Cancer is not just a terrible disease but a strange one. Tumor cells must switch on certain genes in order to thrive and multiply. You might expect that natural selection would have eliminated those genes, because they kill off their owners. Far from it. A number of cancer genes, known as oncogenes, have actually been favored by natural selection over the past few million years. Oncogenes, in other words, have boosted the reproductive success of their owners, and have even been fine-tuned by evolution.
Humans are not alone in getting cancer. In fact, it seems to be a pretty inescapable risk of being an animal. As cells divide and mutate, some mutations may make cells ignore the needs of the body and multiply madly. That’s too bad for other animals, but there’s a silver lining for us: by studying other animals, scientists can get some clues to how cancer evolves in us.
The delicate swordtail (Xiphophora cortezi) is particularly prone to getting melanomas (the bottom picture here shows a fish with a tumor in its tail). When Andre Fernandez and Molly Morris of Ohio University went fishing for delicate swordtails in mountain streams in Mexico, they found six fish with melanomas in a single day’s catch. These melanomas are particularly nasty–instead of striking old fish that are going to die soon anyway, they turn up in young breeders and kill them over a few months.
Melanomas develop from pigment-producing cells in the skin. As these tumors develop, the cells inside them produce lots of extra proteins from a gene called Xmrk. Despite Xmrk’s harm, it has survived in good working order for a long time. Functioning versions of Xmrk exist not just in delicate swordtails, but in related swordtail speices that descend from a common ancestor that lived a few million years ago.
How does such a dangerous gene continue to survive for so long? Fernandez and Morris have just published an experiment that might solve the mystery. A lot of delicate swordtails have large dark spots on their tails, like the one shown on the top fish here. Xmrk is essential for producing those spots. Other fish have been shown to use stripes, spots, and other visual patterns to attract mates. So Fernandez and Morris wondered what the female delicate swordtails thought of the Xmrk spots on males.
Turns out, they like them a lot. When offered a chance to pay a visit to one of two male fish, female delicate swordtails from two populations in Mexico spent more time with spotted males than spotless ones. And they also preferred to consort with males with big spots over males with little ones.
The Xmrk gene definitely imposes an evolutionary cost on fish. But that cost may be erased by the benefit it gives male fish through sexual selection. By the time a male delicate swordtail dies from an Xmrk tumor, he may have mated with a number of females, which will pass down the gene to their young.
We humans may also be shaped by the trade-off between sexual selection and the cost of cancer. Testosterone and related hormones latch onto androgen receptors on the surface of some cells. It’s important for the development of men’s bodies, for example, and the growth of body hair. It also plays a role in the production of sperm. These kinds of traits can affect the success men have in finding mates and having children. But the androgen receptor gene also becomes active during prostate cancer. In fact, versions of the gene that increase sperm count in men also raise the risk of cancer.
For more on the sexy side of cancer, check out my article in Scientific American which, I’m happy to report, was selected by author Sylvia Nasar to be included in The Best American Science Writing 2008, which has just been published. (Take a browse online here.)
September 11th, 2008 at 3:36 am
[...] is somewhat correlated with “objective” physical improvement, but not always. The Loom has an interesting article about a species of fish where cancer is widespread. This cancer kills the fish before they live [...]
September 11th, 2008 at 4:15 am
So what then is driving the females to select mates with spotted tails? For something like peacock feathers or fiddler crabs, females are attracted to a physical trait because it is representative of the males fitness. A peacock male can’t maintain a nice beautiful tail if he can’t feed and clean himself well enough to stay alive and waste energy on a tail, same with the crabs. How does this spot on the fish indicate fitness? Maybe its easier for predator to see, so a fish that has a big target and is still alive must be fit.
September 12th, 2008 at 1:08 am
[...] for sick guys. The fact that these guys have a built in characteristic that leads to an early death is immaterial. It’s all down to selection and possibly one of those pathways that goes no [...]
September 13th, 2008 at 8:19 pm
“So Fernandez and Morris wondered what the delicate swordtail females thought of the Xmrk spots on males.
Turns out, they like them a lot. When offered a chance to pay a visit to one of two male fish, female delicate swordtails from two populations in Mexico spent more time with spotted males than spotless ones. And they also preferred to consort with males with big spots over males with little ones.
The Xmrk gene definitely imposes an evolutionary cost on fish. But that cost may be erased by the benefit it gives male fish through sexual selection. By the time a male delicate swordtail dies from an Xmrk tumor, he may have mated with a number of females, which will pass down the gene to their young.”
Allow me to hazard a wild guess as to a possible significant benefit female offspring of a male with large dark spots might receive.
These fish apparently bear their young alive after the female gestates them for 28 days in her oviducts. The dark spots in question look to be situated so as to be likely to cover the area of the oviducts. The Delicate Swordtail is an inhabitant of fast flowing mountain streams in Mexico, and as such would be exposed to a significant amount of UV radiation. It is so tiny (~2in.) that UV could be expected to be able to penetrate to the growing embryos and potentially harm them– unless they just so happened to be protected by something such as a big dark blot of melanin (which functions in many animal species as a protectant against UV).
If this is the case, it is not classical sexual selection, in which the traits selected for are a severe disadvantage to the male, who shows his superior quality and desirability by being able to bear up under them.
In fact, as I mentioned in a previous comment, true sexual selection may be rarer than is commonly supposed. In the case of the peacock for example, I’ve noticed that the spread feathers of the fully extended tail tend to bend forward toward their tips enough, in my opinion, to form a radar dish-like structure which could focus sound right at the ear of the cock, and which would thereby make it much more difficult for any predator (or rival cock) to sneak up on the bird through cover. The eyespots on the feathers might function to enhance the effectiveness of such a system by provoking involuntary startle responses from hidden predators, especially in combination with the peacock’s harsh cry– which could be directed in a beam by the feather dish too, of course. Moreover, I have seen videos in which peacocks seem to be panning around in a steady circle with their feathers fully extended as if they were scanning just as a radar dish would, or as owls do, for that matter.
And peacocks were, as I recall, used as watchbeasts by Persian nobility.
September 15th, 2008 at 2:14 pm
James, I doubt that the dark spots on the males are going to protect any young in the females.
October 1st, 2008 at 8:13 pm
In response to what is considered attractive and whether it matters—
Why do women prefer tall men and men prefer beautiful women? Why do so many look at looks rather than brains? Seems like it catches their eye (much like a black spot or beautiful tail feathers). If one looks at the plight of the dark melanin spotted male and compare it with dragging along a ludicrously large tail along with the plague called humanity, one can guess that some selection is not driven by survival. The chemistry of the brain can trigger some strange bedfellows.
Killingtonskier