Friday, October 23, 2009


The central question of interest here in the Laboratory for Evolutionary Biodemography is: how does evolution determine demographic patterns (usually individual lifespan) and, secondarily, how do demographic patterns (again usually individual lifespan) influence the evolution of other traits? This questions leads to all manner of difficult sub-questions. One of these, that comes up surprisingly often is, "what is an individual in this case?"

One colleague has been pondering this question in the context of eusocial insects. Eusocial means that some individuals do all the reproducing, and others don't reproduce at all, they just work to increase the survival and reproductive success of the breeders. Queen ants and their workers are a good example. It seems pretty easy to count ants, they have separate little bodies and they are genetically distinct "individuals" but because they don't breed (usually) from the viewpoint of propagating genetic material, their only role is to perform their appointed task within the colony, in order to aid the queen. This has led some ant experts to refer to the ant nest as a super organism, with the queen functioning as the reproductive organ, and the workers, like the cells in our intestines, as merely the body that supports this reproduction. In many organisms reproductive cells can last the whole lifetime, which intestinal cells are disposable, and frequently replaced. Likewise, queens live as long as the colony does, greater than 30 years in some species, while workers usually last only a few weeks or months. So is the colony a single organism, and therefore the workers its sub-parts, or is each worker an individual, and therefore the colony a multiplicity?

Another colleague is studying the demography of hydra, small mostly sessile cnidarians. Hydra are among the most demographically bizarre organisms. For starters, no one has been able to prove that hydra age at all, despite multiple long term attempts. Second, their primary means of reproduction is through budding, where a bump on the side of the organism gradually elongates, grows tentacles, forms a digestive cavity and takes on the form of a fully formed and functional (but somewhat small) hydra before detaching and becoming a separate individual. Add to this that if you mash them up to separate their cells from each other, each cell has the capacity to grow into a new hydra. Yesterday, I spent a few minutes watching through a microscope as a hydra with a large bud sticking off the side, about half the size of the main body, wiggled in a perti dish. Both sets of tentacles, both digestive systems worked, like conjoined twins. As I watched, I wondered if I was looking at one individual, or two, or hundreds. Each cell had the capacity to found a new colony, build a new hydra, and therefore each cell was in a sense an individual. Each stem could be called an individual, by the loose analogy to humans. Or the whole genetically identical, physically attached, coordinated being could be an organism. Depending on what unit we call the individual, we get very different answers as to the lifespan.

A final example I've been wondering about is the giant redwood tree. A single trunk of a redwood seems to the casual observer to be one huge individual. But redwoods bud prolifically from the base, and multiple trunks can grow out of the same stump, the same root system. Large groups of huge trees can be genetically identical, save for the mutations accumulated in their growing tissues over thousands of years of growth. If we consider one stem to be the individual, redwoods can live for thousands of years. But if we consider everything derived from one seed to be the individual, I don't know of any reason not to consider redwoods, like hydra, effectively immortal. Sequoia sempervirens indeed.

So I'm posing the question to you dear reader, what is an individual? What operational rule should be applied? How do we find the individual in a hydra, or in a redwood forest?


jte said...

Seems to have been some text dropped in the third paragraph on hydra. "Hydra are among the most demographically bizarre organisms. For starters, multiple long term attempts. Second..."

What's that "for starters" supposed to say or mean?

Dan Levitis said...

Hmm, that's odd. Well it is fixed now.

Fern said...

For redwood trees maybe it is a question of when reproduction has ended. When is the burl an independent entity, and when is the hydra bud independent? Rather than giving birth like humans, redwood trees and hydra give birth in a way where independence is just not immediately discernible to us because of our human biases.

Liv said...

I think it largely depends on what sort of question is being asked, then using (and being explicit about) the definition that is most useful for this case.

That's sort of a hedgey answer, but given the diversity of reproductive arrangements it doesn't seem like the greatest idea to try to make the organisms fit the terminology, instead of the other way around.