Polymorphism in biology and zoology is the occurrence of two or more different morphs or forms referred to as alternative phenotypes, in the population of a species. To be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population; the term polyphenism can be used to clarify. Genetic polymorphism is a term used somewhat differently by geneticists and molecular biologists to describe certain mutations in the genotype, such as single nucleotide polymorphisms that may not always correspond to a phenotype, but always corresponds to a branch in the genetic tree. See below. Polymorphism is common in nature. Polymorphism functions to retain variety of form in a population living in a varied environment; the most common example is sexual dimorphism. Other examples are mimetic forms of butterflies, human hemoglobin and blood types. According to the theory of evolution, polymorphism results from evolutionary processes, as does any aspect of a species, it is modified by natural selection.
In polyphenism, an individual's genetic makeup allows for different morphs, the switch mechanism that determines which morph is shown is environmental. In genetic polymorphism, the genetic makeup determines the morph; the term polymorphism refers to the occurrence of structurally and functionally more than two different types of individuals, called zooids, within the same organism. It is a characteristic feature of cnidarians. For example, Obelia has the gastrozooids. Although in general use, polymorphism is a broad term. In biology, polymorphism has been given a specific meaning. A more specific term, when only two forms occur, is dimorphism; the term omits characteristics showing continuous variation. Polymorphism deals with forms in which the variation is discrete or bimodal or polymodal. Morphs must occupy the same habitat at the same time; the use of the words "morph" or "polymorphism" for what is a visibly different geographical race or variant is common, but incorrect. The significance of geographical variation is in that it may lead to allopatric speciation, whereas true polymorphism takes place in panmictic populations.
The term was first used to describe visible forms, but nowadays it has been extended to include cryptic morphs, for instance blood types, which can be revealed by a test. Rare variations are not classified as polymorphisms, mutations by themselves do not constitute polymorphisms. To qualify as a polymorphism, some kind of balance must exist between morphs underpinned by inheritance; the criterion is that the frequency of the least common morph is too high to be the result of new mutations or, as a rough guide, that it is greater than 1%. Polymorphism crosses several discipline boundaries, including ecology and genetics, evolution theory, taxonomy and biochemistry. Different disciplines may give the same concept different names, different concepts may be given the same name. For example, there are the terms established in ecological genetics by E. B. Ford, for classical genetics by John Maynard Smith; the shorter term morphism may be more accurate than polymorphism, but is not used. It was the preferred term of the evolutionary biologist Julian Huxley.
Various synonymous terms exist for the various polymorphic forms of an organism. The most common are morpha, while a more formal term is morphotype. Form and phase are sometimes used, but are confused in zoology with "form" in a population of animals, "phase" as a color or other change in an organism due to environmental conditions. Phenotypic traits and characteristics are possible descriptions, though that would imply just a limited aspect of the body. In the taxonomic nomenclature of zoology, the word "morpha" plus a Latin name for the morph can be added to a binomial or trinomial name. However, this invites confusion with geographically variant ring species or subspecies if polytypic. Morphs have no formal standing in the ICZN. In botanical taxonomy, the concept of morphs is represented with the terms "variety", "subvariety" and "form", which are formally regulated by the ICN. Horticulturists sometimes confuse this usage of "variety" both with cultivar and with the legal concept "plant variety".
Three mechanisms may cause polymorphism: Genetic polymorphism – where the phenotype of each individual is genetically determined A conditional development strategy, where the phenotype of each individual is set by environmental cues A mixed development strategy, where the phenotype is randomly assigned during development Selection, whether natural or artificial, changes the frequency of morphs within a population. A genetic polymorphism persists over many generations, maintained by two or more opposed and powerful selection pressures. Diver found banding morphs in Cepaea nemoralis could be seen in prefossil shells going back to t
On the Origin of Species
On the Origin of Species, published on 24 November 1859, is a work of scientific literature by Charles Darwin, considered to be the foundation of evolutionary biology. Darwin's book introduced the scientific theory that populations evolve over the course of generations through a process of natural selection, it presented a body of evidence that the diversity of life arose by common descent through a branching pattern of evolution. Darwin included evidence that he had gathered on the Beagle expedition in the 1830s and his subsequent findings from research and experimentation. Various evolutionary ideas had been proposed to explain new findings in biology. There was growing support for such ideas among dissident anatomists and the general public, but during the first half of the 19th century the English scientific establishment was tied to the Church of England, while science was part of natural theology. Ideas about the transmutation of species were controversial as they conflicted with the beliefs that species were unchanging parts of a designed hierarchy and that humans were unique, unrelated to other animals.
