Natural Selection
Natural selection is treated as the primary—and by some scientists the sole—mechanism producing evolutionary change. In essence it involves the process whereby some aspect of an organism’s environment (either climate, competition, predation, or parasitism) inter-acts with one or more traits of the organism, such that the survival or reproduction of that organism is enhanced (or diminished) relative to organisms that lack those traits. Natural selection is intimately tied to the phenomenon of adaptation: adaptations are honed by natural selection; over time the proportion of organisms with these adaptations will increase in a population because of natural selection.
Natural selection is associated with the idea that nature is red in tooth and claw, the implication being that there is a fierce competitive struggle among all organisms, with natural selection weeding out those less fit organisms while favoring those that are more fit.
Evolution evidences ( Morphological Similarity)
Very suggestive evidence for common descent is also provided by the study of comparative anatomy. It was customary already in the eighteenth century to call certain organisms "related" when they were similar. At that time the French naturalist Comte Buffon described this for horses, donkeys, and zebras. The less similar that two kinds of organisms were, the less closely they were considered to be "related." The systematists, the students of classification, used the degree of similarity to establish a hierarchy of taxonomic categories. The most similar organisms were placed in the same species. Similar species were placed in the same genus, similar genera in the same family, and thus all the way up to the taxa of the highest category. This arrangement of organisms by the degree of their similarity and relationship is called the Linnaean hierarchy , after the Swedish botanist Carolus Linnaeus, who developed the system of binomial classification. Such a classification groups organisms into larger and larger taxa, finally comprising all the animals and all the plants. Beginning with a particular species, let us say the cat, one was able to construct this hierarchy. It was known that there were other species of cats rather similar to the house cat, which Linnaeus also placed in the genus Felts. This group of cats could be combined with the lion, the cheetah, and other genera of cats into the family Felidae. This family of catlike mammals could then be combined with other predatory mammals such as the Canidae (doglike), Ursidae (bears), Mustelidae (weasels), Viverridae (civets), and related groups into the order of Carnivora. In a similar manner, other mammals could be combined into the orders of Artiodactyla (deer and relatives), Perissodactyla (horses, etc.), Rodentia (rodents, etc.), and those of whales, bats, primates, marsupials, and so on to form the class Mammalia (mammals). A similar hierarchy exists for all other kinds of animals, such as birds and insects, and for plants. The nature and causation of this grouping, unless ascribed to creation, was a complete riddle until Darwin showed that it was evidently due to "common descent." Each taxon (group of organisms), Darwin demonstrated, could be explained as consisting of the descendants from the nearest common ancestor, and such descent required evolution.
The Linnaean hierarchy. Each category is nested within the next higher category,
such as the species in the genus.
Evolution evidences (The Fossil Record)
The most convincing evidence for the occurrence of evolution is the discovery of extinct organisms in older geological strata. Some of the remnants of the biota that lived at a given geological period in the past are embedded as fossils in the strata laid down at that period. Each earlier stratum contains the ancestors of biota fossilized in the succeeding stratum. The fossils found in the most recent strata are often very similar to still living species or, in some cases, even indistinguishable. The older the strata are in which a fossil is found—that is, the further back in time—the more different the fossil will be from living representatives. Darwin reasoned that this is to be expected if the fauna and flora of the earlier strata had gradually evolved into their descendants in the later, more recent strata.
Given the fact of evolution, one would expect the fossils to document a gradual steady change from ancestral forms to the descendants. But this is not what the paleontologist finds. Instead, he or she finds gaps in just about every phyletic series. New types often appear quite suddenly, and their immediate ancestors are absent in the earlier geological strata. The discovery of unbroken series of species changing gradually into descending species is very rare. Indeed the fossil record is one of discontinuities, seemingly documenting jumps (saltations) from one type of organism to a different type.
Transitional forms in evolution of the whale are illustrated by Hans Thewissen’s discovery of fossils of the Ambulocetus from 47 million years ago.
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