Evolution Study Guide

Updated on Sep 22, 2011


In general, the term evolution means change over time. In biology, the term evolution refers to the transformation of life from its simplest form to the great diversity we see today over billions of years. Animals are constantly responding to their environments and, in that process, may be successful by producing offspring. In this sense, the organisms have evolved to adapt well to their environment. A process of evolution known as natural selection is a mechanism of evolution.

Charles Darwin and the Origin of Species

The publication of Charles Darwin's On the Origin of the Species by Means of Natural Selection in 1859 gave the world its most clearly stated account of how life developed. His book also addressed the great diversity of living organisms, their interrelationships, their relatedness, and their ability to adapt to various environments. It was a profound, far-reaching theory and forms the basis of modern biology.

Humans have been manipulating other organisms for thousands of years. That's how we ended up with so many dog breeds and most of our crop animals and plants. We notice a trait that we like and want to encourage, and we look for an animal or plant with that characteristic. We then breed that organism with another that has similar traits or other ones we want to encourage. With luck and lots of time, we will eventually end up with an organism that embodies all the traits we want. This is called artificial selection because we are specifically picking the desired traits.

In nature, the process of natural selection occurs. In this case, no one picks the desired characteristics. Instead, an organism has characteristics that are either well suited to its environment or it won't have such characteristics. Those with characteristics that fit well with their environment will survive long enough to reproduce and be able to produce resources for their offspring, thus ensuring continuation of their kind. This is where the idea of "survival of the fittest" comes from. Fittest here doesn't necessarily mean the strongest; it means the organism that is best adapted to its environment. Natural selection is the main mechanism of evolution. The theory of natural selection states that diversity has come about by descent from a common ancestor with some modifications. Apes and humans are not the same organism, but we do share a common ancestor we each descended from and we each have been modified through natural selection.

Generally, Darwin's theory contains these facts:
  • All species have a great ability to reproduce, and the population of each would increase greatly if all individuals reproduced successfully. The birth of more individuals than the environment can support leads to a struggle among individuals with some surviving. This leads to populations relatively stable in size.
  • No two individuals in a population are exactly alike and their variability is inheritable. Therefore, survival depends on inheriting characteristics that best suit the individual to the environment. These individuals will leave more offspring.
  • The unequal ability of members of a population to survive and reproduce will cause a change in the makeup of the population, with favorable characteristics accumulating usually over long periods of time. The theory of punctuated equilibrium, which was not proposed by Darwin but is a more modern idea, states that these changes in a population's makeup can occur relatively quickly.
  • Evolution is not guided; it does not aim toward a final, finished product. It modifies populations over time and will continue to do so, even for our species. All life forms share in this gradual change and have shared in it over vast periods of time so that in a sense, bacteria are as highly evolved in their own very different ways as human beings.
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