Natural Selection - Chapter 4 in the Origin of Species

So far in our tour of the Origin we have seen that pre-Darwinian scientists defined species as distinct entities created by God, and varieties as deviations from the species-type, such deviations occurring due to natural means.  Darwin, in part by observing the human-caused production of domestic organisms argued that varieties evolve within a species, and as they become more distinct they in turn become new species.  There is, therefore, nothing directly divine about the production of species (although God could still be operating behind the scenes, directing the evolution of species).  In chapter three, Darwin argued that organisms produce more offspring than can possibly survive.  Through some mechanism unknown to Darwin, these offspring differ from one another by a small degree; those individuals that have beneficial variations will outcompete those that do not, and will be more likely to survive and pass on their traits to their offspring.  This is the foundation of natural selection.

Struggle for Existence - Chapter 3 in the Origin of Species

Locust swarms involve intense competition for resources

At this point in our tour of Darwin’s Origin, Darwin has questioned the 19^th-century view of species as distinct entities created by God, and varieties as deviations from the species-type.  Instead, Darwin has presented compelling evidence, in large part through domestic organisms, that varieties and species are not different in kind, but only in degree.  That is, varieties, when they become especially well-marked, are identified as species.  A species evolves into different varieties; those varieties, in turn, may evolve into new species.

Darwin has also argued that there is a considerable amount of individual-level variation that exists in wild populations, and it is this variation that is the fuel for what he calls ‘natural selection’, a metaphorical phrase that he took from the phrase ‘artificial selection’.  Artificial selection occurs when humans look at a population and choose the individuals that have the best features for breeding.  Natural selection, although there is no conscious choice occurring, is the equivalent process that occurs in the wild.  Survival of the fittest.

In chapter three, Darwin sets the scene for his next chapter on natural selection.  Here he argues that waste and death is the hallmark of nature and is essential for speciation, rather than direct miraculous intervention from a beneficent God.  Babies die, and they die in large quantities.  This is an essential truth, if we wish to understand evolution. 

Variation under Nature - Chapter 2 in the Origin of Species

The giraffe species is divided into several populations, each exhibiting a region-specific colouration pattern on the hide.  These have become identifiers for dividing giraffes into distinct subspecies.

In the first chapter of the Origin of Species, Darwin looked to domestic animals and concluded that the varieties of domestic breeds were created by man by selecting individuals with slight variants and accumulating these changes over numerous generations.  He referred to this as artificial selection.  In chapter two he begins to show how artificial selection can inform us of how species are formed, through the more powerful ‘natural’ selection.

Variation under Domestication

This semester I am taking a course on Darwin’s first edition of On the Origin of Species.  I thought that I would post chapter summaries of the Origin as I read it, to help keep the material straight in my mind and hopefully introduce you to one of the most important texts of the past 150 years.  If you are a creationist, perhaps you will find less in this book to fear than you had thought; if you are a scientist, you might be surprised by how prescient Darwin was in many ways.  If you are neither, you will still be impressed with the power of Darwin’s arguments.  If you would like, you can follow along by reading the Origin with me – the first edition is free online at http://embryology.med.unsw.edu.au/pdf/Origin_of_Species.pdf. 

I will be skipping over the opening quotes (read my thoughts on them here) and the introduction, and will dive right into the meat of the thing.  This one's a long one...it should be shorter in the future!