Evolutionary Biology Terms Starting With N
Evolutionary Biology Glossary: N
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Neo-Darwinism
/ NEE-oh DAR-win-iz-um / · Greek neos meaning new and Darwinism named after Charles Darwin
Neo-Darwinism is the modern evolutionary synthesis that unifies Darwinian natural selection with Mendelian genetics and population genetics to explain how heritable variation arises and how it changes in frequency over time.
The modern synthesis took shape primarily during the 1930s and 1940s through the work of Ronald Fisher, J.B.S. Haldane, Sewall Wright, Theodosius Dobzhansky, Ernst Mayr, and George Gaylord Simpson, who showed mathematically and empirically that Mendelian inheritance is fully compatible with gradual Darwinian change. Before the synthesis, some biologists believed that large discrete mutations, not gradual selection, drove evolution, and the two camps were largely at odds.
Population genetics provided the mathematical framework showing how mutation, selection, drift, and gene flow alter allele frequencies across generations. Later molecular discoveries, including the structure of DNA in 1953 and the neutral theory proposed by Motoo Kimura in 1968, expanded the synthesis without overturning its core principles.
Theodosius Dobzhansky's 1937 book "Genetics and the Origin of Species" is widely credited with bridging laboratory genetics and field naturalism, making it one of the founding texts of the modern synthesis. Dobzhansky's work on Drosophila pseudoobscura showed that wild populations harbored far more genetic variation than earlier models had assumed.
How To Become Field Biologist? →Darwin already understood how traits were inherited. Darwin had no knowledge of genes or chromosomes; the modern synthesis, built decades after his death, was necessary to explain the mechanism of inheritance that his theory required but could not supply.
Studies of medium ground finches (Geospiza fortis) on Daphne Major island in the Galapagos by Peter and Rosemary Grant tracked beak depth across generations from 1973 onward. During the 1977 drought, average beak depth increased by roughly 0.5 millimeters in a single generation as selection favored birds able to crack larger seeds, providing a direct field measurement of selection acting on heritable variation.
Evolutionary Biology →Neutral Theory
/ NOO-trul THEE-uh-ree / · Latin neutralis meaning neither one nor the other
Neutral theory of molecular evolution is the hypothesis, proposed by Motoo Kimura in 1968, that most genetic variation at the molecular level is selectively neutral and spreads through populations by random genetic drift rather than natural selection.
Kimura argued that the high rate of molecular substitution observed in proteins was too fast to be explained by positive selection without imposing an unsustainable genetic load on populations. Genetic drift can fix neutral alleles at a rate equal to the mutation rate, independent of population size, a prediction that distinguishes neutral theory from selectionist models. Synonymous substitutions, which change a DNA codon without altering the encoded amino acid, accumulate roughly 5 to 10 times faster than nonsynonymous substitutions, consistent with weaker selective constraint on silent sites.
Pseudogenes, which have lost their original function, evolve at rates approaching the neutral expectation because selection no longer acts on their sequences. The nearly neutral theory, developed by Tomoko Ohta in the 1970s, extended Kimura’s framework by showing that slightly deleterious mutations also behave as effectively neutral in small populations.
Kimura's neutral theory provided the theoretical foundation for the molecular clock. Neutral mutations accumulate at a predictable tempo tied to the mutation rate, producing a roughly constant substitution rate over time, the core property that allows researchers to date evolutionary divergence events from DNA sequence differences.
Every DNA difference between individuals is either beneficial or harmful. The vast majority of mutations at the molecular level have no detectable effect on organismal fitness under most environmental conditions, and population genetic analyses consistently find that large fractions of standing genetic variation are selectively neutral.
The alcohol dehydrogenase gene (Adh) in Drosophila melanogaster has been a key test case for neutral theory. Synonymous sites in Adh accumulate substitutions at rates several times higher than nonsynonymous sites, a pattern consistent with neutral evolution at silent positions and purifying selection on amino acid-changing positions.
