Patterns of biological evolution
Biological evolution is said to follow several different patterns over time. Factors such as environment and predation pressures are said to have different effects on the ways in which species exposed to them evolve. Evolutionary biologists have labeled these differing patterns as Divergent, Convergent and Parallel Evolution.
Creationists claim that the evolutionary progressions have not been observed, and that these different developmental pathways are merely post-hoc explanations intended to accommodate observations to the theory of evolution.
- Divergent evolution is when descendants of a species evolve separate traits.
- Parallel evolution is when two unrelated but similar species both develop similar traits.
- Convergent evolution is when to unrelated lineages become more alike by developing similar traits.
When people hear the word "evolution," they most commonly think of divergent evolution, the evolutionary pattern in which two species gradually become increasingly different. Divergent evolution refers to a group from a specific population developing into a new species. In order to adapt to various environmental conditions, the two groups are said to develop into distinct species due to differences in the demands driven by the environmental circumstances. On a large scale, divergent evolution is said to be responsible for the creation of the current diversity of life on earth from the first living cells. On a smaller scale, it is claimed to be responsible for the evolution of humans and apes from a common primate ancestor. On a molecular scale, it is what would describe the appearance of new catalytic functions of enzymes and membrane protein topology.
Divergence and Speciation
If different selective pressures are placed on a particular organism, a wide variety of adaptive traits are said to result. If only one structure on the organism is considered, these changes are said to either add to the original function of the structure, or change it completely. By this means, divergence can lead to speciation, or the development of a new species. Divergence can be invoked when looking at any group of related organisms. The differences are said to be produced from the different selective pressures. Any genus of plants or animals can be said to show divergence. An example can involve the diversity of floral types in the orchids. The greater the number of differences present, the greater the claimed divergence. Scientists speculate the greater the difference between two similar species the longer length of time that the divergence originally took place.
Examples of Divergence
There are many examples of divergence in nature. If a freely-interbreeding population on an island is separated by a barrier, such as the presence of a new river, then over time, the organisms may start to diverge. If the opposite ends of the island have different pressures acting upon it, this may result in divergence. Or, if a certain group of birds in a population of other bird of the same species varies from their migratory track due to abnormal wind fluctuations, they may end up in a new environment. If the food source is such that only birds of the population with a variant beak are able to feed, then this trait will become more common by virtue of its selective survival advantage. The same species in the original geographical location and having the original food source do not require this beak trait and will, therefore, not change in the same way.
Divergence appears to have also occurred in the red fox and the kit fox. While the kit fox lives in the desert where its coat helps disguise it from its predators, the red fox lives in forests, where the red coat blends into its surroundings. In the desert, the heat makes it difficult for animals to eliminate body heat. The ears of the kit fox have a greater surface area so that it can more efficiently remove excess body heat. Their different adaptations are determined primarily by the different environmental conditions and adaptation requirements, not on genetic differences. If they were in the same environment, it is likely that they would evolve similarly. Divergence is supported by DNA analysis where the species that diverged can be shown to be genetically similar.
Evolutionists claim that the human foot evolved to be very different from a monkey's foot, despite their common primate ancestry. It is speculated that a new species (humans) developed because there was no longer was a need for swinging from trees. Upright walking on the ground required alterations in the foot for better speed and balance. These differing traits soon became characteristics that evolved to permit movement on the ground. Although humans and monkeys are genetically similar, their natural habitat required different physical traits to evolve for survival.
Creationists respond that although environmental pressures can cause creatures to adapt, they can only do so within their genetic limits.
Convergent evolution is where creatures are believed to have independently evolved similar features. Convergence causes difficulties in fields of study such as comparative anatomy. Convergent evolution takes place when species of different ancestry begin to share analogous traits because of a shared environment or other selection pressure. Environmental circumstances that require similar developmental or structural alterations for the purposes of adaptation are thought to lead to convergent evolution even though the species differ in descent. These adaptation similarities that arise as a result of the same selective pressures can be misleading to scientists studying the natural evolution of a species. Convergent evolution also creates problems for paleontologists using evolutionary patterns in taxonomy, or the categorization and classification of various organisms based on relatedness. It often leads to incorrect relationships and false evolutionary predictions.
As explained further below, creationists claim that convergence is merely a rationalization for evidence that contradicts divergence.
Examples of Convergent Evolution
An obvious example of convergence would be that of birds, bats, pterosaurs, and flying insects (all different species that are on distinct evolutionary lineages), all of which can fly, but for which their postulated common ancestors could not fly. Thus each species is considered to have developed wings independently. Species do not evolve in order to prepare for future circumstances, but rather the development of flight is considered to be induced by selective pressure imposed by similar environmental conditions, even though they are supposed to be at different points in time. The development potential of any species is not limitless, primarily due to inherent constraints in genetic capabilities. Only changes that are useful in terms of adaptation are preserved. Yet, changes in environmental conditions can lead toward less useful functional structures, such as the appendages that might have existed before wings. Another change in environmental conditions is thought to result in alterations of the appendage to make it more useful, given the new conditions, although changes which involve the generation of new genetic information have never been observed.
