Evolution is the development of physical
attributes or behaviors that provide a benefit in a certain environment. If the environment
changes, or any element of the food chain changes, those attributes may become handicaps or
cease to provide a benefit.
Every organism develops genetic mutations that it
can pass on to offspring. Mutations are random; an organism cannot decide what to change.
Sometimes by chance a mutation proves beneficial to the organism, and in this case the theory of
natural selection proposes that those beneficial traits are more likely to be passed down to
offspring. Other times, also completely by chance, the mutations might be detrimental, in which
case natural selection says they should gradually disappear from the general population. It is
also possible that a mutation has no benefit or detriment and is simply a neutral
characteristic.
One example of an imperfect evolution is in the case of
Sickle Cell Disease. Sickle cells evolvedoriginally as a random mutation, rememberin several
distinct communities, most of which were located in Africa. Red blood cells are usually round,
but sickle cells are crescent-shaped. As a result, malaria parasites cannot attach to them. In
populations where Sickle Cell Disease is common, there was a historic exposure to malaria.
Therefore, there was enough evolutionary benefit for sickle cells to survive and be passed on to
offspring, but it also results in a deadly disease if a child has two copies of the gene. In
this case, clearly, evolution is not working for the benefit of many individuals. You could
definitely use this as an example of the imperfect, even random, nature of
evolution.
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