The political and theological implications were intensely debated, but transmutation was not accepted by the scientific mainstream. The book was written for non-specialist readers and attracted widespread interest upon its publication; as Darwin was an eminent scientist, his findings were taken and the evidence he presented generated scientific and religious discussion. The debate over the book contributed to the campaign by T. H. Huxley and his fellow members of the X Club to secularise science by promoting scientific naturalism. Within two decades there was widespread scientific agreement that evolution, with a branching pattern of common descent, had occurred, but scientists were slow to give natural selection the significance that Darwin thought appropriate. During "the eclipse of Darwinism" from the 1880s to the 1930s, various other mechanisms of evolution were given more credit. With the development of the modern evolutionary synthesis in the 1930s and 1940s, Darwin's concept of evolutionary adaptation through natural selection became central to modern evolutionary theory, it has now become the unifying concept of the life sciences.
Darwin's theory of evolution is based on key facts and the inferences drawn from them, which biologist Ernst Mayr summarised as follows: Every species is fertile enough that if all offspring survived to reproduce, the population would grow. Despite periodic fluctuations, populations remain the same size. Resources such as food are limited and are stable over time. A struggle for survival ensues. Individuals in a population vary from one another. Much of this variation is heritable. Individuals less suited to the environment are less to survive and less to reproduce; this effected process results in populations changing to adapt to their environments, these variations accumulate over time to form new species. In editions of the book, Darwin traced evolutionary ideas as far back as Aristotle. Early Christian Church Fathers and Medieval European scholars interpreted the Genesis creation narrative allegorically rather than as a literal historical account. Nature was believed to be unstable and capricious, with monstrous births from union between species, spontaneous generation of life.
The Protestant Reformation inspired a literal interpretation of the Bible, with concepts of creation that conflicted with the findings of an emerging science seeking explanations congruent with the mechanical philosophy of René Descartes and the empiricism of the Baconian method. After the turmoil of the English Civil War, the Royal Society wanted to show that science did not threaten religious and political stability. John Ray developed an influential natural theology of rational order. In God's benevolent design, carnivores caused mercifully swift death, but the suffering caused by parasitism was a puzzling problem; the biological classification introduced by Carl Linnaeus in 1735 viewed species as fixed according to the divine plan. In 1766, Georges Buffon suggested that some similar species, such as horses and asses, or lions and leopards, might be varieties descended from a common ancestor; the Ussher chronology of the 1650s had calculated creation at 4004 BC, but by the 1780s geologists assumed a much older world.
Wernerians thought strata were deposits from shrinking seas, but James Hutton proposed a self-maintaining infinite cycle, anticipating uniformitarianism. Charles Darwin's grandfather Erasmus Darwin outlined a hypothesis of transmutation of species in the 1790s, Jean-Baptiste Lamarck published a more developed theory in 1809. Both envisaged that spontaneous generation produced simple forms of life that progressively developed greater complexity, adapting to the environment by inheriting changes in adults caused by use or disuse; this process was called Lamarckism. Lamarck thought there was an inherent progre
Evolutionary developmental biology
Evolutionary developmental biology is a field of biological research that compares the developmental processes of different organisms to infer the ancestral relationships between them and how developmental processes evolved. The field 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 noted that having similar embryos implied common ancestry, but little progress was made until the 1970s. Recombinant DNA technology at last brought embryology together with molecular genetics. A key early discovery was of homeotic genes; the field is characterised by some key concepts. One is deep homology, the finding that dissimilar organs such as the eyes of insects and cephalopod molluscs, long thought to have evolved separately, are controlled by similar genes such as pax-6, from the evo-devo gene toolkit; these genes are ancient, being conserved among phyla. Another is that species do not differ much in their structural genes, such as those coding for enzymes.
