The reason that they look so similar is that some body plans have been particularly effective, allowing the creatures possessing them to survive and multiply across the globe. But, if they're so similar, why doesn't one species, over the course of time, drive the other to extinction? Even if one isn't even just marginally better at doing whatever it does than the other (and, since they aren't literally identical, that's unlikely) then sheer bad luck ought to hit one or the other of them eventually.
Often, the reason for this is that they live in different places, and so never have to compete directly with one another. But it doesn't take long to confirm that many, very similar, species, do, in fact, share the same habitat and geographical locality. In their case, there has to be some difference between them, perhaps one that isn't very obvious, that leads to them being good at slightly different things. That way, they can happily live alongside one another, without having to compete for limited resources, at least until times get really bad. How such subtle differences manifest naturally varies quite a lot between different groups.
Let's take a look at a wild animal that's probably quite familiar to many Americans, even if they haven't actually seen one in the wild: the cottontail rabbit. There are over sixty different species of rabbit in the world, and, while some of them do have distinctive appearances, many of them do look quite similar, and, partly as a result of that, have similar lifestyles. So, while jackrabbits and hares, for example, are larger, longer-limbed animals with a knack for fast running, cottontails look an awful lot like the rabbits you would see in Europe.
There are differences, of course, most notably in that cottontails don't dig burrows, but, in any event, the European animals aren't a competitive threat anyway, the Atlantic Ocean being what it is. There are, however, no less than seventeen recognised species of cottontail rabbit. Some live only in Mexico, or Central or South America, but that still leaves ten found in the United States. There are, for example, different species of cottontail found in New England, the Appalachian Mountains, Florida, Louisiana, and California, among others.
Three species are particularly widespread. The eastern cottontail (Sylvilagus floridanus) is found across pretty much the entire eastern half of the US, while both desert (S. audubonii) and mountain cottontails (S. nuttallii) are found across most of the western half. As one might expect, these ranges overlap with those of many of the other species, so that two (or more) different kinds of rabbit can be found in the same general area.
One such place is the northern edge of the Great Basin, in particular southern Idaho. Desert cottontails aren't found in this area, but mountain cottontails are, and they share it with the pygmy rabbit (Brachylagus idahoensis). To be fair, this is not a cottontail, since it lacks the white underside to the tail that characterises that particular genus. But, nonetheless, molecular studies have shown that pygmy rabbits are the closest living relatives of cottontails, and they both clearly look like "rabbits".
So, in those places where they live together in exactly the same environment, what is it that enables them to do so without one of them out-competing the other? Well, since the pygmy rabbit isn't actually a cottontail there are differences, and some of these may help explain that. There's the colour of the tail I mentioned above, although it's debatable how significant that might be. Pygmy rabbits also dig burrows, which cottontails (and jackrabbits) don't, which certainly suggests some difference in lifestyle. And, as one might expect from the name, they're smaller.
That last one is perhaps the most significant, since it might suggest that they eat different plants, which would certainly solve the problem. The thing is, mountain cottontails eat a very wide range of plants, including various kinds of grass and bush that are found in this part of the world. There are a number of ways of competing with an animal that does this, without changing the diet more than would be reasonable while still remaining a rabbit, but one of them is to focus on one very specific kind of food. And this is what the pygmy rabbit does.
If an animal is going to survive on one particular, narrow, kind of food, there are two important requirements it needs to fulfil. Firstly, the food needs to be widely available wherever they live, and moreover, must be available year-round. Secondly, they need to pick something that animals with a more generalist diet tend to avoid, or there really isn't much point.
Since generalists will, by definition, eat pretty much anything, that usually means that animals with a specialist diet will either eat something that isn't very nutritious, something that's otherwise just plain hard to eat (because it's covered in thorns, say), or that's outright poisonous. That last one is particularly common, because, while it does require that the animal in question develop some effective means of not killing itself with its diet, it does mean that there's likely to be plenty of the food left for them. This, for instance, it why koalas eat eucalyptus.
In the case of the pygmy rabbit, what it eats is sagebrush (Artemisia), an aromatic shrub found throughout the Great Basin that happens to be chock full of toxic terpene compounds (including the main one also found in eucalyptus). Unsurprisingly, not many animals will eat this, if they have a choice, with the similarly specialised greater sage-grouse being one of the few non-rabbit exceptions.
Having said which, there are times when animals such as mountain cottontails don't have much of a choice. Sagebrush grows year-round, or the pygmy rabbits would die out, but in winter, most of the foods that cottontails would normally eat either aren't around at all, or are difficult to reach beneath the snow. That may leave them with few alternatives.
Indeed, a recent study on the diet of the two species suggested that, at least in winter, as much as 40% of the diet of cottontails in southern Idaho consisted of sagebrush, far more than previously suspected. However, it turned out that the cottontails typically ate only the stems of the plants, going as far as to cut off and leave the growing shoots, where most of the toxins seem to be present. In contrast, the pygmy rabbits tended to eat the leaves, which are more toxic than the stems, leaving the latter alone.
This points to the kind of difference between closely related animal species that you can't observe simply by looking at their shape or colour. They may look much the same, but these two rabbit species have digestive systems that function rather differently. Cottontails are able to digest and extract nutrition from, highly fibrous foods that pygmy rabbits find more problematic. That's why they eat the stems, which are high in fibre, while the pygmy rabbits, even when they eat the same plant, eat different parts of it.
In contrast, the pygmy rabbits have a far greater ability to tolerate the specific toxins found in sagebrush, and, in moderate quantities, probably like the taste. (Even in summer, when other food is available, they eat more sagebrush than anything else, and in winter, it's about 99% of their diet). This is probably mostly down to their livers, which have evolved to process and remove terpenes from the diet, something that has been observed to result in more acidic urine, as well as a higher proportion of the substances passing straight through the gut and coming out in the droppings.
It turns out that the sagebrush benefits from this, too. The loss of plants due to being eaten is more than offset by the fact that pygmy rabbits defecate and dig their burrows, thus both fertilising and stirring up the soil, close to where they expect their food to be found. The end result is faster and more effective seed growth by the plants that they feed on, thus contributing to the preservation of the great sagebrush-dominated steppelands of the northwestern USA.
[Photo by Tom Koerner of the USFWS. In the public domain.]