Summary
“Can the principal of selection, which we have seen so potent in the hands of man, apply to nature? I think we shall see it can act most efficiently”.
After earlier examining man’s power of selection, Darwin looks to the power of Natural Selection. Where man has the power of a breeder’s hand, nature has the power to kill off individuals with qualities that are not advantageous to life, while those with more advantageous qualities will be more likely to live and reproduce. Qualities that are neither useful or injurious would not be affected by natural selection and would remain fluid throughout the species or could eventually become fixed “owing to the nature of organisms and the nature of conditions” (p64).
Darwin points out a key advantage that nature had over man’s selections. That is that man can only consciously select based on external visible qualities while nature selects based on “the whole machinery of life” (65) Darwin cautions against the misinterpretation by some of Natural Selection, where Nature becomes overly personified as a deity. Nature should be seen as the aggregate of many natural laws and sequences of events.
So far Darwin has focused on traits that make species a better chance of survival, but here he discusses Sexual Selection, or traits which may make an individual more attractive to the opposite sex. In this way, though a trait may not help an individual survive it may give them more offspring. In male alligators, salmon, and beetles Darwin looks at the violent fights to win the female. In some birds of paradise the female is not won by violence but in the most gregarious showing of the male’s plumage. This could be one factor contributing to the differentiation often seen between sexes of the same species.
Using flowers, bees and birds as examples Darwin discusses how variations in one species may aid another and from shared advantageous traits species could evolve in tandem.
Next Darwin tackles the difficulty, but possibility of the formation of new species through Natural Selection. This is a difficult task, in part, because where there is a variation that is advantageous and passed down to offspring, in nature and without the breeders hand, there will be a tendency to reversion to the previous and dominate form. Isolation can often be a factor in overcoming this tendency to reversion. The first type of isolation Darwin discusses is differing birth and breeding seasons. Geography is the next form of isolation discussed. Some believed isolation and migration so important that, absent them, evolution was impossible. Darwin doubted this but saw isolation, such as happens on islands a strong help in allowing new variations to find definition and species to arise. Small isolated areas may be the most beneficial for the production of new species, but the health of an existing species is helped in a large area where there may be many individuals and much opportunity for variation which may over time benefit the species as a whole.
As Natural Selection favors individuals with traits most advantageous for survival and reproduction, species that fail to advance or are crowded out by others may become extinct. The risk of extinction is greater with rarer species than those more common because within their population pool there is less chance for variation and modifications will happen slower.
“The truth of the principle, that the greatest amount of life can be supported by great diversification of structure, is seen under many natural circumstances. In an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be severe, we always find great diversity in its inhabitants.” (85)
The divergence of character that may define varieties may over the course of many generations and thousands of years grow to differentiate species. Traits that are advantageous will grow over time, while others may die out and varieties become extinct. Darwin lays the effect of this out in a chart which shows two hypothetical families of species over many generations.
Looking at how species become more advanced, Darwin looks at how any “lowly organized forms” still exist in nature. Though he admits that there is much to be discovered in this question, Darwin points out that in some conditions of life “a high organization of life would be of no service,- possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and injured.” (p96)
In this chapter much is discussed about divergence of species, but in closing Darwin talks about convergence of species, a phenomenon where related variations may converge into one. Some naturalists see this as an important force and the only answer to why there is not an infinite number of species roaming the earth. Darwin sees extinction through climate, geography, competition from within a species or with another species as a much stronger force.
