Finally, we get to natural selection, the big show, the process by which Darwin saw evolution.
"This preservation of favorable variations and the rejection of injurious variations, I call Natural Selection," (Darwin, 144). Well put Darwin. Natural selection can be basically boiled down to the simple concept of traits that allow an individual to survive the best will be passed onto the next generation.
Darwin explains that if there is a sudden change in environment, it's inhabitants must adapt or perish. Also important is the fact that different species have an impact on each other, whether it is a predator/prey relationship or a competition for space or food. This complex relationship seems to have fascinated Darwin as he has mentioned it in previous chapters. "Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be," (Darwin, 145). A modern example of this can be seen in invasive species that come to an area and disrupt the local ecology.
Darwin believed that direct changes in climates affected variability, "A change in the conditions of life, by specially acting on the reproductive system, causes or increases variability," (Darwin 145). It is difficult to access the role that the environment plays in variability. Variations are rooted in genetics, but Darwin obviously had no understanding of modern genetics. So yes, while the expression of some genes is determined by factors such as temperature, this is not the case for all genes. Therefore, the directed change of natural selection is acting on existing individuals survival, and not the variations of developing individuals.
"Every selected character is fully exercised by [nature]; and the being is placed under the well-suited conditions of life," (Darwin 146). Darwin cited the example of birds and insects being the same color as their habitat as an example of natural selection. This makes sense: if a bug lives on red leaves, variations of the bug that are red are camouflaged better than variations of another color. Since the other colors would be easier to spot, they would be eaten more and the red ones would be left with more food and access to mates.
Darwin also points out that variations that occur during specific life stages are repeated in offspring. For instance, if a goat is going to grow horns, it will do so at around the same age that it's parents first grew horns. Unknown to Darwin, such developmental features are determined genetically.
Sexual Selection is mentioned in this edition as a process through which selection is determined by mate choice. This is mostly observed in males, as Darwin notes, because they are most often the sex competing for mates. Sexual selection rightly deserves it's own blog post so look for that soon. This sort of selection creates divergence between the sexes (sexual dimorphism) and focuses mostly on contest-competition and displays. Darwin notes examples of deer and birds.
Darwin gives many examples of natural selection in this chapter with animals and plants. However, the evidence he uses is mostly circumstantial and not derived from direct observation. Yet, because evolution is such a long process it is not surprising he could not directly observe many of his theories in action (although he did take 30+ years to publish his ideas, think of how many generations of fruit flies he could have gone through!). As a side note, there was a very interesting study of Galapagos finches, which were regarded as highly inspirational for Darwin, and the effect of climate change on their beaks over many years.
Moving on, Darwin talks briefly about intercrossing (hybridization gets it's own chapter, look forward to that). He also has a tangent about hermaphrodites and their reproductive prospects. Incorrectly, Darwin assumes that no animal species reproduces asexually. Wrong. While sexual reproduction is the norm, there are examples of asexual species and those that can reproduce through parthenogenesis (when an egg develops without fertilization; this has been observed in some fish, reptiles, and insects). I'll let Darwin pass on this one, I don't think he ever encountered any such species in his travels and if he did I doubt he lifted up the legs on each lizard to check it's sex (they'd all be girls)... Basically, what Darwin is saying is that even if a plant or animal can reproduce itself, pairing of animals is essential to create variation and increase diversity.
Darwin explains how different factors impact natural selection, such as isolation and large areas. Darwin's view was that natural selection seeks to fill every niche. Also the layout of the land, and whether or not it is isolated impact natural selection. If no new species are moving into an area, the native species are not challenged and not subject to strong selection due to a change in predators, space or food sources. Darwin favored the idea of species doing better over large areas that they could spread out and populate. Confined, isolated areas stagnate change and create what Darwin referred to as living fossils -- species that have not changed very much over thousands of years because they are so well adapted to their isolated spot and nothing has invaded to force for selection.
Extinction is the next topic Darwin tackles, basically summing it up as varieties that do not do as well become rarer and if they are not selected for over a long enough time, they go extinct. Brilliant insight Darwin. "Consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them," (Darwin 163). Therefore, Darwin believed that extinctions were caused not by extremely different species (with presumable different needs) but rather similar ones competing for the same resources.
The idea of extinction is followed by the important concept of divergence of character. This idea is that as variations become more pronounced, they move towards a category of sub-species, and then fully differentiated species. Darwin never really defines what a species is, which again for purposes of these posts, I am defining as groups that do not/cannot mate with each other and thus do not exchange any genetic material that result in viable offspring. Since Darwin viewed selection as trying to optimize the use of the environment, divergence would allow for the maximization of species in different environments. "As a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their progeny will increase," (Darwin, 170). In this section, Darwin draws an imagined phylogeny tracing many different species back to a common origin. This is one of the most important concepts to be hammered home: species are descendant from common ancestors. The more species diverge, the less related they are to each other.
The importance of this chapter is insurmountable. Darwin ends the chapter quite poetically, "As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications," (Darwin, 177).