Generally, evolution is any process of change over time. In the context of life science, evolution is a change in the traits of living organisms over generations, including the emergence of new species. Since the development of modern genetics in the 1940s, evolution has been defined more specifically as a change in the frequency of alleles in a population from one generation to the next. Darwin's theory of evolution describes the descent of all living organisms from a common ancestor. Natural Selection is the principal mechanism that causes evolution. In common parlance the word "evolution" is often used as a shorthand for both the modern theory that all extant species share a common ancestor as well as the mechanisms through which natural selection acts to change populations over time. This theory was updated in the twentieth century through the incorporation of Gregor Mendel's theory of inherited characteristics, now called genes. The combination of Darwin's theory of natural selection and Mendel's theory of genetics is called the modern synthesis. In the modern synthesis, "evolution" means a change in the frequency of an allele within a gene pool. This change may be caused by a number of different mechanisms: natural selection, genetic drift or changes in population structure (gene flow). |
The theory underlying the modern synthesis has three major aspects: 1. The common descent of all organisms from a single ancestor. 2. The origin of novel traits in a lineage.
3. The mechanisms that cause some traits to persist while others perish. The modern synthesis, like its Mendelian and Darwinian antecedents, is a scientific theory. In plain English, people use the word "theory" to signify "conjecture", "speculation", or "opinion". In contrast, a scientific theory is a model of the world (or some portion of it) from which falsifiable hypotheses can be generated and be verified through empirical observation. In this sense, "theory" and "fact" do not stand in opposition, but rather exist in a reciprocal relationship. Currently, the modern synthesis is the most powerful theory explaining variation and speciation, and for use in the science of biology, it has replaced other explanations for the origin of species, including creationism and Lamarckism. Ancestry of organisms
A phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains bacteria, archaea, and eukaryotes as described initially by Carl Woese. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, presumably owing to rapid rRNA evolution. The exact relationships of the three domains are still being debated.| |
A group of organisms is said to have common descent if they have a common ancestor. In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool. Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds — even those which do not fly — have wings. Today, the theory of evolution has been strongly confirmed by the science of DNA genetics. For example, every living thing makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. Because the selection of these traits is somewhat arbitrary, their universality strongly suggests common ancestry. In addition, abiogenesis — the generation of life from non-living matter — has never been observed, indicating that the origin of life from non-life is either extremely rare or only...