Natural selection is the process in which organisms with certain traits survive and reproduce while organisms that are less able to adapt to their environment die off. As Darwin pointed out, natural selection does not necessarily produce evolutionary progress, much less perfection. The limits to the effectiveness of natural selection are most clearly revealed by the universality of extinction. More than 99.9 percent of all evolutionary lines that once existed on Earth have become extinct. Mass extinctions remind us forcefully that evolution is not a steady approach to an ever-higher perfection but an unpredictable process in which the best-adapted organisms may be suddenly exterminated y a catastrophe and their place taken by lineages that prior to the catastrophe seemed to be without distinction or prospects. There are numerous constraints, or limits, on the power of natural selection to bring about change. First, the genetic variation needed to perfect a characteristic may not be forthcoming. Second, during evolution, the adoption of one among several possible solutions to a new environmental opportunity may greatly restrict the possibilities for subsequent evolution. For instance, when a selective advantage for a skeleton developed among the ancestors of the vertebrates and the arthropods, the ancestors of the arthropods had the prerequisites for developing an external skeleton, and those of the vertebrates had the prerequisites for acquiring an internal skeleton. The entire subsequent history of these two large groups of organisms was affected by the two different paths taken by their remote ancestors. The vertebrates were able to develop such huge creatures as dinosaurs, elephants, and whales. A large crab is the largest type that the arthropods were able to achieve. Another constraint on natural selection is developmental interaction. The different components of an individual organism – its structures and organs – are not independent of one another, and none of them responds to selection without interacting with the others. The whole developmental machinery is a single interacting system. Organisms are compromises among competing demands. How far a particular structure or organ can respond to the forces of selection depends, to a considerable extent, on the resistance offered by other structures and organs, as well as components of the genotype (the totality of an individual's genes). The structure of the genotype itself imposes limits on the power of natural selection. The classical metaphor of the genotype was that of a beaded string on which the genes were lined up like pearls in a necklace. According to this view, each gene was more or less independent of the others. Not much is left of this previously accepted image. It is now known that there are different functional classes of genes, some charged to produce material, others to regulate it, and still others that are apparently not functioning at all. There are single coding genes, moderately repetitive DNA, highly repetitive DNA, and many other kinds of DNA. Discovering exactly how they all interact with one another is still a rather poorly understood area of genetics. A further constraint on natural selection is the capacity for nongenetic modification. The more plastic the organism's body characteristics are (owing to developmental flexibility), the more this reduces the force of adverse selection pressures. Plants, and particularly microorganisms, have a far greater capacity for individual modification than do animals. Natural selection is involved even in this phenomenon, since the capacity for nongenetic adaptation is under strict genetic control. When a population shifts to a new specialized environment, genes will be selected during the following generations that reinforce and may eventually largely replace the capacity for nongenetic adaptation. Finally, which organisms survive and reproduce in a population is partly the result of chance, and this also limits the power of natural selection. Chance operates at every level of the process of reproduction, from the transmission of parental chromosomes to the survival of the newly formed individual. Furthermore, potentially favorable gene combinations are often destroyed by indiscriminate environmental forces such as storms, floods, earthquakes, or volcanic eruptions, without natural selection being given the opportunity to favor these genotypes. Yet over time, in the survival of those few individuals that become the ancestors of subsequent generations, relative fitness always plays a major role.