I'm finally posting another chapter summary for On the Origin of Species 2009 Penguin Classics edition (it's taken me long enough).
Chapter 2 is called “Variation under Nature” to differentiate what Darwin discussed in the previous chapter, “Variation under Domestication.” This chapter is short but packed full of info. Darwin takes the ideas from the first chapter and applies them to the natural world of undomesticated plants and animals.
The chapter starts off by discussing the nature of species. There’s not a clear cut line between different species as many people suppose. There are lots of gray areas, and scientists during Darwin’s time, and even today, argue about where one species ends and another begins. Sometimes one plant or animal can be defined as a separate species by one scientist only to be defined as a variety (of another species) by a different scientist. This can get confusing to the lay person. You have to keep in mind that we’re looking at constant transitions and changes in living things as opposed to clear cut distinctions and separations. Variety is a constant in life on Earth and very important to evolution:
These individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the same manner as a man can accumulate in any given direction individual differences in his domesticated productions. (50)
Darwin goes on to give examples of various species/varieties that are contested, including Rubus, Rosa, Hieracium, and Brachiopods. He also mentions the primrose and cowslip (Primula veris) and the several varieties of oak trees. He considers the contested species/varieties to be invaluable:
Those forms which possess in some considerable degree the character of species, but which are so closely similar to some other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species, are in several respects the most important for us. (51)
He refers to highly contested varieties as incipient species. These varieties have the potential to one day become distinct species. Here’s another quote from the second chapter, which I think is important, even if it is long:
Hence I look at individual differences, though of small interest to the systematist, as of high importance for us, as being the first step towards such slight varieties as are barely thought worth recording in works on natural history. And I look at varieties which are in any degree more distinct and permanent, as steps leading to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to species. The passage from one stage of difference to another and higher stage may be, in some cases, due merely to the long-continued action of different physical conditions in two different regions; but I have not much faith in this view; and I attribute the passage of a variety, from a state in which if differs very slightly from its parent to one in which it differs more, to the action of natural selection in accumulating (as will hereafter be more fully explained) differences of structure in certain definite directions. Hence I believe a well-marked variety may be justly called an incipient species; but whether this belief be justifiable must be judged of by the general weight of the several facts and views given throughout this work. (55-6)
Darwin spends the rest of the chapter discussing observations he and others have made concerning the patterns found in various genera. He also makes it a point to mention that not all incipient species will actually make it to the level of distinct species. Some will retain their current features because it is most beneficial, and some will simply go extinct.