In population genetics, gene flow is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough two populations are considered to have equivalent allele frequencies and therefore be a single population, it has been shown that it takes only "One migrant per generation" to prevent populations from diverging due to drift. Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants change the distribution of genetic diversity within the populations, by modifying the allele frequencies. High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. For this reason, gene flow has been thought to constrain speciation by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to full speciation. In some cases migration may result in the addition of novel genetic variants to the gene pool of a species or population.
There are a number of factors. Gene flow is expected to be lower in species that have low dispersal or mobility, that occur in fragmented habitats, where there is long distances between populations, when there are small population sizes. Mobility plays an important role in the migration rate, as mobile individuals tend to have greater migratory prospects. Although animals are thought to be more mobile than plants and seeds may be carried great distances by animals or wind; when gene flow is impeded, there can be an increase in inbreeding, measured by the inbreeding coefficient within a population. For example, many island populations have low rates of gene flow due to geographic isolation and small population sizes; the Black Footed Rock Wallaby has several inbred populations that live on various islands off the coast of Australia. The population is so isolated that lack of gene flow has led to high rates of inbreeding. Decrease in population size leads to increased divergence due to drift, while migration reduces divergence and inbreeding.
Gene flow can be measured by using the effective population size and the net migration rate per generation. Using the approximation based on the Island model, the effect of migration can be calculated for a population in terms of the degree of genetic differentiation; this formula accounts for the proportion of total molecular marker variation among populations, averaged over loci. When there is one migrant per generation, the inbreeding coefficient equals 0.2. However, when there is less than 1 migrant per generation, the inbreeding coefficient rises resulting in fixation and complete divergence; the most common F s t is < 0.25. This means. Measures of population structure range from 0 to 1; when gene flow occurs via migration the deleterious effects of inbreeding can be ameliorated. F s t = 1 / The formula can be modified to solve for the migration rate when F s t is known: N m = 1 / 4, Nm = number of migrants; when gene flow is blocked by physical barriers, this results in Allopatric speciation or a geographical isolation that does not allow populations of the same species to exchange genetic material.
Physical barriers to gene flow are but not always, natural. They may include oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of China, which has hindered the gene flow of native plant populations. One of these native plants, Ulmus pumila, demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba, Heteropappus hispidus, Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of China where Ulmus pumila grows; this is because Ulmus pumila has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects. Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools. Barriers to gene flow need not always be physical. Sympatric speciation happens when new species from the same ancestral species arise along the same range.
This is a result of a reproductive barrier. For example, two palm species of Howea found on Lord Howe Island were found to have different flowering times correlated with soil preference, resulting in a reproductive barrier inhibiting gene flow. Species can live in the same environment, yet show limited gene flow due to reproductive barriers, specialist pollinators, or limited hybridization or hybridization yielding unfit hybrids. A cryptic species is a species that humans cannot tell is different without the use of genetics. Moreover, gene flow between hybrid and wild populations can result in loss of genetic diversity via genetic pollution, assortative mating and
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the evolutionary relationships among various biological species or other entities—their phylogeny —based upon similarities and differences in their physical or genetic characteristics. All life on Earth is part of a single phylogenetic tree. In a rooted phylogenetic tree, each node with descendants represents the inferred most recent common ancestor of those descendants, the edge lengths in some trees may be interpreted as time estimates; each node is called a taxonomic unit. Internal nodes are called hypothetical taxonomic units, as they cannot be directly observed. Trees are useful in fields of biology such as bioinformatics and phylogenetics. Unrooted trees illustrate only the relatedness of the leaf nodes and do not require the ancestral root to be known or inferred; the idea of a "tree of life" arose from ancient notions of a ladder-like progression from lower into higher forms of life. Early representations of "branching" phylogenetic trees include a "paleontological chart" showing the geological relationships among plants and animals in the book Elementary Geology, by Edward Hitchcock.
