Evolutionary Biology Terms Starting With W
Evolutionary Biology Glossary: W
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Wallace Line
/ WAHL-is LYN / · Named after naturalist Alfred Russel Wallace
Wallace Line Wallace Line is a sharp biogeographic boundary running through the Indonesian archipelago that separates the distinct fauna of Asia from that of Australasia.
The line runs through island Southeast Asia. Deep water channels limited animal dispersal even during lower sea levels. It became important evidence that geography shapes evolution and species distribution.
The Wallace Line separates islands that are close together but have very different animal communities. Deep water channels limited animal movement even when sea levels were lower.
Nearby islands must have nearly the same wildlife. Deep barriers can keep faunas distinct despite short distances.
Borneo has many Asian placental mammals, while nearby Sulawesi has a more mixed and distinctive fauna. This contrast helps show the biogeographic importance of the Wallace region.
Weismann Barrier
/ VICE-mahn BARE-ee-er / · Named after German biologist August Weismann, plus Latin barra meaning barrier or obstruction
Weismann Barrier is the principle that genetic information flows only from germline cells to somatic cells, not vice versa, preventing inheritance of acquired characteristics in multicellular organisms.
The Weismann barrier, proposed by August Weismann in the 1890s, established that hereditary information passes only through specialized germline cells while somatic cell changes cannot be inherited, definitively rejecting Lamarckian evolution. This principle explains why exercising to build muscle does not produce muscular offspring, as muscle cell modifications cannot alter germline DNA. Weismann famously cut tails off mice for 22 generations, showing offspring still developed normal tails, demonstrating that somatic modifications are not inherited.
The barrier arises because germline cells separate early in development and remain isolated from somatic cell influences, maintaining genome integrity across generations. Modern molecular biology confirms this principle while revealing nuances, as epigenetic modifications can sometimes cross the barrier through mechanisms like DNA methylation patterns transmitted via gametes.
Plants lack a strict Weismann barrier because they do not set aside a germline early in development, instead producing gametes from somatic tissues, allowing occasional transmission of somatic mutations. Some evidence suggests that environmental stresses can produce heritable epigenetic changes that cross the barrier in mammals, though DNA sequences themselves remain isolated.
Epigenetic inheritance breaks the Weismann barrier because it transmits information across generations without changing DNA sequence. Epigenetic marks are largely reset during gametogenesis and early embryogenesis through a process called epigenetic reprogramming, meaning that even those marks that do escape erasure transmit regulatory states rather than new genetic instructions, leaving the core Weismann principle , that acquired somatic changes cannot alter heritable DNA sequence , intact.
In humans, germline cells segregate from somatic cells by the second week of embryonic development, establishing the Weismann barrier early. Even if an adult develops cancer-preventing mutations in liver cells through environmental exposure, these beneficial changes cannot be transmitted to offspring because liver cells are somatic and isolated from germline eggs or sperm.
Wright Effect
/ RITE ih-FEKT / · Named after Sewall Wright, American evolutionary biologist who described genetic drift in small populations in 1931
Wright Effect Wright Effect describes random fluctuations in allele frequencies within small populations, causing evolutionary changes independent of natural selection.
The Wright Effect occurs when population size becomes sufficiently small that chance events override selective forces in determining which alleles persist across generations. Sewall Wright mathematically demonstrated in 1931 that populations under 100 breeding individuals experience significant genetic drift, where random sampling during reproduction causes certain alleles to increase or decrease in frequency regardless of adaptive value. In populations of 10-20 individuals, a single generation can shift allele frequencies by 10-15 percent purely by chance.
This effect becomes especially pronounced during population bottlenecks, founder events, or when populations fragment into isolated subgroups. Wright’s models showed that genetic drift intensity varies inversely with population size, meaning smaller populations experience stronger random fluctuations that can fix neutral or even slightly deleterious alleles while eliminating beneficial ones.
Computer simulations of the Wright Effect show that neutral alleles in populations of 50 individuals have roughly a 1 in 100 chance of becoming fixed purely by random drift, a process that would take thousands of generations in larger populations. Some island populations have lost flight ability in insects not through selection but through random fixation of flightlessness alleles via the Wright Effect.
Genetic drift does not only affect harmful mutations. The Wright Effect acts on all alleles regardless of their fitness impact, and neutral or even beneficial alleles can be lost from small populations by chance.
The greater prairie chicken populations in Illinois experienced the Wright Effect during the 1990s when numbers dropped below 50 birds. Genetic studies revealed that random drift eliminated nearly 30 percent of genetic diversity within just five generations, fixing several alleles that reduced hatching success from 93 percent to 38 percent, demonstrating how chance can override selection in small populations.