We know for a fact that with age, individual starts to degrade. And this is true with all individuals of all species. Simply, the odds of defect increases with age.
For example, a computer which worked for 5 years is statistically more likely to break down than a new computer.
Human who is 80 years old is statistially much more likely to die in next 10 years than a young individual.
So it seems that two rules are certain:
- Individual has limited life span
- Limited life span is due to change which is actually a defect and causes harm
So, a simple conclusion is drawn: Aging happens due to negative change which becomes defect.
But are species as a whole immune to this same rule?
For a long time, it was believed that natural selection makes sure only good traits are passed.
However, this couldnt be further from truth.
A negative trait which is harmful, but isnt too damaging to reproduction ability, actually gets passed on.
We can see that animals and humans with defects often do reproduce, and even punish their offspring with same defect they have.
So natural selection actually only applies to reproduction related big defects. It does not apply to small defects related to reproduction, and it does not apply to defects irrelevant to reproduction.
Small defects which reduce reproduction success by 1% arent really going to be ever removed by natural selection, because:
- Defect would still get passed on a lot
- Even individuals without defect could produce offspring with defect, because random mutations happen even when original genetic trait remains.
- When this defect happens in females, it is certain it will get passed on, because defect which reduces male's chance to reproduce might not affect female in same way
So we have 4 types of defects which arent removed by natural selection:
- Small harm to reproduction defect
- Defect which doesnt harm reproduction
- Random mutations which happen in offspring even when parents are not defective.
- Defect in females which doesnt affect female reproduction ability, but affects male offspring if inherited
Because of these 4, not only that natural selection fails to remove small defects, but given enough time, number of small defects within species greatly increases.
Because even by 3 alone, the more time passes, the more likely it becomes that at some point, most offspring experiences random small defective mutations, and these then must get passed on or species will lose most of its population.
So number of negative defects, given enough time, actually increases.
And this is negative change, or the aging of species.
the only way this can be avoided is if natural selection and random mutation remove defects faster than they are created, but this actually isnt possible. Random mutation is probably as likely to produce a negative change as it is to produce a positive change, and once it produces mass of negative change, such change becomes inherited on mass scale.
In other words, aging of species is something which happens, and it is very likely that given enough time, species at some point lose ability to reproduce, and thus perish.