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The confusing part is that we are used to thinking about evolution as spanning multiple generations: a gene increases fitness, so its prevalence increases over

The confusing part is that we are used to thinking about evolution as spanning multiple generations: a gene increases fitness, so its prevalence increases over the generations. Or the flip side: it decreases fitness, so the members that have it reproduce less, its prevalence decreases. But this study takes a snapshot of a population, and looks at gene prevalence as a function of age. So what you see are not effects of reproduction, but rather on survival. It finds the Alzheimer's gene less in 70 year olds, because those with the gene are more likely to have died earlier, and thus not appear in the study. Thus your statement "APOE-Alzheimer-propensity gene is a new gene" is likely not true. It is likely just as prevalent in the current generation as two generations before; just those individuals from two generations before who had it are more likely to be already dead. So the study looks at how genes influence survival rates -- something that is very obviously related to evolution if it occurs before reproduction. If survival rates only depend on some genes in higher age, they have only small, second-order effects on reproduction (like a child being more likely to survive if it has healthy grandparents or parents). (Disclaimer: I'm not a biologist, so take this with a grain of salt).

Udik 9y ago
> It finds the Alzheimer's gene less in 70 year olds, because those with the gene are more likely to have died earlier, and thus not appear in the study. So wouldn't the study be: "Those carrying genetic disease have a tendency to die early(er)"? What does evolution have to do with it? BTW, it's perfectly possible for a gene to be wildly successful from the evolutionary point of view, while at the same time killing its owner invariably by the age of 50. How would this study have classified such a gene?
riazrizvi 9y ago
I suppose that does explain the drop off in expression of the gene: Alzheimer's risk of death/year 1-3% [1]. That annual mortality rate is in the right ballpark to explain the frequency of gene expression decrease by about 50% between ages 40 and 80 [2]. Agree the study does look at genes influencing survival rates. I still don't see the link to the headline 'humans are evolving!'. Small second-order effects, okay, sure. So how about, 'Massive Genetic Study shows genetics affect mortality rates, plus maybe there's a small secondary effect on reproduction, who knows?'. [1]. 83,000 (i) or 3,349 (ii) Alzheimer's deaths per year out of 2mm deaths (iii) => 1-3% i) http://www.nydailynews.com/life-style/health/alzheimer-kills-previously-thought-study-article-1.1712078 http://www.nydailynews.com/life-style/health/alzheimer-kills... ii) http://www.alz.org/mglc/in_my_community_60862.asp http://www.alz.org/mglc/in_my_community_60862.asp iii) https://www.cdc.gov/nchs/fastats/deaths.htm https://www.cdc.gov/nchs/fastats/deaths.htm [2] See graphs in paper.
- Retric 9y ago
  People over 40+ still have kids. So, there would be a direct link if people started to die at 41.
- mirimir 9y ago
  I'm confused. Sure, they identified two alleles with occurrence that decreases with age. And sure, that does suggest that those alleles reduce survival rate. But to demonstrate evolutionary change, they'd need data on occurrence of these alleles as a function of birth date, stratified by age. It'll take some time to get such data. Probably at least a few decades.

Udik 9y ago

> It finds the Alzheimer's gene less in 70 year olds, because those with the gene are more likely to have died earlier, and thus not appear in the study. So wouldn't the study be: "Those carrying genetic disease have a tendency to die early(er)"? What does evolution have to do with it? BTW, it's perfectly possible for a gene to be wildly successful from the evolutionary point of view, while at the same time killing its owner invariably by the age of 50. How would this study have classified such a gene?

riazrizvi 9y ago

I suppose that does explain the drop off in expression of the gene: Alzheimer's risk of death/year 1-3% [1]. That annual mortality rate is in the right ballpark to explain the frequency of gene expression decrease by about 50% between ages 40 and 80 [2]. Agree the study does look at genes influencing survival rates. I still don't see the link to the headline 'humans are evolving!'. Small second-order effects, okay, sure. So how about, 'Massive Genetic Study shows genetics affect mortality rates, plus maybe there's a small secondary effect on reproduction, who knows?'. [1]. 83,000 (i) or 3,349 (ii) Alzheimer's deaths per year out of 2mm deaths (iii) => 1-3% i) http://www.nydailynews.com/life-style/health/alzheimer-kills-previously-thought-study-article-1.1712078 http://www.nydailynews.com/life-style/health/alzheimer-kills... ii) http://www.alz.org/mglc/in_my_community_60862.asp http://www.alz.org/mglc/in_my_community_60862.asp iii) https://www.cdc.gov/nchs/fastats/deaths.htm https://www.cdc.gov/nchs/fastats/deaths.htm [2] See graphs in paper.

Retric 9y ago
People over 40+ still have kids. So, there would be a direct link if people started to die at 41.
mirimir 9y ago
I'm confused. Sure, they identified two alleles with occurrence that decreases with age. And sure, that does suggest that those alleles reduce survival rate. But to demonstrate evolutionary change, they'd need data on occurrence of these alleles as a function of birth date, stratified by age. It'll take some time to get such data. Probably at least a few decades.