Genetic Diversity and Selection Pressure as Driving Forces for Evolution
As indicated in chap. 5.2 the selection is a very strong driving force for evolution. However selection additionally needs genetic diversity. Misleading concepts in history which tried to argue that any evolutionary process might be accelerated by extinguishing actively a part of the genetic diversity are wrong. Hopefully in any case of selection pressure the genetic diversity grows quickly to its old values and above them. Supporting a population's resistance and evolution always means that the genetic diversity has to increase as we will see in the following.
The concept of the infinite typing monkey showed that next to mutations especially genetic diversity (the number of text variations present when all monkeys are typing) are important if a predator shows up that is extinguishing a part of the text fragments. This picture is a symbol for a new selection pressure that induces directed evolution. Reproduction is strongly correlated with the selection pressure as selection most generally is not represented by an extinction of individuals in the population but by disadvantage in the reproduction speed for a certain genotype. A broad genetic diversity together with new selection mechanisms can easily lead to a leap of evolution as it was for example observed by Stuart et al. from the University of Austin, Texas when they investigated quick evolution of lizard feet (Stuart et al., 2014).
Figure 80. Rapid evolution of a native lizard species caused by pressure from an invading lizard species. The native species had much smaller feet in average (left side) than observed 15 years later after their feet evolved to better grip branches (right side).
The authors observed that Anolis carolinensis, otherwise known as the green anole had made notable physical changes within just 20 generations (15 years). The study discovered that the rapid evolution of a native lizard species was caused by pressure from an invading lizard species, introduced from Cuba. After contact with the invasive species, only the native lizards which were able to perch higher into the trees than others were able to escape. In such way the population began perching higher in trees, and, generation after generation, their feet evolved to become better at gripping the thinner, smoother branches. So only after a few generations the size and phenotype of the lizard's feet had changed significantly (Stuart et al., 2014). The scientists were astonished by the speed the adaption took place as significant changes occurred within a few months when native lizards shifted to higher perches and showed more sticky scales on their feet. “We did predict that we'd see a change, but the degree and quickness with which they evolved was surprising,” said Yoel Stuart, author of the study (Stuart et al., 2014).
This latest study is a well-documented example of what evolutionary biologists call “character displacement”, i.e. when similar species in competition with each other evolve differences to take advantage of different ecological niches. Charles Darwin observed a similar example when two species of finch in the Galapagos Islands diverged in beak shape as they adapted to different food sources. Probably this observation was a key element for Darwin to develop his understanding for evolution as the origin of species (Darwin, 1859). The cruel fact that induced the observed character displacement is represented by a new selection pressure due to the invading species.
Similarily the most severe selection pressure ever might have been the “big bang” in the biosphere when the atmosphere changed from reducing to an oxidizing environment as evolution developed oxygenic photosynthesis. The release of oxygen led to the aerobic atmosphere thus opening the road for a much more efficient exploitation of the Gibbs free energy through the aerobic respiration of heterotrophic organisms who had a large advantage and quickly reproduced in large amount while ROS became powerful selectors extinguishing the large majority of the former population.