How do "instincts" work?

I’ve never heard a clear explanation of the mechanism by which a species can develop innate fears and behavioral patterns. How exactly is it that revulsion of snakes, herding, ant colony architecture plans, spider webs, and other such complex behavioral patterns come to be encoded and passed down to other generations? From a cursory glance, it almost sounds like Lamarckism.

Lamarck as it turns out wasn’t completely wrong. See the ‘epigenetic inheritance’ section under neo-Lamarckism.

Survival.

Very helpful. :rolleyes:

I think nowadays when people think of brains and intelligence they straight away go to a computer analogy, and it can be very misleading.
For most animals, much of their behaviour is instinctive, so it’s not like a general-purpose computer running the “evade predator” program, it’s more like your knee-jerk reflex but with a much more complex pathway.

So instincts are something like wiring patterns, and as long as offspring inherit in some sense their parent’s patterns, possibly with differences due to mutations, then you’ll get evolution over time.
That said, the genetics of this we don’t understand yet. The “connectome” is far more information than the genome could contain. But of course the same thing applies to our circulatory system. Also large changes to behaviour can be brought about chemically so this is another way that instincts can occur.

There’s no reason why there couldn’t be genetic coding that created an increased level of activity in the amygdala (fear) when certain patterns of neurons in the sensory cortex were activated (i.e., certain visual, auditory, olfactory, etc, patterns). That’s just an example. The real issue is transferring that new genetic code to germ cells. That’s really the trick.

I’m not sure, but I don’t think the ability to create genetic changes in any given generation of a species as the result of either mutation or environmental pressures is really in dispute. It’s the latter part - the ability to repackage them for propagation that’s in dispute.

The reason it’s unclear is that we still don’t understand all that well how brains work. So, we can understand that instincts and behavior can evolve like any other trait - individuals are born with slight variations in their instinct genes, some are better at surviving than others and so pass on their variants, and the behavior evolves. It seems weird only because we don’t have a good feel for how a gene can cause a behavior. That genes can and do cause behavior is incontrovertable. Behavior is controlled by the brain, the brain is made of cells, and cells are governed by their genes. Behavioral geneticists have been linking specific genes to specific behaviors via specific neurons in model organisms forever. But our understanding of how, precisely, that works is still in its infancy. We’re working on it, though.

What sounds like Lamarckism? You have not outlined a mechanism, or referred to any example explanations, so I do not know what you think sounds like anything. Anyway, I see no difficulty in explaining, in general terms, how instincts could evolve via impeccably Darwinian processes. A mutation causes some alteration in an organism’s brain structure such that it reacts more fearfully towards (let us say) spiders than do others of its kind. Since spiders are dangerous to this species, the individual with an increased fear of spiders is more likely to survive and reproduce than are its fellows, and the mutated gene spreads. Nothing Lamarckian there. What’s your problem?

If the real question is “How do genetic changes lead to behavioral changes?” then I think the real answer is that we do not really understand very much about how genes affect behavior (although we have lots of good reasosn to think that they can do so). Presumably it works via subtle changes in brain structure and electrochemical function, but we still understand very little either about how the genome determines how the brain is structured, and how it works, or about how brains control the details of behavior. We have some possible clues about what we ought to be looking for, though. See, for instance, Rakison & Derringer (2008), “Do infants possess an evolved spider-detection mechanism?” [PDF version].

Some behaviors such as nest building for instance, birds of the same species will often build the same kind of nest, or spiders will spin the same web. One that has always puzzled me if I have it correctly is the migration of the monarch butterfly. Supposedly it will migrate south and then die, its off spring will return to the same trees its parents bred in without ever having seen that tree. I believe this is what the OP is reffering to.

“How exactly is it that revulsion of snakes, herding, ant colony architecture plans, spider webs, and other such complex behavioral patterns come to be encoded and passed down to other generations? From a cursory glance, it almost sounds like Lamarckism.”

The change isn’t created during the creatures life and then passed down (generally), the creature is born with a slight tendency in that direction due to a semi-random variation in the creatures genes. The tendency helps the creature survive and have little critters. Some of the new little critters have the same tendency, and eventually their little critters semi-randomly have an even greater tendency which helps them even more.

Lather, rinse, repeat.

Divergent ants just aren’t going to hack it, so once an ant’s habits become successful, they aren’t going to change very much, and for what it is they do, they don’t really need to.

Right. People who by chance happened to have an innate fear of snakes outproduced people who didn’t, because the latter were bitten and died.

Now, we don’t know nearly enough about how the brain works or how genes are expressed to understand how genes can create instinctive behavior. Fear of snakes is a pretty good example, since it’s unlikely there’s a gene coding for recognizing snakes per se. That is, there certainly isn’t a gene coding for every concept, because we don’t have nearly enough genes.

In any case, it doesn’t have to be lamarkism, because learned things don’t need to be passed on.

I think the other reason we have a hard time understanding instinct is because we like to believe that we have free will. If I take the time to, say, weave a basket, I know my own mental processes. I know that I deliberately set out to weave the basket, and how I planned it, collected the materials, and worked through each step with a goal in mind. So when I see a bird making a nest, it’s really hard for me to understand how it could have done that with what I would call mindless instinct. MY brain is run by ME; animal brains are run by DNA. Now, obviously, I’m playing fast and loose with terminology, and edging dangerously close to very deep waters, but I think this is another reason the idea of instinct is so foreign to us. We know perfectly well that our brains are completely under our control. It’s almost scary to think that there might be some other force at work.