Charles Darwin produced one of the first illustrations and crucially popularized the notion of an evolutionary "tree" in his seminal book The Origin of Species. Over a century evolutionary biologists still use tree diagrams to depict evolution because such diagrams convey the concept that speciation occurs through the adaptive and semirandom splitting of lineages. Over time, species classification has become more dynamic; the term phylogenetic, or phylogeny, derives from the two ancient greek words φῦλον, meaning "race, lineage", γένεσις, meaning "origin, source". A rooted phylogenetic tree is a directed tree with a unique node — the root — corresponding to the most recent common ancestor of all the entities at the leaves of the tree; the root node serves as the parent of all other nodes in the tree. The root is therefore a node of degree 2 while other internal nodes have a minimum degree of 3; the most common method for rooting trees is the use of an uncontroversial outgroup—close enough to allow inference from trait data or molecular sequencing, but far enough to be a clear outgroup.
Unrooted trees illustrate the relatedness of the leaf nodes without making assumptions about ancestry. They do not require the ancestral root to be inferred. Unrooted trees can always be generated from rooted ones by omitting the root. By contrast, inferring the root of an unrooted tree requires some means of identifying ancestry; this is done by including an outgroup in the input data so that the root is between the outgroup and the rest of the taxa in the tree, or by introducing additional assumptions about the relative rates of evolution on each branch, such as an application of the molecular clock hypothesis. Both rooted and unrooted phylogenetic trees can be either bifurcating or multifurcating, either labeled or unlabeled. A rooted bifurcating tree has two descendants arising from each interior node, an unrooted bifurcating tree takes the form of an unrooted binary tree, a free tree with three neighbors at each internal node. In contrast, a rooted multifurcating tree may have more than two children at some nodes and an unrooted multifurcating tree may have more than three neighbors at some nodes.
A labeled tree has specific values assigned to its leaves, while an unlabeled tree, sometimes called a tree shape, defines a topology only. The number of possible trees for a given number of leaf nodes depends on the specific type of tree, but there are always more multifurcating than bifurcating trees, more labeled than unlabeled trees, more rooted than unrooted trees; the last distinction is the most biologically relevant. For labeled bifurcating trees, there are:!! =! 2 n − 2! for n ≥ 2 total rooted trees and!! =! 2 n − 3! for n ≥ 3 total unrooted trees, where n represents the number of leaf nodes. Among labeled bifurcating trees, the number of unrooted trees with n leaves is equal to the number of rooted trees with n − 1 leaves. A dendrogram is a general name for a tree, whether phylogenetic or not, hence for the diagrammatic representation of a phylogenetic tree. A cladogram only represents a branching pattern.
Evolutionary game theory
Evolutionary game theory is the application of game theory to evolving populations in biology. It defines a framework of contests and analytics into which Darwinian competition can be modelled, it originated in 1973 with John Maynard Smith and George R. Price's formalisation of contests, analysed as strategies, the mathematical criteria that can be used to predict the results of competing strategies. Evolutionary game theory differs from classical game theory in focusing more on the dynamics of strategy change; this is influenced by the frequency of the competing strategies in the population. Evolutionary game theory has helped to explain the basis of altruistic behaviours in Darwinian evolution, it has in turn become of interest to economists, sociologists and philosophers. Classical non-cooperative game theory was conceived by John von Neumann to determine optimal strategies in competitions between adversaries. A contest involves players. Games can be a single round or repetitive; the approach a player takes in making his moves constitutes his strategy.
Rules govern the outcome for the moves taken by the players, outcomes produce payoffs for the players. Classical theory requires the players to make rational choices; each player must consider the strategic analysis that his opponents are making to make his own choice of moves. Evolutionary game theory started with the problem of how to explain ritualized animal behaviour in a conflict situation; the leading ethologists Niko Tinbergen and Konrad Lorenz proposed that such behaviour exists for the benefit of the species. John Maynard Smith considered that incompatible with Darwinian thought, where selection occurs at an individual level, so self-interest is rewarded while seeking the common good is not. Maynard Smith, a mathematical biologist, turned to game theory as suggested by George Price, though Richard Lewontin's attempts to use the theory had failed. Maynard Smith realised that an evolutionary version of game theory does not require players to act rationally —– only that they have a strategy.
