechanisms allow the exchange of genes between two populations and is a mean to make new alleles enter a population. The more important the gene flow is between two populations, the less differences there will be between the two gene pools.
Genetic drift: It occurs when a small part of the population leaves the original one to form a new one. This small part do not have the same gene frequency as the original population and will not experience the same mutations and gene flows. Consequently, the gene pool will differ from the original one and make the group evolve in a different way.
Natural selection: This is originated by environmental pressures on the phenotype. It means that two identical populations in different environments will evolve differently (the alleles selected by the environment will differ). The natural selection will favour the combination of alleles that give a survival or reproductive advantage. Over time, the most successful alleles will spread in the population, representing the main part of the gene pool.
The most famous example of natural selection is the moth .