Monday, August 26, 2013

Could we have evolved traits that are detrimental to our survival?

Let us assume that we collected data on the presence or absence of a trait (e.g., propensity toward risky behavior) in a population of individuals, as well as on intermediate effects of the trait, downstream effects on mating and survival success, and ultimately on reproductive success (a.k.a. “fitness”, in evolutionary biology).

The data would have been collected over several generations. Let us also assume that we conducted a multivariate analysis on this data, of the same type as the analyses employing WarpPLS that were discussed here in previous posts (). The results are summarized through the graph below.

Each of the numbers next to the arrows in the graph below represents the strength of a cause-effect relationship. The number .244 linking “a” and “y” means that a one standard deviation variation in “a” causes a .244 standard deviation increase in “y”. It also means that a one standard deviation variation in “a” causes a 24.4 percent increase in “y” considering the average “y” as the baseline.

This type of mathematical view of evolution may look simplistic. This is an illusion. It is very general, and encompasses evolution in all living organisms, including humans. It also applies to theoretical organisms where multiple (e.g., 5, 6 etc.) sexes could exist. It even applies to non-biological organisms, as long as these organisms replicate - e.g., replicating robots.

So the trait measured by “a” has a positive effect on the intermediate effect “y”. This variable, “y” in turn has a negative effect on survival success (“s”), and a strong one at that: -.518. Examples: “a” = propensity toward risky behavior, measured as 0 (low) and 1 (high); and “y” = hunting success, measured in the same way. (That is, “a” and “y” are correlated, but “a”=1 does not always mean “y”=1.) Here the trait “a” has a negative effect on survival via its intermediate effect on “y”. If I calculate the total effect of “a” on “w” via the 9 paths that connect these two variables, I will find that it is .161.

The total effect on reproductive success is positive, which means that the trait will tend to spread in the population. In other words, the trait will evolve in the population, even though it has a negative effect on survival. This type of trait is what has been referred to as a “costly” trait ().

Say what? Do you mean to say that we have evolved traits that are unhealthy for us? Yes, I mean exactly that. Is this a “death to paleo” post? No, it is not. I discussed this topic here before, several years ago (). But the existence of costly traits is one of the main reasons why I don’t think that mimicking our evolutionary past is necessarily healthy. For example, many of our male ancestors were warriors, and they died early because of that.

What type of trait will present this evolutionary pattern – i.e., be a costly trait? One answer is: a trait that is found to be attractive by members of the other sex, and that is not very healthy. For example, a behavior that is perceived as “sexy”, but that is also associated with increased mortality. This would likely be a behavior prominently displayed by males, since in most species, including humans, sexual selection pressure is much more strongly applied by females than by males.

Examples would be aggressiveness and propensity toward risky behavior, especially in high-stress situations such as hunting and intergroup conflict (e.g., a war between two tribes) where being aggressive is likely to benefit an individual’s group. In warrior societies, both aggressiveness and propensity toward risky behavior are associated with higher social status and a greater ability to procure mates. These traits are usually seen as male traits in these societies.

Here is something interesting. Judging from our knowledge of various warrior societies, including American plains Indians societies, the main currency of warrior societies were counts of risky acts, not battle effectiveness. Slapping a fierce enemy warrior on the face and living to tell the story would be more valuable, in terms of “counting coup”, than killing a few inexperienced enemy warriors in an ambush.

Greater propensity toward risky behavior among men is widespread and well documented, and is very likely the result of evolutionary forces, operating on costly traits. Genetic traits evolved primarily by pressure on one sex are often present in the other (e.g., men have nipples). There are different grades of risky behavior today. At the high end of the scale would be things that can kill suddenly like race car driving and free solo climbing (, ). (If you'd like to know the source of the awesome background song of the second video linked, here it is: Radical Face's "Welcome Home".)

One interesting link between risky behavior and diet refers to the consumption of omega-6 and omega-3 fats. Risky behavior may be connected with aggressive behavior, which may in turn be encouraged by greater consumption of foods rich in omega-6 fats and avoidance of foods rich in omega-3 fats (, ). This may be behind our apparent preference for foods rich in omega-6 fats, even though tipping the balance toward more foods rich in omega-3 fats would be beneficial for survival. We would be "calmer" though - not a high priority among most men, particularly young men.

