Evolution of the Tropics

by Allen Simmons
Northeastern State University

Introduction:

Biological evolution is the driving force that has shaped all living organisms on Earth. Put bluntly evolution is the genetic change in a given population from one generation to the next. It is a cornerstone theory in the biological sphere and is probably the most important discovery ever made. The theory of evolution is built on several observations made by Charles Darwin during his voyage aboard the H.M.S. Beagle. Darwin came to realize that species have great fertility and produce more offspring than can grow to adulthood in any given environment with populations remaining roughly the same size having only modest fluctuations in numbers and that the food sources have definite limits but are mostly constant. Due to these three observations Darwin concluded that there will always be a struggle for survival among individual members of the species and that in the reproducing species, variation is rampant and no two members are identical. More importantly he determined that some of this variation is heritable from generation to generation.

From these interpretations Darwin inferred his "natural selection" theory in which only the best characteristics for feeding and for reproducing would be indefinitely inherited and passed on. The heritable characteristics of the individuals who got the most or the best food, reproduced with the best or the most mates, would pass on their favorable characteristics, causing the species to change for the better over time.After two hundred years Darwin’s theory still applies, given that we’ve discovered it is far more complicated than just natural selection. Now we understand more fully the vectors of evolutionary change and how it manifests itself in the form of deoxyribonucleic acid. Organisms are subjected to evolutionary pressures under more circumstances and scenarios than we as researchers and observers can put together. There are several factors that are known to apply evolutionary pressures upon organisms such as, climate, inter- and intra-special competition, predation, and even sexual selection.

Nowhere, but in the tropics, is evolution so apparent and abound. In the 1950s, environmentalist Theodosius Dobzhansky describes it as best that words can: “Plants and animals of temperate lands seem to us somehow easy to live with, and this is not only because many of them are long familiar. Their style is for the most part subdued, delicate, often almost inhibited. Many of them are subtly beautiful; others are plain; few are flamboyant. In contrast, tropical life seems to have flung all restraints to the winds. It is exuberant, luxurious, flashy, often even gaudy, full of daring and abandon, but first and foremost enormously tense and powerful.” This couldn’t be more true! The tropics have a staggering amount of biodiversity and an incredible number of different species. Studies have shown that in the Amazon there can be up to 150 – 280 different species of trees alone in as little as one-tenth of a hectare (Gillman, 2007). Such biological diversity is a certain indication of evolutionary pressure working in tandem at exponential levels.

One such causative agent of evolutionary pressure can be the climate in which these organisms dwell. It should be apparent from our own habits alone that animals and plants prefer a particular climate. It can be said that the bio-productivity of an area can be heavily dependent upon the area’s climate. In the tropics, the climate seems to be the most conducive to living things than the climate at higher latitudes. In the tropics the temperatures are relatively high and stable with a steady supply of rainwater, which in turn allows lush vegetation to be available to herbivorous animals at a near non-stop rate (Kricher, 1997). Biologists have found that metabolic rate (the speed of chemical reactions occurring within the body) is directly related to temperature. The higher the temperature, the higher organisms' metabolism will be simply because biochemical and enzymatic reactions occur more quickly at higher temperatures (to a point of course). Two observations can be made here; that tropical species evolve more quickly than cold-weather species because higher temperatures lead to higher mutation rates (Gillman, 2007). This is a reasonable hypothesis since warm-weather organisms likely have higher metabolic rates, and some substances involved in metabolic reactions can cause DNA damage, potentially leading to a mutation. Mutations, in turn, increase genetic variation, the raw material of evolution. So warm weather means higher metabolism, which means higher mutation rates, which may mean that warm-weather species evolve more quickly than cold-weather species.

Competition, especially for resources, is also an important factor influencing plant and animal adaptation, and that competition plays a substantial role in evolution by natural selection. The tropics offer the most bizarre and ingenious species with adaptations to combat one another. Competition does not occur if the resource is too plentiful to limit the growth, distribution or abundance of at least one of the populations. Competition can occur between individuals that are members of the same species. This is called intraspecific competition. Intuitively, intraspecific competition seems likely to be intense because two members of the same species are likely to have very similar resource needs. This sharing of resources among individuals of a single population ultimately limits how large a population can be in a given area (Purves, 2008) Interspecific competition on the other hand, occurs between members of different species. The intensity of interspecific competition should be related to the similarity between the two species. The amount of overlap in resource utilization is called niche overlap (Purves, 2008). Darwin’s time on the Galapagos Islands exhibits prime examples of evolutionary differentiation due to competition for food sources. Take the Saddle-Back tortoise for example, its shell is curved over the back of the neck allowing the tortoise to lift its head and neck up high and reach for higher foliage. Its counterpart, the Dome-Shelled tortoise does not have this curvature of the shell and therefore is limited to foraging at ground level.
Competition is a powerful force of natural selection. There are many examples of closely related species that differ just enough to reduce niche overlap and permit coexistence (Purves, 2008). Genetic variation in a character that influences competition, coupled with selection to reduce competitive overlap, make a strong argument for the evolutionary divergence of characters. Competition likely affects species diversity and should cause a reduction in the number of species living within an area, preventing very similar species from co-occurring (Purves, 2008). In the long run though competition is likely to increase species diversity, by acting as a force for specialization and divergence.