The results of a game shows how good that strategy was, just as evolution tests alternative strategies for the ability to survive and reproduce. In biology, strategies are genetically inherited traits that control an individual's action, analogous with computer programs; the success of a strategy is determined by how good the strategy is in the presence of competing strategies, of the frequency with which those strategies are used. Maynard Smith described his work in the Theory of Games. Participants aim to produce as many replicas of themselves as they can, the payoff is in units of fitness, it is always a multi-player game with many competitors. Rules include replicator dynamics, in other words how the fitter players will spawn more replicas of themselves into the population and how the less fit will be culled, in a replicator equation; the replicator dynamics models heredity but not mutation, assumes asexual reproduction for the sake of simplicity. Games are run repetitively with no terminating conditions.
Results include the dynamics of changes in the population, the success of strategies, any equilibrium states reached. Unlike in classical game theory, players do not choose their strategy and cannot change it: they are born with a strategy and their offspring inherit that same strategy. Evolutionary game theory encompasses Darwinian evolution, including competition, natural selection, heredity. Evolutionary game theory has contributed to the understanding of group selection, sexual selection, parental care, co-evolution, ecological dynamics. Many counter-intuitive situations in these areas have been put on a firm mathematical footing by the use of these models; the common way to study the evolutionary dynamics in games is through replicator equations. These show the growth rate of the proportion of organisms using a certain strategy and that rate is equal to the difference between the average payoff of that strategy and the average payoff of the population as a whole. Continuous replicator equations assume infinite populations, continuous time, complete mixing and that strategies breed true.
The attractors of the equations are equivalent with evolutionarily stable states. A strategy which can survive all "mutant" strategies is considered evolutionarily stable. In the context of animal behavior, this means such strategies are programmed and influenced by genetics, thus making any player or organism's strategy determined by these biological factors. Evolutionary games are mathematical objects with different rules and mathematical behaviours; each "game" represents different problems that organisms have to deal with, the strategies they might adopt to survive and reproduce. Evolutionary games are given colourful names and cover stories which describe the general situation of a particular game. Representative games include hawk-dove, war of attrition, stag hunt, producer-scrounger, tragedy of the commons, prisoner's dilemma. Strategies for these games include Hawk, Bourgeois, Defector and Retaliator; the various strategies compete under the particular game's rules, the mathematics are used to determine the results and behaviours.
The first game that Maynard Smith analysed is the classic Hawk Dove game. It was conceived to analyse a contest over a shareable resource; the contestants can be either Dove. These are two subtypes or
An extinction event is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms, it occurs. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty; these differences stem from the threshold chosen for describing an extinction event as "major", the data chosen to measure past diversity. Because most diversity and biomass on Earth is microbial, thus difficult to measure, recorded extinction events affect the observed, biologically complex component of the biosphere rather than the total diversity and abundance of life. Extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. Marine fossils are used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals.
The Great Oxygenation Event, which occurred around 2.45 billion years ago, was the first major extinction event. Since the Cambrian explosion five further major mass extinctions have exceeded the background extinction rate; the most recent and arguably best-known, the Cretaceous–Paleogene extinction event, which occurred 66 million years ago, was a large-scale mass extinction of animal and plant species in a geologically short period of time. In addition to the five major mass extinctions, there are numerous minor ones as well, the ongoing mass extinction caused by human activity is sometimes called the sixth extinction. Mass extinctions seem to be a Phanerozoic phenomenon, with extinction rates low before large complex organisms arose. In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five mass extinctions, they were identified as outliers to a general trend of decreasing extinction rates during the Phanerozoic, but as more stringent statistical tests have been applied to the accumulating data, it has been established that multicellular animal life has experienced five major and many minor mass extinctions.
The "Big Five" cannot be so defined, but rather appear to represent the largest of a smooth continuum of extinction events. Ordovician–Silurian extinction events: 450–440 Ma at the Ordovician–Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 60% to 70% of all species. Together they are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that became extinct. Late Devonian extinction: 375–360 Ma near the Devonian–Carboniferous transition. At the end of the Frasnian Age in the part of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera and at least 70% of all species; this extinction event lasted as long as 20 million years, there is evidence for a series of extinction pulses within this period. Permian–Triassic extinction event: 252 Ma at the Permian–Triassic transition. Earth's largest extinction killed 57% of all families, 83% of all genera and 90% to 96% of all species.
