In modern evolutionary biology, teleology is considered to be an irrelevant explanation for what caused things to be. Nevertheless, you may find many biologists speaking in teleological terms - "The animal evolved wings so it could fly away from predators." or "Our brains became bigger to allow us to function better socially." This is a trick that evolutionary biologists use: It turns out that most language is clumsy and inconvenient for talking about the dynamics of evolution by selection, and evolution really looks almost as if there was teleology.
The key word is "almost". There are subtle differences between how an evolution with teleology and without teleology would look like. Biologists use the "trick" because they don't like being wordy and think that they are smart enough to never overlook the teleological fallacy.
Teleology is regarded as irrelevant because nature has no mechanism for deciding to obtain certain outcomes beforehand. Evolution is not a means to an end, it is a consequence of some individuals making imperfect copies of themselves more often than other individuals. Whenever one has a population of things, and the following claims hold for this population:
- The things make copies of themselves. (reproduction)
- Some the things reproduce more than others. (differential fitness/selection)
- Copies can end up being is slightly better or worse at copying than the thing they were copied from. (mutation)
One will observe, after following this population, that the nature of the things making up the population changes over time. This is a bit of "duh" conclusion because, well, of course you will end up with a lot of the fast-reproducing ones and few of the slow-reproducing ones, and of course you may get a lot of much faster reproducing things than any in the initial population, because the fast-reproducing ones make more copies of themselves (even if some are a bit mutated). This also applies to theoretical constructs such as numbers as well as actual biological organisms, and everything in between (and is supported by ample empirical evidence at different levels). Copious repeated applications of this theme, adapted to individual cases, are pretty much the business of evolutionary biology, and these days, it's rare to find a biology that isn't even a little bit evolutionary biology as well.
It isn't hard to see that in the above example, nobody tried to make the fast-reproducing things dominate, and nobody wanted this outcome. It's just what happened, because of different rates of reproduction they have. If you get down to the differential systems describing the system, you will see that setting it up in this way literally implies that the number of fast-reproducers will increase over time (and this is really what it means "to reproduce rapidly").
Moreover, the outcome of an event cannot also be its cause, because the cause is often understood to precede the outcome in time. My investment of money in stocks last month cannot have been caused by the profit I made today, because last month, I hadn't yet made the profit. The cause can be my anticipation that I will profit, but again, evolution does not require nor seems to exhibit evidence of any "anticipation" - it will continue to repeatedly generate mutants that are unfit even though they are "clearly" not going to work out well, and evolution still works in spite of this. Teleological arguments may explain the actions of rational actors, but who wants to prove that nature is rational and/or that God exists without making circular references to evolution?
The question "why did this feature of this organism evolve?" comes up again and again in biology. Besides simple curiosity, it also has practical implications, such as "what if this feature stops working?" or "what features would, if they stopped working, produce this effect?" (for causes of disease).
When you are trying to to figure out the function of something by asking why that something is there (to clear away some of the philosophical issues, by function I mean how it helps and how its absence hurts), if your explanation starts with "it came to be so that it could..." then you know you are wrong, because nature cannot anticipate what will provide which benefit. It simply tries everything, and keeps the ones that work.
Therefore, to explain why something in biology exists and is the way it is, you have two options:
- Assert that it arose by random chance. (this is actually the case in reality often enough, it is known as genetic drift) The challenge with this option is explaining how it arose by chance despite being unlikely. If you succeed, then you can expect that removing the feature, or at least replacing it with an alternate version will have no effect (for example, it doesn't really affect your life if you are capable of rolling your tongue or not).
- Assert that it evolved. There are two challenges. The first is to explain how individuals which possessed the feature reproduced more rapidly than those who did not. The second is to explain the same for every intermediate form, because evolution happens in small steps - it is not enough to show that having wings is better than not having wings, you must also show that having wing stumps is better than not having them, because the wing had to have come from "somewhere". If you succeed, you can predict what problems having a defect in that feature will create, and after building a large enough database of such defects, you can then take any defect and immediately narrow down its cause to a handful of features known to lead to it. You can also suggest how to improve the feature further.
Some obvious ways in which teleological explanations can fail are:
- Without knowing, you may come to believe that a feature came to be to achieve a goal, when in reality it has no function ("why do men have nipples?"). You will then end up chasing after "red herrings" when looking for causes of effects.
- You will be puzzled why seemingly desirable features do not exist ("why don't animals have wheels?").
- You will assume that every feature is "as good as it gets", and then you will be surprised when some of them later manage to get better.
- You will accept as true some fantastical explanations which are impossible because the intermediate forms are too unfit, since you will assume that nature will "grin and bear it" through those "tough times" of very low fitness "for the sake of" the final outcome with very high fitness. (more broadly, this problem is known as "climbing a hill" in optimization)
That's not to say you don't have to look hard to find pitfalls of teleological thinking (although it's clearly illogical after considering the nature of evolution). That's why the "trick" exists - most of the time, you can carry on happily thinking teleological thoughts and never see any ill-consequences. But that one time when it causes you to be wrong is a possibility scientists do not like to accept. Being a heuristic, the pseudo-teleology trick is not very persuasive when you are trying to settle one argument and move on to the next gene already.