Some thoughts on aging and the phenotype

I have been gone a while. Something of a creative meltdown after the Evolution meetings. Perhaps one to many Caparinha, at what might have been the best party ever at an Evolution meeting. Leave it to the Brazilians to throw a party with enough food and liquor and the wackiest live music ever. In any case, I am back in Vermont, eating kale and other healthy things that us aging hippies do, and thinking about the pain of having to ply my profession as a teacher. Time to get back to work.

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Admittedly a terrible picture, but the party was one of the best.   If you missed the Evolution meetings this year you made a mistake.

Jake Moorad started a discussion with me about how aging affects individuality. My first thought was that I have no idea. As usual, such an answer means that it is a really interesting question. After giving it more thought I have come to realize that it has no effect at all. After all, an individual is the level at which we assign fitness, which is potentially quite arbitrary. In most cases the “individual” will be an organism and its associated symbionts. Thus, despite the fact that an individual changes as it ages, I think it should have little influence on what we call an individual. What it does change, however, is it complicates what we think of as the phenotype. The fact that the phenotype changes over time is not a trivial issue, and it is one that needs to be given some attention.

How to view the phenotype as a vector through time is a topic I have discussed before, and one that is a general issue for the phenotypic view of evolution. My solution is to treat the phenotype of an individual as a vector through time that begins at formation of the individual, and ends at their dissolution. There are a couple of interesting things in that sentence. Note that I am specifying the “individual”. It seems to me that, although perhaps not essential, it makes sense to assign phenotypes at the same level at which we assign fitness. I could be argued out of that, but for the moment it seems right (which is not terribly convincing to me, let alone anybody else). Second, I speak of the “formation of the individual” and “dissolution” of the individual. If we assign fitness at the level of the organism, this will be at conception or birth, depending on your perspective, and ends when the organism dies. But remember two things: First, if we assign fitness at a level other than the organism, “birth” and death may not be an appropriate terms. However formation and dissolution will always be appropriate, since our definition of evolution involves the gain and loss of evolution, individuals must of necessity have a beginning and an ending if they are to be considered to evolve.

If we take a classic genic perspective of the individual as a single species organism it is easy to ignore the time dependent aspects of the phenotype. Most importantly under a genic perspective genes are the only important heritable effects on the phenotype. These all enter at the time of formation (birth) and are unchanging through time (this last ignores somatic mutations of course).   This is not true from a phenotypic perspective. Culture is a prime example. The language you speak whether you are most comfortable with a fork and knife or with chopsticks are all heritable aspects of phenotype that are added after birth. Similarly, your cultural parents will not necessarily be the same as your genetic parents. Children learn their earliest language from their parents, but the vast majority of their language comes from peers and children that are slightly older than they are. This acquisition of heritable elements is not limited to culture, of course. We acquire much of our microbiome from our parents, and other individuals with which we live, and in many social insects, such as termites, trophallaxis is essential for their survival.

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The phenotype as a vector through time. Effects entering from the top are possibly heritable inputs, effects leaving below the lines are products of the phenotype. Note that the phenotype ends at death. This is why products such as beaver dams and human produced things like books are part of the phenotype, and not part of the distended (or is it extended, I always get it wrong) phenotype.

However, it is not just the acquisition of heritable elements that affect the need to consider the time element in the phenotype. Traits also change over time.   At a simple level every trait will have a time element. One way that this is handled is to simply to measure traits at a time when they are stabilized. For example, most vertebrates have targeted growth. Thus, there is a period between when adulthood is reached, and before senescent decline that traits are stable enough that we can effectively ignore the time element. In reality, of course, we should always include the age component, thus, it should be considered weight at age X, not adult body weight. However, speculating about how things ought to be done is different than doing things, and well, I for one will not be angry at people who simply measure adult body weight.

This does raise one additional interesting point. That is we can measure the time element of a trait to whatever precision we choose. Thus, in principle we could measure a trait such as whether an animal is inhaling or exhaling, or whether their heart is in systole or diastole. From an evolutionary perspective it would be silly to measure such highly time dependent transient traits, nevertheless it emphasizes the point that traits are aspects of the phenotype we choose to measure, and as such can be measured to whatever precision is appropriate.

More interesting, however, is the expression of traits with non-genetic inheritance. In some instances traits might not be expressed at all until the causal elements are acquired. For example, the trait of speaking a language cannot be expressed until the language is learned. Language is not acquired at birth, and if it is not the mother tongue, it may be acquired quite late in life. Further, if this person goes on to teach others their newly acquired language we can say that they have a heritable trait (the language understanding) that they acquired late in life.

Finally, some traits we may be interested in might be rate of change over time. Such traits might be the slope of the decline in fertility with age since puberty. To be honest I am not sure of the language to use to describe such traits, since such traits are explicitly incorporating the time element, and do not fit well with my phenotype as vector metaphor. If anybody has any ideas I would be pleased to hear them.

The point of this is that the evolution of aging remains an important issue. However, I am inclined to think it will have at most a small effect on our understanding of individuality. Where I think it will have its big impact will be on how we think about the phenotype, and the necessity to think of traits as being age dependent.

 

 

One Response to “Some thoughts on aging and the phenotype”

  1. Jacob Moorad says:

    Interesting thoughts as usual. I agree pretty much with what you say about how aging affects our notation of individuality (i.e., not much). However, I think the more interesting question is subtly different: How does choosing a particular definition of individuality affect our evolutionary predictions of aging?

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