Editing Speciation (section)

=== Darwin's dilemma: why do species exist? ===

In ''[[On the Origin of Species]]'' (1859), Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species.<ref name="OofS">{{harvnb|Darwin|1859}}</ref> Chapter 6 of Darwin's book is entitled "Difficulties of the Theory". In discussing these "difficulties" he noted

{{blockquote|Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?|''[[On the Origin of Species]]'' (1859), chapter 6<ref name="OofS"/>}}

This dilemma can be described as the absence or rarity of transitional varieties in habitat space.<ref>{{cite book |last=Sepkoski |first=David |title=Rereading the Fossil Record: The Growth of Paleobiology as an Evolutionary Discipline |chapter-url=https://books.google.com/books?id=jNfJyXKSDRcC&pg=PA9 |date=2012 |publisher=University of Chicago Press |isbn=978-0-226-74858-0 |pages=9–50 |chapter=1. Darwin's Dilemma: Paleontology, the Fossil Record, and Evolutionary Theory |quote=One of his greatest anxieties was that the "incompleteness" of the fossil record would be used to criticize his theory: that the apparent "gaps" in fossil succession could be cited as negative evidence, at the very least, for his proposal that all organisms have descended by minute and gradual modifications from a common ancestor.}}</ref>

Another dilemma,<ref>{{cite journal |last1=Stower |first1=Hannah |title=Resolving Darwin's Dilemma |journal=Nature Reviews Genetics |date=2013 |volume=14 |issue=747 |pages=747 |doi=10.1038/nrg3614 |s2cid=45302603 |quote=The near-simultaneous appearance of most modern animal body plans in the Cambrian explosion suggests a brief interval of rapid phenotypic and genetic evolution, which Darwin believed were too fast to be explained by natural selection. |doi-access=free }}</ref> related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed", and wondered "why do we not find them embedded in countless numbers in the crust of the earth". That clearly defined species actually do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species.<ref name="OofS"/>