Turned up to eleven: Fair and Balanced

Wednesday, September 04, 2002

Back to science stuff
Now that I got that out of my system, I want to comment (gently!) on Charlie Murtaugh's bloggage about "Homeland Insecurity". I read an excerpt of that article, and agree with his assessment of Scheier's analysis, but I wanted to comment on the immune system analogy. Charlie says "I'm sure there's an evolutionary expert out there who will shoot me down, but I strongly suspect that we owe both aspects of our lifestyle to our flexible immune system."

I won't shoot down that assertion, because I think it is true, to a great extent (leaving aside the many other things that play into our lifestyle). Dr. Murtaugh contrasts vertebrates adaptive immunity with the innate immunity that invertabrates possess, suggesting that this may explain the shorter lifespan of insects, and their smaller size. I have to disagree here. 1) there are some fairly large invertebrates, just not insects (outside scary dinosaur movies, that is). You have to look in the ocean, though, because gravity gets in the way (think squid and octopus). Land based invertebrates, insects in particular, utilize an "exoskeleton" to allow for structural integrity; this holds their innards in, and holds their shape. Insects, therefore, have to go through a metabolically costly process of molting to develop (the stages are called instars, I believe). This combination of factors leads to the size limitations (for flying insects, the costs of flight also probably are important). Another thing to remember is that bigger is not necessarily better; factors such as body heat maintenance, predator prey relations, cost of reproduction (ranging from exposure to predation when copulating or raising young to the sheer physical cost of brooding) may play a role in keeping an organism small. Plus, insects have been around a very long time, and have thus developed to fit their particular niche very well, in some sense.

Thus, I would probably take the converse argument to Charles, and suggest that the adaptive immunity that vertebrates utilize is not useful to a small, short-lived, chitinous exoskeleton bearing insect, and thus confers no advantage over the cheap but effective innate system. Incidentally, humans and vertebrates have an extensive innate immune system themselves, which provides a strong first line of defense against infection while the adaptive system is, well, adapting. Evolution does not follow any one path, but rather organisms diverge into different survival and reproduction strategies. For large, endoskeletal vertebrates that have offspring in very small numbers and are slow to mature, a comprehensive immunity to unpredictable disease is a viable survival strategy. Conversely, evolving for rapid maturation, large broods, and short generation times obviates the need for this level of protection against infection.