Evolution is the idea that all existing animals and plants are descended from some one ancestor many millions of years ago or at least a small number of ancestors many millions of years ago. It is “descent with modification” according to Charles Darwin. In a modern view, evolution is change in genetic frequencies between generations.
Two major theories to explain the evolution process: Darwin’s natural selection theory and Sewell Wright’s synthetic theory of evolution. The key point about natural selection theory is: more individuals are produced each generation that can survive; phenotypic variation exists among individuals and the variation is heritable; individuals with heritable traits better suited to the environment will survive; when reproductive isolation occurs new species will form. The key points about synthetic theory are: a species evolves when gene frequencies changes and the species moves it to a higher level of adaptation for a specific ecological niche; mutation of alleles and migration of individuals with those new alleles will create variation in the population; selection will then chose the better adapted individuals and the population will have evolved. This theory attempts to explain evolution in terms of changes in gene frequencies.
Natural selection is the process by which the organisms with the best or most favorable genetic adaptations out-compete other organisms in a population, tending to displace the less-adapted organisms. If the fitness and selection coeeficient is known, the allele frequency after selection can be calculated. If a heterozygote has higher fitness than the homozygotes, both alleles are maintained in the population because both are favored by the heterozygote genotype.
Species and Speciation
Speciation is the rise of new species. There are four modes of speciation: allopatric, perrpatric, parapatric and sympatric; two major causes: geographic separation and reduced gene flow. Two theories are used to explain how speciation occur: gradualism and punctuated equilibrium.
Molecular Evolution and phylogenetic tree
Evolution can always dated to nucleotide sequence change in DNA. Mitochodnria and chloroplast evolve faster than nuclear genome. Evolution can be described by phylogenetic trees which depict the genetic distance and relationships among the species.