These genes are reused, 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 conserved, as any change would have many adverse consequences which natural selection would oppose. New morphological features and 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 consolidated at gene level, something that may have been important early in the history of multicellular life. 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 in turn like that of a reptile and mammal 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 recognised four distinct animal body plans: radiate, like starfish. Zoologists largely abandoned recapitulation, though Ernst Haeckel revived it in 1866. From the early 19th century through most of the 20th century, embryology faced a mystery. Animals were seen to develop into adults of differing body plan through similar stages, from the egg, but zoologists knew nothing about how embryonic development was controlled at the molecular level, therefore little about how developmental processes had evolved. Charles Darwin argued; as an example of this, Darwin cited in his 1859 book On the Origin of Species the shrimp-like larva of the barnacle, whose sessile adults looked nothing like other arthropods.
Darwin noted Alexander Kowalevsky's finding that the tunicate, was not a mollusc, but in its larval stage had a notochord and pharyngeal slits which developed from the same germ layers as the equivalent structures in vertebrates, should therefore be grouped with them as chordates. 19th century zoology thus converted embryology into an evolutionary science, connecting phylogeny with homologies between the germ layers of embryos. Zoologists including Fritz Müller proposed the use of embryology to discover phylogenetic relationships between taxa. Müller demonstrated that crustaceans shared the Nauplius larva, identifying several parasitic species that had not been recognised as crustaceans. Müller recognised that natural selection must act on larvae, just as it does on adults, giving the lie to recapitulation, which would require larval forms to be shielded from natural selection. Two of Haeckel's other ideas about the evolution of development have fared better than recapitulation: he argued in the 1870s that changes in the timing and changes in the positioning within the body of aspects of embryonic development would drive evolution by changing the shape of a descendant's body compared to an ancestor's.
It took a century. In 1917, D'Arcy Thompson wrote a book on the shapes of animals, showing with simple mathematics how small changes to parameters, such as the angles of a gastropod's spiral shell, can radically alter an animal's form, though he preferred mechanical to evolutionary explanation, but for the next century, without molecular evidence, progress stalled. In the so-called modern synthesis of the early 20th century, Ronald Fisher brought together Darwin's theory of evolution, with its insistence on natural selection and variation, Gregor Mendel's laws of genetics into a coherent structure for evolutionary biology. Biologists assumed that an organism was a straightforward reflection of its component genes: the genes coded for proteins, which built
History of paleontology
The history of paleontology traces the history of the effort to understand the history of life on Earth by studying the fossil record left behind by living organisms. Since it is concerned with understanding living organisms of the past, paleontology can be considered to be a field of biology, but its historical development has been tied to geology and the effort to understand the history of Earth itself. In ancient times, Herodotus and Strabo wrote about fossils of marine organisms, indicating that land was once under water. During the Middle Ages, fossils were discussed by Persian naturalist Ibn Sina in The Book of Healing, which proposed a theory of petrifying fluids that Albert of Saxony would elaborate on in the 14th century; the Chinese naturalist Shen Kuo would propose a theory of climate change based on evidence from petrified bamboo. In early modern Europe, the systematic study of fossils emerged as an integral part of the changes in natural philosophy that occurred during the Age of Reason.
The nature of fossils and their relationship to life in the past became better understood during the 17th and 18th centuries, at the end of the 18th century, the work of Georges Cuvier had ended a long running debate about the reality of extinction, leading to the emergence of paleontology- in association with comparative anatomy- as a scientific discipline. The expanding knowledge of the fossil record played an increasing role in the development of geology, stratigraphy in particular. In 1822, the word "paleontology" was used by the editor of a French scientific journal to refer to the study of ancient living organisms through fossils, the first half of the 19th century saw geological and paleontological activity become well organized with the growth of geologic societies and museums and an increasing number of professional geologists and fossil specialists; this contributed to a rapid increase in knowledge about the history of life on Earth, progress towards definition of the geologic time scale based on fossil evidence.
