[eli5] If genetics passed down from generation carry the anatomy of the creature, how does genetics pass down behaviors/instincts?
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Core behaviour, instincts and reflexes are to do with your nervous system. Like the rest of your body, this is coded by your genes. Your brain is different to a dolphin’s because your DNA makes it grow and develop differently.
Of course a behaviour is hard to distill into a gene. Say, finding vermin disgusting. This will help you survive and pass on your genes to your offspring. But rats, roaches, flies etc. all look different and there isn’t a gene that codes all of these animals into your brain for automatic recognition. But you could have a gene that means you find the smell of rotting food disgusting. This would mean that your smell receptors tell your brain to be disgusted when they detect the chemical compounds found in rotten food.
That means you avoid rotting food, and thus avoid vermin, and thus survive longer, and so are more likely to pass on your genes so that the human race will evolve to find rotten food repulsive.
If Sally in the next hut lacks this gene, and leaves rotten food out and attracts vermin and disease, she will likely die before she can have offspring. But your children will have the rotten-smell aversion gene, and so will their children etc. And thus over time the human population will only consist of descendants of people who have the rotten-smell aversion gene. That is evolution.
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