Evolution Lecture 6 • Directional selection: selection in favor of an advantageous allele. Directional selection could lead to the substitution of one allele for the other even if the selection is weak, even if the fitness advantage given by the advantageous allele is very small. If fitness differences are very big, the substitution for an allele could happen very quickly in like 3-5 generations. DDT: retained its action to kill insects for many weeks and months. Very effective in controlling malaria. After the use of DDT, resistance to it evolved in every insepct species exposed to it. It occurred in very few generations very predictiably. If the insect population survives at all after exposure to DDT, you can be sure that resistance to DDT will evolve. The predictability of this event tells us that mutation to DDT resistance are already present in the population. The variation already had to be there for natural selection to work so quickly. Herbicide resistance evolves very quickly. It is so frequent,
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