As knowledge of life's history continued to improve, it became obvious that there had been some kind of successive order to the development of life. This would encourage early evolutionary theories on the transmutation of species. After Charles Darwin published Origin of Species in 1859, much of the focus of paleontology shifted to understanding evolutionary paths, including human evolution, evolutionary theory; the last half of the 19th century saw a tremendous expansion in paleontological activity in North America. The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection, as demonstrated by a series of important discoveries in China near the end of the 20th century. Many transitional fossils have been discovered, there is now considered to be abundant evidence of how all classes of vertebrates are related, much of it in the form of transitional fossils; the last few decades of the 20th century saw a renewed interest in mass extinctions and their role in the evolution of life on Earth.
There was a renewed interest in the Cambrian explosion that saw the development of the body plans of most animal phyla. The discovery of fossils of the Ediacaran biota and developments in paleobiology extended knowledge about the history of life back far before the Cambrian; as early as the 6th century BC, the Greek philosopher Xenophanes of Colophon recognized that some fossil shells were remains of shellfish, which he used to argue that what was at the time dry land was once under the sea. Leonardo da Vinci, in an unpublished notebook concluded that some fossil sea shells were the remains of shellfish. However, in both cases, the fossils were complete remains of shellfish species that resembled living species, were therefore easy to classify. In 1027, the Persian naturalist, Ibn Sina, proposed an explanation of how the stoniness of fossils was caused in The Book of Healing, he modified an idea of Aristotle's. Ibn Sina modified this into the theory of petrifying fluids, elaborated on by Albert of Saxony in the 14th century and was accepted in some form by most naturalists by the 16th century.
Shen Kuo of the Song Dynasty used marine fossils found in the Taihang Mountains to infer the existence of geological processes such as geomorphology and the shifting of seashores over time. Using his observation of preserved petrified bamboos found underground in Yan'an, Shanbei region, Shaanxi province, he argued for a theory of gradual climate change, since Shaanxi was part of a dry climate zone that did not support a habitat for the growth of bamboos; as a result of a new emphasis on observing and cataloging nature, 16th century natural philosophers in Europe began to establish extensive collections of fossil objects, which were stored in specially built cabinets to help organize them. Conrad Gesner published a 1565 work on fossils that contained one of the first detailed descriptions of such a cabinet and collection; the collection belonged to a member of the extensive network of correspondents that Gesner drew on for his works. Such informal correspondence networks among natural philosophers and collectors became important during the course of the 16th century and were direct forerunners of the scientific societies that would begin to form in the 17th century.
These cabinet collections and correspondence networks played an important role in the developme
Abiogenesis, or informally the origin of life, is the natural process by which life has arisen from non-living matter, such as simple organic compounds. While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity that involved molecular self-replication, self-assembly and the emergence of cell membranes. Although the occurrence of abiogenesis is uncontroversial among scientists, there is no single accepted model for the origin of life, this article presents several principles and hypotheses for how abiogenesis could have occurred. Researchers study abiogenesis through a combination of molecular biology, astrobiology, biophysics and biochemistry, aim to determine how pre-life chemical reactions gave rise to life; the study of abiogenesis can be geophysical, chemical, or biological, with more recent approaches attempting a synthesis of all three, as life arose under conditions that are strikingly different from those on Earth today.
Life functions through the specialized chemistry of carbon and water and builds upon four key families of chemicals: lipids, amino acids, nucleic acids. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers think that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to have existed; the classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Scientists have proposed various external sources of energy that may have triggered these reactions, including lightning and radiation. Other approaches focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.
Complex organic molecules occur in the Solar System and in interstellar space, these molecules may have provided starting material for the development of life on Earth. The biochemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the age of the universe was only 10 to 17 million years. The panspermia hypothesis suggests that microscopic life was distributed to the early Earth by space dust, meteoroids and other small Solar System bodies and that life may exist throughout the universe; the panspermia hypothesis proposes that life originated outside the Earth, but does not definitively explain its origin. Earth remains the only place in the universe known to harbour life, fossil evidence from the Earth informs most studies of abiogenesis; the age of the Earth is about 4.54 billion years. In May 2017 scientists found possible evidence of early life on land in 3.48-billion-year-old geyserite and other related mineral deposits uncovered in the Pilbara Craton of Western Australia.
