The only requirement is that the change has to enhance survival and/or reproduction now. Evolution is not forward-looking, heading in the direction of whatever would be a cool idea. (It'd be nice if it were; I've wanted wings for ages. Big, feathery falcon wings from my shoulders. It'd make wearing a shirt impossible, but let's face it, I hate wearing shirts anyway so that's really not much of a sacrifice.)
Anyhow, the trick sometimes is figuring out what the benefit is, because it's not always obvious. The extravagant tail of the peacock is clearly an attractant for females, although at this point the male peacocks may have maxed out -- reached the point where the tail's advantage of attracting females is counterbalanced by the disadvantage of being so cumbersome that it makes it harder to escape predators. When two competing selecting agents hit that balance point, the species -- with respect to that trait, at least -- stops evolving.
A good bunch of the wild colorations you find in nature have to do with sex. Not only attracting mates in animals, but colorful flowers attracting a specific pollinator -- because pollination is (more or less) plant sex. But not all; the stripes of the Bengal tiger are thought to break up its silhouette in the dappled sunlight of its forest home, making it less visible to prey. The bright colors of the dart-poison frogs are warning colorations, advertising the fact that they're highly poisonous and that predators shouldn't even think about it if they know what's good for them. A recent study concluded that one advantage of stripes in the zebra is that it confuses biting flies, including the dangerous tsetse fly (carrier of African sleeping sickness) -- horses that were draped with striped cloth (mimicking the zebra's patterns) were far less susceptible to horsefly bites. It's probable that the stripes also confuse predators such as lions, which frequently try to target one animal in a fleeing herd and separate it from the rest, a task that's difficult if the stripes make it hard to tell where one zebra begins and the other ends. So zebra stripes may be a twofer.
Sometimes, though, the reason for a bright coloration isn't obvious. In the summer here in upstate New York we often see brilliant little tiger beetles, named not for stripes (most of them don't have 'em) but for their role as a voracious predator of other insects. The ones we have here are a glistening emerald green, which I always figured camouflaged them on plant leaves -- but there are ones that are an iridescent blue, and one species is green and blue with orange spots.
Hard to call that camouflage.
[Image licensed under the Creative Commons Ryan Hodnett, Six-spotted Tiger Beetle (Cicindela sexguttata) - Mississauga, Ontario 01, CC BY-SA 4.0]
Turns out that even the non-green ones might be using their sparkling colors as camouflage, however implausible that sounds. A study that appeared this week in Current Biology, led by Karin Kjernsmo of the University of Bristol, concluded that the iridescence itself confuses predators, as much as it seems like it would attract attention.
Kjernsmo was studying the aptly-named Asian jewel beetles, which like our North American tiger beetles come in a wide range of glittering colors. She took the wing cases of jewel beetles, both the iridescent and the matte species, and baited them with mealworms to see if birds had a preference. 85% of the targets with matte wings (of various colors) were picked off by birds, while only 60% of the iridescent ones were.
"It may not sound like much," Kjernsmo said, "but just imagine what a difference this would make over evolutionary time."
Her next question, though, was why. This is much harder to determine, mostly because you can't ask a bird why it picked a particular insect for lunch. (Well, you can ask.) So what she did was a simple but suggestive experiment using human subjects -- she stuck various-colored wing cases to leaves at eye level on a forest trail, and had thirty-six human subjects walk the trail and see how many they could find. They found 80% of the matte ones -- and only 17% of the iridescent ones!
It's a surprising result. It may be that the shifting, sparkling surface of an iridescent insect confounds the ability of your visual cortex to make sense of what it's seeing by rendering it more difficult to perceive the edges, and therefore the shape, of what you're looking at. The result: you can see the colors, but you don't recognize it as a beetle. It's a plausible guess, but it will take more research to find out if it's the correct one, and if the reason the humans couldn't see iridescent wings is the same as why birds didn't eat them.
But once again, we're left with a slight difference in selection by a predator leading to what Darwin called "endless forms most beautiful and most wonderful." The natural world is deeply fascinating, and is even more wonderful when you not only can appreciate its beauty -- but understand where that beauty may have come from.
The brilliant, iconoclastic physicist Richard Feynman was a larger-than-life character -- an intuitive and deep-thinking scientist, a prankster with an adolescent sense of humor, a world traveler, a wild-child with a reputation for womanizing. His contributions to physics are too many to list, and he also made a name for himself as a suspect in the 1950s "Red Scare" despite his work the previous decade on the Manhattan Project. In 1986 -- two years before his death at the age of 69 -- he was still shaking the world, demonstrating to the inquiry into the Challenger disaster that the whole thing could have happened because of an o-ring that shattered from cold winter temperatures.
James Gleick's Genius: The Life and Science of Richard Feynman gives a deep look at the man and the scientist, neither glossing over his faults nor denying his brilliance. It's an excellent companion to Feynman's own autobiographical books Surely You're Joking, Mr. Feynman! and What Do You Care What Other People Think? It's a wonderful retrospective of a fascinating person -- someone who truly lived his own words, "Nobody ever figures out what life is all about, and it doesn't matter. Explore the world. Nearly everything is really interesting if you go into it deeply enough."
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