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Evolution Preserving Maladaptive Traits

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This is a concept I've been having a bit of a hard time with, as it seems there are several examples of traits that should have been selected against a long time ago. There are an abundance of traits that seem to be selectively maladaptive, yet have maintained an existence in species-populations (i.e. microcephaly, achondroplasia, hemophilia, color-blindness would have been devasting in the environment of evolutionary adaptiveness in humans, etc.).

 

I suppose you could argue that many of these traits are neutral, or the onsent of nearly all genetic disorders could occur past the point of conception, meaning the carrier could reproduce before the gene is expressed in the phenotype. However, some diseases, such as cystic fibrosis, should have been immediately selected against; primarily because the majority of individuals with the disease are infertile. Also, some genetic disorders could be adaptive (i.e. bipolar disorders - the manic stages could provide more energy for a hunter-gatherer to hunt and horde, and then rest while in the depressive stages. Also, sickle-cell anemia carries who reproduce with a mate with a healthy version of the gene could produce children more resistant to malaria), and thus even though they are viewed through a maladaptive lens in society today, they could have served an adaptive purpose in our past.

 

Considering the vast body of hereditary diseases, and different causal factors expressing these genes, I must ask: do you know of any explicit genetic diseases (or class of genetic diseases) that don't fit well with our modern conception of evolution?

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A common mistake is to think that evolution is some sort of god, that it can eliminate EVERY single deleterious gene. It's not the case. Many genes that are causing genetic diseases are very rare, and as most of the time they are recessive, they don't reduce fitness "enough".

 

Also we often find that a gene increases the likelihood of having a certain disease, but the gene can do a lot more so the benefits outweigh the risks, especially if the frequency of the gene is low in the population. I'm really not an expert of genetic diseases, but I've heard that the gene causing cystic fibrosis was providing a protection against typhoid. Sickle-cell disease is another well known example...

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Phil.....your quote- is absolutely marvelous. As a poet-published poet....I promise to mention you in my upcoming composition as an inquisitor of emotions as a feasible foundation for science.

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Also note that selection occurs on the phenotype and not on the genotype level. Most genetic diseases are the result of recessive alleles. And recessive alleles will usually not be eleminitated from the gene pool as there are usually heterocygous carriers of these alleles which do not express the disease.

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Some diseases that might seem 'maladaptive' are really evolutionarily neutral because they usually strike after an organism has ceased to reproduce. Evolutionary success isn't measured in lifespan, it's measured in number of reproducing offspring. If you have two women, one who lives to be 100 but has no children and another who dies at 50 of Alzheimer's but has 10 children, all of whom grow up to breed, then the latter woman is by far the more evolutionarily successful.

 

Armand Marie Leroi's Mutants explores this idea more fully, postulating that much of what we see as normal aging may simply be the combined results of genetic disorders that are 'evolutionarily invisible' due to the fact that they strike after spawning is imposible or no longer likely.

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Some diseases that might seem 'maladaptive' are really evolutionarily neutral because they usually strike after an organism has ceased to reproduce. Evolutionary success isn't measured in lifespan, it's measured in number of reproducing offspring. If you have two women, one who lives to be 100 but has no children and another who dies at 50 of Alzheimer's but has 10 children, all of whom grow up to breed, then the latter woman is by far the more evolutionarily successful.

 

Armand Marie Leroi's Mutants explores this idea more fully, postulating that much of what we see as normal aging may simply be the combined results of genetic disorders that are 'evolutionarily invisible' due to the fact that they strike after spawning is imposible or no longer likely.

 

That is true for most organisms but with some caveats. Man, like many organisms... wolves, ants, termites, etc. is a social animal. The passing on of genetic material is not invested in the individual but in the group. It's a numbers games and percents. Non-offspring producing individuals in social organisms can be more successful if their genetic material is passed on through collective effort. In some primate societies...some large mammals (elephants, whales, dolphins) the older non-producing individuals still have a role in the collective raising of the gene pool. A spinster aunt has a stake in the genetic material of her nieces and nephews being passed on successfully....as does a matriach elephant in a herd of younger fertile cows. The sterile uncle still hunts for the nomdic tribe...the grandfather knows where to find water...the female cousins make mocassins for their hunting male cousins, and so on.

 

The lady who lives to 100 and has no children may not be all that less successful in passing on genetic material than the one who dies at 50 with 10 child-bearing offspring. The odds are the 100 year old in our western society has a stake in the successful raising of thousands of individuals that she never gave birth to (let alone knows). An unproductive mother in a modern society could very well be much more genetically successful than a very fertile one in a hunter-gatherer society.

 

What we call empathy, caring, altruism and so are the strategy of the 'selfish' gene.

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This is a concept I've been having a bit of a hard time with, as it seems there are several examples of traits that should have been selected against a long time ago. There are an abundance of traits that seem to be selectively maladaptive, yet have maintained an existence in species-populations (i.e. microcephaly, achondroplasia, hemophilia, color-blindness would have been devasting in the environment of evolutionary adaptiveness in humans, etc.).

