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Random Mutations and Evolutionary Change:
For 70 years after the publication of the Origin of Species, it seemed as if Lamarcks shadow would loom forever over Darwin. On the one hand, most biologists came to the reality of evolutionthat living species shared a common ancestry and had been transformed over time. But natural selectionthe engine of evolution, according to Darwinremained controversial. Many biologists argued that there must be some built-in direction to the variation that arose in each generation, helping to push each lineage towards its current state.
Many of these first geneticists who rediscovered Mendels insights around 1900 also opposed natural selection. After all, Darwin had talked of natural selection gradually altering a species by working on tiny variations. But the Mendelists found major differences between traits encoded by alleles. A pea was smooth or wrinkled, and nothing in between. In order to jump from one allele to another, evolution must make giant jumpsan idea that seemed to clash with Darwin.
Natural Selection in a Mendelian World
Small, not Drastic Changes
Wright introduced the most compelling metaphor in population genetics, known as the adaptive landscape (see figure, left). You can imagine the varying fitness of different combinations of genes as a hilly landscape, in which the valleys represent less-fit combinations of genes and the peaks represent the fitter ones. Natural selection tends to move the populations towards the peaks of the hills. But since the environment is always changing, the peaks shift, and the populations follow after them in a never-ending evolutionary journey.
Fisher image courtesy of the School
of Mathematics and Statistics, University of St. Andrews, Scotland
Haldane image courtesy of the American Philosophical Society.
Wright image © Hildegard Adler.
Adaptive landscape image after a graphic by Rodney Dyer, Iowa State University.
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