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The correct answers are B, C and D; genetic drift, mutation, gene flow, and natural selection. The definition of evolution is change in population over time. There are four mechanisms by which populations of organisms can evolve. It is important to remember that evolution only occurs if genotypes within the population change. The process, therefore, depends on the presence of genetic variation, which has been described as raw material for evolutionary forces to act on. Genetic drift and genetic mutations are both random processes that occur purely by chance. Genetic mutations are alterations in the genome that occur in both sexually and asexually reproducing individuals. Such mutations can produce novel genotypes which can sometimes be beneficial to the survival of an organism. A Genetic drift is when a random event removes individuals and particular genotypes from the population. This mechanism is more powerful in small populations where genotypic variation is more limited. Gene flow is due to migration of individuals. Some individuals may bring new genotypes into the population, while others leave the population. Since this changes relative frequencies of genes, it can lead to evolution over time. Natural selection is a potent force of evolution that has been demonstrated to occur. Individuals with stronger genotypes are better able to survive and hence reproduce. Over time, their genotypes then become more frequent in the population, which then evolve.
Genetic drift is another random mechanism that occurs in which some unexpected event, possibly natural disaster, eliminates a portion of the population. This may then also eliminate or drastically reduce the frequency of particular genotypes in the population. The likelihood of genetic drift occurring is not as high in large population as it is in small population. This is because small populations will have less genetic variation compared with a population that has a lot of individuals present. Since frequency of genotypes is so important in influencing evolution, this means that gene flow is an important mechanism that can lead to change over time. Individuals may move into or out of the population, which means there will be change in what genotypes are present. Gene flow is most limited in island populations which experience limited movement of individuals. Instead, such populations may show adaptive radiation over time as founder populations adapt and colonize different niches that are present.
These traits are expression of genes that are copied and passed on to offspring during reproduction. Mutations in these genes can produce new or altered traits, resulting in heritable differences between organisms. New traits can also come from transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. Evolution occurs when these heritable differences become more common or rare in the population, either non - randomly through natural selection or randomly through genetic drift. Natural selection is a process that causes heritable traits that are helpful for survival and reproduction to become more common, and harmful traits to become more rare. This occurs because organisms with advantageous traits pass on more copies of these heritable traits to the next generation. Over many generations, adaptations occur through combination of successive, small, random changes in traits, and natural selection of those variants that best suit their environment. In contrast, genetic drift produces random changes in the frequency of traits in the population. Genetic drift arises from the role chance plays in whether an individual will survive and reproduce. One definition of species is a group of organisms that can reproduce with one another and produce fertile offspring. However, when species are separated into populations that are prevented from interbreeding, mutations, genetic drift, and selection of novel traits cause accumulation of differences over generations and emergence of new species. Similarities between organisms suggest that all known species descend from common ancestor through this process of gradual divergence. The theory of evolution by natural selection was proposed roughly simultaneously by both Charles Darwin and Alfred Russel Wallace, and set out in detail in Darwin's 1859 book on the Origin of Species. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form modern evolutionary synthesis, in which connection between units of evolution and mechanism of evolution was make. This powerful explanatory and predictive theory has become a central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.
Pathogenic fungal species are often organized into discrete populations. Population genetics usually assumes simple model of N populations, each of which is equally likely to receive and give migrants to and from each of other populations. Under this model, providing additional simplifying assumptions, relationship between N e M e and F can be derive: F 1 /. This approach has been severely criticized by some authors 95 96 who raise concerns about unrealistic assumptions under the N - island model. Even though they do not provide reliable estimates of rates of gene flow, measures of population differentiation can nonetheless be used to gain information on the history of dispersal. Several studies report very low differentiation among samples of fungal pathogens of agricultural crops or forestry trees from different localities across the continent. Coalescent theory 100 relates patterns of common ancestry within a set of genes to the structure of populations from which they were sample. In coalescent models, patterns of relationships among genes are represented by genealogy, and the structure of the population is represented by parameters such as population size, rates of population growth, orwhat is relevant to present discussion rates and directions of gene flow. Both genealogy and parameters are generally unknown, and one usually wants to estimate parameters of model. It is generally impossible to jointly consider all possible ancestral relationships and parameter values and to search for combinations that maximize the probability of model. Instead, approaches have been developed that simultaneously explore many relatively probable genealogies and parameter values. These approaches are collectively referred to as coalescent genealogy samplers. Several methods relying on coalescent genealogy samplers were designed to estimate, among other parameters, rates of gene flow between species or populations. 103 104 These methods offer the advantage of allowing less restrictive models than more traditional methods presented earlier. These methods have been successfully applied to infer ancestral routes of colonization for several fungal globally distributed plant pathogens such as barley scald pathogen Rhynchosporium secalis, 105 and apple scab pathogen V. Inaequalis. 97 methods based on coalescent genealogy samplers remain computationally demanding. For many datasets and models of population structure, they even remain computationally intractable. As a result, there is increasing interest in developing alternative approaches that are faster and easier to implement. One of the most promising approaches is approximate Bayesian computation 106; it has been shown to be particularly powerful to determine origin and routes of introduction of invading pest species, 107 - 109 and it is very likely that it will also provide important insights into the history of fungal pathogens.
The probability that any adaptive allele will eventually become fixed or lost depends on the demography of the population in which it arise. One indicator of demographic influences is N e, which can be influenced by factors such as sex ratio, variation in offspring number, inbreeding, mode of inheritance, age structure, changes in population size, spatial structure and genetic structure. In humans and other species with expanding populations, very large samples are needed to identify rare, young polymorphisms that reflect current N e values and that may contribute to disease or adaptation from new mutations 7 - 9. Smaller studies sample older, more common polymorphisms, reflecting historical N e values. Such standing variation may contribute to soft sweeps. Populations with large N e values are likely to fix adaptive alleles regardless of how strongly they are favor. They also encounter many mutations each generation6 and therefore have shorter waiting times for adaptive alleles to arise. Small populations, by contrast, sample far fewer mutations per generation and lose many favorable alleles to drift, even those with large selective advantages.
Adaptive alleles may be derived from new mutations or from pre - existing standing variation. In classic selective sweep, new mutations appear on single haplotype that quickly go to fixation. Therefore, when the genome is scanned after hard sweep, pattern of reduced nucleotide variation may be found near adaptive mutation. By contrast, in soft sweeps, favor variants occur on multiple genetic backgrounds, and variation among haplotypes obscures reduction in polymorphism around favor variants 111, making soft sweeps difficult to detect. Such soft sweeps can occur if old, standing genetic variation has recombine onto several genetic backgrounds before it becomes advantageous or if independent advantageous mutations occur in parallel. Finally, signatures of selective sweeps may be obscured by population structure, and variation may resemble selection on standing variation 112. Indeed, identifying soft selective sweeps has been one of the most challenging problems in molecular population genetics. However, recent analysis of changes in allele frequency at height - associate polymorphisms finds clear evidence for selection on pre - existing polygenic variants analytical approach that should be broadly applicable to 38. Several population characteristics influence the probability of hard versus soft sweep. Soft sweeps are more likely when populations are large or when mutation rates are high 113, whereas small populations or low mutation rates favorr hard sweeps from single, new mutation. In addition, soft sweeps are more likely in widely distributed species with low migration rates 27, facilitating parallel sweeps in different parts of the species range.
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