Evolution is descent with modification.
I'm specifically talking about
biological or
organic evolution here. It's necessary to make this distinction because the word itself precedes its biological usage. Anyone who adheres to the traditional sense of the word might be misled into thinking that
individual organisms
transform themselves from one form to another! We can avoid this by making a second
bold declaration:
Populations evolve, not individuals.
Even exceptions to this broad rule can be subsumed when start to see everything as a population; an individual is, after all, a population of cells. Let's skirt around the edges of this issue of levels of evolution (which just begs for a write-up of its own) and stick to the task at hand.
Evolution happens to populations of living things that reproduce. (If there was a population of critters that didn't reproduce, they'd eventually die off. Even immortal critters have to knock off eventually. So, conveniently enough, any living thing that happens to still be walking around is probably reproductive. Even those noding E2.)
It isn't enough to simply reproduce for there to be biological evolution, however. There must also be some system of genetic inheritance. At least some of the generative information contained in the parent must get passed onto the offspring.
Reproduction and genetic inheritance: it so happens that having these two ingredients is enough for biological evolution. How evolution applies strictly to populations may become clearer now: because some individuals may leave more offspring than others (by dumb luck or otherwise -- genetic drift and natural selection) causes the membership of the population to change between generations.
We've had to assume that there are genetic differences between families. One exception to the bare minimum for evolution I gave above is when everyone is genetically identical. The changing makeup of a population that is evolution is strictly genetic change. Pierced noses and dyed hair don't count.
I feel comfortable about making this assumption because genetic variation is virtually guaranteed. Mutations are always being introduced in the genetic code because of mistakes made during replication. Mutation is the fuel of the engine that is evolution.
Where does this leave us? There are two more things that I want to mention about evolution: speciation and evolution's place in society.
Did you notice that the fundamental processes that I mentioned above don't account for the proliferation of species? Heck. Neither did the Origin of Species.
Speciation is a special case of evolution.
This write-up is dedicated to the late Stephen Jay Gould, who would have done a much better job of it.
What follows below is the original content of this write-up. Because it has since migrated to the top of the nodeshell, and because it refers to write-ups since deleted, an overall upgrade became necessary.
There is a fundamental and common mistake being made here that cmmike has touched upon; and it is also worth elaborating on. As a biological term, evolution refers to heritable change. This can be caused by selection as pi has described, but also by mutation and sampling error (drift). A good analogy for the latter process comes in the form of black and white beans in an urn. A handful of beans will not necessarily reflect the colour proportions within the urn.
Because these process involve genetic change and because genetic change is
heritable, all these processes are components of evolution.
Ever since the development of evolution as a principal biological discipline, other disciplines have drawn analogies from evolutionary processes including history, sociology and the computational sciences. It is an important caveat that transmission and reproduction within these systems will have very different constraints than a genetic (flesh and blood) system. Hence, these analogies are often inaccurate and misleading, succeeding only in generating excitement, misplaced legitimacy and confusion.