Some people think that evolution is all about progress and improvement. I’ve even heard the ridiculous argument that if evolution is true, then mankind should reach moral perfection (showing just how out of touch people’s ideas about evolution are). But, really it’s about differential reproductive success – which 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 their full natural lifespan. And, apparently, certain genotypes are prone to this cancer. So, why hasn’t evolution wiped-out the cancer-prone genotypes? It seems that this particular cancer has a side effect: it makes males very sexy to the females of the species. As a result, the cancer-prone genotype proliferated and this species of fish is prone to this type of cancer.
Fish B: definitely a lot more sexy that Fish A, thanks to that dark tumor in its tail.