> because faced with the same environment, eating grass, quadripedal, horses wouldn't turn into eagles over time because there would be no need for a drastic change to survive.
Depends on what you mean by "time." As environemtnal pressures and mutations kick in, you could have horses that evolve into tall giraffe-like critters, some that bulk up into things like rhinos, some that smallerize into small deer-like critters. The deer-like critters could over the eons turn into even smaller rodent-like critters. Those pseudo-rodents could become runners, shifting more and more to bepidalism; the forelegs could become more like arms. The fur could mutate into something like feathers, the arms could strengthen to wings, and you're on your way to something vaguely eagle-like.
Nobody, except for some creationists using dishonest tactics to try to portray evolution as a farce, would suggest that horses would, should or even could give birth directly to eagles. But genetics and the fossil record have shown several times that one type of animal *can* become something very different... over a substantial length of time. Each step along the way tends to be fairly small, but with a vast number of small steps, massive changes can occur.
> I don't see that gorillas really have much use or need for strength. For all their show when frightened, they seldom use their strength according to what I've read.
Perhaps. But when they have need of that strength (say, fighting off predators or males competing for gorrilla-chicks), those who don't have it lose out. Those who do have it reproduce more.
> They are herbivores primarily and live in areas where there aren't many natural predators of any size that attack them.
And that's largely due to their size. Gorrilla territroy is not devoid of predators, including several species of large cats; being big makes the adults less vulnerable.
> So can we assume that they are on their way to evolving into smaller animals that won't need as much fodder to survive?
Perhaps they are *now*, since human activity is greatly reducing their turf. However, they'll likely simply go extinct in the wild logn before substantial evolution occurs. But remove humans from the equation and there's no need for gorrillas to get smaller... they have all the gorrilla-chow (bamboo and such... not rare or endangered) they could ever want.
For a mutation to "kick in" as you put it, would it only be a one-time deal where the gene was sympathetic to the original species as far as being able to procreate, but would maintain the separate/different DNA lineage and breed true?
I can't quite fathom a simultaneous mutation in a breeding pair or within a group.
I know that some scientists are trying to trace the DNA of certain groups back to an original ancestor. Saw a National Georgrahic special on their work. ANd I think they have DNA kits available which are working to trace individual's links back to certain ethnic lines.