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.

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.