What is DEPRESSION EXTREMITY? What does EXTENDED DEPRESSION mean? DEPRESSION OF EXPULSION meaning – DEPRESSION OF EXTREMITY definition – explanation DEPRESSION OF EXPLOITATION.
Source: Wikipedia.org article, adapted under license.
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In biology, exogamy depression is when the progeny resulting from crosses between genetically distant individuals (crossing) shows a lower fitness in the parental environment than either their parents, or that the progeny of crosses between individuals are more closely related. The concept is opposed to inbreeding depression, although the two effects can occur simultaneously. Inbreeding depression manifests itself most significantly in two ways:
The intermediate genotypes are not adapted to the parents' habitat. For example, selection in one population could favor a large body size, while in another population small body size could be more advantageous, while people with intermediate body sizes are comparatively disadvantaged in both populations. As another example, in the Tatra Mountains, the introduction of Ibex in the Middle East resulted in hybrids that produce calves in the coldest time of the year.
Breakdown of biochemical or physiological compatibility. Within isolated reproductive populations, the alleles are selected in the context of the local genetic background. Because the same alleles can have quite different effects on different genetic backgrounds, there is the potential evolution of different locally adapted gene complexes. The crossed crossing between individuals with differently adapted genetic complexes can cause the interruption of this selective advantage, which results in a loss of physical capacity.
The different mechanisms of depression of exogamy can operate at the same time. However, determining which mechanism is more important in a particular population is very difficult. In general, the first mechanism will be more prevalent in the first generation (F1) after the initial crossing when the majority of individuals are formed by the intermediate phenotype. An extreme case of this type of exogamy depression is sterility and other fitness reduction effects that are often seen in interspecific hybrids (such as mules), which involve not only different alleles of the same gene but even different orthologous genes .
The second mechanism may not appear until two or more generations later (F2 or more), when recombination has undermined the positive epistasia of vitality. The hybrid vigor in the first generation may, in some circumstances, be strong enough to mask the effects of exogamy depression. An example of this is that plant breeders will produce purebred F1 hybrids, which will improve the uniformity and vigor of the offspring, however, the F1 generation is not used for further breeding due to unpredictable phenotypes in their offspring. Unless there is strong selective pressure, the depression of exogamy may increase in subsequent generations as the co-adapted gene complexes fragment without the counterfeit of new co-adapted gene complexes to take their place.
If the crossing is limited and the populations are large enough, the selective pressure acting on each generation can restore fitness. Unless the generation of F1 hybrids is sterile or of very low aptitude, the selection will act in each generation using the greatest diversity to adapt to the environment. This can lead to the recovery of fitness to the baseline, and sometimes even to a better physical condition than the original parental types in that environment. However, as the hybrid population is likely to experience a decline in physical capacity for some generations, it must persist long enough to allow the selection to act before they can recover.
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