Evolution is the change in the heritable characteristics of biological populations over successive generations. It is the central unifying idea of biology. It explains both what all living things have in common and how varied they are. Life is understood to come from a single line of descent with modification. Through that history, organisms from bacteria to vertebrates share genetic machinery, while repeated splitting of lineages, adaptation and extinction have produced Earth's biodiversity. Evolutionary theory has two parts. One is the historical claim that organisms share common ancestry. The other is a set of mechanisms, chief among them natural selection, that explain how populations change.
History of the idea
People were speculating about how living forms change and where they come from as early as ancient Greece. However, Aristotle and most later naturalists treated species as fixed. In the 18th century, Carl Linnaeus organised species into a nested hierarchy. Biology today recognises that hierarchy as a reflection of shared ancestry. In the early 19th century, Jean-Baptiste Lamarck proposed one of the first comprehensive theories of species change. It rested partly on the inheritance of acquired characteristics, an idea later rejected.
The decisive shift came in 1859, when Charles Darwin published On the Origin of Species. Darwin gathered a large body of evidence that organisms descend with modification from common ancestors. He also proposed natural selection as the main mechanism. Alfred Russel Wallace reached the idea of natural selection independently, and the two men's ideas were first presented together in 1858. Darwin did not know how traits are inherited. The work of Gregor Mendel on inheritance was largely overlooked until 1900.
Between roughly 1918 and 1947, scientists combined Mendelian genetics with Darwinian selection. The result became known as the modern synthesis (also called neo-Darwinism). R. A. Fisher, J. B. S. Haldane and Sewall Wright built the mathematical framework of population genetics. Theodosius Dobzhansky, Ernst Mayr, George Gaylord Simpson and Julian Huxley then linked it to field biology, systematics and paleontology. Under the synthesis, evolution came to be defined as change in allele frequencies within populations. The discovery of the structure of DNA in 1953 and the rise of molecular biology later revealed the physical basis of heredity and variation.
Mechanisms
Evolution depends on heritable variation. Variation comes ultimately from mutation, meaning changes in the DNA sequence of a gene or in larger chromosomal structure. In sexually reproducing species, recombination shuffles that variation into new combinations. Several processes then change how common different variants are in a population:
- Natural selection. Individuals carrying certain heritable traits survive and reproduce more successfully than others in a given environment, so those traits become more common. Over many generations this produces adaptations: traits that suit organisms to their surroundings. Sexual selection, which favours traits that improve mating success, is often treated as a special case.
- Genetic drift. Random changes in allele frequencies happen because of chance in which individuals reproduce. Drift is strongest in small populations and can fix or remove variants whatever their effect.
- Gene flow. Genes move between populations through migration and interbreeding, which tends to make populations more alike.
- Mutation pressure. Recurrent mutation changes allele frequencies, though usually slowly.
The neutral theory of molecular evolution holds that much change at the DNA level is driven by drift acting on variants with little effect on fitness. Selection, by contrast, is the main explanation for adaptive traits. Among microorganisms, horizontal gene transfer between unrelated lineages is also a major source of new traits.
Speciation and macroevolution
Speciation is the process by which one lineage splits into two or more species. It typically happens when populations become reproductively isolated, for example because of geographic separation, and then diverge through selection and drift. Over long timescales, repeated speciation and extinction produce the branching pattern often called the tree of life. Large-scale change of this kind is sometimes called macroevolution, as distinct from microevolution within populations. Most biologists regard the two as the same processes working over different timescales. Evolutionary rates vary a great deal. Some lineages stay outwardly stable for long periods, and others change quickly, for instance after mass extinctions open up ecological opportunities.
History of life
Earth formed about 4.54 billion years ago. The oldest widely accepted fossils of microbial life, including stromatolites from Western Australia, are about 3.5 billion years old. Chemical traces in older rocks may point to even earlier life. All living organisms are thought to descend from a last universal common ancestor (LUCA), a single-celled organism or population. Most studies place LUCA at least 3.5 billion years ago, and some molecular-clock estimates put it around 4.2 billion years ago, although these dates are uncertain.
Major transitions followed. Oxygenic photosynthesis in cyanobacteria gradually changed the atmosphere. The complex eukaryotic cell arose, multicellularity appeared, and animals diversified rapidly in the Cambrian period. Plants and animals then colonised land. Five mass extinctions have reshaped life's history, including the one about 66 million years ago that ended the non-avian dinosaurs. Human evolution is one recent branch of this history: modern humans share a common ancestor with chimpanzees that lived several million years ago.
Evidence
Several independent lines of evidence support evolution:
- Fossils. The fossil record shows organisms changing through geological time, including transitional forms between major groups. Radiometric dating fixes the timescale.
- Comparative anatomy and development. Homologous structures, vestigial organs and shared patterns of embryonic development point to common ancestry.
- Biogeography. The distribution of species, especially on islands, matches descent from nearby ancestral populations.
- Molecular biology. All known organisms use DNA and nearly the same genetic code to build proteins. Genome comparisons yield nested patterns of similarity that agree with trees drawn from anatomy.
- Direct observation. Evolution has been seen in the wild and in the laboratory. Examples include changes in the beaks of Galápagos finches, long-term experiments with bacteria, and the spread of antibiotic resistance and pesticide resistance.
Significance and applications
Evolutionary biology shapes ecology, taxonomy, genetics and medicine. It helps explain the emergence of pathogens and guides the yearly updating of influenza vaccines. It also informs agriculture, conservation and the study of inherited disease. Phylogenetic methods developed to reconstruct evolutionary relationships are used to trace epidemics and to study the history of languages. Ideas borrowed from biological evolution have influenced fields such as computer science, where evolutionary algorithms are a technique in machine learning. In the scientific community, the fact of evolution and common descent is not in dispute. Research continues on the relative importance of different mechanisms, the origin of life, and how development, ecology and inheritance interact.
References
- Modern Synthesis - an overviewsciencedirect.com
- The Modern Synthesisphilsci-archive.pitt.edu
- Modern Synthesisencyclopedia.com
- The Modern Synthesis of Genetics and Evolution — TalkOrigins Archivetalkorigins.org
- Common Descent - an overviewsciencedirect.com
- Last universal common ancestoren.wikipedia.org
- All Life on Earth Today Descended From a Single Cell. Meet LUCA.quantamagazine.org
- History of Life on Earthnaturalhistory.si.edu
- Assessing the Earliest Evidence for Life in the Geologic and Genomic Records - PubMedpubmed.ncbi.nlm.nih.gov
- graphsearch.epfl.chgraphsearch.epfl.ch