For example, the wings of all flying animals are very similar because the same laws of aerodynamics apply. These laws determine the specific criteria that govern the shape for a wing, the size of the wing, or the movements required for flight. All these characteristics apply irrespective of the animal involved or the physical location, except for insects, which have wings that have more marked differences. Evolutionists think that understanding the reason why each different species developed the ability to fly relies on an understanding of the possible functional adaptations, based on the behavior and environmental conditions to which the species was exposed. Although only theories can be made about extinct species and flight, since these behaviors can be predicted using by fossil records, these theories can often be tested using information gathered from their remains. Evolutionists believe that perhaps the wings of bird or bats were once appendages used for other purposes, such as gliding, sexual display, leaping, protection, or arms to capture prey. Creationists, on the other hand, point out that the joints and muscles required for capturing prey, for example, are quite different to those required for flying.
In various species of plants which share the same pollinators, many structures and methods of attracting the pollinating species to the plant are similar. These particular characteristics enabled the reproductive success of both species due to the environmental aspects governing pollination, rather than similarities derived by being genetically related by descent.
Evolutionists believe that convergent evolution is supported by the "fact" that these species come from different ancestors, but that is itself part of the evolutionary story. They claim that this has been proven by DNA analysis, although such analysis does not exclude the possibility that similar creatures were created with similar DNA. Evolutionists admit, however, that understanding the mechanisms that brings about these similarities in characteristics of a species, despite the differences in genetics, is more difficult.
Parallel evolution occurs when unrelated organisms develop the same characteristics or adaptive mechanisms due to the nature of their environmental conditions. Or stated differently, parallel evolution occurs when similar environments produce similar adaptations. The morphologies (or structural form) of two or more lineages evolve together in a similar manner in parallel evolution, rather than diverging or converging at a particular point in time. Parallel evolution can be difficult to distinguish from convergent evolution.
Examples of Parallel Evolution
One example is the complex plumage patterns that seem to have evolved independently among many very different bird species.
A molecular example of Parallel Evolution is the ligand specificity of repressors and periplasmic sugar-binding proteins.
Parallel evolution is exemplified in the case of the tympanal and atympanal mouthears in hawkmoths, or Sphingidae species. These insects have developed a tympanum, or eardrum, similar to humans as a means to communicate through sound. Sounds induce vibrations of a membrane that covers the tympanum, known as the tympanic membrane. These vibrations are detected by small proteins at the surface of the tympanic membrane called auditory receptors. Within the Sphingidae species, two differing subgroups acquired hearing capability by developing alterations in their mouthparts by a distinctly independent evolutionary pathway.
Investigating the biomechanics of the auditory system reveals that only one of these subgroups has a tympanum. The other subgroup has developed a different mouthear structure that does not have a tympanum, but has a mouthear with functional characteristics essentially the same as the subgroup with the tympanum. The evolutionary significance of how hearing capabilities developed in parallel in two different subgroups of a species reveals that distinct mechanisms can exist leading to similar functional capabilities with differing means for acquiring the same functional attribute. For both subgroups, hearing must have been an important characteristic for the species to survive given the environmental conditions.
Parallel Evolution and Speciation
Parallel speciation is a type of parallel evolution in which reproductive incompatibility in closely related populations is determined by traits that independently evolve due to adaptation to differing environments. These distinct populations are reproductively incompatible and only populations that live in similar environmental conditions are less likely to become reproductively isolated. In this way, parallel speciation suggests that there is good evidence for natural selective pressures leading to speciation, especially since reproductive incompatibility between to related populations is correlated with differing environmental conditions rather than geographical or genetic distances.
All three types of evolution presume that evolution has actually occurred, but this basic premise is disputed by creationary scientists, who point out that a vital requirement of evolution—the generation of new genetic information—has never been observed and is even contrary to observations that show that genetic information is only ever lost through mutations.
Furthermore, they argue that convergent and parallel evolution are nothing more than attempts to explain evidence that doesn't fit the standard evolutionary story of a diverging "family tree" of living things.
As Walter ReMine points out so clearly, the evolutionist has a seemingly endless smörgåsbord of stories which can be used post-facto. If organisms not related by a phylogenic evolutionary tree show dramatically similar features one reads this is due to ‘evolutionary convergence’. However, if organisms in very similar environments fail to display apparent similarities, convergence had not occurred. Such rationalizations are free of substance and an inadequate substitute for hard evidence.
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