TROPICAL RAINFORESTS: Rainforest Diversity
Galapagos Tortoise. (Photo by Rhenda Glasco)


Today is is generally accepted that species form through the process of evolution. Evolution was first formally described by Charles Darwin and Alfred Henry Wallace in the late 1850s. Darwin's publication of The Origin of Species shook the foundation of Christian doctrine: that all species were divinely created. The work was immediately condemned by Catholics, Jews, and Protestants alike marking a rare consensus in a long bloody history of divisiveness.

The effort of these eminent naturalists and others, along with the bounty of hitherto unknown creatures found in newly discovered lands undermined the belief in the literal Ark as presented by the Scripture. Such an ark, which housed a pair of all species, was becoming quite crowded in the eighteenth century. Darwin and Wallace challenged the existing view that a supreme being created each species for a specific habitat (which required no evidence), and presented the concept of evolution by natural selection, for which they supplied evidence from their travels in the tropics.

The theory of evolution through natural selection is relatively simple. Individuals within any population, whether it be rhinoceros beetles, goanas, or barnacles, show variations in certain inheritable characteristics. These genetic variations are often random or chance changes in the genetic makeup of an individual that occur during reproduction. These differences in genetic characteristics mean some individuals are better suited to the conditions of their environment and leave more offspring than those less fit. Over long periods of time, the genetic composition of the population will change and the species essentially adapts (not consciously, but by "survival of the fittest") to changes in their environment. When a population has undergone so much genetic change that it is no longer capable of breeding with the original stock it is considered a new species.

In order for one species to become two or more species some form of geographic isolation is usually required (see the elephant example under "Ice Ages/Glaciation). New species can also form through adaptive radiation which occurs when a single species enters a habitat where niches remain unfilled. Gradually, those individuals with certain characteristics, say a longer beak or stronger claws, will have the advantage over other individuals with different traits. The individuals with these characteristics with thrive and produce more offspring and eventually fill the open niche. The most famous example of adaptive radiation are Darwin's finches of the Galapagos Islands, but many better examples exist like the lemurs of Madagascar (see chapter 4 under "Mammals") and the Honeycreepers of Hawaii. Honeycreepers, a bird the size of finch, have radiated into many different niches on the Hawaiian islands so that before human interference, some 30 species existed.

Convergent or parallel evolution is an interesting phenomenon whereby different species with different ancestry evolve to occupy the same niche, usually on separate land masses. These phylogenetically different species often develop very similar physical traits and behaviors, because those characteristics are superb for their niche.

Relatively new evidence suggests that evolution may not be the slow, continuous process, of Darwin's reasoning, but a rather sporadic process that occurs in bursts of rapid change. This more chaotic view of evolution is encompassed by the theory of punctuated equilibrium which proposes that most structural change occurs during the speciation events, when a species originates and that very little further evolutionary change occurs thereafter. Therefore we expect relatively rapid speciation after an extinction event or geographic isolation, but relatively little or no evolution once the previously unoccupied niches are filled. Studies in the late 1990s uncovered a mechanism in fruit flies that could help explain the rapid formation of new species in response to a changing environment. So-called "Doomsday genes" may enable species that have existed with the same appearance and behavior for thousands of years to undergo radical structural changes in mere generations in response to sudden environmental changes.

Population Diversity

Continued: Rainforest diversity

Other pages in this section: