How come you never see midget animals?
Dear Straight Dope:
There's really no "politically correct" [read: oversensitive whiny bullshit] way to ask this, so straight out: Why are there no animal midgets? You never see a monkey that's half as tall as his monkey compatriots, or a golden retriever the size of a chihuahua. Same question for plants: Why, in the middle of a cornfield, are there no tiny corn plants, or oak trees that never grow taller than me?
Actually, mutations like this appear spontaneously from time to time in most organisms--just not very often, so you're unlikely to encounter them. More to the point, mutations usually are quite detrimental, and an organism so affected will only rarely reach adulthood, and even more rarely reproduce, and even MORE rarely will their offspring carry the same mutation. The odds, then, are stacked heavily against such mutations persisting in nature, meaning you will almost never see them unless you spend a long time looking.
Still, while mutations are rare, dwarfism in general is not--you probably see a lot more examples of it than you think. It's more common in simpler forms of life--physically and developmentally complex organisms are more likely to suffer from being abnormally small, while simpler organisms can more readily adapt. In fact, in many species of plants and insects, individuals routinely adapt their growth (and thus their mature size) to external conditions. In these species, development and size are under environmental control (rather than genetic control--if you remember your biology, this is the distinction between "phenotype" and "genotype"). Individuals can and often do become dwarfed without any mutation being involved, and are much more likely to survive when they do (compared to genetic dwarfism). If you have a bonsai tree, for instance, and are able to get seeds from it, you can grow a perfectly normal tree from those seeds--the dwarfed condition in this case isn't genetic nor a serious impediment to the plant, as it would be to a vertebrate. That said, there are hundreds of dwarf plant varieties where the condition is genetic-- in fact such plants make up a substantial portion of the world's gardens, having been purposely cultivated for their size.
Dwarfism is less common but far from unknown in vertebrates. One of the best-known examples of environmentally-controlled dwarfism is the goldfish, which is tiny when kept in a bowl but many times larger if raised in a pond with abundant food. As for genetic animal midgets--sorry, but contrary to what you say you see them all the time in domesticated species. You yourself cited an example, the chihuahua, which is a fraction of the size of a golden retriever, even though they're the same species, Canis familiaris. The chihuahua didn't get that small by accident; rather, it was the result of the occasional undersize mutation purposely bred. It's simply that in domestic animals (and plants), we intentionally keep mutants alive and artificially enhance their breeding success, whereas an identical mutation appearing out in the wild would generally--though not always--mean the end of the genetic line for the animal carrying it. That's the difference between artificial selection and natural selection, even though the underlying process is fundamentally the same--in fact, much of Charles Darwin's On the Origin of Species was devoted to examples from domestic breeding rather than examples from nature.