A Little Help from Rainforest Friends

 

Some of the defense mechanisms that organisms evolve as adaptations involve other organisms—a sort of “co-evolution.” Lianas and epiphytes rely on symbiosis or partnerships. We say a relationship is commensalistic if one partner benefits while the other is relatively neutral. Other kinds of adaptations are more like “shadow dances”—an organism might use another’s traits without any contact at all. Consider these examples:

 

Batesian Mimicry  (named for Henry Walter Bates, a British expert on Amazon butterflies)  is when a species with few protections resembles a related species. The Heliconid butterfly is toxic; just looking like it discourages potential predators.

 

Muellerian Mimicry (named for German biologist Fritz Mueller) is when several unrelated animal species resemble one another.  For example, yellow and black stripes (like those of the poison arrow frog) seem to be common warning signs in nature. Any organism with yellow and black stripes is likely to be avoided.

 

Self-mimicry involves developing a pattern that resembles a deceptive body part, like giant eye spots on a butterfly’s wings.  A variation on self-mimicry is an auditory display called “Beau Gesting.” (Remember the movie?) A bird (like the mockingbird) shows off its creativity by sounding like a whole bunch of birds; it may intimidate a predator this way or impress a future mate.

 

Camoflauge involves looking like your surroundings, like a chameleon or walking stick. Some rainforest organisms (like toads) can change their coloration.

 

On first glance, it’s hard to imagine how “natural selection” could be behind the sorts of adaptations on this page. How could a butterfly suddenly look like another, or suddenly develop spots that look like big, nocturnal eyes? The answer, of course, is that it can’t. The traits that make a species resemble another species, a tree or a moss, are complex. They can’t just appear. Sometimes the traits develop gradually; each generation is a little darker, a little greener, a little rounder or flatter. Just a little change might give the organism just a little edge on survival. Evolution is a long process of probabilities, not a sudden, and magical event. And sometimes a group of genes may actually have a perfectly good function and suddenly become useful for another function, when the environment changes. The genes don’t suddenly appear, but the way they are expressed might change in a relatively short period of time. Photo credit for frog: USGS

 

Mechanisms of Evolution

 

To understand how evolution works, it’s helpful to have “X-ray vision.” Don’t look at the organisms in a population, but look through their skin and cell membranes, to their genes! Of course, it’s impossible, but that’s the mental model that biologists use to envision how the ratios among genes can change depending upon a number of different situations.

 

Start by imagining a very large population in a fairly open area—no barriers of any kind to interbreeding. The population is well-adapted, and the climate isn’t changing. The threats have existed for a long time, and the population has adaptations to resist them. In these conditions, the ratio of gene forms (alleles) in the population is likely to remain pretty constant.

 

But imagine a few of the organisms in this population get blown (literally) down the river, and raft their way to a new island.  (That’s probably how South American organisms got to the Galapagos Islands.) Or organisms get trapped in an isolated oxbow lake as the main channel river meanders another way. They may have randomly carried one form of some genes rather than another. Their offspring on the island or the lake would resemble their parents and the reason wouldn’t be natural selection. We call this founder effect.

 

Or a major environmental disaster might hit the entire population. Most of the organisms might be killed, and only a very few with certain traits might have survived. This one time, very sudden change in the gene pool would be called bottleneck effect.

 

If the range of the population is very large and there are few barriers, it might actually be impossible for organisms at one end of the range to ever reach and interbreed with organisms on the other end of the range. The entire population might vary across the range; we call that genetic drift.

 

A common question that biology students ask: “If a trait is a great disadvantage to an organism, does it disappear from the population completely?” The answer is “perhaps.” Remember, some genes can remain “masked” and not expressed in an organism. (Mendel called them recessive.) A recessive gene might remain in a population indefinitely, because it would not result in natural selection. And some genes (like human sickle cell anemia) are very deleterious in the homozygous condition (if you inherit two of them) but may actually be beneficial in the heterozygous condition (if you inherit only one of them, and then are more resistant to malaria.)

 

An evolutionary puzzle--what's that Astrocrom tree doing above? Watch for it at ExplorNapo and hypothesize on the Discussion Boards.

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