The explicit assumption of the article is right here:
On Soay Island, off the western coast of Scotland, wild sheep are apparently defying the theory of evolution and progressively getting smaller.
The Reporter, Bryan Walsh, is incorrect. Though growth in size is typically encouraged by evolution, it is by no means a law of evolution that things can't go either way.
I recently saw a science program, I forget what it's called, which talked about the case of a Mammoth species on an island off California, which shrank after its population became trapped on it when the end of the ice age cut it off from the mainland. Not only was there not enough food to manage the larger size, but the smaller size gave the dwarf mammoths an advantage in the angle of slopes they could climb.
Or to put it plainly, the advantages and disadvantages of the size pressured the size of the Mammoth to go down, to reach a new point of equilibrium.
Same thing with the sheep.
Now for some species, that new equilibrium might not come fast enough. And basically those species are screwed.
But shrinking sheep? That's consistent with the laws of evolution, because those laws speak not to the promotion of idealized traits, but the adaptation of a species to an environment. Nothing law-breaking about that.