However, a number of discoveries suggest that life may have appeared on Earth earlier. As of 2017, microfossils, or fossilised microorganisms, within hydrothermal-vent precipitates dated from 3.77 to 4.28 billion years old found in Quebec, Canadian rocks may harbour the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 billion years ago. According to biologist Stephen Blair Hedges, "If life arose quickly on Earth … it could be common in the universe." The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds—methane and water vapour—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. According to models, suggested by study of ancient minerals, the atmosphere in the late Hadean period consisted of water vapour and carbon dioxide, with smaller amounts of carbon monoxide and sulfur compounds.
During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining. As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, lost it during the Hadean period, along with the bulk of the original inert gases; the solution of carbon dioxide in water is thought to have made the seas acidic, giving it a pH of about 5.5. The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory." It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry. Oceans may have appeared first in the Hadean Eon, as soon as two hundred million years after the Earth was formed, in a hot 100 °C reducing environment, the pH of about 5.8 rose towards neutral. This has been supported by the dating of 4.404 Ga-old zircon crystals from metamorphosed quartzite of Mount Narryer in the W
In biology, extinction is the termination of an organism or of a group of organisms a species. The moment of extinction is considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point; because a species' potential range may be large, determining this moment is difficult, is done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "reappears" after a period of apparent absence. More than 99 percent of all species, amounting to over five billion species, that lived on Earth are estimated to have died out. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. In 2016, scientists reported that 1 trillion species are estimated to be on Earth with only one-thousandth of one percent described. Through evolution, species arise through the process of speciation—where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche—and species become extinct when they are no longer able to survive in changing conditions or against superior competition.
The relationship between animals and their ecological niches has been established. A typical species becomes extinct within 10 million years of its first appearance, although some species, called living fossils, survive with no morphological change for hundreds of millions of years. Mass extinctions are rare events. Only have extinctions been recorded and scientists have become alarmed at the current high rate of extinctions. Most species that become extinct are never scientifically documented; some scientists estimate that up to half of presently existing plant and animal species may become extinct by 2100. A 2018 report indicated that the phylogenetic diversity of 300 mammalian species erased during the human era since the Late Pleistocene would require 5 to 7 million years to recover. A dagger symbol placed next to the name of a species or other taxon indicates its status as extinct. A species is extinct. Extinction therefore becomes a certainty when there are no surviving individuals that can reproduce and create a new generation.
A species may become functionally extinct when only a handful of individuals survive, which cannot reproduce due to poor health, sparse distribution over a large range, a lack of individuals of both sexes, or other reasons. Pinpointing the extinction of a species requires a clear definition of that species. If it is to be declared extinct, the species in question must be uniquely distinguishable from any ancestor or daughter species, from any other related species. Extinction of a species plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge. In ecology, extinction is used informally to refer to local extinction, in which a species ceases to exist in the chosen area of study, but may still exist elsewhere; this phenomenon is known as extirpation. Local extinctions may be followed by a replacement of the species taken from other locations. Species which are not extinct are termed extant; those that are extant but threatened by extinction are referred to as threatened or endangered species.
An important aspect of extinction is human attempts to preserve critically endangered species. These are reflected by the creation of the conservation status "extinct in the wild". Species listed under this status by the International Union for Conservation of Nature are not known to have any living specimens in the wild, are maintained only in zoos or other artificial environments; some of these species are functionally extinct, as they are no longer part of their natural habitat and it is unlikely the species will be restored to the wild. When possible, modern zoological institutions try to maintain a viable population for species preservation and possible future reintroduction to the wild, through use of planned breeding programs; the extinction of one species' wild population can have knock-on effects, causing further extinctions. These are called "chains of extinction"; this is common with extinction of keystone species. A 2018 study indicated that the 6th mass extinction started in the Late Pleistocene could take up to 5 to 7 million years to restore 2.5 billion years of unique mammal diversity to what it was before the human era.
