The notion fitness, widely used in genetics usually serves to measure a relative rate of organism reproduction. Another important character of an organism is its ecological resistance which is basically the product of macroevolution. It can be determined as a probability of an organism survival and participation in reproduction of the species. Ecological resistance determines the level of the accidental death of organisms that are genetically valuable. For the comparison of ecological resistance in different organisms and species the negative meanings of the Malthusian parameter can be used. Ecological resistance depends on the presence in genomes of essential genes and fairly complete sets of nonessential, or adaptive, genes which can reside in genomes both as "plus" and "minus" alleles. The recovery of complete sets of adaptive genes lost as a result of mutations and, thus, of a high level of ecological resistance in organisms is provided by genetic exchange between them. With respect to mutations leading to the increase in fitness the effect of genetic exchange is negative since it leads to the formation of recombination load, i.e. a decrease in fitness of the offspring. In microevolutionary processes, the elevation in ecological resistance level does not take place since it requires a long time for the formation of new genes and new elements of organization in the process of positive selection. At the same time, a constant recovery of a high level of ecological resistance of the species decreased as a result of mutations takes place in some individuals due to genetic exchange. Mutations affecting ecological resistance of an organism, as a rule, cause a decrease in its viability and they are usually excluded from populations as a result of negative (stabilizing) selection.