Evolutionary Biology Terms Starting With B

Background Extinction is the normal, gradual rate at which species disappear from Earth over geological time between mass extinction events, driven by ordinary ecological pressures rather than global catastrophes.

Background extinction represents the baseline rate of species loss produced by competition, predation, disease, and minor environmental fluctuations. Paleontologists estimate this rate at roughly 0.1 to 1 species per million species per year during stable periods, a figure derived from the fossil record of well-preserved groups such as marine invertebrates. Mass extinction events stand apart by eliminating 75 percent or more of all species within geologically brief intervals, as occurred at the end-Cretaceous boundary 66 million years ago.

Paleontologists use the background rate as a benchmark to identify intervals of anomalously high extinction in the stratigraphic record. Current extinction rates, estimated at 100 to 1,000 times above background levels, have led many researchers to propose that Earth is entering a sixth mass extinction driven by habitat loss, overexploitation, and climate change.

Batesian mimicry is a form of mimicry in which a harmless or palatable species evolves a resemblance to a harmful or unpalatable species, gaining protection from predators that have learned to avoid the model.

Batesian mimicry exploits the learned avoidance behavior of predators: once a predator has experienced a noxious model, it avoids similar-looking prey. The mimic gains protection without producing toxins or defensive structures, while the model may suffer increased predation if mimics become too common and predators begin encountering harmless individuals more frequently. Henry Bates first described this phenomenon in 1862 after observing harmless butterflies in the Amazon rainforest that closely resembled toxic Heliconius species.

The effectiveness of the strategy depends on mimics remaining less abundant than models; when mimics outnumber models, predators lose the learned association and begin attacking both. This selective pressure has driven Batesian mimicry to evolve independently across insects, snakes, fish, and spiders.

Behavioral Evolution is the process by which heritable behavioral traits change across generations through natural selection, genetic drift, and other evolutionary mechanisms acting on variation in behavior within populations.

Behavioral evolution encompasses changes in innate and learned behaviors that affect survival and reproduction, from migration routes to mating displays to cooperative foraging. Many behaviors have strong genetic components that respond to selection rapidly: laboratory experiments selectively breeding fruit flies (Drosophila melanogaster) for geotaxis, the tendency to move up or down, produced measurable divergence within fewer than 20 generations. Game theory models, developed in part by John Maynard Smith in the 1970s, explain the persistence of cooperative and altruistic behaviors by showing that strategies such as reciprocal altruism can increase inclusive fitness even when they appear individually costly.

Molecular approaches have since identified specific gene variants, including the foraging gene in Drosophila and the arginine vasopressin receptor gene in voles, that connect genetic variation directly to behavioral differences between individuals and populations.

Biogeographic evidence is the use of species distributions across geographic regions to support conclusions about evolutionary relationships and common ancestry.

Species do not spread across Earth at random. Oceans, mountain ranges, and continental boundaries act as barriers that split populations, allowing them to diverge into distinct lineages over time. Matching distribution patterns frequently reinforce fossil, anatomical, and genetic evidence of shared ancestry.

Alfred Russel Wallace mapped these patterns in the 1850s and 1860s, identifying a sharp faunal boundary between Asian and Australian species now called the Wallace Line.