Why do the birds,and other animals have instinct ?

Question

From the genetics point of view ?

Answer 1

Instincts are basically reactions to situations or conditions that don't require thought. A cat instinctively arches its back and hisses in the face of danger; a fly automatically takes flight if you swat at it; a mother bear will not think twice (or at all) before attacking an intruder who might threaten her cubs.

Humans have instincts too. Maternal instincts include being prone to hearing a crying baby when asleep (something many men lack altogether); infants have a sucking instinct, and many other less obvious examples exist.

Instincts define behavior, and behavior originates in the brain, so instinct is effectively a change in brain chemistry favoring a certain behavior. Brain chemistry, like all bodily chemistry, is primarily modified by proteins, which are in turn coded (ultimately) by genes. A mutation in a gene can cause a change in protein structure, which almost always causes a change in protein function and therefore behavior.

Take the classic example of knockout mice, mice that have a certain gene made inactive to see how it behaves differently. It turns out that knocking out a certain gene in the mouse genome causes mice to lose their fear of cats, their natural predator. Let's assume that mice initially lacked this gene, meaning they didn't run away at the mere scent of a cat. Such mice would get eaten very often.

Most of these mice died, but one mouse had a mutation that caused it to react in a certain way to cats, not run away but at least change direction. Such a mouse had a significant advantage over mice that were indifferent to cats, and would be much more likely to survive, mate and pass on the mutant gene that caused its change in behavior. Soon any mouse that lacked the gene would be eaten and not pass on its genes.

At some point, a different mouse has another mutation on the same gene which causes it to not simply change direction but to turn the other way entirely. This would once again be an advantage over its peers and the gene would eventually dominate all its alleles (same gene without the new mutation) in the gene pool.

Such small incremental steps are how evolution works. After millions of years of such small steps, mice have a fully function "fear-of-cats" gene (which may be an oversimplification of how fear actually works in mice).

Answer 2

They have it in order to survive, be it in obtaining food, shelter, a mate, etc, etc, etc.

From the genetics side of things it is mainly going to be the survival of the fitest. In other words just allowing the strongest blood line to be passed on.

Answer 3

natural selection