Why does evolution not make life longer for humans or any other species?
Why do we age is a classical question in Evolutionary Biology. There are several things to consider when we think of how genes that cause disease, aging, and death to evolve.
One explanation for the evolution of aging is the mutation accumulation (MA) hypothesis. This hypothesis by P. Medawar states that mutations causing late life deleterious (damaging) effects can build up in the genome more than diseases that cause early life disease. This is because selection on late acting mutations is weaker. Mutations that cause early life disease will more severely reduce the fitness of its carrier than late acting mutations. For example, if we said in an imaginary species that all individuals cease to reproduce at 40 years old and a mutation arises that causes a fatal disease at 50 years old then selection can not remove it from the population - carriers will have as many children as those who do not have the gene. Under the mutation accumulation hypothesis it is then possible for mutations to drift through the population.
Another hypothesis which could contribute to aging is the antagonistic pleiotropy (AP) hypothesis of G.C. Williams. Pleiotropy is when genes have more than one effect, such genes tend to cause correlations between traits, height and arm length probably have many of the same genes affecting them, otherwise there would be no correlation between arm length and height (though environment and linkage can also cause these patterns)... Back to AP as an explanation for aging, if a gene improves fitness early in life, but causes late life disease it can spread through the population via selection. The favourable early effect spreads well because of selection and, just as with MA, selection can not "see" the late acting disease.
Under both MA and AP the key point is that selection is less efficient at removing late acting deleterious mutations, and they may spread more rapidly thanks to beneficial early life effects. Also if there is extrinsic mortality (predation etc.) then the effect of selection is also weakened on alleles that affect late life. The same late-life reduction in the efficacy of selection also slows the rate at which alleles increasing lifespan spread.
A third consideration is the disposable-soma model, a description by T. Kirkwood of life-history trade-offs which might explain why aging and earlier death could be favoured. The idea is that individuals have a limited amount of resources available to them - perhaps because of environmental constraints or ability to acquire/allocate the resources. If we then assume that individuals have to use their energy for two things, staying alive via repair and maintenance (somatic-maintenance) and making offspring (reproductive-investment), then any energy devoted to one will take away from the other. If an individual carries a gene that makes it devote all of its energy to somatic maintenance then its fitness will be very low (probably 0!) and that gene will not spread. If the level of maintenance required to live forever costs more energy than an individual can spare without suffering from low fitness (very likely) or can even acquire and efficiently convert in the first place (also very likely) then high-maintenance alleles will not spread (and aging & death will continue to occur).
To go a little further, it is common for sexes to age differently (this is what I work on) and one possible explanation is that the sexes favour different balances of the trade off between somatic-maintenance and reproductive investment, this can lead to conflict over the evolution of genes affecting this balance and slow the rates of evolution to sex specific optima. This paper provides a good review of the area.
To summarise, evolution has not managed to get rid of death via genetic disease etc. (intrinsic mortality) because the effect is only weakly selected against, and those alleles may provide some early life benefit, and resource limitation may also reduce the potential to increase lifespan due to trade-offs with reproductive effort. Evolution is not about the survival of the fittest but the reproduction of the fittest - the fittest allele is the one which spreads the most effectively.
EDIT: Thanks to Remi.b for also pointing out some other considerations.
Another thought is that of altruistic aging - aging for the good of the population (the population is likely to contain related individuals, you are related to all other humans to some degree). In this model aging is an adaptive process (unlike in MA where it is just a consequence of weak selection). By dying an individual makes space for it's offspring/relatives to survive (because resources are then less likely to limit populations). This will stop excessive population growth which could lead to crashes in the population and so, by dying earlier, an individual promotes the likelihood that its progeny will survive. Arguments of altruistic sacrifice are often hard to promote but recent work suggests that this is a more plausible model than once thought.
Evolvabilty theories also suggest that aging is an adaptive process. These suggest that populations, composed of a mixture of young and old, have biases in how well adapted the members of the population are - where younger individuals are better adapted (because they were produced more recently it is likely that the environment is similar to the environment they are favoured in). Thus by removing the less well adapted individuals from a population via senescence and freeing up resources for younger better adapted individuals, a population evolves more rapidly towards it optimal state.
This is a very good question.
There is a big ongoing field of research called "evolution of aging/senescence" that tackles this question. I won't give you a complete overview of the different hypothesis the could explain why we age but here is a fundamental concept that is to know.
