Lynn Margulis , author of Luminous Fish: Tales of Science and Love  and Mind, Life, and Universe: Conversations with Great Scientists of Our Time, recently sat for an interview  with Suzan Mazur at Scoop , an independent news site from New Zealand. Here’s an excerpt.
Suzan Mazur: Can you shed some light on what’s going on regarding the status and meaning of natural selection?
Lynn Margulis: I think I see the problem clearly. There is absolutely no doubt that natural selection itself can be measured every minute of the day in every population of organisms. Darwin was brilliant to make “natural selection” a sort of godlike term, an expression that could replace “God”, who did it — created life forms. However, what is “natural selection” really? It is the failure of biotic potential to be reached. And it’s quantitative.
Biotic potential is the intrinsic ability of any population to overgrow its environment by production of too many offspring. Whether born, hatched, budded or sporulated, all organisms potentially produce more offspring than can survive to reproduce themselves. Natural selection is intrinsically an elimination process. I’ll give you some specific examples.
My favorite one – I show this in a film and people just gasp. An ordinary bacterium – Proteus vulgaris – divides at the rate of every 15 minutes.
I have a time-lapse view of Proteus vulgaris where I show two hours of growth – 1, 2, 4, 8, 16, 32, 64, etc., until it fills the screen. I explain that if Proteus vulgaris continued to grow at this rate, not once a minute or once every 10 seconds, but once every 15 or 20 minutes, i.e., the way it really grows when it’s not limited, this bacterium would reach the mass of the Earth over a weekend.
It’s easy to show that the biotic potential measured as “number of offspring per unit time” (convertible of course into its equivalent “number of offspring per generation”) is never reached. Ever.
Darwin said the whole Earth could be covered by the progeny of a single pair of elephants.