Evolutionary Fitness

31 01 2007

To tell the truth, I’ve never liked the term “fitness” as it pertains to evolutionary science. When I was in elementary school, evolution was “survival of the fittest” (and indeed, Darwin himself was enamored with the phrase for a time), but ever since I started reading the literature and thinking about it I started to realize that “fitness” is little more than an abstraction. John Wilkins has already done a good job of contemplating the concept over at Evolving Thoughts, perhaps the most important aspect he mentions being the need to understand the ecological setting in which evolution is occurring.

I feel there needs to be a new synthesis between ecological studies and evolution as how can we possibly hope to understand how one works if we don’t understand the other? The other night I started in on the dated, but informative book Serengeti: Dynamics of an Ecosystem and one of the early chapters deals specifically with the evolutionary/ecological relationship between the ungulates of the area and the plant life. Although grasses and plant cover aren’t exactly the most interesting topics in the world to me, what I came away with is the obvious, but important finding that in areas that are not grazed by animals like Wildebeest (the primary grazing “army” of the Serengeti) there is a higher proportion of plants that grow from the stem (top) of the plant, whereas in areas that are heavily grazed there are more grasses that grow from the roots. On this same theme, grasses that are moderately grazed actually grow faster after being munched a bit than if they were left alone, the activity of the wildebeest (followed by gazelle shortly after) directly influencing the growth and competition between plant life. Such is a relationship that may not seem very special at the moment, but if studied in detail over the long term it could help scientists understand more about evolution through the interplay of organisms in an environment.

Another point that Wilkins makes note of is that not all environments are the same; lions in the Kalahari desert are not all living on equally productive real-estate. Oftentimes, animals are said to live in a particular environment, but seldom do what stop to think about what happens with animals that live on fringes or that migrate over long distances, the dynamics of the locations varying. There are more subtle factors at work than may be possible to study, and beyond basic ecological changes, animal behavior also plays an important role. Last night I was reading some more of the fantastic book Cry of the Kalahari where two male lions in a coalition (Muffin and Moffett) fought a very large and powerful rival male named Satan. If either Muffin or Moffett had faced Satan alone, they certainly would have been pummeled if not killed, but being that they formed a coalition they were successful in immobilizing their foe, who soon died. Given the intricacies of animal intelligence and behavior, I wouldn’t think that all male lions are equally open to forming coalitions to hold territory and females, the likelihood of such groupings forming being exceedingly complex to start with. You could have an exceedingly fit male who would form a coalition but is never given the appropriate opportunity (i.e. no male siblings) like Satan, but you could have two less-fit lions like Muffin and Moffett pool their strength to overcome the aggressor who is more “fit” on a 1:1 basis.

Speaking of African cats, cheetahs also come to mind in this discussion. Some time between 12,000 and 10,000 years ago, cheetah populations plummeted to the point where inbreeding became the rule, thus making extant cheetahs exceedingly vulnerable to disease. Perhaps more nomadic animals or animals whose populations did not fall as drastically (as in East Africa) would fare better, but even so the amount of inbreeding has gone so far as to cause minor asymmetries in skull structure, one of the tell-tale signs (beyond near-instant epidemics, that is). According to ideas about fitness, these cheetahs should be getting more fit over time in their populations but the opposite seems to be, needing human intervention and studbooks to ensure their survival. Even our current efforts are not enough, cheetahs being so neurotic that reintroduction to the wild is difficult, thus current depleted populations may continue to inbreed. What will occur in natural populations in the future is anyone’s guess, but hopefully if I get the chance I would like to monitor who is mating with who, survival rate of the offspring, incidence of nomads, etc. so that we can see what happens to a population that becomes almost hopelessly inbred; will there be some new reservoir of variation or will the downward trend continue until extinction? What will happen if a population of cheetahs becomes more isolated from a larger population, will they die out faster from an even higher incidence of inbreeding? The point of these questions is that “fitness” doesn’t always necessarily go up in a population, detrimental attributes not always quickly snuffing out groups.

Ecological change/succession, time, and weather patterns are also subject I find sometimes lacking in evolutionary discourse. You can be exceedingly well-adapted to your environment but if there’s a harsh drought one year when there was none, you’re out of luck. How can a creature adapt to pressures that were not previously there? Dinosaurs didn’t adapt to survive extraterrestrial impacts as the pressure to fuel such changes was not there; the impact 65 million years ago was a one-time crap shoot for the entire group and they lost big time, but I would be hard-pressed to say that they were not well adapted. Indeed, I get frustrated when evolutionary scientists think of animals evolving almost in a vaccuum, or at least a sealed environment where every square foot of grass has equal productive value and whoever are the fittest will become apparent. It’s a crappy slogan, but if anything it seems to be “survival of the luckiest”, adaptive attributes needing the ability to increase in frequency and maladaptive ones not always signaling the immediate end of a population or species. Thus, I generally consider fitness to be an arbitrary, abstract term that doesn’t hold much value scientifically, especially since so many biotic and abiotic factors play into what survives and what does not.




5 responses

6 02 2007

[…] Tuesday February 06th 2007, 11:25 am Filed under: Ecology, Evolution, Behavior After writing my previous posts, I started thinking more and more about the idea of “fitness” in organisms, and […]

2 04 2007

Uhm, nice piece, but… isn’t the alliance-building skill to be counted in your definition of fitness? therefore, the two less physically fit male lions would be fitter than Satan.

2 04 2007

Thank you for the comment luca. If you look at what I wrote, however, I do suggest this same point (that alliance building can increase the chances of the males getting food, holding territory, etc., thus being more “fit”) when I say;

“I wouldn’t think that all male lions are equally open to forming coalitions to hold territory and females, the likelihood of such groupings forming being exceedingly complex to start with. You could have an exceedingly fit male who would form a coalition but is never given the appropriate opportunity (i.e. no male siblings) like Satan, but you could have two less-fit lions like Muffin and Moffett pool their strength to overcome the aggressor who is more “fit” on a 1:1 basis.”

Also, what one may mean by fitness may differ. The way I normally think of the time is an overall life-history sort of fitness, i.e. the ability to survive and reproduce. Others consider fitness in terms of genes or merely mating success (as your genes won’t get passed on if you don’t mate). Regardless of these discussions, I find the term overall to be a bit subjective, or at least easily misunderstood by the general public. Hopefully more integrated approaches will develop in the future that will expand fitness considerations beyond data-crunching and look at altruism, alliance-building, and other behavior aspects as well as reproductive success.

3 04 2007

good point laelaps. I read the bit you cite, but since you didn’t made explicitly the connection between alliance-building and fitness, I thought I’d point out. As for the “fitness” definition, I always interpreted solely as “reproductive fitness” (over the whole life, as you say) – but this may be a bias due to my previous experience with genetic programming. This fits the hypothesis that the two males may have a higher fitness than satan, in case they keep their territories and their ability to mate – or lower, if even keeping their territories are denied mating by their females because they’re perceived as not adequate. After all, intelligence (in this case alliance-building) isn’t always a winning trait.

I’ll continue reading up your other posts 😉

3 04 2007

Thanks again luca. I’m a bit biased myself, getting into paleontology/behavior/anatomy/etc. side of biology and I still struggle with genetics. Hopefully those in th field of genetics and those in more “traditional” fields will work together more to help understanding, as I don’t think either is entirely adequate on its own.

You’re also right about the coalition issue; forming a coalition doesn’t guarantee mating (many young males form coalitions before even getting a territory and mates within it), and in the case of more than one male per pride, one may be more reproductively “fit” than the other, i.e. sire more offspring.

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