Courtesy : Bachelor of Science Biology (CBZ) – Chemistry, Botany, Zoology Library

Genes, development, and evolution

Further information: Evolutionary developmental biology

Model of concentration gradient building up; fine yellow-orange outlines are cell boundaries.

Development is the process by which a multicellular organism (plant or animal) goes through a series of a changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle. There are four key processes that underlie development: Determination, differentiation, morphogenesis, and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells from less specialized cells such as stem cells. Stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. Cellular differentiation dramatically changes a cell’s size, shape, membrane potential, metabolic activity, and responsiveness to signals, which are largely due to highly controlled modifications in gene expression and epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome. Morphogenesis, or the development of body form, is the result of spatial differences in gene expression. Specially, the organization of differentiated tissues into specific structures such as arms or wings, which is known as pattern formation, is governed by morphogens, signaling molecules that move from one group of cells to surrounding cells, creating a morphogen gradient as described by the French flag model. Apoptosis, or programmed cell death, also occurs during morphogenesis, such as the death of cells between digits in human embryonic development, which frees up individual fingers and toes. Expression of transcription factor genes can determine organ placement in a plant and a cascade of transcription factors themselves can establish body segmentation in a fruit fly. # ISO certification in India

A small fraction of the genes in an organism’s genome called the developmental-genetic toolkit control the development of that organism. These toolkit genes are highly conserved among phyla, meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the Hox genes. Hox genes determine where repeating parts, such as the many vertebrae of snakes, will grow in a developing embryo or larva. Variations in the toolkit may have produced a large part of the morphological evolution of animals. The toolkit can drive evolution in two ways. A toolkit gene can be expressed in a different pattern, as when the beak of Darwin’s large ground-finch was enlarged by the BMP gene, or when snakes lost their legs as Distal-less (Dlx) genes became under-expressed or not expressed at all in the places where other reptiles continued to form their limbs. Or, a toolkit gene can acquire a new function, as seen in the many functions of that same gene, distal-less, which controls such diverse structures as the mandible in vertebrates, legs and antennae in the fruit fly and eyespot pattern in butterfly wings. Given that small changes in toolbox genes can cause significant changes in body structures, they have often enabled convergent or parallel evolution. # ISO certification in India

Evolution

Evolutionary processes

Further information: Evolutionary biology

Natural selection for darker traits

A central organizing concept in biology is that life changes and develops through evolution, which is the change in heritable characteristics of populations over successive generations. Evolution is now used to explain the great variations of life on Earth. The term evolution was introduced into the scientific lexicon by Jean-Baptiste de Lamarck in 1809. He proposed that evolution occurred as a result of inheritance of acquired characteristics, which was unconvincing but there were no alternative explanations at the time. Charles Darwin, an English naturalist, had returned to England in 1836 from his five-year travels on the HMS Beagle where he studied rocks and collected plants and animals from various parts of the world such as the Galápagos Islands. He had also read Principles of Geology by Charles Lyell and An Essay on the Principle of Population by Thomas Malthus and was influenced by them. Based on his observations and readings, Darwin began to formulate his theory of evolution by natural selection to explain the diversity of plants and animals in different parts of the world. Alfred Russel Wallace, another English naturalist who had studied plants and animals in the Malay Archipelago, also came to the same idea, but later and independently of Darwin. Both Darwin and Wallace jointly presented their essay and manuscript, respectively, at the Linnaean Society of London in 1858, giving them both credit for their discovery of evolution by natural selection. Darwin would later publish his book On the Origin of Species in 1859, which explained in detail how the process of evolution by natural selection works. # ISO certification in India

To explain natural selection, Darwin drew an analogy with humans modifying animals through artificial selection, whereby animals were selectively bred for specific traits, which has given rise to individuals that no longer resemble their wild ancestors. Darwin argued that in the natural world, it was nature that played the role of humans in selecting for specific traits. He came to this conclusion based on two observations and two inferences. First, members of any population tend to vary with respect to their heritable traits. Second, all species tend to produce more offspring than can be supported by their respective environments, resulting in many individuals not surviving and reproducing. Based on these observations, Darwin inferred that those individuals who possessed heritable traits that are better adapted to their environments are more likely to survive and produce more offspring than other individuals. He further inferred that the unequal or differential survival and reproduction of certain individuals over others will lead to the accumulation of favorable traits over successive generations, thereby increasing the match between the organisms and their environment. Thus, taken together, natural selection is the differential survival and reproduction of individuals in subsequent generations due to differences in or more heritable traits. # ISO certification in India

Darwin was not aware of Mendel’s work of inheritance and so the exact mechanism of inheritance that underlie natural selection was not well-understood until the early 20th century when the modern synthesis reconciled Darwinian evolution with classical genetics, which established a neo-Darwinian perspective of evolution by natural selection. This perspective holds that evolution occurs when there are changes in the allele frequencies within a population of interbreeding organisms. In the absence of any evolutionary process acting on a large random mating population, the allele frequencies will remain constant across generations as described by the Hardy–Weinberg principle. # ISO certification in India

Another process that drives evolution is genetic drift, which is the random fluctuations of allele frequencies within a population from one generation to the next. When selective forces are absent or relatively weak, allele frequencies are equally likely to drift upward or downward at each successive generation because the alleles are subject to sampling error. This drift halts when an allele eventually becomes fixed, either by disappearing from the population or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. # ISO certification in India