Extinction of a parent species where daughter species or subspecies are still extant is called pseudoextinction or phyletic extinction. The old taxon vanishes, transformed into a successor, or split into more than one. Pseudoextinction is difficult to demonstrate unless one has a strong chain of evidence linking a living species to members of a pre-existing species. For example, it is sometimes claimed that the extinct Hyracotherium, an early horse that shares a common ancestor with the modern horse, is pseudoextinct, rather than extinct, because there are several extant species of Equus, including zebra and donkey. However, as fossil species leave no genetic material behind, one cannot say whether Hyracotherium evolved into more modern horse species or evolved from a common ancestor with modern horses. Pseudoextinction is much easier to demonstrate for larger taxonomic groups; the coelacanth, a fish related to lungfish and tetrapods, was consi
Introduction to evolution
Evolution is the process of change in all forms of life over generations, evolutionary biology is the study of how evolution occurs. Biological populations evolve through genetic changes that correspond to changes in the organisms' observable traits. Genetic changes include mutations, which are caused by damage or replication errors in organisms' DNA; as the genetic variation of a population drifts randomly over generations, natural selection leads traits to become more or less common based on the relative reproductive success of organisms with those traits. The age of the Earth is about 4.54 billion years. The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago. Evolution does not attempt to explain the origin of life, but it does explain how early lifeforms evolved into the complex ecosystem that we see today. Based on the similarities between all present-day organisms, all life on Earth is assumed to have originated through common descent from a last universal ancestor from which all known species have diverged through the process of evolution.
All individuals have hereditary material in the form of genes received from their parents, which they pass on to any offspring. Among offspring there are variations of genes due to the introduction of new genes via random changes called mutations or via reshuffling of existing genes during sexual reproduction; the offspring differs from the parent in minor random ways. If those differences are helpful, the offspring is more to survive and reproduce; this means that more offspring in the next generation will have that helpful difference and individuals will not have equal chances of reproductive success. In this way, traits that result in organisms being better adapted to their living conditions become more common in descendant populations; these differences accumulate resulting in changes within the population. This process is responsible for the many diverse life forms in the world; the modern understanding of evolution began with the 1859 publication of Charles Darwin's On the Origin of Species.
In addition, Gregor Mendel's work with plants helped to explain the hereditary patterns of genetics. Fossil discoveries in paleontology, advances in population genetics and a global network of scientific research have provided further details into the mechanisms of evolution. Scientists now have a good understanding of the origin of new species and have observed the speciation process in the laboratory and in the wild. Evolution is the principal scientific theory that biologists use to understand life and is used in many disciplines, including medicine, conservation biology, forensics and other social-cultural applications; the main ideas of evolution may be summarized as follows: Life forms reproduce and therefore have a tendency to become more numerous. Factors such as predation and competition work against the survival of individuals; each offspring differs from their parent in random ways. If these differences are beneficial, the offspring is more to survive and reproduce; this makes it that more offspring in the next generation will have beneficial differences and fewer will have detrimental differences.
These differences accumulate over generations. Over time, populations can branch off into new species; these processes, collectively known as evolution, are responsible for the many diverse life forms seen in the world. In the 19th century, natural history collections and museums were popular; the European expansion and naval expeditions employed naturalists, while curators of grand museums showcased preserved and live specimens of the varieties of life. Charles Darwin was an English graduate trained in the disciplines of natural history; such natural historians would collect, catalogue and study the vast collections of specimens stored and managed by curators at these museums. Darwin served as a ship's naturalist on board HMS Beagle, assigned to a five-year research expedition around the world. During his voyage, he observed and collected an abundance of organisms, being interested in the diverse forms of life along the coasts of South America and the neighboring Galápagos Islands. Darwin gained extensive experience as he collected and studied the natural history of life forms from distant places.
Through his studies, he formulated the idea that each species had developed from ancestors with similar features. In 1838, he described; the size of a population depends on how much. For the population to remain the same size year after year, there must be an equilibrium, or balance between the population size and available resources. Since organisms produce more offspring than their environment can support, not all individuals can survive out of each generation. There must be a competitive struggle for resources; as a result, Darwin realised. Instead, survival of an organism depends on the differences of each individual organism, or "traits," that aid or hinder survival and reproduction. Well-adapted individuals are to leave more offspring than their less well-adapted competitors. Traits that hinder survival and reproduction would disappear over generations. Traits that help an organism survive and reproduce would accumulate over generations. Darwin realised that the unequal ability of individuals to survive and reproduce could cause gradual changes in the population and used the term natural selection to describe this process.
Observations of variations in animals and plants formed the basis of the theory of natural sele