Finally, we get to selection pressure. Selection pressure simply means there is pressure for one variant of a species to survive better than the rest, because one is better adapted than the rest.
Selection pressure is probably the easiest of the 3 tenets to understand, because it’s what we naturally think of when we think of evolution. Another name for it is “survival of the fittest”. Giraffes with long necks are more fit because they can reach higher branches. T. rexes with stronger jaws are more fit because they are more likely to win a bait-the-triceratops contest. Apes that display more intelligence are likely to be more fit, and thus reproduce more, than their less-intelligent brethren.
Likewise, resistant zucchini-hoppers survive the spraying regime much better than non-resistant zucchini hoppers. So, most of the hoppers that get to reproduce are immune. That means that over time the proportion of immune hoppers increases.
Copyright University of Maryland, 2007
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