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Evolving: Microevolution to Macroevolution
There are many changes that the earth undergoes. Microevolution, a small change in the gene frequency of a population, is one of these changes that scientist have explored. Evolution at this scale can be observed over short periods of time. Theses changes are due to factors such as natural selection of genes, new populations are genes mixing with genes, gene mutation, and genetic drift to name a few.
A population is a group of the same species that coexist in the same area. Populations share traits that show qualitative differences. These qualitative differences from generation to generation make up a gene pool for a population. A gene pool is the genetic make-up of a specific population, and is the combination of all the alleles for all traits members of the population exhibit. There are two types of mutations, lethal and neutral, that can change the gene pool of a population. Lethal mutations lead to the death of the individual specie. Death does not have to occur immediately, it may take several months or years, but if the expected longevity of a being is significantly reduced, the mutation is considered lethal. On the other hand, neutral mutation does not affect the particular phenotype whose expression is under the genetic control of the mutated gene. Also, neutral mutation does not help nor affects you in its environment. Scientist calculate the allele frequency, genetic diversity of a species population, by starting with the genetic equilibrium. Genetic equilibrium happens when allele frequencies are not changing and therefore the population is not evolving.
The Hardy-Weinberg Formula explores genetic equilibrium further. Godfrey Hardy and Wilhem Weinberg state that gene pools can remain stable only when no mutations are occurring, the population is very large, the population is isolated from other populations of the same species, all members survive and reproduce, and mating is completely random.
There are various selection processes of evolution that affect the allele frequency in nature. They can also alter the population or species so much that they become a new species. One is natural selection. Natural selection is a process that states populations varying in details of heritable traits survive and reproduce with differing success. Ultimately, if there is variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome. Next, directional selection favors one extreme phenotype. Then there is stabilizing selection that favors a median phenotype. Finally, there is disruptive selection which occurs whentwo or more phenotypes on opposite end dominate leaving none for the median. This usually results in two or more new populations
There is another form of evolution selection that maintains variation which is sexual selection, a type of natural selection. Sexual selection is an organism’s ability to successfully obtain and reproduce with a mate. Sexual dimorphism is the difference in form between male and female members of the same species. Sexual dimorphism is the existence of physical differences between the sexes, other than differences in the sex organs. Sexual dimorphism includes differences in size, coloration, or body structure between the sexes. In addition to sexual dimorphism is balanced polymorphism. This is a genetic polymorphism that is stable and maintained in a population by natural selection, because the heterozygotes for particular alleles have a higher adaptive value fitness.
In small populations, random changes in allele frequency can lead to genetic diversity. Genetic drift is a random change in the gene pool due to chance. In each generation, some organisms may by chance leave behind a few more descendents than other organisms. The genes of the next generation will be the result of the lucky organisms, not necessarily the better organisms. The genetic drift can have two effects, bottleneck or founder. The bottleneck effect is reduction due to natural disasters and hunting, which reduces the size of a population. The founder effect is when a few individuals colonize an isolated island, lake or some other new habitat. The smaller the colony, the less its genetic makeup will represent the original colony and will reduce genetic variation. Genetic drift is high in inbred populations. Inbreeding occurs when individuals mate with a close relative. The offspring may be homozygous for a copy of an allele which is identical by descent from one of the ancestors:
However, individuals of the same species do not always stay in the same geographic location or population. Gene flow is any movement of genes from one population to another by immigration and emigration. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If genes are carried to a population where those genes previously did not exist, gene flow can be a very important source of genetic variation.
In nature, there are prezygotic and postzygotic isolating mechanisms to sexual reproducing species and maintaining different identifies. The Pre-mating isolating mechanisms are temporal, mechanical, behavioral, ecological, and gamete incompatibility. Temporal isolation is when individuals of different species do not mate because they are active at different times of day or in different seasons. Mechanical isolation is when reproduction is attempted, but transfer of sperm does not take place. Behavioral isolation happens when potential mates meet, but choose members of their own species. Ecological isolation occurs when individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences. Gamete incompatibility is when sperm transfer takes place, but the egg is not fertilized. The postzygotic isolation mechanisms are hybrid inviability and hybrid sterility. Hybrid inviability is when the hybrid embryo forms but the embryo dies early or dies before reproduction. Hybrid sterility is when hybrid survives but cannot make a functional gamete. Conclusively, an isolating mechanism, whether prezygotic or postzygotic, is any property of two species that stops them from interbreeding.
A physical barrier that ends gene flow between populations is called a speciation. Allopatric speciation is geographic isolation. Isolation might occur because of great distance or a physical barrier, such as a desert or river. In this mode of speciation, something physically foreign to the organisms prevents two or more groups from mating with each other regularly, eventually causing that lineage to divide, adapt, and not be able to interbreed again. Sympatric speciation does not require a large geographic distance to reduce gene flow between parts of a population. Sympatric speciation is the formation of two or more descendant species from a single ancestral species all occupying the same geographic location. This setting can cause polyploidy. Polyploidy is the process of genome doubling that gives rise to organisms with multiple sets of chromosomes. In general, polyploid organisms contain a multiple sets found in the same or a closely related species. In parapatric speciation there is no specific barrier to gene flow. The population is continuous, but does not mate randomly. Individuals are more likely to mate with their geographic neighbors rather than with individuals in a different part of the population’s range.
Conclusively, macroevolution can be defined simply as evolution on a larger scale. It includes the patterns and changes of one species giving rise to multiple species, the origin of major populations, and major extinction events. Coevolution is used to describe cases where two or more species affect each other’s evolution process, and at times these evolution processes are stasis, meaning that they do not change much for a long time. Some organisms develop exaptation, a feature that performs a function that was not produced by natural selection for its current use. Next, some evolve by adaptive radiation by an event in which a lineage rapidly diversifies, and with the newly formed lineages evolving different adaptations. Different factors may trigger adaptive radiations, but each is a response to an opportunity. These triggers are called key innovations and these open new niches to an organism and provide the opportunity for an adaptive radiation. No two species can occupy the same niche in the same location for any long period of time. They compete for food and space, until the death of one species occurs otherwise known as extinction. Each massive loss of species has been followed by adaptive radiation of some survivors. Mass extinctions provide surviving organisms with new opportunities.
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