The Elements of Life
In biology, the elements of life are the essential building blocks that make up living things. They are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. The first four of these are the most important, as they are used to construct the molecules that are necessary to make up living cells. These elements form the basic building blocks of the major macromolecules of life, including carbohydrates, lipids, nucleic acids and proteins. Carbon is an important element for all living organisms, as it is used to construct the basic building blocks of life, such as carbohydrates, lipids, and nucleic acids. Even the cell membranes are made of proteins. Carbon is also used to construct the energy-rich molecules adenosine triphosphate [ATP] and guanosine triphosphate [GTP]. Hydrogen is used to construct the molecules water and organic compounds with carbon. Hydrogen is also used to construct ATP and GTP. Nitrogen is used to construct the basic building blocks of life, such as amino acids, nucleic acids, and proteins. It is also used to construct ATP and GTP. Oxygen is used to construct the basic building blocks of life, such as carbohydrates, lipids, and nucleic acids. It is also used to construct ATP and GTP. Phosphorus is used to construct the basic building blocks of life, such as carbohydrates, lipids, and nucleic acids.
Convergent evolution occurred in horses, where equine chorionic gonadotropin [eCG] is secreted by a special population of trophoblast cells. From: Pathobiology of Human Disease, 2014
L. Gabora, in
Brenner's Encyclopedia of Genetics [Second Edition], 2013Convergent Evolution
Introduction
Convergent evolution refers to the evolution in different lineages of structures that are similar or ‘analogous’, but that cannot be attributed to the existence of a common ancestor; in other words, the fact that the structures are analogous does not reflect homology. A similarity may reside at the phenotypic level, in which case the lineages share the overt trait, but the underlying DNA sequences are different. Convergent evolution occurs when species occupy similar ecological niches and adapt in similar ways in response to similar selective pressures. Traits that arise through convergent evolution are referred to as ‘analogous structures’. They are contrasted with ‘homologous structures’, which have a common origin. The opposite of convergent evolution is ‘divergent evolution’, whereby related species evolve different traits.
Well-documented cases of convergent evolution of similar DNA sequences are not plentiful; such cases are usually restricted to a few amino acids. Convergent evolution can mislead phylogenetic inference because it mimics shared ancestry. Standard phylogenetic methods are not equipped to differentiate between the two. When convergent evolution is mistaken for homology, this produces a phylogenetic tree that is falsely reticulate or bushy in appearance, that is, species appear to originate from a common ancestor when in fact that is not the case.
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Evolutionary Convergences
Nelson R. Cabej, in Epigenetic Principles of Evolution, 2012
Epigenetic Explanation of Evolutionary Convergences
As pointed out earlier, Darwin himself believed in the possibility of convergent evolution based on directed change “without the aid of any form of selection” rather than on random variability [gene mutations in modern terminology]:
If the varying individual did not actually transmit to its offspring its newly acquired character, it would undoubtedly transmit to them, as long as the existing conditions remained the same, a still stronger tendency to vary in the same manner.
Darwin [1872]
Evolutionary phenotypic changes in metazoan morphology may imply modification of existing developmental pathways, reactivation of ancestral pathways, or evolution of new pathways. It is important to bear in mind that neither modification of existing pathways nor evolution of new developmental pathways implies obliteration or irreversibility of previous or ancestral developmental pathways; gene products involved in these pathways generally are still present and functionally unchanged. Only the spatiotemporal pattern of their expression is epigenetically changed and regulated.
The epigenetic paradigm would relate the occurrence of evolutionary convergences with the similarity of solutions to problems arising from the adverse effects of environmental conditions, with the limited number of developmental algorithms [signal cascades] and constraints on modification of these algorithms. Since these signal cascades start with neural signals, one basic prediction from the view of the epigenetic paradigm would be that the frequency of evolutionary convergences would dramatically increase with the evolution of the nervous system, coinciding with the Cambrian explosion.
This prediction seems to have been validated in a recent study by Vermeij showing that before the Cambrian explosion, evolutionary convergences have been rare events, but later evolutionary innovations appear repeatedly. Vermeij estimated that only 23.3% of first 56 convergences occurred during the first 2.5 billion years of life on Earth, and 76.7% during the last 0.5 billion years since the Cambrian explosion. Only 4% of the singular nonconvergent innovations have occurred during the last