The successful marine arthropod, the trilobite, became extinct. The evidence regarding plants is less clear; the "Great Dying" had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles. The recovery of vertebrates took 30 million years, but the vacant niches created the opportunity for archosaurs to become ascendant. In the seas, the percentage of animals that were sessile dropped from 67% to 50%; the whole late Permian was a difficult time for at least marine life before the "Great Dying". Triassic–Jurassic extinction event: 201.3 Ma at the Triassic–Jurassic transition. About 23% of all families, 48% of all genera and 70% to 75% of all species became extinct. Most non-dinosaurian archosaurs, most therapsids, most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments; the Temnospondyl lineage of large amphibians survived until the Cretaceous in Australia.
Cretaceous–Paleogene extinction event: 66 Ma at the Cretaceous – Paleogene transition interval. The event called the Cretaceous-Tertiary or K–T extinction or K–T boundary is now named the Cretaceous–Paleogene extinction event. About 17% of all families, 50% of all genera and 75% of all species became extinct. In the seas all the ammonites and mosasaurs disappeared and the percentage of sessile animals was reduced to about 33%. All non-avian dinosaurs became extinct during that time; the boundary event was severe with a significant amount of variability in the rate of extinction between and among different clades. Mammals and birds, the latter descended from theropod dinosaurs, emerged as dominant large land animals. Despite the popularization of these five events, there is no definite line separating them from other extinction events.
In evolution, co-operation is the process where groups of organisms work or act together for common or mutual benefits. It is defined as any adaptation that has evolved, at least in part, to increase the reproductive success of the actor's social partners. For example, territorial choruses by male lions discourage intruders and are to benefit all contributors; this process contrasts with intragroup competition where individuals work against each other for selfish reasons. Cooperation exists not only in other animals as well; the diversity of taxa that exhibits cooperation is quite large, ranging from zebra herds to pied babblers to African elephants. Many animal and plant species cooperate with both members of their own species and with members of other species. Cooperation in animals appears to occur for direct benefit or between relatives. Spending time and resources assisting a related individual may at first seem destructive to an organism's chances of survival but is beneficial over the long-term.
Since relatives share part of the helper's genetic make-up, enhancing each individual's chance of survival may increase the likelihood that the helper's genetic traits will be passed on to future generations. However, some researchers, such as ecology professor Tim Clutton-Brock, assert that cooperation is a more complex process, they state that helpers may receive more direct, less indirect, gains from assisting others than is reported. These gains include protection from increased reproductive fitness. Furthermore, they insist that cooperation may not be an interaction between two individuals but may be part of the broader goal of unifying populations. Prominent biologists, such as Charles Darwin, E. O. Wilson, W. D. Hamilton, have found the evolution of cooperation fascinating because natural selection favors those who achieve the greatest reproductive success while cooperative behavior decreases the reproductive success of the actor. Hence, cooperation seemed to pose a challenging problem to the theory of natural selection, which rests on the assumption that individuals compete to survive and maximize their reproductive successes.
Additionally, some species have been found to perform cooperative behaviors that may at first sight seem detrimental to their own evolutionary fitness. For example, when a ground squirrel sounds an alarm call to warn other group members of a nearby coyote, it draws attention to itself and increases its own odds of being eaten. There have been multiple hypotheses for the evolution of cooperation, all of which are rooted in Hamilton's models based on inclusive fitness; these models hypothesize that cooperation is favored by natural selection due to either direct fitness benefits or indirect fitness benefits. As explained below, direct benefits encompass by-product benefits and enforced reciprocity, while indirect benefits encompass limited dispersal, kin discrimination and the greenbeard effect. One specific form of cooperation in animals is kin selection, which involves animals promoting the reproductive success of their kin, thereby promoting their own fitness. Different theories explaining kin selection have been proposed, including the "pay-to-stay" and "territory inheritance" hypotheses.
