This is something of a complex question with multiple answers, each completely true and therefore none the complete truth.
The following material is flagged Green Level. It is intended to reflect material that the author believes to be a matter of consensus among experts in the field. This belief may be incorrect, however; and as the author is not an expert and does not have an expert fact-checking the article, errors may creep in.First, let us consider a simple organism. Let us consider the simplest of organisms, a single cell. Each of these cells can either photosynthesize and eat nutrients (say, sulfates) that bubble up from a volcanic vent far below.
So, all of these cells are coexisting happily. Well, not really. There's a limited amount of light, and a limited amount of nutrients, so the cells are competing for these resources, and frequently a cell is outcompeted and starves to death. (Not to mention that, lacking nervous systems, it isn't exactly easy for a cell to be happy anyway.)
Now, let us suppose that the area covered by the cells increases.
So, it seems as though nothing would happen here, right?
In the future, suppose that the descendants of these cells are neighbors. In the interface between the areas occupied by the two strains, interactions between the cells show up. Sometimes, the two strains fight one another. Sometimes, they ignore one another.
And, once in a great while, they cooperate. Sometimes, the two cells will grow together, starting to share resources. Sometimes, the cells will join, sharing resources so that when one does poorly, the other covers its needs.
Of course, in the environment described, the joined-cell pair must still compete with single cells. Its resource requirements are twice those of a single cell, but so is its resource production.
Usually, anyway. Suppose that the environment is not constant. Sometimes, a cloudy day means that there is less light. Sometimes, changes in the volcanic vent's output change the nutrients available. So, sometimes the ratio of light to nutrients changes. Sometimes, light-specialized cells outcompete nutrient-specialized cells. Sometimes, the opposite happens. But which is the joined-cell pair?
Both, obviously. In a completely dark area, the pair will do half as well as a nutrient-focused cell, and in nutrient-less water, the pair will do half as well as a light-focused cell. But, in each case, it will do infinitely better than a cell focused on what is absent. So, the cell pair is better able to handle changing environments. So, with the changing environment, eventually the cell-pairs outcompete the single cells.
We can even apply this (we can apply it better, in fact) using the assumption that in the beginning, each cell is able to handle both resources. In any population of cells, there will be mutations. In this case, some of the mutants are better able to handle one resource than the general population. So, sometimes one mutant will join to a mutant able to handle a different resource better than itself. And remember, each cell pair must compete with single cells.
Now, suppose that there is a limit on "safe resource absorption". There is a certain level at which more or better resource absorption "crowds out" some other mechanism. For instance, suppose that each type of resource-absorber takes up a certain amount of space in the cell, and a better absorber takes more space in the cell. In a normal cell, one absorber eventually collides with another, so past a certain point, the cell must "decide" between not increasing its efficiency or making itself more vulnerable to changes in its environment.
A cell-pair, though, need not make that decision. Since there are two cells, each can specialize in a different resource.
(Now, on to blog stuff. Unless I get comments telling me to continue evolution, next week it's back to game theory. Depending on what you tell me I ought to be doing, I'll be alternating between Topics each week.)