We'll assume that there is some extrinsic mortality, mortality against which a lineage won't never be able to escape and I think it is not an assumption that is hard to meet. Therefore, an allele that has age-specific effect such as decreasing the fecundity at age 3 will undergo a higher selection pressure and will faster get eliminated from the population than another allele causing the same decrease in fecundity but at age 4 because some individuals would have died between age 3 and age 4. In other words, natural selection is more efficient at lower age than at higher age. Now, imagine in humans an allele that increases the reproductive success of an individual at age 20 by decreasing the survival at age 78. This allele will easily spread in the population. Such alleles are said to have age-specific antagonist pleiotropic effect. And empirical studies have shown that alleles that have antagonist age-specific antagonist pleiotropic exist.
In short, it is because there is some extrinsic mortality that natural selection acts with a different strength at different ages allowing some deleterious allele at old age to fix in the population especially if those alleles have age-specific antagonist pleiotropic effect. You'll find in this book mathematical formulation and more complete discussion of this effect
Other hypotheses exist which are based on lineage selection, group selection or on the mutation-selection balance.
Actually, genetically, there is no reason for animals to continue to exist after they have procreated.
If you look at salmon, they die immediately after procreating, which is probably the most efficient way to carry the best genes to the next generation.
In the case of mammals, they need to teach their offspring where to find food, where to find water and how to avoid dangers.
In the case of humans, that goes into the third generation, so most humans know their parents and their grandparents, and even they live with them in some cultures, since their experiences and ideas are taken as very important.
So maybe the question should be the opposite, why do we live to see our grandchildren to grow?
Taking your same question to the opposite: why do people die? Why don't they live forever instead of reproducing. I can think of various reasons for this. Imagine a race that live forever and another race who have a lot of children early on and then die. Now imagine a contagious disease that spreads among both populations. Which race has the most probability of survival?
Of course the race that reproduces rapidly has a better chance.
And also, evolution works wonders for the race that reproduces rapidly instead of the race that lives forever. Meaning the number of years an species live has carefully been "designed out" by evolution.
Evolution would not work if it didn't stabilize around the best genes. And that's exactly what happens with humans. Most humans have the same traits: live around the same number of years, and have more or less the same abilities, most differences are almost irrelevant.
Because evolution isn't about individuals: it's about species. What matters to natural selection isn't how long you live, but how many grandchildren you have. A long lifespan can be an evolutionary advantage, but like any trait, it's only an advantage to the extent that allows you to reproduce more.
It would seem that a longer lifespan would be advantageous anyway, because it would give you more time to reproduce. However, for reasons we don't yet fully understand, it doesn't seem to work out that way in practice. Most organisms (assuming they live long enough) eventually reach an age where they stop reproducing. Even humans do this, and although we've managed to increase the average lifespan quite a bit over the course of recorded human history (to say nothing of the millennia before that), the average age at which people stop reproducing apparently hasn't changed very much.
Why not? What makes this an evolutionary advantage over having more reproductive years? This is one of those things that we haven't really figured out yet. There are a number of competing theories, and the other answers here go into some of them. But the most direct way to answer your question is fairly simple: longer lifespans (and/or reproductive years) haven't given us, or our children, any more success at reproducing. Thus, there is no pressure on the species to live longer, and so it doesn't happen.
If you take the line of "The Selfish Gene - Richard Dawkins". Evolution doesn't care about individuals, it cares about genes. So as long as the genes are passed along reliably into the future, evolution may do it with 4 generations per 100 years or 100 generations per 100 years.
There is no selection mechanism that would favor high age.
By the time it's apparent whether or not an individual can reach a high age healthily, they'll have ceased all reproductive activity.
Conversely, people who get cancer at 45 will have likely reproduced already.
To an extent it does; in that we live longer than our mouse-like ancestors. So the question becomes: why not keep extending it to immortality.
The key thing is that evolution cares only about the survival of your genes; so if you live for 1000 years or if 10 generations of your family have 1 individual's worth of your genes in each generation (each living for 100 years) this is equivalently successful.
But this assumes it's either-or that an organism can reproduce or live a long time, could it not do both? In principle it could but the resources of a particular niche are limited so in order to avoid mass starvation the reproductive rate must reduce as the average age of the individuals go up.
So this suggests having very long lived individuals is no better than having short lived individuals, but is it at least equal? Sadly not, if fewer new individuals are born then the rate of evolution of that species is reduced. A very long lived species is less able to respond to environmental changes. As such over an evolutionary timescale an extraordinarily long lived animal is likely to be outcompeted by a shorter lived species.
Of course there is a natural breakeven point; there is a considerable cost in bringing an individual from infant to reproductive adult so once they've got there it makes sense to keep them around for a while, but not indefinitely.
protected by Community♦ May 2 at 20:20
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