An Introduction to Evolution

The Definition:
Biological evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.

The Explanation:
Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't involve descent through genetic inheritance.

Short-term change w/o inheritance
Leaves on trees change color and fall over several weeks.
Long-term change w/o inheritance
Mountain ranges erode over millions of years.

The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother.

Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we're all distant cousins: humans and oak trees, hummingbirds and whales.

Short-term change w/ inheritance
A genealogy illustrates change with inheritance over a small number of years.
Long-term change w/ inheritance
Over a large number of years, evolution produces tremendous diversity in forms of life.
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