Three ways selection reshapes a bell curve.

A bell curve of a trait under three fitness rules: stabilising narrows it, directional shifts it, disruptive splits it.

Turn one peak into two clear peaks within 20 generations.

Confusing directional with disruptive selection; thinking selection always narrows variation.

• Hardy-Weinberg equilibrium is upset by five factors: natural selection, mutation, genetic recombination, gene migration (gene flow) and genetic drift.
• Gene migration: when part of a population moves to another place and population, gene frequencies change in both. The new population gains alleles that the old one loses. Migration repeated many times is gene flow.
• Genetic drift: the same change in frequencies happening by chance.
• Sometimes the drifted sample differs so much that it becomes a different species. The drifted population becomes the founders, and this is the founder effect.
• Experiments on microbes show that when an advantageous mutation already present is selected, new phenotypes appear; within a few generations this can lead to speciation.
• Variation from mutation, from recombination during gametogenesis, from gene flow or from genetic drift changes the frequencies of genes and alleles in later generations. Coupled with greater reproductive success, natural selection makes the population look different.
• Stabilising selection: more individuals come to have the mean character value; the curve narrows around the mean.
• Directional selection: more individuals come to have a value other than the mean; the peak shifts to one side.
• Disruptive selection: more individuals come to have the extreme values at both ends of the curve; two peaks form.

Teaching this idea? Put the simulation on your own page or LMS with this code.