What does evolutionary trend mean?

The subtitle of this blog is no foresight, no way back. For those who don’t know it, this quote if from the famous British biologist Maynard Smith. I think it’s time for explaining a little better what this phrase means, because it is an essential introduction to the subject being treated in this very post: biological evolution is not teleological, i. e., no evolutionary changes occur aiming an ultimate end, or a predetermined destination whatsoever. Evolutionary changes occur, that’s it. And sometimes not even that: changes themselves are not a natural need, that is, a population can be maintained for an indefinite period of time without major evolutionary changes, even if the environmental conditions have been modified in this period. Evolution can occur: it is not, however, an obligation.

What I just said is clear to most evolutionary biologists. But, then, what about evolutionary trends? This is an extremely common concept in biology, which most of us use constantly. I remember when I started teaching zoology in high school, talking about evolutionary trends in the Kingdom Animalia: development of a nervous system, development of a muscular system, specialization of sensory organs and so on…

First of all, we must point out the mistake that most lay people make when they think of an “evolutionary trend”: they associate trend to teleology.That is, they imagine that there is a force “pulling” that taxon in a particular direction: the birds have reduced their body weight because there is a tendency in the group to reduce it; grasses have reduced perianth because there is a tendency in the group losing it; primates have increased their cephalic mass because there is a tendency in the group to increase it. But this is wrong. There is no teleology, no force pulling anything towards a certain direction. Evolution has no foresight.

Another incorrect thing, and that bothers me a lot, is what might be called “speculative biology”. I never liked thinking about evolutionary biology as a science of predictions: the number of factors and interrelations in the real world is frighteningly large so we can try to predict the evolutionary paths of a taxon, even in relatively short time intervals. Some time ago, Animal Planet presented a miniseries called “The future is wild” (in my country it was broadcasted by Discovery Channel). The episodes contained computer animations showing what life would be like on Earth in 5, 100 and 200 million years from now. An interesting mental exercise, but to what extent valid? To what extent it showed, for the spectators, a correct view of evolutionary biology? For me, it was not a very good idea. I disliked that miniseries. It bothers me a lot this attempt to predict the future, even more in such a big lapse of time.

Animal Planet's "The future is wild"

Animal Planet’s “The future is wild”

The path that an object has made from the past to the present provides important information for us to predict its future trajectory. Imagine a movie of a quarterback throwing a ball or a goalie taking a goal kick, and when the ball is in mid-air I pause the film. By the trajectory travelled until then you can easily predict the ball’s future trajectory, and predict approximately where it will drop. But imagine that, right after I play the movie again, a sudden and strong gust of wind has changed the trajectory of the ball: it won’t fall where you predicted.

Let me explain my idea with a chart: given the trajectory of an object to the present, you can easily trace its future trajectory; given a curve in a char, you think you can guess where it is going. In the graph below, it is the blue dashed line.

The measure of a given feature is in the vertical axis, and time in the horizontal axis.

The measure of a given feature is in the vertical axis, and time in the horizontal axis.

But imagine now that this is not a physical example, but rather a biological one, and the vertical axis represents the measure of any given feature, from the past to the present. This feature has a value that has been decreasing to the present. However, what assures us that it will continue to decrease? And if conditions change? And if it starts to increase (red dashed line)? By the way, (almost) everyone who works with charts and data analysis knows that you cannot guess where a curve is going to just by looking its past.

So, finally, what we should understand by “evolutionary trend”? What does this concept, which Futuyma uses abundantly in his textbook on evolutionary biology, actually mean? I think the answer can be given by Maynard Smith, creator (not alone) of the concept of Evolutionarily Stable Strategy (ESS).

Once a certain path was taken by a population, a certain strategy was choose, and once most of the members of the population follow that strategy, natural selection is capable of preventing the emergency of alternative strategies. Thus, the population will find itself increasingly compromised with that initial strategy, deepening it and making it more and more fixed (using a population genetics lingo). This, however, does not demand a destination previously outlined.

Expanding the boundaries of time and populations, we can think in a similar way: the evolutionary history of birds, in which flight led to weight loss, doesn’t open much room for an evolutionary line where the weight will increase. The evolutionary history of grass, losing the perianth and relying on the wind for pollination, doesn’t open much room for the resurgence of a beautiful and showy flower. Among animals, selection favours a nervous system (associated with muscular system) increasingly complex. But even here we have to be careful: who, seeing the light and emaciated ancestor of flying birds, could have imagined that one of their lineages would lead to the heavy ostrich, or the heavy moa? And among the animals, how many lineages did not abandon the nervous system altogether throughout evolution?

There are certain terms that we should use carefully, judiciously. I think “evolutionary trend” is one of those terms.

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