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!

Genetics and the Origin of Species

The mule is a famous example of a sterile hybrid.  Photo circa 1937

We are officially almost finished our summer-long read through Dobzhansky’s evolutionary classic, Genetics and the Origin of Species.  We have spent an awful lot of time talking about genetics – about mutations, and how their value is context-dependent; about chromosomes, and how breakdowns during cell replication and gamete production can lead to large-scale changes; about natural selection, and how it affects gene frequencies; but now it is time to actually roll up our sleeves and delve into the ‘origin of species’ part of Dobzhansky’s book title.  Last week we explored polyploidy, a form of instant speciation that is solely due to reproductive incompatibilities brought about by chromosomal mismatch.

In other words, last week we saw that a simple genetic cause (chromosomal duplication) led to instant speciation, simply because the polyploid and its parental form could not successfully reproduce (or, more precisely, their offspring could not reproduce); that is, the polyploidy event caused a reproductive barrier to form between the parent and its polyploid offspring.  They were reproductively isolated.

Even You Can Create a New Species - The Curious Case of Polyploidy

There are currently 1.7 million named species of plants, animals and algae on this planet.  This does not include the vast array of bacteria and other such single-celled creatures.  And those are only the ones we know about – based on rates of discovery, the National Science Foundation estimates that we have only described 10% of the world’s true diversity.  Most of these organisms will likely be small or marine, but not all; twenty-five species of primates have been discovered since 2000 alone.  And then this diversity really only scrapes the surface of the total diversity that has ever existed, with a mind-boggling number of plant and animal species known only from the fossil record.

Theodosius Dobzhansky, in Genetics and the Origin of Species (1937), presents, in chapters 7-9, an introductory treatment on the topic of speciation.  He brings genetics and natural selection and various observations together to provide a compelling account of how new species are formed.  This would be the groundwork used by Enrst Mayr, who would develop the modern Biological Species Concept.