Evolutionary Biology Terms Starting With S

Sedimentation is the process by which particles of rock, mineral, shell, and organic material settle out of water or air and accumulate in layers that can compact and cement over time into sedimentary rock.

Rivers carry eroded material downstream, sorting particles by mass so that coarse sand drops near the river mouth while fine clay travels farther and settles slowly in deeper, calmer water. On ocean floors, this layering is continuous: the deep-sea sediment record off Antarctica preserves an unbroken sequence stretching back more than 65 million years. Pressure from overlying material compacts lower layers to densities exceeding 2 grams per cubic centimeter, and mineral-rich groundwater flowing through pore spaces precipitates calcite or silica that cements grains together into rock.

Because younger sediment always accumulates on top of older sediment under undisturbed conditions, the resulting strata provide a chronological archive in which fossil-bearing layers can be dated by their position, by radiometric methods applied to interbedded volcanic ash, or by the known ages of index fossils found within them.

Selective pressure is any environmental or biological factor that causes individuals with certain heritable traits to survive or reproduce at higher rates than others in the same population, shifting the frequency of those traits across generations.

Selective pressures take many forms: predation, parasitism, competition for food or mates, temperature extremes, and human-introduced chemicals all alter which phenotypes leave the most descendants. These pressures carry no intention or direction; they simply filter the variation already present in a population. The peppered moth (Biston betularia) in industrial England provides a well-documented example: soot from coal burning darkened tree bark during the nineteenth century, and dark-colored moths survived predation at higher rates than pale moths, shifting population color composition within decades.

When clean-air legislation reduced pollution after the 1950s, pale moths recovered, demonstrating that selective pressure can reverse direction as environments change.

Sexual selection is a mode of natural selection in which traits that improve an individual's ability to attract mates or defeat rivals for mating opportunities are favored, even when those traits reduce survival.

Charles Darwin introduced the concept in 1871 in “The Descent of Man, and Selection in Relation to Sex,” distinguishing two mechanisms: intersexual selection, in which members of one sex choose among potential mates, and intrasexual selection, in which members of the same sex compete directly with one another. Intersexual selection can produce elaborate ornaments such as the elongated tail feathers of the long-tailed widowbird (Euplectes progne), which Malte Andersson experimentally manipulated in 1982, showing that males with artificially lengthened tails attracted significantly more females than control males. Intrasexual selection drives the development of weapons such as the antlers of male elk (Cervus canadensis), which can span more than 1.2 meters and are shed and regrown each year at considerable metabolic cost.

Both mechanisms can push traits to extremes that would be disadvantageous in the absence of mate competition.

Speciation is the evolutionary process by which one or more populations diverge genetically and reproductively until they can no longer interbreed and produce fertile offspring.

Populations begin to diverge when gene flow between them is reduced or eliminated, allowing mutation, genetic drift, and natural selection to push them in different directions. Allopatric speciation, the most thoroughly documented mode, occurs when a geographic barrier such as a mountain range or ocean channel physically separates populations; the Isthmus of Panama, which closed roughly 3 million years ago, split marine populations on the Pacific and Caribbean sides, producing dozens of sister-species pairs now called “geminate species.” Sympatric speciation, by contrast, occurs without geographic separation, driven instead by ecological specialization, host shifts, or chromosomal changes such as polyploidy. Reproductive isolation accumulates gradually, and populations can pass through a stage of partial isolation, producing some viable hybrids, before becoming fully distinct species.

Stabilizing selection is a form of natural selection that favors individuals with intermediate trait values and removes those at both extremes, keeping the average expression of a feature relatively constant across generations.

This mode of selection operates when intermediate phenotypes have higher survival or reproductive success than either extreme, a condition that applies to many traits in stable environments. Human birth weight provides a well-studied example: babies born below about 2.5 kilograms or above about 4 kilograms historically faced higher mortality than those born near the population mean of roughly 3.4 kilograms, because very small infants lack physiological reserves and very large infants risk complications during delivery. Stabilizing selection does not eliminate genetic variation entirely; it reduces the frequency of extreme phenotypes while leaving substantial variation near the mean.

Because it preserves the status quo rather than shifting trait averages, it is the most common selective regime acting on most traits in most populations during periods of environmental constancy.

Sympatric speciation is the divergence of a new species from an ancestral population while both occupy the same geographic area, without physical separation driving the split.

Populations can diverge sympatrically when disruptive selection favors individuals at opposite ends of a trait distribution, when mate choice becomes correlated with ecological preference, or when chromosomal changes such as polyploidy instantly prevent interbreeding with the parent population. Demonstrating sympatric speciation requires ruling out cryptic geographic barriers, which makes it harder to confirm than allopatric speciation. Cichlid fishes in Lake Victoria provide a frequently cited case: more than 500 species evolved within the lake over roughly 15,000 years, with divergence in male coloration and female color preference apparently driving reproductive isolation between populations living in the same water.

Host-shifting in phytophagous insects offers another line of evidence; Rhagoletis pomonella flies in North America shifted from native hawthorns to introduced apples in the nineteenth century, and the two host-associated populations now differ genetically and mate at different times of year.

Synapomorphy is a shared derived trait inherited from the most recent common ancestor of a group and present in all members of that group, distinguishing it from ancestral traits shared more broadly across the tree of life.

Cladistics, the method of classifying organisms by shared evolutionary history, depends on identifying reliable synapomorphies to define natural groups called clades. A trait qualifies as a synapomorphy only relative to a specific group: hair and mammary glands are synapomorphies of mammals, but the same features are ancestral characters when comparing mammals to other amniotes. This context-dependence means the same trait can be a synapomorphy at one level of the tree and a plesiomorphy at another.

Molecular data have expanded the search for synapomorphies beyond anatomy to include shared gene sequences, chromosomal rearrangements, and the insertion sites of transposable elements, which are particularly powerful because independent insertion at the same genomic location is vanishingly unlikely. Shared retroviral insertions, for example, confirmed the close relationship of humans and chimpanzees in the 1990s.