Evolution, Evidence of

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Evolution, Evidence of

That all existing varieties of life have evolved from simpler, interrelated ancestors is one of the most solidly established facts known to modern science, and is the basis of all modern biological science. Evidence of evolution takes numerous forms, including distribution of species (both geographically and through time), comparative anatomy, taxonomy, embryology, cell biology, molecular biology, vestigial organs and genes, DNA patterns, and paleontology (the study of fossils).

The English naturalist Charles Darwin (1809–1882) formulated the theory of evolution through natural selection in his ground-breaking publication The Origin of Species by Means of Natural Selection, (1859). The first observations that prompted Darwin to become a pioneer of evolutionary thinking were made on his journey aboard the HMS Beagle as a naturalist (what today would be called a biologist). Prior Darwin’s work, most people, including most scientists, accepted a literal interpretation of the Biblical account of creation, according to which all animals and plants were brought into the world suddenly. (Today, this view is called creationism.) Darwin himself held this view as a young scientist, until he was convinced otherwise by his own developing ideas. On his journal aboard the Beagle, Darwin made extensive collections of the plants and animals that he came across wherever the ship stopped, and very soon he started to notice patterns within the organisms he studied.

Similarities emerged 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; however, these mammals are different, 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, adaptive changes guided by natural selection occurred separately within each population, altering them in divergent ways. This process is known as adaptive radiation. From the same basic origin, many different forms have evolved. Each environment is slightly different, and slightly different forms are better suited to survive.

The theory of evolution—a “theory” not because scientists are uncertain as to whether evolution has occurred, but because in scientific jargon any well-established system of explanations is called a “theory”—helps us understand why islands in the Galápagos are inhabited by organisms that are similar to those on the South American mainland, and many similar cases around the world. The diversity of related species in isolated habitats, and their relationships to species on nearby continents, is a consequence of evolutionary mechanisms acting on a small number of common ancestors.

If it is true that widely separated groups of organisms have ancestors in common, then such organisms should have certain basic structures in common as well. The more structures they have in common, the more closely related they would presumably be. The study of evolutionary relationships based on similarities and differences in the structural makeup of certain species is called comparative anatomy. What scientists look for are structures that may serve entirely different functions, but are basically similar. Such homologous (similar) structures suggest a common ancestor. A classic example of this is the pentadactyl (five digits, as in the hand of humans) limb, which in modified forms can be seen in all mammals.

Evolutionary relationships are reflected in taxonomy. Taxonomy is the study of how species can be assigned to nested categories based on specific defining characteristics. Each level within the taxonomic system denotes a greater degree of relatedness to a particular the organism if it is closer in the hierarchical scheme. Taxonomy predates evolutionary theory, but evolutionary evidence from fossils and DNA has largely confirmed the taxonomic relationships derived from the study of present-day structures.

In embryology, the developing fetus is studied, and similarities with other organisms are observed. For example, annelids and mollusks are very dissimilar as adults. If, however, the embryo of a ragworm and a whelk are studied, one sees that for much of their development they are remarkably similar. Even the larvae of these two species are very much alike. This suggests that they both belong to a common ancestor. It is not, however, true (as was long believed) that a developing organism replays all its evolutionary stages as an embryo. There are some similarities with the more conserved characteristics, but embryonic development is subject to evolutionary pressures as much as other parts of the life cycle. Embryology provides strong evidence for evolution but embryonic development does not preserve a simple record of evolutionary history.

The analysis of developmental stages of various related species, particularly involving reproduction suggests common descent and supports evolution. Sexual reproduction in both apes and humans, for example, is very similar. Molecular biology also produces evidences supporting evolution. Organisms such as fruit flies have similar gene sequences that are active in specific times during development and these sequences are very similar to sequences in mice and even humans that are activated in similar ways during development.

Even in cell biology, at the level of the individuals cell, there is evidence of evolution in that there are many similarities that can be observed when comparing various cells from different organisms. Many structures and pathways within the cell are important for life. The more important and basic to the functioning of the tissues in which cells contribute, the more likely it will be conserved. For example, the DNA code (the genetic material in the cell) is the very similar in comparing DNA from different organisms.

In molecular biology, the concept of a molecular clock has been suggested. The molecular clock related to the average rate in which a gene (or a specific sequence of DNA that encodes a protein) or protein evolves. Genes evolve at different rates the proteins that they encode. This is because gene mutations often do not change the protein. But due to mutations, the genetic sequence of a species changes over time. The more closely related the two species, the more likely they will have similar sequences of their genetic material, or DNA sequence. The molecular clock provides relationships between organisms and helps identify the point of divergence between the two species. Pseudogenes are genes that are part of an organism’s DNA but that have evolved to no longer have important functions. Pseudogenes, therefore, represent another line of evidence supporting evolution, which is based on concepts derived from molecular genetics.

One of the most massive repositories of evidence for evolution is the fossil record. Paleontology (the study of fossils) provides a record showing that many species that are extinct. By techniques such as carbon dating and studying the placement of fossils within the ground, an age can be assigned to the fossil. By placing fossils together based on their ages, gradual changes in form can be identified. Although fossil records are incomplete, with many intermediate species missing, some sequences of fossils preserve a record of the appearance of new species (speciation) and intermediate forms between higher taxonomic levels are numerous. The claim, sometimes made by opponents of evolution, that there are no intermediate fossils, is incorrect. Careful analysis of habitat, environmental factors at various timepoints, characteristics of extinct species, and characteristics of species that currently exist supports theories of evolution and natural selection. Finally, evolution has been observed in the field in living species, as in the Grants’ famous studies of Darwin’s finches in the Galápagos Islands. The Grants tracked whole populations of finches, measuring the characteristics of every individual bird and tracking which birds had offspring. Their data showed that natural selection does modify species in the real world. There is no fundamental distinction between the action of natural selection in slightly modifying finchs’ beaks and its action over a longer time in producing the more dramatic differences between species.