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The theory of natural selection has it that individuals with better genes tend to survive and reproduce, passing their genes to their offspring. This gradual process results in a population more adapted at survival. However, due to the advancement in medical science, humans with poorer genes tend to survive and reproduce just as well.

For example, in the past, many people, excluding those with natural immunity due to some genetic mutations, would have succumbed to illnesses such as malaria and typhoid. But with better hygiene and medical treatment, these patients tend to survive.

Now, imagine a future where all illnesses including cancer can be treated. How would it impact the survival of the human race? Would we become more and more vulnerable such that our survival hinges heavily on medical technology, analogous to how an astronaut's survival is dependent on his spacesuit?

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First, healthcare after reproductive age should not affect the survival of the humans as species. Second, if healthcare is here to stay, not just some temporary episode, then even healthcare for those under reproductive age shall not harm survival of humans, no more than knives and fire would harm survival. Think of healthcare as another tool like fire and knives and there should be no reason it would harm the survival. –  Andrei Nov 20 '12 at 21:15

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Gene frequencies (frequencies of each allele at a given loci) in populations are affected by many things divided in to systematic and dispersive processes. Systematic process (migration, mutation, selection) affect gene frequencies in an often quite predictable manner and strength. Dispersive process (Random drift, differentiation between sub-populations, uniformity within sub-populations, increased homozygosity) are random in their direction and only predictable in their amount. Your question focusses on the systematic process called selection.

In a large population (if we ignore dispersive processes for the time being) the gene frequencies will reach an equilibrium called Hardy-Weinberg equilibrium. This means, with constant rates of the systematic processes, each locus will reach a point of stable equilibrium. Mutation and migration bringing more variation, and selection reduces it (over simplified - sometimes selection maintains variation, see my answer about why variation exists here). The frequency of each allele at equilibrium will be where these effects balance out. If selection becomes weaker against one allele it will become more frequent in the population.

By using medicine etc. to cure diseases we reduce the selection acting on a deleterious mutation thereby putting the gene frequencies in to a state of flux during which they move to new equilibrium points. So in short the answer is yes, modern medicine is decreasing our genetic fitness by allowing deleterious mutations to become more common.

I raise the question, will we ever be able to cure all genetically caused disease? In my reckoning, no, at least certainly not in our lifetimes.

It only represents a real risk if we suddenly are unable to cure diseases. This would increase selection against that allele and the gene frequencies would move back towards the equilibrium point which existed before the selection was reduced. What this means for the population is the loss (death) of those with the low fitness genotype.

The best reading you can do on this is the opening chapters of Falconer and Mackay's Introduction to Quantitative Genetics. Also read about Hardy-Weinberg equations.

HTH

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I have to read a few times to understand. But this is certainly a great answer! –  Question Overflow Oct 31 '12 at 13:52
    
I know how you feel, I have read the chapters in that book many times and they are not easy! condensing it down was hard! –  GriffinEvo Oct 31 '12 at 15:42

The theory of natural selection has it that individuals with better genes tend to survive and reproduce, passing their genes to their offspring.

Yes, under selective pressure, that is. The stronger the selective pressure, the more the population will change. Without a selective pressure, there isn't an impetus to change.

However, due to the advancement in medical science, humans with poorer genes tend to survive and reproduce just as well.

You're making a mistake. "Poor" and "Good" are purely contextual. You might say that Sickle Cell Anemia is a poor lot to be stuck with... Unless, of course, you're in an area ravaged by Malaria. Then it's a boon.

What would be more accurate is that there aren't any strong forces acting upon humans that causes selective pressure and the proceeding adaptation. Of course, we are the result of such evolution, so by a basic standard all modern humans who reproduce are fit in a Biological sense.

For example, in the past, many people, excluding those with natural immunity due to some genetic mutations, would have succumbed to illnesses such as malaria and typhoid. But with better hygiene and medical treatment, these patients tend to survive.

The brain is the one of the finest products of evolution. Because we're no longer dying en masse at the whims of bacteria or virii (usually) does not mean we haven't adapted. Our ability to build advanced tools and perform science is an adaptation. One for which we pay heavily; we're practically helpless until our teens.

Now, imagine a future where all illnesses including cancer can be treated. How would it impact the survival of the human race?

We'd live longer, and assuming the rate of reproduction stays the same, there'd be a lot more of us.

Would we become more and more vulnerable such that our survival hinges heavily on medical technology, analogous to how an astronaut's survival is dependent on his spacesuit?

Well... Vulnerable to what? To disease? Probably not. Current Immunizations and treatments often utilize the body's own immune system; basically getting sick and all the benefits of encountering the disease without the side-effects of significant death.

To genetic diseases? No. The human population has exploded in the last hundred or so years, and our genetic diversity has been greater than at any point in the last 40,000 years when a major bottlenecking event occurred. For a genetic disease to deal significant damage on a species, it would have to be suffuse throughout the population. That's not happening.

To a post-apocalyptic world where technology breaks down and suddenly we're facing off against parasites and predators? Well, if the event ever occurs, then a lot of us won't be ready, that's for sure. The stress such a life put on us in the past limited our lifespan to nearly half what it is now, but unless there's a complete lack of knowledge many basic safety habits should persist. Cooking, boiling water, dressing wounds, how illnesses spread - these are all things which dramatically increase quality of life when utilized correctly.

Will we ever reach a point where we, quite literally, cannot live without technological intervention? No idea. Humans have very, very strong reactions to things which aren't human but appear human (The Uncanny Valley is a great example). It's just as possible that those instincts will prevent us from such a fate as the possibility that we'll be completely fine with becoming cyborgs. Nobody can even begin to give you a time-frame, though. As-is, there's no reason to become cyborgs (no Selective Pressure), and won't be for the foreseeable future.

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