 

I think you will find that many of these are NOT "traits". Instead, they are spontaneous mutations and a few of them pop up every generation. I know that is the case for achondroplasia and cystic fibrosis.

 

Others are defects in embryological development -- microcephaly -- and don't relate directly to the alleles of the individual.

 

Others are shielded from the full effects of natural selection because human technology can compensate for them. That is the case for hemophilia. Individuals can be protected from the consequences of poor blood clotting by altering their environment so that injury happens rarely.

 

However, some diseases, such as cystic fibrosis, should have been immediately selected against; primarily because the majority of individuals with the disease are infertile.

 

Which means you are dealing with new mutations per generation.

 

There is also another consideration: most traits are polygenic. What this means is that there are several alleles at each loci (gene) that contribute to the trait. As you noted for bipolar disorders, the ability to provide energy is beneficial. That allele stays. It is only when that allele is present with 2 or more other alleles (maybe not common) do you get bipolar disorder. And, as you noted, the person is likely to produce offspring.

 

Also, sickle-cell anemia carries who reproduce with a mate with a healthy version of the gene could produce children more resistant to malaria), and thus even though they are viewed through a maladaptive lens in society today, they could have served an adaptive purpose in our past.

 

Sickle cell is a classic example of the superiority of the heterozygote. In a climate with malaria, both homozygotes are deleterious: homozygote normal gets malaria and dies. Homozygote sickle cell dies from anemia. So 50% of the offspring die. BUT, the 50% that are heterozygote have an advantage in that particular environment. Therefore the sickle cell allele stays embedded in the population because everyone has one such allele.

 

Considering the vast body of hereditary diseases, and different causal factors expressing these genes, I must ask: do you know of any explicit genetic diseases (or class of genetic diseases) that don't fit well with our modern conception of evolution?

 

No, for the several reasons I gave above. Mostly, your problem is:

1. Not realizing that the same mutations can occur in each generation. Thus, achondroplasia is not always inherited but can be a spontaneous mutation.

2. You are thinking of very simple Mendelian genetics of 1 gene = 1 trait. That is the very rare exception. Most traits are polygenic and most genes contribute to more than one trait.

 

Below is a more detailed discussion of sickle cell:

 

Douglas Futuyma Evolutionary Biology, pages 384-385.

 

"If the heterozygote has higher fitness than either homzygote, both alleles are necessarily propagated in successive generations, in which, of course, union of gametes yields all three genotypes among the zygotes. Heterozygote advantage is also termed overdominance or heterosis for fitness. If the fitness of AA, AB, and BB are 1-s, 1, and 1-t respectively, selection wil bring the allele frequences from any initial value to the stable equilibrium

p = t/(s+t, q = s/(s+t) where p and q are the equilibrium frequencies of A and B respectively. The equilibrium frequencies of the alleles and genotypes thus depend on the balance of fitness of the two homozygotes."

 

"Single locus heterozygote advantage has been documented in a few cases, including WAtt's study of PGI in Colias butterflies. The best known case is is the beta-hemoglobin locus in some African and Mediterranean human populations. One allele at the locus is normal hemoglogine, the other allele is for sickle-cell hemoglobin (S) ... The relative finesses have been estimated as W(aa) = 0.89, W(as) =1, W(ss) =0.2 [where aa is homozygote normal and as is heterozygote, and ss is homozygote sickle

cell]. The heterozygote advantage therefore arises from a balance of opposing selective factors: anemia and malaria. In the absence of malaria, balancing selection yields to directional selection, because then the AA genotype has the highest fitness. In the African-American population, the frequency of S is about 0.05 and is declining due to mortality."

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Rhinos are very short sighted because they don't need good sight so individuals with bad sight were not weeded out and were able to pass their genes on.

 

Humans have a very poor sense of smell because we don't really need one, and individuals with poor senses of smell....blah blah blah.

 

Wearing glasses is no longer a hindrance to passing genes on (and are now even worn as a fashion item) so humans probably will get worse sight in the future (if technology doesn't stop it happening).

 

Unless it's a hindrance to passing genes on, 'maladaptive' traits will increase as they won't get selected against. The term 'maladaptive' is a fuzzy one though as has been mentioned above.

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Unless it's a hindrance to passing genes on, 'maladaptive' traits will increase as they won't get selected against. The term 'maladaptive' is a fuzzy one though as has been mentioned above.

 

That's the problem: the idea that some traits are absolutely "maladaptive".

 

In the technological environment we have now, poor eyesight is not "maladaptive" for humans. In the environment that rhinos live in, poor eyesight is not "maladaptive".

 

Now, if we were all fighter pilots engaged in combat every day, then poor eyesight would indeed be "maladaptive". In that environment

 

What everyone tends to forget is that "maladaptive" and "adaptive" always apply to specific environments. Change the environment and you change what is "adaptive" and "maladaptive".

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