It's ok if you don't believe me yet. Darwin wouldn't have, either. He and his contemporaries viewed parasites as degenerates who, at best, violated the progressive nature of evolution. Even in The Origin of Species, Darwin refers to parasites as regressive instead of progressive. But truly, no group of species is a better choice for evolution's finest.
First off, let's talk numbers. Parasitism is the most popular lifestyle on earth - over 40% of all known species are parasitic, and the number of parasitic species rises daily1. Sure, you might say, but they tend to be small. In that case, let's talk biomass - weight, just to be clear. One group of parasites, the flukes, have been found to be equal in weight to fish in estuarine habitats, and three to nine times the weight of the top predators, the birds - estimates which are thought to be conservative for the earth as a whole2. Though they're largely ignored when we study food webs, they've been estimated to be involved in over 75% of inter-species interactions1. Clearly, by the numbers, they are the most prolific and successful organisms on earth.
Soybean Nematode and Egg
But even that is not why I would argue they are evolution's finest. They, more than any other group out there, both exhibit extreme evolutionary adaptations and spur them onward in other species.
No matter how complex or how impressive any other species may be, it has parasites. We do - lots, actually. Every species we might hold as a masterpiece of evolutionary complexity cannot out maneuver their parasites. Not one. Even parasites, marvelous as some are, have parasites - like a crazy russian doll. They have evolved amazing abilities to survive host defense systems, manipulate host behavior and boost heir own reproductive success. They've even been implicated in major cultural differences in people. It turns out that a rat parasite, Toxoplasma gondii, needs to be eaten by a cat to complete its lifestyle. Somehow it developed a trick to make rats unafraid of cat smells. When it accidentally ends up in people, it does the same kind of mind-altering, making people more guilty and insecure, even more frugal, mild-tempered, and complacent3. Other parasites do far more intricate manipulations of behavior, turning males into females, creating walking zombies, even forcing suicide. If parasites can not only break into and survive the most complex assortments of systems available, even with modern medicine fighting against them, and manipulate those complex organisms to slave to their bidding, how can we not credit them as masters at what they do?
But even more impressively, I would argue, is that no other group has so dramatically impacted how other species have evolved. They don't just affect their hosts immune systems, either. If you read much into evolutionary theory, you realize it's riddled with parasites. Why are some birds very colorful? Oh, because if they've got a lot of parasites they can't be, so it's a signal of a healthy male4. Why are we attracted to certain people? Because their immune genes are different from ours, giving our children the best chance to fight off the next generation of parasites. Almost everywhere you look, evolutionary changes are spurred on by parasites. It's even suggested that sex itself evolved as a response to parasites. It's a way of better shuffling our genes so that we have better odds at fighting off parasites.
left: Malaria and red blood cells.
left: Malaria and red blood cells.
© Prof David Ferguson
right: Haemonchus contortus.
Even we, as "ideal" or "complex" as we are, owe much to parasites. Some even argue that we are worse off without them. The argument, as it goes, is that our immune system evolved in the presence of unkillable parasites, particularly the parasitic worms. These worms, or Helminths as they are called as a group, were too costly to try and eradicate. Attacking foreign invaders, after all, is energetically expensive, and always runs the risk of over-activating our immune system, leading to self-inflicted injuries and diseases. So the best strategy, instead, was to have an immune system that functioned optimally against other issues, like the fatal viruses or bacteria, despite the mostly benign worm infections5. Since worms secrete anti-inflammatory compounds to fight off our defenses, we were better off with systems that overcompensated for that. Now, since we have drugs which kill them off, our immune system is out of balance. Many cite the rising rates of auto-immune and inflammatory diseases like allergies, arthritis, irritable bowel, type 1 diabetes, and even cancer in developed nations as evidence that ridding ourselves of helminths has damaged our health6. They're backed up with multiple studies that show unexpected results, like that mice genetically predisposed to diabetes never develop it if infected with flukes at an early enough age.7
Parasites are uniquely capable of out-evolving their hosts and adapting to whatever changes go on in them. Simply put, they evolve better. They change their genes faster and keep up with a barrage of host defense systems, often like it's effortless, spurring onward dramatic changes in other species. If Darwin had only known how amazingly complex the barriers these creatures have to overcome and the extent to which they have affected the species he'd encountered on his travels, he would not have labeled them "degenerates".
As far as evolution is concerned, no group of species demonstrates it, causes it, and is so capable of it as the parasites. While disgusting or even cruel, they are truly evolutionary masterpieces. So while you may find them vile or detestable, you have to admit they're good at it. Can you really argue that some other group is more deserving of the title of Evolution's Finest?
1. A. Dobson, K. D. Lafferty, A. M. Kuris, R. F. Hechinger, W. Jetz (2008). Colloquium Paper: Homage to Linnaeus: How many parasites? How many hosts? Proceedings of the National Academy of Sciences, 105 (Supplement_1), 11482-11489 DOI: 10.1073/pnas.0803232105
2. Armand M. Kuris, Ryan F. Hechinger, Jenny C. Shaw, Kathleen L. Whitney, Leopoldina Aguirre-Macedo, Charlie A. Boch, Andrew P. Dobson, Eleca J. Dunham, Brian L. Fredensborg, Todd C. Huspeni, Julio Lorda, Luzviminda Mababa, Frank T. Mancini, Adrienne B. Mora, Maria Pickering, Nadia L. Talhouk, Mark E. Torchin, Kevin D. Lafferty (2008). Ecosystem energetic implications of parasite and free-living biomass in three estuaries Nature, 454 (7203), 515-518 DOI: 10.1038/nature06970
3. Kevin D. Lafferty (2006). Can the common brain parasite, Toxoplasma gondii, influence human culture? Proceedings of the Royal Society B: Biological Sciences, 273 (1602), 2749-2755 DOI: 10.1098/rspb.2006.3641
4. Jesús Martínez-Padilla, François Mougeot, Lorenzo Pérez-Rodríguez, Gary R. Bortolotti (2007). Nematode parasites reduce carotenoid-based signalling in male red grouse Biology Letters, 3 (2), 161-164 DOI: 10.1098/rsbl.2006.0593
5. Joseph A. Jackson, Ida M. Friberg, Susan Little, Janette E. Bradley (2009). Review series on helminths, immune modulation and the hygiene hypothesis: Immunity against helminths and immunological phenomena in modern human populations: coevolutionary legacies? Immunology, 126 (1), 18-27 DOI: 10.1111/j.1365-2567.2008.03010.x
6. Joel V. Weinstock, David E. Elliott (2009). Helminths and the IBD hygiene hypothesis Inflammatory Bowel Diseases, 15 (1), 128-133 DOI: 10.1002/ibd.20633
7. Anne Cooke (2009). Review series on helminths, immune modulation and the hygiene hypothesis: How might infection modulate the onset of type 1 diabetes? Immunology, 126 (1), 12-17 DOI: 10.1111/j.1365-2567.2008.03009.x