Building Blocks of Proteins →Niche Construction
/ NICH kun-STRUK-shun / · From French nicher meaning to nest, and Latin constructio meaning a putting together
Niche construction is the process by which organisms modify their own environment or that of other species, generating selective pressures that feed back to influence the evolution of the constructing organism and others in the community.
Niche construction challenges the traditional view that environments select organisms passively, showing instead that organisms actively reshape the selective pressures they and their neighbors experience. Beavers (Castor canadensis) exemplify this process by building dams that convert streams into wetlands, altering hydrology, vegetation, and predator-prey dynamics for hundreds of cohabiting species. Earthworms modify soil structure and chemistry by ingesting and excreting organic matter, changing conditions for soil microbes, fungi, and plant roots in ways that feed back on earthworm fitness.
This process creates ecological inheritance, where modified environments persist across generations and influence descendant populations even without genetic transmission. Human niche construction through agriculture, beginning roughly 10,000 years ago, drove rapid evolutionary changes in domesticated crops, livestock, and commensal species such as house mice and rats.
Termite mounds built by Macrotermes species in sub-Saharan Africa can persist for centuries and regulate internal temperature to within 1 degree Celsius despite external temperature swings exceeding 20 degrees Celsius. These structures support specialized communities of fungi, beetles, and other invertebrates found nowhere else, demonstrating that a single species' construction activity can generate entirely new ecological niches.
Niche construction is rare or limited to humans and social animals. Virtually all organisms modify their environments to some degree, from bacteria that alter local pH and oxygen levels to plants that change soil chemistry, light availability, and water retention in ways that affect the evolution of neighboring species.
The African mound-building termite (Macrotermes bellicosus) constructs ventilated mounds that can reach 9 meters in height and house colonies of several million individuals. These structures persist for decades and maintain stable internal humidity and gas concentrations, selecting for specialized fungal cultivars (Termitomyces species) that cannot survive outside the mound environment.
Non-Darwinian Evolution
/ non dar-WIN-ee-un ev-uh-LOO-shun / · From Latin non meaning not, plus Darwinian from Charles Darwin, and Latin evolutio meaning unrolling
Non-Darwinian evolution is evolutionary change at the molecular level driven by random genetic drift fixing selectively neutral mutations rather than by natural selection favoring advantageous variants.
Motoo Kimura formalized this concept in his 1968 neutral theory of molecular evolution, arguing that most molecular substitutions observed between species are selectively neutral and spread through populations by drift alone. The theory predicts that the rate of neutral substitution equals the mutation rate, independent of population size, a prediction confirmed across many DNA sequence comparisons. Synonymous substitutions, which change a DNA codon without altering the encoded amino acid, accumulate 5 to 10 times faster than nonsynonymous substitutions, consistent with relaxed selective constraint at silent sites.
Pseudogenes, which have lost their original function, evolve at rates approaching the neutral expectation because purifying selection no longer acts on their sequences. Population genetic studies estimate that 30 to 50 percent of amino acid substitutions in many proteins are effectively neutral or nearly neutral, meaning selection cannot distinguish them from truly neutral changes.
The molecular clock hypothesis depends on non-Darwinian evolution: only neutral mutations accumulate at rates predictable enough to serve as a clock. Positively selected substitutions occur in bursts tied to environmental change, making them unreliable timekeepers, while neutral substitutions tick forward at a rate set by the mutation rate alone.
All genetic changes must be either beneficial or harmful. The vast majority of mutations at the molecular level are selectively neutral, having no detectable effect on organismal fitness under most environmental conditions, and they spread or disappear through populations by chance rather than selection.
The GULO pseudogene, which once encoded the enzyme for vitamin C synthesis, has accumulated mutations at neutral rates in humans and other haplorhine primates for roughly 40 million years. Because ancestors of this group obtained sufficient vitamin C from fruit, the gene lost selective constraint and now stands as a molecular record of neutral substitution accumulating without functional consequence.