The following is a question (rephrased somewhat) by a friend and reflects a bias many people have when thinking about organismal behaviour.
''How can behaviour be hard-wired into an organism? For example, how can a parasitiod wasp exhibit the sophisticated behaviours that it does when finding the ant nest that harbours a blue butterfly caterpillar? A human baby needs constant care by its parents, but a wasp already knows what to do when it emerges from a caterpillar. How is it able to do this?''
The basic answer is that all organisms have behavioural sub-routines that are appropriate to the demands of their environments (that is, the criteria that determine whether individual organisms will prosper or die), because these environments have selected for these sub-routines (alternative sub-routines were "purged" from the population, because they didn't have what it took, given the aforementioned criteria, to get into the next generation). Humans are utterly dependent at birth on their parents/guardians, but they can afford to be; these wasps, on the other hand, can hit the ground running, and this has to do with the particular life style they have evolved to excel at. The question, "How do they know where to go and what to do?" can be answered in different ways, depending upon what aspect of the behaviour you're interested in - or, more precisely, what level of abstraction you're interested in. If you're interested in the actual physical biochemical processes going on in the brain of the wasp, then you need to ask a question about what enzymes and neural patterns are being used, and so forth. This "low level" approach, while interesting, is often not what evolutionary biologists are centrally concerned about. The question they ask (or at least, what behavioural biologists ask) is: "why" do these wasps behave in this way? "Why" is this behavioural routine, as opposed to some other, the one being maintained in the population? This is a more high-level, abstract question that separates the actual nuts-and-bolts happenings from what can be classified as a "functional" perspective (what is this behaviour "for"?). However, the two questions are of course not entirely separate, but in order to link them, it's important to understand something from the outset: natural selection, in its most general formulation, is simply this: certain configurations of matter are more effective at perpetuating themselves than other configurations, by virtue of the consequences they have in particular environments. Once you've understood this, the conceptual hurdle of thinking about how wasps "know what to do" becomes much less formidable.
As a thought experiment, imagine the following: a mutation arises, which happens to change the amount of some enzyme that was being manufactured in the wasp's cells, which has an effect on the expression of some compound that is used in the synapse connections in the brain, which...and so on, which has the consequence that the search pattern of the wasp is changed in some way. It's not important to know exactly what the particular pathway is if we're focusing our attention at the high-level functional mode of analysis, only that there are such pathways. By virtue of the eventual behavioural consequence of the mutation, the latter will automatically become more prevalent in the population (because it does better than "rival" - alternative - mutations at helping the wasp carry out some task). All we need to assume is that there are genetic changes that result in the brain being altered in some way, and that these changes have consequences for survival and reproduction. This is how natural selection "programs" the behaviour of organisms: by indirectly selecting among genetic changes via the proxy of some high-level effect (in this case, organismal behaviour) that is "visible" to it. Thus in this sense does the genome "code for" these behaviours. The genes themselves of course have no idea about anything; they're just strings of nucleotides. But due to the effects they have on the physical patterns of the brain during the development of the wasp, we can treat them as though they knew what they were doing. And so, by extension, the wasp "knows what it's doing" - because it has neural structures that result from this coded program, and these structures process information (or rather, environmental stimuli that are represented to the wasp in an appropriate pattern) in a very particular, highly effective way.
That's really all there is to it. Again, we must always be able to translate back and forth between our low level, nuts-and-bolts explanation ("protein A induces protein B to...etc") with our high-level, functional, more abstract explanation ("Wasps do this because, in this environment, this behavour allows the wasp to find food more easily...etc"). In practise, biologists always implicitly know what they mean when they talk about a behaviour (or a gene) "for" something. This more abstract way of talking is simply less cumbersome than having to always revert back to "gene language".
The mistake people often make is to be mystified by behaviour, because they think that the animal has to "know" what it's doing. And, of course, at some level this is true. The animal has to be able to process information and act on that information in a highly sophisticated way (sometimes, anyway). But it's not necessary to imagine that the animal consciously has any conception of what it's doing, any more than the genes that code "for" that behaviour know. The animal might, as a matter of fact, know what it's doing, or it might not, but this isn't a prerequisite for it actually doing something. A similar question could be asked of cells: how do they "know" how to manufacture proteins (and the myriad other things they do)? And again, the answer is that selection has favoured certain configurations of matter that have the effect that the entity in question behaves in some way appropriate to the demands of the environment, and that some of these ways are more appropriate to the needs of coping with the overall environment than others. It's simply following a set of instructions and rules-of-thumb which have been programmed by natural selection over many generations, given the average set of constraints and challenges encountered in that environment by an individual's predecessors. In the case of cells, we're interested in how proteins are made and delivered to other parts of the cell. In the case of behavour, we're interested in how complex, multicellular systems (that is, organisms - and people!) interact with their environments. Of course, there are more "steps" along the way when considering complex organisms than there are when considering single cells - but this just means that the causal chain is more tortuous. There is still an effect that reaches out from the original, biochemical goings-on in the cell to the outward manifestations that are expressed by the organism as a whole. If we take the abstract, high-level view, we can "cut out" the biochemical middlemen and focus on the functional significance of some behaviour over another (that is, its relevance to the organism's prospects of survival and reproduction), and simply ask "Why does a wasp do this?".
This template can be used for thinking about any complex biological feature, whether morphological or behavioral. A favourite of mine is the examples of mimicry that many insects exhibit. Some insects look just like dried leaves, right down to the veins and ragged edges. Others look like other insects, as a means of infiltrating their dwellings. As a kid, I marvelled at such things. I always had a sense that some historical process had built these structures, but I was mystified as to how the insect could ''know'' to build such a structure, just as I was mystified as to how Triceratops could ''know'' to have horns to defend from Tyrannosaurs. I knew that the animals in question didn't actually need to know in the conscious sense, that they weren't actually thinking about their needs in that particular environment, but nevertheless there had to be some design process, as it were, present (and no, I didn't gravitate towards God. I don't know why that was; perhaps the atheism of my parents unduly influenced me. Religion always seemed to me too dopey to take seriously, perhaps because of the pious injunction to believe or face the torment of Hell, which perhaps led my young mind to suspect that control, rather than explanation, was at the core of religious doctrine). The scientific and fully satisfying answer is that the design work has been distributed over many generations, with modest increments that aided survival and reproduction being preserved here and there, accumulating through time and resulting in something that looks uncannily like the result of a conscious design process. This is how the universe manufactured us.
Once you've made this conceptual inversion of the world - that historical, impersonal material forces can produce things that exhibit, in some important sense, function - then you've come to a profound philosophical insight: conscious purpose need not precede form.
Evolution is the process that changes systems that are themselves endowed with a series of internal processes to navigate an environment space. As the systems change, so does the broader configuration that it is a part of. These systems filter information about the environment, come to a determination of what to do, and then, based upon the fit of the response with the severity of the environmental demand, differentially propagate themselves - and change the environment while doing so, partly because they are themselves simply part of the environment, and partly because they craft the chemistry that permeates the biosphere. In this context can systems enter into cooperative relationships with other systems, merge with them, purge them, and parasitise them. No organism can be understood in isolation, and the biosphere, the totality of all the organisms and their interrelations, must be understood as a gigantic web that is itself changing through time.
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