There wouldnt be genetic freaks in a pronghorn population....in the same "way" that you see them in human populations.
...pronghorns have had,..and in some regards still hav selective presure towards speed.
particularly now, in the way in which female basicly choose the "fastest" , and "strongest" males...
its very complicated....and i dont have time...but its very fasinateing...and if you have a chance you should read John Byers book.
anyway...the point is that humans no longer have selective pressure...in physiological aspects(ok, so not completely true, but certainly FAR less than other species).
so there is in a sence....more variation..in the human population..more outliers..
Tergat, Halie, El G. are outliers in the human population.....But....So are weight lifters(dont know names sorry).
think of pronghorns has a species of ...Tergat's(or whoever). AND NOTHING ELSE...no gymnasts, swimmers, high jumpers....just the best distance runners in the world..
some maybe fater than othere....BUT NOT SIGNIFICANTLY...and IF they are significantly slower they will most likly NOT reproduce..
as a side note...there is good evidence to suggest that reproduction is THE selective pressure that kep pronghorns from slowing significantly since ...c.10,000years ago...when the north american cheehtah, long-limbed hyena...whatever else died off...cool stuff...
read that book i posted earlier