Developmental processes are the series of biological changes associated with information transfer, growth, and differentiation during the life cycle of organisms. Information transfer is the transmission of DNA and other biological signals from parent cells to daughter cells. Growth is the increase in size due to cell expansion and cell division. Differentiation is the change of unspecialized cells in a simple body pattern to specialized cells in more complex body pattern. While nearly all organisms, even single-celled bacteria, undergo development of some sort; the developmental process of complex multicellular organisms is emphasized here. In these organisms, development begins with the manufacture of male and female sex cells. It proceeds through fertilization and formation of an embryo. Development continues following birth, hatching, or germination of the embryo and culminates in aging and death.
People have long been interested in the connection between the development of an organism, its ontogeny, and the evolutionary ancestry of the species, its phylogeny. Anaximander, a philosopher of ancient Greece, noted that human embryos develop inside fluid-filled wombs and proposed that human beings evolved from fish as creatures of the water.
This early idea was a progenitor to recapitulation theory, proposed in the 1800s by Ernst Haeckel, a German scientist. Recapitulation theory is summarized by the idea that the embryological development of an individual is a quick replay of its evolutionary history. As applied to humans, recapitulation theory was accepted by many evolutionary biologists in the 1800s. It also influenced the intellectual development of other disciplines outside of biology, including philosophy, politics, and psychology.
By the early 1900s, developmental biologists had disproved recapitulation theory and had shown that the relationship between ontogeny and phylogeny is more complex than proposed by Haeckel. However, like Haeckel, modern biologists hold that the similarities in the embryos of closely related species and the transient appearance of certain structures of mature organisms early in development of related organisms indicates a connection between ontogeny and phylogeny. One modern view is that new species may evolve when evolution alters the timing of development, so that certain features of ancestral species appear earlier or later in development.
Nearly every multicellular organism passes through a life cycle stage where it exists as a single undifferentiated cell or as a small number of undifferentiated cells. This developmental stage contains molecular information which specifies the entire course of development encoded in its many thousands of genes. At the molecular level, genes are used to make proteins, many of which act as enzymes, biological catalysts which drive the thousands of different biochemical reactions inside cells.
Adult multicellular organisms can consist of one quadrillion (a one followed by fifteen zeros) or more cells, each of which has the same genetic information. (There are a few notable exceptions, such as the red blood cells of mammals, which do not have DNA, and certain cells in the unfertilized eggs of amphibians, which undergo gene amplification and have multiple copes of some genes.) F.C. Steward first demonstrated the constancy of DNA in all the cells of a multicellular organism in the 1950s. In a classical series of experiments, Steward separated a mature carrot plant into individual cells and showed that each cell, whether it came from the root, stem, or leaf, could be induced to develop into a mature carrot plant which was genetically identical to its parent. Although such experiments cannot typically be done with multicellular animals, animals also have the same genetic information in all their cells.
Many developmental scientists emphasize that there are additional aspects of information transfer during development which do not involve DNA directly. In addition to DNA, a fertilized egg cell contains many proteins and other cellular constituents which are typically derived from the female. These cellular constituents are often asymmetrically distributed during cell division, so that the two daughter cells derived from the fertilized egg have significant biochemical and cytological differences. In many species, these differences act as biological signals which affect the course of development. There are additional spatial and temporal interactions within and among the cells of a developing organism which act as biological signals and provide a form of information to the developing organism.
Organisms generally increase in size during development. Growth is usually allometric, in that it occurs simultaneously with cellular differentiation and changes in overall body pattern. Allometry specifically studies the relationships between the size and morphology of an organism as it develops and the size and morphology of different species.
A developing organism generally increases in complexity as it increases in size. Moreover, in an evolutionary line, larger species are generally more complex that the smaller species. The reason for this correlation is that the volume (or weight) of an organism varies with the cube of its length, whereas gas exchange and food assimilation, which generally occur on surfaces, vary with the square of its length. Thus, an increase in size requires an increase in cellular specialization and morphological complexity so that the larger organism can breathe and eat.
Depending on the circumstances, natural selection may favor an increase in size, a decrease in size, or no change in size. Large size is often favored because it generally makes organisms faster, giving them better protection against predators, and making them better at dispersal and food gathering. In addition, larger organisms have a higher ratio of volume to surface area, so they are less affected by environmental variations, such as temperature variation. Large organisms tend to have a prolonged development, presumably so they have more time to develop the morphological complexities needed to support their large size. Thus, evolutionary selection for large size leads to a prolongation of development as well as morphological complexity.
Differentiation is the change of unspecialized cells in a simple body pattern to specialized cells in a more complex body pattern. It is highly coordinated with growth and includes morphogenesis, the development of the complex overall body pattern.
Below, we emphasize molecular changes in organisms which lead to development. However, this does not imply that external factors have no role in development. In fact, external factors such as changes in light, temperature, or nutrient availability often elicit chemical changes in developing organisms which profoundly influence development.
The so-called "Central Dogma of Biology" says that spatial and temporal differences in gene expression cause cellular and morphological differentiation. Since DNA makes RNA, and RNA makes protein, there are basically three levels where a cell can modulate gene expression: 1) by altering the transcription of DNA into RNA; 2) by altering the translation of RNA into protein; and 3) by altering the activity of the protein, which is usually an enzyme. Since DNA and RNA are themselves synthesized by proteins, the gene expression patterns of all cells are regulated by highly complex biochemical networks.
A few simple calculations provide a better appreciation of the complexity of the regulatory networks of gene expression which control differentiation. Starting with the simplifying assumption that a given protein (gene product) can be either absent or present in a cell, there are at least ten centillion (a one followed by 6,000 zeros) different patterns of gene expression in a single typical cell at any time. Given that a multicellular organism contains one quadrillion or more cells, and that gene expression patterns change over time, the number of possible gene expression patterns is enormous.
Perhaps the central question of developmental science is how an organism can select the proper gene expression pattern among all these possibilities. This question has not yet been satisfactorily answered. However, in a 1952 paper, Alan Turing showed that simple chemical systems, in which the component chemicals diffuse and react with one another over time, can create complex spatial patterns which change over time. Thus, it seems possible that organisms may regulate differentiation by using a Turing-like reaction-diffusion mechanism, in which proteins and other molecules diffuse and interact with one another to modulate gene expression. Turing's original model, while relatively simple, has been a major impetus for research about pattern development in biology.
Lastly, aging must also be considered a phase of development. Many evolutionary scientists hold that all organisms have genes which have multiple effects, called pleiotropic genes, that increase reproductive success when expressed early in development, but cause the onset of old age when expressed later in development. In this view, natural selection has favored genes which cause aging and death because the early effects of these genes outweigh the later effects.
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