17.2 Evolution as Genetic Change in Populations. Pages 487 - 497. Natural Selection in Action. Peppered Moths. White moths against a light colored background. White moth against a dark background White. As a predator, which moth would be easier prey????. Industrial Revolution in England.
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Pages 487 - 497
In the mid-twentieth century, controls were introduced to reduce air pollution and as the air quality improved tree trunks became cleaner and lichen growth increased. Once again the normal pale Peppered Moths were camouflaged and the black forms were more noticeable. Now the situation in urban areas has again become the same as in the countryside, with normal pale Peppered Moths being far more common than the black forms. So natural selection has been seen to work in both directions, always favouring the moth that is best suited to the environmental conditions. The same thing has been observed throughout Europe and the USA.
In a normal population without selection pressure, individual traits, such as height, vary in the population. Most individuals are of an average height, while fewer are extremely short or extremely tall. The distribution of height falls into a bell curve
eliminates extreme individuals. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures act to favor plants of medium height.
selects against one extreme. In the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.
eliminates intermediate individuals. For example, imagine a plant of extremely variable height that is pollinated by three different pollinator insects: one that was attracted to short plants, another that preferred plants of medium height, and a third that visited only the tallest plants
If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against, and the population would tend toward both short and tall plants, but not plants of medium height.
From the outside, the fin of a whale may look like the flipper of a penguin, but the bone structure of a whale fin is still more similar to the limbs of other mammals than it is to the structure of penguin flippers. More importantly, convergent evolution never results in two species gaining the ability to interbreed; convergent evolution can’t take two species and turn them into one.