One of the observations made by Darwin in the course of constructing his theory of natural selection is that although many more offspring are produced than would be necessary to replace the parents were all to survive, population sizes tend to remain stable over extended periods of time. This led him to the insight that, on average, a significant portion of every generation never lived to reproduce; in turn, whatever characteristics increased the chances of an individual surviving would tend to be passed on in greater numbers to the next generation.
Whatever organism is picked, it is obvious that survival of all
offspring is impossible over extended periods. Using our Southwestern Deer Mouse as an
example and being conservative with the numbers, over 15,000 mice would potentially be
produced, starting with a single pair, over a time span of only 5 years. If that's
not enough to boggle the mind, the figure would be over 20 million in less than 10
years. On average, though, we would have only two descendants of the original pair.
Mind-boggled yet?
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.