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Saddleback Tortoise © 2012 Scholastic Inc.
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Dome-Shelled Tortoise © 2012 Photostuff.org

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Predation is probably one of the most easily understood selection pressures. Organisms simply must adapt to capture and consume or hide and avoid being eaten, a fairly simple scenario. The predatory adaptations of claws and teeth are relatively straight forward, in that it is much easier to kill your prey with razor sharp claws than with stubby hooves. It is the defensive adaptations that a truly astounding. Adaptations to camouflage oneself are the most abundant and is known as crypsis, or cryptic coloration. This adaptation depends strictly upon the creatures environment because without it, the animal would become apparent. Kricher (1997) discusses birds in the tropics and how the males are generally brightly colored and tend to stand out while the females deviate to pigmentation with more earth tones. It is a good hypothesis that this is because the females are the ones who tend to the nest, which holds the mating couple’s ultimate investment. Aposmatic coloration is just the opposite of crypsis, the organism stands out of the crowd to send a message, “You don’t want to mess with me.” The most common example of this is tropical poison-dart frog. Their skin is extremely toxic to the touch and to ingestion. Their bold patterning and coloration of bright red, blues, oranges, and yellows serve as an easy to remember warning sign to predators that the frog is very toxic. The defensive adaptations of organismc in the tropics are numerous and in some cases very inventive.

It isn’t just animals that have evolved to predate on other animals. There are species of tropical plants that have adapted to capture and consume insects (and occasionally small animals) Carnivorous plants have the most bizarre adaptations to low-nutrient environments. They make use of traps to catch their prey and they work in a variety of ways. Pitfall traps of pitcher plants are leaves folded into deep, slippery pools filled with digestive enzymes; 
Flypaper (or sticky or adhesive traps) of sundews and butterworts are leaves covered in stalked glands that exude sticky mucilage; Snap traps of the Venus flytrap and waterwheel plant are the most well known and have hinged leaves that snap shut when trigger hairs are touched (Raven, 2005). Below is a video from the World Book showing a flytrap doing what it does best. The other is an example of a pitcher plant displaying that not even small mammals are exempt from being consumed.


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Sexual selection is often difficult to understand. Sexual selection makes many organisms go to extreme lengths for sex and the opporotunity to reproduce and pass on their genes. Peacocks maintain elaborate tails, wolves fight over territories, fruit flies perform dances, and some species deliver persuasive gifts. It was Charles Darwin who originally proposed that the so-called secondary sexual characteristics of male animals , such as the elaborate tails of peacocks, bright plumage or expandable throat sacs in many birds, large racks in mooses, & deep voices in men evolved because females preferred to mate with individuals that had those features (Shermer, 2009) Sexual selection occurs when some individuals out-reproduce others, and those that have more offspring differ genetically from those that have fewer. I believe the best (and possibly most extreme) example of how sexual selection can drive evolutionary change between sexes can be seen in the birds of paradise, found only in the Indo-Pacific. Most are distinguished by striking colors and bright plumage of yellow, blue, scarlet, and green. These colors distinguish them as some of the world's most dramatic and attractive birds. Males often sport vibrant feathered ruffs or amazingly elongated feathers, which are known as wires or streamers (Jones, 2011). Some species have enormous head plumes or other distinctive ornaments, such as breast shields or head fans. Males put their bright colors and unusual ornaments to good use when they display for females. Their elaborate dances, poses, and other rituals accentuate their appearance and put on a phenomenal show for both female birds and any humans lucky enough to be in the vicinity. Such displays can last for hours, and in many species they consume a significant part of the male's time (Jones, 2011). Below is an excerpt from the documentary Birds of the Gods produced by Harvey Jones.












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Citations: (Does not conform to APA)
Kricher, J. (1997). A Neotropical Companion. Princeton, N.J.: Princeton University Press.
Raven, P. H., Evert, R. F., & Eichhorn, S. E. (2005). Biology of plants. New York: W.H. Freeman and Co
Gillman, Len N.; Wright, Shane D. (2007), Molecular Evolution has Wheels in the Tropics, Biologist. Nov 2007, Vol. 54 Issue 4, p195-199.
Dobzhansky, T. (1950), Evolution in the Tropics, American Scientist, 1950, Vol. 38, p209-221
Shermer, M. (2009). A Skeptic's Take on the Public Misunderstanding of Darwin. Scientific American, Retrieved from http://www.scientificamerican.com/article.cfm?id=darwin-misunderstood
Darwin, Charles (1859). The Origin of Species. New York, New York: Random House., 1979 Reprint, Kindle Edition
Purves, W.K., G.H. Orians and H.C. Heller. (2008) Life: The Science of Biology. Sinauer, Sunderland MA. Retrieved from http://www.globalchange.umich.edu/globalchange1/current/lectures/competition/competition.html
Jones, H. (2011). Birds of the Gods [DVD]. Available from http://www.pbs.org/wnet/nature/episodes/birds-of-the-gods/introduction/6229/