User:Thunderbear65/Evolutionary development

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Evolutionary developmental biology (informally, evo-devo) a field of biology that combines the disciplines of evolutionary biology and embryonic development to study the relationship between evolution and developmental processes and mechanisms

In order to understand evolutionary developmental biology an understanding of embryology must be made first. The field of evolutionary developmental biology grew from 19th-century beginnings, where embryology faced a mystery: zoologists did not know how embryonic development was controlled at the molecular level. Charles Darwin suggested that animals with similar embryos might have a common ancestor, but little progress was made prove this until the 1970s. By using enzymes and various laboratory techniques to manipulate and isolate DNA segments of interest, recombinant DNA technology at last brought embryology together with molecular genetics. Through recombinant DNA technology the discovery of homeotic genes was made. Homeotic genes are any of a group of genes that control the pattern of body formation during early embryonic development of organisms. These genes encode proteins called transcription factors that direct cells to form various parts of the body and are are responsible for determining an animals anterior, posterior, dorsal and a ventral side, and specific regions along the body axis.

The field of Evolutionary developmental biology is composed of multiple core evolutionary concepts. One is deep homology, the finding that dissimilar organs that have long thought to have evolved separately are actually controlled by similar genes. These similar genes are a subset of genes in an organism's genome whose products control the organism's embryonic development which is also known as Toolkit genes. For example the eyes of insects, vertebrates and cephalopod molluscs, which have thought to have evolved separately, are controlled by a similar genes known as pax-6, from the evo-devo gene toolkit. These genes are ancient, being highly conserved among phyla; they generate the patterns in time and space which shape the embryo, and ultimately form the body plan of the organism. Furthermore, species do not differ much in their structural genes, such as those coding for enzymes; what does differ is the way that gene expression is regulated by the toolkit genes. These genes are reused, unchanged, many times in different parts of the embryo and at different stages of development, forming a complex cascade of control, switching other regulatory genes as well as structural genes on and off in a precise pattern. This multiple pleiotropic reuse explains why these genes are highly conserved, as any change would have many adverse consequences which natural selection would oppose.

New morphological features and ultimately new species are produced by variations in the toolkit, either when genes are expressed in a new pattern, or when toolkit genes acquire additional functions. Another possibility is the neo-Lamarckian theory that epigenetic changes are later consolidated at gene level, something that may have been important early in the history of multicellular life.

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Recapitulation[edit]
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Main article: Recapitulation theory

A recapitulation theory of evolutionary development was proposed by Étienne Serres in 1824–26, echoing the 1808 ideas of Johann Friedrich Meckel. They argued that the embryos of 'higher' animals went through or recapitulated a series of stages, each of which resembled an animal lower down the great chain of being. For example, the brain of a human embryo looked first like that of a fish, then in turn like that of a reptile, avian, and a mammal meaning they climb up the staircase before becoming human. The embryologist Karl Ernst von Baer opposed this, arguing in 1828 that there was no linear sequence as in the great chain of being, based on a single body plan, but a process of epigenesis in which structures differentiate. Von Baer instead recognized four distinct animal body plans: radiate, like starfish; molluscan, like clams; articulate, like lobsters; and vertebrate, like fish. Zoologists then largely abandoned recapitulation, though Ernst Haeckel revived it in 1866. Recapitulation provided a precursor towards evo-devo in many fields including zoology, but considering it was during the 19th century the concept was still new before it evolved in the present day.