Evolution In A Nutshell

Evolution -- What is it and how does it work?

Lots of people don't understand evolution or natural selection. Even some writers of essays on evolution don't seem to have a firm grasp of it. Heck, I probably don't completely understand it myself.

But I think I can summarize the key facts. If you absorb these facts you will understand more about evolution than most people do, even more than most high-school science teachers.

1. The organisms we see on Earth today are different from those of past times. Some that once were common have disappeared. New forms unknown in the past have come into existence. This is what is called “evolution” -- change over time. Everybody agrees that evolution occurs.


2. The organisms in a population are not all perfectly the same. There are slight differences among individuals. Except for clones (like identical twins) no two individuals are exactly alike. This is common experience. Everybody agrees that this is so.


3. Offspring tend to resemble their parents, at least in what are termed “heritable” traits. Some traits, such as what language you speak, are not inherited. Other traits, such as degree of skin pigmentation in humans, are clearly influenced by heredity (people tend to resemble their parents and grandparents). In Darwin's time nobody understood how this worked, but today everybody knows about genes, DNA, and stuff. Everybody agrees that many traits are inherited.
   


4. In most types of organisms many more offspring are produced than can survive to produce offspring of their own. That is, some individuals die without leaving any progeny, or at least not as many progeny as others -- their traits are not passed on as widely. Everybody who has looked at the natural world at all agrees that this is true for most creatures. (I am trying to think of some kind of plant or animal that isn't capable of producing enough offspring to overtax the resources it needs from its environment in just a few generations. Pandas?)
   

All of the above points are true and widely accepted. Here is the new idea Darwin and Wallace had:

• Individual organisms' inherited traits can influence their success in leaving progeny. Some traits will help the individual leave more, and more successful, offspring. Such traits might include resistance to disease, attractiveness to mates, efficiency at finding or making food, or ability to avoid being eaten before reproducing. Those traits will be passed on to more progeny than other traits which don't help their possessors survive and reproduce.


• In fact some traits may actually hurt their owners' chances of leaving offspring. Individuals with these less-helpful traits (perhaps susceptibility to disease, inefficient food-finding, or less ability to avoid predators) will leave fewer progeny, and thus those traits will not be passed on to as many members of the next generation.
  

The selection of which traits are favored and which are unhelpful is made by the natural environment, in the determination of how many progeny each individual leaves and how widely its traits are passed on to the next generation. This is “natural selection”.

Thus the differences among individuals, plus pressures from the environment which limit the total numbers of progeny that can survive, will lead to gradual change in the commonness of specific heritable features. That is what is meant by “evolution by means of natural selection”.

If you accept 1 through 4 above, then natural selection seems a very logical and interesting hypothesis to explain how life on Earth changes over time. A century and a half of intense research in all fields of biology (biogeography, paleontology, genetics, molecular biology, ecology, plant and animal breeding, and others) has provided a mountain of evidence that this basic “theory of evolution” about how populations of organisms can change is correct, and accounts for the diversity of life on Earth.

Any questions?

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David Wheat's Science In Action site has articles about science and math in the real world, weird science, science news, unexpected connections, and other cool science stuff. There is an index of the articles by topic here.

tags: science, evolution, science education, natural selection, education, Science In Action