If widely separated groups of organisms are originated from a common ancestry, they are expected to have certain basic features in common. The degree of resemblance between two organisms should indicate how closely related they are in evolution:
Groups with little in common are assumed to have diverged from a common ancestor much earlier in geological history than groups with a lot in common;
In deciding how closely related two animals are, a comparative anatomist looks for structures that are fundamentally similar, even though they may serve different functions in the adult. Such structures are described as homologous and suggest a common origin.
In cases where the similar structures serve different functions in adults, it may be necessary to trace their origin and embryonic development. A similar developmental origin suggests they are the same structure, and thus likely derived from a common ancestor.
When a group of organisms share a homologous structure that is specialized to perform a variety of functions in order to adapt different environmental conditions and modes of life are called adaptive radiation. The gradual spreading of organisms with adaptive radiation is known as divergent evolution.
Divergent evolution is the accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same species to different and isolated environments which blocks the gene flow among the distinct populations allowing differentiated fixation of characteristics through genetic drift and natural selection. Primarily diffusion is the basis of molecular division can be seen in some higher-level characters of structure and function that are readily observable in organisms. For example, the vertebrate limb is one example of divergent evolution. The limb in many different species has a common origin, but has diverged somewhat in overall structure and function.Alternatively, “divergent evolution” can be applied to molecular biology characteristics. This could apply to a pathway in two or more organisms or cell types, for example. This can apply to genes and proteins, such as nucleotide sequences or protein sequences that derive from two or more homologous genes. Both orthologous genes (resulting from a speciation event) and paralogous genes (resulting from gene duplication within a population) can be said to display divergent evolution. Because of the latter, it is possible for divergent evolution to occur between two genes within a species.
In the case of divergent evolution, similarity is due to the common origin, such as divergence from a common ancestral structure or function has not yet completely obscured the underlying similarity. In contrast, convergent evolution arises when there are some sort of ecological or physical drivers toward a similar solution, even though the structure or function has arisen independently, such as different characters converging on a common, similar solution from different points of origin. This includes analogous structures.
Divergent species are the direct consequence of adaptive radiation. Divergent species occur when a part of the species is separated from the population by a physical barrier (flood waters, mountain range, deserts). Once separated, the species begins to adapt to their new environment (natural selection). After many generations and continual evolution of the separated species, the population eventually becomes two separate species to such an extent where they are no longer able to interbreed with one another.An example of divergent species is the apple maggot fly. The apple maggot fly once infested the fruit of a native Australian hawthorn. In the 1860s some maggot flies began to infest apples. They multiplied rapidly because they were able to make use of an abundant food supply. Now there are two distinct species, one that reproduces when the apples are ripe, and another that continues to infest the native hawthorn. Furthermore, they have not only evolved different reproductive timing, but also now have distinctive physical characteristics.
Nested Hierarchies and Classification
Taxonomy is based on the fact that all organisms are related to each other in nested hierarchies based on shared characteristics. Most existing species can be organized rather easily in a nested hierarchical classification. This is evident from the Linnaean classification scheme. Based on shared derived characters, closely related organisms can be placed in one group (such as a genus), several genera can be grouped together into one family, several families can be grouped together into an order, etc. The existence of these nested hierarchies was recognized by many biologists before Darwin, but he showed that his theory of evolution with its branching pattern of common descent could explain them. Darwin described how common descent could provide a logical basis for classification:“All the foregoing rules and aids and difficulties in classification are explained, if I do not greatly deceive myself, on the view that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, …”
—Charles Darwin, On the Origin of Species, page 577
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