Evolution, Evidence Of
Evidence of evolution can be observed in a number of different ways, including distribution of fossils of species both geographically and through geologic time. Evolution is a major scientific theory. As such, it has a tremendous amount of supporting evidence and no clearly contradicting evidence. If new evidence appears to refute it, then a new theory must be formulated. Any evidence requiring a totally new theory, however, would have to be staggering in its scope and strength. The new evidence that has been forthcoming in recent scientific studies supports the theory of evolution and merely fine tunes scientific understanding of the mechanisms involved.
A clear and strong argument in favor of evolutionary theory is found in the fossil record. Paleontology (the study of fossils) provides an unarguable record that many species no longer exist. By such techniques as carbon dating and studying the placement of fossils within the ground, an age can be given for fossils. By placing fossils together based on their ages, a gradual change in form can be surmised which can be followed and extrapolated to the species that exists today. Although the fossil record is incomplete, and many intermediate species are missing, the weight of evidence from those that do exist favors the theories of evolution and natural selection.
Extremely strong evidence supporting evolutionary theory is found in the strata of fossils. No fossils more ancient than those found in underlying layers have ever been found. This correlation of the geologic record with the biological evolutionary record is profound.
English naturalist Charles Darwin (1809–1882) formulated the theory of organic evolution through natural selection in his groundbreaking publication The Origin of Species by Means of Natural Selection, published in 1859. One of the first pieces of evidence that started the young scientist thinking along evolutionary lines was given to him on his journeys aboard the H.M.S. Beagle. Darwin made extensive collections of plants and animals that he came across wherever the ship stopped, and soon he started to notice patterns within the organisms he studied.
There were similarities between organisms collected from widely differing areas. As well as the similarities, there were also striking differences. For example, mammals are present on all of the major landmasses, but Darwin did not find the same mammals even in similar habitats. One explanation of this is that in the past when the landmasses were joined, mammals spread over all of the available land. Subsequently this land moved apart, and the animals became isolated. As time passed, random variation within the populations was acted upon by natural selection. This process is known as adaptive radiation—from the same basic stock, many different forms have evolved. Each environment is slightly different, and slightly different forms are better suited to survive there. An example of this, which is seen at a formative stage, is the case of the finches on the Galápagos Islands. All of the Galápagos finches bear similarities to the mainland finches, but each species has evolved to fill a particular niche, which is not already filled by an animal on the islands even though there are species filling these ecological openings on the mainland.
If it is true that widely separated groups of organisms have ancestors in common, then logic dictates that they would have certain basic structures in common as well. The more structures they have in common then the more closely related they must be. The study of evolutionary relationships based on commonalties and structural differences is termed comparative anatomy. What scientists look for are structures that may serve entirely different functions but are basically similar. Such homologous structures suggest a common ancestor. A classic example of this is the pentadactyl limb, which in suitably modified forms can be seen in all mammals. A greater modified version of this can also be seen amongst birds. This limb has been used by different groups for slightly different purposes and so provides an example of divergent evolution.
These evolutionary relationships are reflected in taxonomy. Taxonomy is an artificial, hierarchical system showing relationships between species. Each level progressed within the taxonomic system denotes a greater degree of relatedness for the organism in that group to the level above.
In embryology, the developing fetus is studied, and similarities with other organisms are observed. This adds evidence to a past recent common ancestor. It is not, however, true that a developing organism replays its evolutionary stages as an embryo; there are some similarities with the more conserved regions, but embryonic development is subjected to evolutionary pressures as much as other areas of the life cycle.
Cell biology is an area where many similarities can be seen between organisms. Many structures and pathways within the cell are vital for the continuance of life. The more important and basic to the whole structure of life a pathway or organelle is, the more likely it is to be observed. For example, the DNA code is the same in virtually all living organisms, as are such structures as mitochondria. These are virtually ubiquitous throughout known life. Most scientists hold that it is inconceivable that each of these things arose separately for each species of living organism. The conclusion advanced by science is that all life forms arose from the same basic source many millennia ago. The examples visible today have survived, and the organisms carrying these processes on have adapted, yielding the diversity of forms seen today.
Cosmology; Dating Methods; Fossils and Fossilization; Stratigraphy
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