Over at scienceblogs.com, there is an ongoing series of posts covering basic concepts in science. [John Wilkins (who was the one who had the idea for the series in the first place) is maintaining a list of posts in the series.](http://scienceblogs.com/evolvingthoughts/2007/01/basic_concepts_in_science_a_li.php) If you are interested in evolution, you will probably appreciate all of the posts in the biology section of the list. The following post is my first contribution to this series. I'm crossposting it here because the concept - fitness - has come up in the comments in [both of](http://scienceblogs.com/authority/2007/01/how_fast_can_evolution_work.php) [my two](http://scienceblogs.com/authority/2007/01/more_on_evolutionary_speed_lim.php) most recent posts here.
In 1862, the British philosopher Herbert Spencer used the phrase "survival of the fittest" to describe Darwin's concept of natural selection. It's not a bad phrase, really, and it doesn't do a bad job of describing natural selection - the individuals in any population that are "fittest" - best suited to reproduce - are the ones most likely to reproduce successfully. If this is correct (and it is), we can expect that "fitness" would be a very important concept in evolutionary biology. It is, of course, and [John Wilkins has already provided a good explanation of the concept in general](http://scienceblogs.com/evolvingthoughts/2007/01/fitness.php). I'm going to look at something a little more specific - how can we measure fitness.