The "pay-to-stay" theory suggests that individuals help others rear offspring in order to return the favor of the breeders allowing them to live on their land. The "territory inheritance" theory contends that individuals help in order to have improved access to breeding areas once the breeders depart. Studies conducted on red wolves support previous researchers' contention that helpers obtain both immediate and long-term gains from cooperative breeding. Researchers evaluated the consequences of red wolves' decisions to stay with their packs for extended periods of time after birth. While delayed dispersal helped other wolves' offspring, studies found that it extended male helper wolves' life spans; this suggests that kin selection may not only benefit an individual in the long-term through increased fitness but in the short-term through increased survival chances. Some research suggests; this phenomenon is known as kin discrimination. In their meta-analysis, researchers compiled data on kin selection as mediated by genetic relatedness in 18 species, including the western bluebird, pied kingfisher, Australian magpie, dwarf mongoose.
They found that different species exhibited varying degrees of kin discrimination, with the largest frequencies occurring among those who have the most to gain from cooperative interactions. Cooperation exists not only in animals but in plants. In a greenhouse experiment with Ipomoea hederacea, a climbing plant, results show that kin groups have higher efficiency rates in growth than non-kin groups do; this is expected to rise out of reduced competition within the kin groups. The inclusive fitness theory provides a good overview of possible solutions to the fundamental problem of cooperation; the theory is based on the hypothesis that cooperation helps in transmitting underlying genes to future generations either through increasing the reproductive successes of the individual or of other individuals who carry the same genes. Direct benefits can result from simple by-product of cooperation or enforcement mechanisms, while indirect benefits can result from cooperation with genetically similar individuals.
This is called mutually beneficial cooperation as both actor and recipient depend on direct fitness benefits, which are broken down into two different types: by-product benefit and enforcement. By-product benefit ari
Timeline of the evolutionary history of life
This timeline of the evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet Earth. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, individual organisms and molecules, such as DNA and proteins; the similarities between all present day organisms indicate the presence of a common ancestor from which all known species and extinct, have diverged through the process of evolution. More than 99 percent of all species, amounting to over five billion species, that lived on Earth are estimated to be extinct. 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. However, a May 2016 scientific report estimates that 1 trillion species are on Earth, with only one-thousandth of one percent described.
While the dates given in this article are estimates based on scientific evidence, there has been controversy between more traditional views of increased biodiversity through a cone of diversity with the passing of time and the view that the basic pattern on Earth has been one of annihilation and diversification and that in certain past times, such as the Cambrian explosion, there was great diversity. Species go extinct as environments change, as organisms compete for environmental niches, as genetic mutation leads to the rise of new species from older ones. Biodiversity on Earth takes a hit in the form of a mass extinction in which the extinction rate is much higher than usual. A large extinction-event represents an accumulation of smaller extinction- events that take place in a brief period of time; the first known mass extinction in earth's history was the Great Oxygenation Event 2.4 billion years ago. That event led to the loss of most of the planet's obligate anaerobes. Researchers have identified five major extinction events in earth's history since: End of the Ordovician: 440 million years ago, 86% of all species lost, including graptolites Late Devonian: 375 million years ago, 75% of species lost, including most trilobites End of the Permian, "The Great Dying": 251 million years ago, 96% of species lost, including tabulate corals, most extant trees and synapsids End of the Triassic: 200 million years ago, 80% of species lost, including all of the conodonts End of the Cretaceous: 66 million years ago, 76% of species lost, including all of the ammonites, ichthyosaurs, plesiosaurs and nonavian dinosaurs Smaller extinction-events have occurred in the periods between these larger catastrophes, with some standing at the delineation points of the periods and epochs recognized by scientists in geologic time.
The Holocene extinction event is under way. Factors in mass extinctions include continental drift, changes in atmospheric and marine chemistry and other aspects of mountain formation, changes in glaciation, changes in sea level, impact events. In this timeline, Ma means "million years ago," ka means "thousand years ago," and ya means "years ago." 4000 Ma and earlier. 4000 Ma – 2500 Ma 2500 Ma – 542 Ma. Contains the Palaeoproterozoic and Neoproterozoic eras. 542 Ma – present The Phanerozoic Eon the "period of well-displayed life," marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic and Cenozoic, which are divided by major mass extinctions. 542 Ma – 251.0 Ma and contains the Cambrian, Silurian, Devonian and Permian periods. From 251.4 Ma to 66 Ma and containing the Triassic and Cretaceous periods. 66 Ma – present Dawkins, Richard. The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company.
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