This evolved preference may also be behind the appeal of industrial foods that are very rich in omega-6 fats. These foods seem to be particularly bad for us in the long term. But when the sources of omega-6 fats are unprocessed foods, the negative effects seem to become "invisible" to statistical tests.


LeonRover said...

With respect to a usually exact author, I would have expressed the title of this post as:

Has the Environment changed so that Traits which have not changed for 15,000 years are now Detrimental to our Survival ?

Anonymous said...

LeonRover missed the point.

Great post!

LeonRover said...

Guilty, Anon.

I hadn't realised that Ned was doing "The Red Queen" 'cos I hadn't read the post. Head hang in shame.

Nice state diagram of within species dynamics


Anonymous said...

So Omega 6 fatty acids cause aggression?
That give a new meaning to "going nuts"!

JamesSteeleII said...

Great post. All adaptations come with a risk reward ratio themselves and will have subsequent downstream effects on untold other traits. It all comes down to whether the end result is on average a net positive for reproductive success as to whether a seemingly negative trait might persist in a species. Bipedalism is a good example az it comes with its own set of negatives yet as a trait has persisted in spite of them. Evidently the positive effects upon reproductive success outweighed the negative effects associated with such a trait. I find it surprising how many people, even established evolutionary scientists, I've heard dismiss the relation of some negative trait to something they see as being only positive often with the statement that we would have died out long ago if that were the case.

Ned Kock said...

Hi James. This type of understanding of costly traits, which is more mathematical, is in my view much more clear than verbal descriptions of costly traits – most famously put forth by Zahavi. I’ve seen a lot of confusion in the past about what are referred to as Zahavian traits, and also strange conclusions – e.g., that these traits are very common, even more so than costless traits.

Ned Kock said...

Btw, the s > m is in the direction shown and positive because one must be alive to mate. This applies even to species where death often follows mating – e.g., spider species.

js290 said...

With the perspective of this blog post, it's interesting to think that GMO tries to disregard the cost of a particular trait. All these traits are coupled to each other. It's incorrect to decouple them.

dearieme said...

What would Uncle Genghis do?

Zorica Vuletic said...

Today I watched "20 Most Shocking..."

It consisted of mostly young men doing the stupidest things, and then getting hurt. LOL

I thought of this post the whole time. haha

For example, one guy holds his arm out so that a crocodile can be tickled and then it bites his arm. What?! Another tries to crawl under a trampoline while people are jumping on it---his head gets smashed to the ground. Ahhhh boi.

Ned, can you please explain a little bit about the phenomenon where guys like to be more effeminate? aka the skinny jeans wearing kind of guys? I have some ideas, but I'd like to hear yours.


Ned Kock said...

Hi Zorica. The phenomenon you described is a form of costly display that may well be a distortion of an evolved instinct, as violence today is institutionalized and we don’t hunt. Although some of those stunts may have been designed for sale (e.g., Jim Carrey’s character in “The Incredible Burt Wonderstone”).

The propensity toward display in men is consistent with what we see in other species, quite visibly in birds – most of the display is performed by males.

As for the new trends you mentioned, they may be simply a healthier form of display. I don’t see wearing jeans that have a tighter fit as necessarily effeminate. Even other related things like eyelining have been used by male warriors in the past and rock stars (and Johnny Depp) today.

I see those as forms of display that are healthier than the more risky forms of behavior you described.

Zorica Vuletic said...

I understand what you mean, but I did not mean only about wearing tight jeans, but I meant about how many of these men dressing as such also tend to be low in muscle tone and/or skinny-fat and/or skinny in a wirey sense. This probably does not change what you explained to me.

Yes, it's better to practice costly traits in a healthier way. hehe.

Perhaps it's still 'costly' in that it can signify that despite appearing softer, that they can 'afford' to look softer while still having an underlying healthy reproductive fitness. As in only the fittest can 'get away' with looking soft/feminine.

Thanks for your reply. :)

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