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Evolving non "life-or-death" but useful characteristics

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Evolving non "life-or-death" but useful characteristics

Postby stephen502 » Sun Jul 01, 2012 7:44 pm

Hi all,

Forgive me if this question has been answered before, but I've spent some time looking around for answers and haven't really found a satisfactory one.

My questions has to do with the evolution of non-"life or death" but nonetheless useful characteristics, and how the model of natural selection explains why they persist. One example of such a characteristic in my mind is eyebrows. While eyebrows have a clear functional purpose of keeping dirt out of our eyes (which has obvious benefits), I find it hard to believe that keeping dirt out of one's eyes would ever have made a difference between life and death for an individual (ie, because our ancestors could see without the impediment of dirt in their eyes, the cheetah did not eat them-----or because our ancestors could see without impediment, they were able to successfully hunt the cheetah, which they would not have been able to do otherwise). And forgive me if there is a blatant hole in my reasoning, but to me, it seems like this is an instance of some characteristic that is not randomly persisted through generations (it has a specific and practical purpose--to keep dirt out of our eyes) but at the same time, if it's not going to make any substantial impact on the survival of an individual, then how can it be selected for?

One might offer the explanation that eyebrows were persisted "randomly," that it was in fact other useful characteristics which helped our ancestors thrive, and that those who thrived might just happened to have had eyebrows. While I can't dismiss this answer, I think the fact that there is a very practical and straightforward reason why we'd want to retain hair above our eyes suggests that it is unlikely that it was just a characteristic that randomly "tagged along" for the ride through generations.

I'm sorry to be so focused on this one specific manifestation...I think there are a lot of others, like some of our fight-or-flight responses like yelling when we're startled or feeling the urge to defecate when we're frightened. Both of these traits theoretically could help us survive, but again, I find it difficult to imagine a specific instance when having these characteristics would make a difference between life or death. I understand if you look at an organism holistically, at the sum total of its useful characteristics, why it survives will make more sense. But, I guess my question boils down to if a particular trait is not useful in and of itself, how does the model of natural selection explain why there would be any pressure for such a characteristic to be passed down? (and again, I'm not talking about random not-useful or even detrimental characteristics....I get that these will show up due to the randomness of NS....I'm talking about characteristics that in fact have some practical use, but that really wouldn't make much of a difference in life or death)

Thanks for listening to rambling. And sorry if this has already been answered, or if I'm missing something incredibly obvious

Best,
Stephen
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Postby Rap » Tue Jul 03, 2012 6:13 am

I think a complete way to say it is that a trait will be present in a population if it is (or was) a trait that increases the probability that the gene (or genes) that are responsible for that trait are propagated, or is a trait that is genetically associated with such a trait. So its not simply a matter of life or death. For sexually reproducing organisms, its about the probabilities of life or death of the gene up to and including sexual maturity of the organism only, and then reproductive success of that gene. Or, its life or death of that gene (probably in close relatives) up to and including sexual maturity, and then reproductive success of that gene. Or the same thing for genes that are somehow "linked" to that gene. Its more complicated than simply "survival of the fittest".

I don't know about eyebrows, but there are some possibilities. I think humans are one of the few species that show the "whites of their eyes", because humans are social animals, and eyes are (or were) very important in communication, being able to see which way a person's eyes were directed. Predatory animals, like cats and dogs especially, don't show the whites of their eyes, their eyes are part of their camouflage. Maybe eyebrows also fall into this "communication" category. What about eyebrows on dogs and cats? I wonder if domesticated dogs and cats have more expressive eyebrows than wild dogs and cats? Dogs are more social than cats, do they have more expressive eyebrows than cats? Pick a really non-social animal and check them for eyebrows.

Also, consider the eyes on a peacock's tail. I have heard the theory (don't know if its true) that perhaps in the past, peacocks as a population had to deal with some eye disease, and females that developed the ability to assess the health of a male based on the appearance of their (actual) eyes and choose ones that were uninfected, perhaps due to a genetically developed resistance to the disease, were more sexually successful. Males which developed big, clear "eyes" on their tails took advantage of this by "fooling" the more fit females into accepting them. Eventually the population as a whole became resistant to the disease, but the eye game was still on. Eyebrows may not be a signal of health, but they may be part of some similar kind of game. People without eyebrows are less sexually attractive, and the reason for that doesn't have to exist right now, the reason may be long gone, but the eyebrow game is still on.
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Postby JackBean » Tue Jul 03, 2012 10:12 am

It may not be so crucial for us, since we have pretty movable fingers, but for other animals may be keeping-dirt-out-of-eyes quite crucial.

To me, your examples look exactly what (can) make(s) the difference. Because if you surprise the predator with yelling, you may survive. If you have problems with eyes, you will not survive for long.
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Re: Evolving non "life-or-death" but useful characteristics

Postby stephen502 » Wed Jul 04, 2012 7:47 am

Thank you both for your answers.

@Rap, I take your point about NS being more than just survival of the fittest. I realize that traits that are sexually beneficial will help an organism become more "successful" in terms of gene propagation (independent of "life or death" traits.) I like your possible explanation about the eyebrows....I have heard a similar theory. That said, I still question whether or not such advantages would have been enough of a "game-changer" for the individual to distinguish itself from its peers and allow its characteristic to be widely propagated. But I still don't know if my question has been answered on a fundamental level (eyebrows I think might have been a poor example--see below)

@JackBean Okay, yelling and eyebrows aside, how about the urge to defecate when frightened? How do you select for such a trait? Can you ever imagine an instance when defecating while being pursued by a predator would make any difference between life or death? Even if it did in one instance, statistically how could it be significant enough to make a difference on the population level? Maybe I'm being a bit myopic....I still think there is something here, though. I will try and think of some more examples. In the mean time, any explanation for this example? Thank you
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Postby canalon » Thu Jul 05, 2012 3:18 am

First, as pointed earlier, life and death is not the only selection. It is more different reproductive success (directly linked to survival, but not only). And in addition you have to think about all the non selective aspect of life (all the random events that can alter a population without much surviving)

Second, you have to understand your example a bit better. In fact the need to defecate linked when terrorized, might be just a by product of a mechanism that acts on all the smooth muscles. Or more generally, some things are selected not because they have any intrinsic value, but because they are part/respond to a wider mechanism that cannot be easily broken down.

And this is just a simplification of all the parameters that can affect what we observe. It is a bit more complicated than it appears.
Patrick

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Postby stephen502 » Thu Jul 05, 2012 5:51 am

@canalon, thanks for your response. It was definitely helpful for me. Yes, your points are good ones...it's very difficult/risky to look at any of these examples in isolation. I think I can accept that these traits are probably part of a broader and more complicated mechanism or system that was selected for. I guess that means I've reached a satisfactory conclusion to my question...thank you all for your help
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