Adaptation is a term used to describe the process by which specific manifested characters, determined by populations of alleles (forms of a gene) change in a population through succeeding generations. The changes are specifically related to characteristics that improve the organism's ability to successfully reproduce (i.e., to produce offspring that are capable of reproducing). Often, adaptation is erroneously related to survival, but this relationship is secondary to the ability to produce offspring that can, in turn, reproduce. Obviously, however, organisms must survive at least until they are able to reproduce--and a certain degree of longevity may be required to successfully raise offspring so that they may reproduce. With adaptation a species undergoes modification that favor its reproducibility and survival in a given set of environmental condition. There is no adaptation at the level of the individual. Adaptation is found only in population genetics.
Fitness is a measure of reproductive success of a particular genotype and the phenotype expressed. Often fitness is mathematically (quantitatively) related to the fitness of one genotype in contrast to the most common genotype found in the population (presumably the most fit genotype). The relative reproductive ability of any genotype phenotype combination is term Darwinian fitness.
Species survival depends on the ability of organisms to adapt to environmental changes. Radical climate changes and human activity may cause disruption in the food chain, water resources, and climate, affecting fauna, and flora. Human activity and human overpopulation are presently the leading causes of the dramatic population decrease of thousands of species and the fast extinction of others. Independent of radical environmental changes, genetic traits and the characteristics they determine (phenotype), may either favor survival of a given population or may be the direct cause of lethal diseases or an increase the susceptibility of a given group to pathogenic factors present in the environment.
Genes are always exposed to environmental challenges, and such pressures may induce addition mutations in addition to the normal rate of mutations. Phenotype is the expression of a genotype in a given environment and new phenotypes may favor the survival of the more fit groups in a population as well as the gradual disappearance of the less fit ones. This process is termed natural selection. However, fitness does not purely imply survival of a given group or generation, being better defined by Charles Darwin as the capacity of a given group of the population to produce offspring in higher rates than others that carry a competing genotype. In other words, Darwinian fitness is the ability of organisms to better contribute to the next generation, thus ensuring the species continuation.
In genetics there are two levels for the concept of fitness: individual and population. At the individual level, fitness is better measured by larger and healthier offspring that survive to reproduce, whereas at the level of population the most fit are the large ones, with a great variation of genotypes/phenotypes among the groups, which offers the species a wide range of possibilities for adaptation and survival in the presence of challenging contingencies and changes in environmental conditions.
Adaptation is the result of three factors: behavior, acclimation, and developmental plasticity. Behavior is a controversial issue, since different academic trends tend to define it in different ways. Some field studies of animal behavior report culturally learned behaviors that are taught by one generation to the next, whereas others define behavior as a mere transient response to a stimulus. Acclimation implies organic adaptation to new food sources, or to radical differences in temperature and altitude, and other environmental challenges as well. Therefore, acclimation involves reversible changes in metabolism and structure, such as thicker or thinner fur, changes in skin/fur color, increase or decrease of hemoglobin levels to adapt to higher or lower altitudes, longer or shorter beaks, etc. Such changes are therefore the direct result of environmental pressures on the species genome, leading to both mutation and selection in a given population to produce higher numbers of "fit" individuals. Advantageous mutations lead to species survival and are presumably an indication of past natural selection in a given environment.
Mutations may occur in a gene that encodes a functional product, thus causing the loss of such product. Mutations may also occur in a gene that lost the capacity of encoding a functional product, thus leading again to the synthesis of such product. The first event is known as forward mutation and the second, as reverse mutation. Population genetics studies genotype frequencies, trying to determine the frequency of mutation occurrence and recurrence in a given population. Studies of human population genetics, for example, have registered an increase of alleles with forward mutations because they are 10 times more frequent than the reverse mutations that could correct them. As the number of individuals carrying a forward mutation as one of their two alleles for a gene increases, the mutation recurrence tends to increase in the next generation, unless selection acts against it.
An example of recurrent mutation in human beings is the ABO blood group, where types A and B are functional alleles that control enzymes that link glucosamine or n-acetyl glucosamine to a common precursor. Type O is a nonfunctional allele, because it does not recognize the enzyme substrate. Although type O blood was rare in the population a century ago, today it is the most frequent allele.
Another mutation that presents a high frequency in human populations is a varied range of discrete mutations termed polymorphism, that can occur in genes such Cytochrome P450, N-acetyl transferase, red blood cells, etc.
Heterozygote polymorphisms are sometimes fitter than either homozygote, as illustrated by sickle anemia in Africa. African rain forests have a high incidence of malaria and the heterozygous sickle cell mutation provides a better resistance to the disease. Therefore in Africa, three different genotype phenotype combinations are found. The first is a normal genotype and phenotype with no mutation. The second is a heterozygous genotype but normal phenotype, carrying one normal allele and a mutated sickle cell allele. The third is the homozygous sickle cell mutation, manifesting the sickle cell anemia phenotype. Individuals with normal and heterozygous genotypes are both infected by falciparum that causes malaria fever. However, a heterozygote individual shows milder forms of the disease, being more fitted to survive the infection than an individual with the normal genotype, if both are left untreated. The carriers of homozygous sickle cell genotype, which present mutation in both alleles, usually die during childhood, especially in areas with inadequate medical treatment. Therefore, the heterozygous polymorphic genotype confers a selective advantage in such an environment and the sickle cell gene--although deleterious in the homozygous individual conveys a selective advantage in the heterozygous population and is thus retained in the gene pool.
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