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Principles of Natural Selection
Darwin’s theory was based on several observations:
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Individual organisms within a species differ from each other - there is variation.
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Offspring resemble their parents - characteristics are inherited.
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Organisms produce more offspring than can survive, and many that do survive do not
reproduce.
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Organisms compete for limited resources in the environment.
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Individuals that are best suited to their environment are more likely to survive and
reproduce (a.k.a. “natural selection”). They pass on their traits to their offspring. This
process of natural selection causes species to change over generations.
*Darwin wasn’t the first to suggest evolution of species. He was the first to suggest natural
selection as a mechanism for evolution, and to provide many kinds of evidence for it.
*The alleles of a species that are favored in the environment will become more frequent in the
gene pool of the species, due to the organisms’ higher likeliness of surviving as part of the
species at large.
*“Descent with modification:” Species today are descended with modifications from ancestral
species. Species can diverge from common ancestors.
Question: Why do giraffes have long necks?
Lamarck’s Explanation (Theory of Acquired Characteristics): When a giraffe stretches its
neck to reach high branches, it can grow longer. Then, it can pass on the long-neck trait to its
offspring. Over time, this can lead to a change in the species.
Darwin’s Explanation (Theory of Evolution by Natural Selection): In a population of giraffes
there is random genetic variation in neck length. In an environment where there are trees and
bushes, the longer-necked animals are better adapted and so compete well compared to their
shorter-necked relatives. These animals live longer, through more breeding seasons, and so they
can have more offspring. In the next generation, there are more long-neck genes than short-neck
genes in the population. If this continued over very many generations, then in time the average
neck length would increase.
Modern-Day: Lamarck’s hypothesis has been proven to be incorrect. Only genetic traits can be
passed on to offspring. Scientists have collected many more kinds of evidence to support
Darwin’s theory. Today, scientists think it is more likely that giraffe natural selection was for
long legs to run away from predators faster, and if you have long legs you need a long neck to
drink. Either way, the natural selection process works the same.