There are a lot of tag wikis that start with some variation on "The study of / a discipline that", "Questions related to / questions about", or "[tag] refers to / deals with / is /". It seems to me that it would be simpler just to get to the point, at least for the excerpt.
That is, it seems more concise and clear to, when you're hovering over [evolution] to see:
The observed changes in successive generations of biological organisms due to heritable components (genes and DNA).
rather than seeing:
Evolution refers to the observed changes in successive generations of biological organisms due to heritable components (genes and DNA).
But maybe I'm wrong – perhaps to newcomers to the site it's more clear to repeat the word up front.
Regardless, it seems like some tidying is in order such that the tags reflect more or less the same format. So, what is the best way to write tag wikis?
For what it's worth, it looks like SO prefers the first clause to be "[tag] is" as in
PHP is an open-source server-side scripting language widely used in web development. Use this tag for questions about PHP classes, methods, functions, syntax and use.