Awhile back, on a web list, a person doubted that the wild horses of North America were derived from escaped (or stolen) horses brought over by the Spaniards. His thesis was that they never did become extinct at the end of the last ice age, though he didn't mention that they don't show up in fossil and archaeological sites dated to the time between the end of the Pleistocene and the entry of Europeans into North America.
One line of evidence he implied was that even if a few horses did break
loose, they'd not have time to form vast herds. Unfortunately for his thesis,
animal populations tend to grow exponentially when given plenty of resources and few
environmental impediments. If you follow an exponential growth curve for even a
relatively few generations, it's easy to see how a few animals can quickly take
over an area. Even if every pair has only four surviving offspring every generation, a
population that starts out with one male and one female will have over a million
individuals in 20 generations.
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.