In approaching evolutionary puzzles it is helpful to have certain thought-tools in mind. These are approaches to life, literally, including issues of survival, mating, and so on. My grad school advisor, Irv DeVore. used to tell his students to imagine what an animal would be thinking on first waking up in the morning (or evening if nocturnal) to get through the day: “How will I find food today? …. how will I avoid being someone else’s food? … Will I find a mate, or will one find me? …” (In writing that just now I can’t help but think DeVore may have been cribbing Jack London just a little.)
Anyway, here are some rules of thumb useful for thinking about both physical and behavioral systems.
Romer’s Rule. Romer’s rule is simply stated as “A frog is simply a fish trying desperately to remain a fish” (the original formulation uses amphibian instead of frog, but this avoids explaining what an amphibian is). What this means is that a well adapted animal (we’re talking species here, not individual) is generally not going to change because, as we know, almost all change is bad (selected against). But if the environment (broadly speaking) changes then there may be selection for whatever variant features are available from mutation or hidden variation. But there may be certain features that are so important that they are conserved while other features change in a way that allows that continuity. With the frog/fish example, consider that the water is a really good place to lay your eggs. You get an environment buffered from dramatic change in temperature, that is not arid (because it’s water) so the eggs can’t dry out, and that has the potential of being well oxygenated. For animal eggs to work out of the water all sorts of adaptations have to be in place.
But the water may have the problem of being ephemeral. So a fish capable of leaving the water, not to leave the water per se but to go find other bodies of water in which to lay its eggs, is following Romer’s Rule.
The Energetics Approach. This is simple. There is a limited amount of energy in the environment. Organisms need this energy for maintenance of their soma, reproduction, etc. So the question you have to ask is how does an organism go about garnering energy from the environment and turning it into babies?
Life History Theory (LHT). This approach is so powerful that there are many biologists who do nothing else and manage to study a very wide range of systems. There are ways I like to use to describe LHT. One is the static model using a funny little graphs that look like this:
(click on the image for a larger size)
All of the energy that an organism has can be thought of as being partitioned into three areas: Maintenance, growth, and reproduction. Each of these areas gets one of the three legs in the graph. The total length of the three legs cannot be increased (and would not normally be decreased) so if one gets longer, the others have to change. Perhaps growth happens mainly early in life and reproduction happens once later one, or iteratively in the adult. Maintenance is ongoing, but the cost of maintenance increases with growth. (Maintenance includes homeostasis of temperature for warm blooded animals, the immune system, locomotory energy, etc.).
The trick is that for a given organism, there is no “free lunch” of energy. Therefore, say, the reproduction leg is going to be longer during a certain season, the reproduction leg lengthens, so one or both of the other needs to be shorter. Ideally growth is already shorter. One could say that this is why growth stages generally precede reproductive stages.
This graph can be drawn as a total lifetime expenditure for a given morph of a particular species (female mona monkeys, or male crows, adult monarchs) or it can be drawn in a way that illustrates a particular life stage or situation.
This three part system is an oversimplification for some purposes, but it allows generalization and the study of large scale patterns.
The second common way of thinking of LHT is exemplified in this graph:
This is a version I happen to like, and again it is an oversimplification, but a useful one. An individual organism is conceptualized as a line beginning with a fertilized egg and ending with death. Along the way the individual (I’ve drawn a female here) spawns off offspring.
Imagine this graph drawn for a Pacific Salmon, which will swim upstream to spawn and die, vs. a gorilla female, which will have a few offspring over her lifetime and spend a fair amount of energy investing in each one. Again, large scale patterns emerge.
The most important thing about using LHT ways of thinking is that one needs to account for the organisms’ overall budget. Almost as important is being able to measure which changes in a species pattern have greater or lesser effects. For instance, is changing interbirth interval more or less important than changing lifespan, with respect to total number of offspring an organism will have?
These approaches may be most useful because they allow a certain amount of anthropomorphizing, a useful creative tool in thinking about adaptations (like it or not) but at the same time constrain that way of thinking. Also, by abstracting the basic variables and constants related to biological systems, we can make comparisons … and predictions … that might otherwise be difficult.