Let's go back. That's sort of Why UV and Ecology. Let's go back to Why UV and Evolution. Well, natural selection consists of two stages. Generation of heritable variability, in other words, you have to have some sorts of changes that are inherited. Everyone here should know that, of course. And then you have a differential selection of the offspring. Some of them make it and some of them don't and the reasons they make it or don't you're assuming has something to do with their heredity, not just because they happen to be standing under a tree when it was knocked over by lightening, because that doesn't make any change in the next generation in terms of something that would be of any advantage. It was just bad luck.

So there's some sort of hope that this differential selection is tied in with the generation of heritable variability. What's interesting is that I'll argue that UV radiation has an effect on both of these issues. I promised I'd talk a little bit about UV through time. Here's that picture of the fossil record I showed you before. It's squished over on the side. Now, this yellow panel gives you some idea of what's gone on with the sun over the last 4.5 billion years. The sun is a star. As stars age, they burn hotter and hotter and therefore they're putting out more solar radiation. The estimate is that our sun put out about 71% of its current level of radiation about 4.5 billion years ago and that it's been going up, up, up to modern levels, particularly during this period where the protists were dominant, at the same time as knowing that some stars like the sun can actually put out more UV radiation when they're younger.

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