Tuesday, January 24, 2012

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.

‘Species’, says Darwin, is an ill-defined concept that ‘includes the unknown element of a distinct act of creation’, while a variety is defined by ‘community of descent.’  Thus God formed the wild sheep species, which in turn through domestication and artificial selection was modified into the current and multitudinous breeds of sheep.  Domesticated sheep are thus a variety of the wild sheep species – they are linked by common descent to their ancestral form.  A goat, however, and a sheep are distinct species and thus distinct acts of creation – there is no line of descent between a goat and a sheep.  At least, that was the thinking of many biologists in Darwin’s day.  Darwin defines species and variety in this manner because it was the common definition of the time.  By the end of the chapter he will have a much different definition, as he shows the inadequacy of believing species to be specially created. 

Just as among domestic animals we see different artificially-developed breeds of the same species, so to in nature do we see different varieties of a single species.  One population of tree may be slightly different from another population of tree, but those differences are so slight that they are still classified as the same species (creations of God), but different varieties of that species (having been modified somewhat from God’s original template for that species).

Often, argued Darwin, so-called varieties were classified as distinct species by some scholars, and as varieties by others.  For instance, one scientist in Darwin’s day said that there were 251 species of plants in Britain, while another said there were only 112, with the difference being made up by varieties – how arbitrary the species/variety distinction must be!  ‘It must be admitted that many forms, considered by highly-competent judges as varieties, have so perfectly the character of species that they are ranked by other highly competent judges as good and true species.  But to discuss whether they are rightly called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat the air.’

Some divided a flower into four different varieties of a single species, others argued they were four different species!  And yet, if a species is a creation by God, and a variety is a modification united by descent, shouldn’t it be very easy to tell where a variety ends and a species begins? 

One method scientists used to determine if something was a species or a variety was to look for intermediates.  If a chain of intermediates could be found that linked two species, then those species would most likely be varieties that descended from a single species. 

The classifier’s dilemma was elegantly posed by Darwin in an autobiographical account of a young naturalist (likely himself) who, upon describing a group of organisms previously unknown to him (Darwin classified barnacles) is, at first, greatly impressed by the number of different individuals and is lost in a flurry of variations.  As the investigator observes more and more organisms, he comes to know which differences are individual-specific, and which are specific to a variety or species.  He uses this information to divide the organisms up into a great number of species.  But then, upon investigating creatures from other lands, he finds intermediates that cast his classification scheme into doubt, and begins to unite organisms based on similarities rather than differences, lumping species together into fewer species and creating more varieties.  If so many intermediates exist, and intermediates of intermediates, then the special status of a species begins to wane.  ‘These differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage.’

‘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…I attribute the passage of a variety, from a state in which it differs very slightly from its parent to one in which it differs more, to the action of natural selection in accumulating differences of structure in certain definite directions.  Hence I believe a well-marked variety may be justly called an incipient species…’

The species definition has to change, if the above is true, such that a species is ‘a set of individuals closely resembling each other.’  It is therefore an arbitrary term – varieties and species must be produced in the same manner, via the accumulation of successive slight variations by natural selection.

The remainder of the chapter is spent discussing botanical arithmetic, a Victorian vogue that compared the numbers of species and genera (groupings of species) of plants in Britain, with the hope of uncovering patterns.  Among other things, Darwin here notes that:

1.       Plants with wide ranges tend to have many varieties (normally explained by the different environmental conditions in which these varieties find themselves)
2.       Dominant genera (large or widespread) contain species that are more variable than species in small genera.
3.       Dominant genera have dominant species.
4.       Large genera have more varieties per species than small genera - This should not be the case under the hypothesis of special creation.  If each species is distinct, there is no reason why large genera should have species with more varieties than small genera.  It only makes sense if the large genera are large because they have been speciating, with the current varieties being the species of the future.
5.       ‘Species of larger genera resemble varieties, more than do the species of smaller genera.’
6.       ‘Little groups of species are generally clustered like satellites around certain other species.’ Why would this be, if each species was a distinct special creation?
7.       Closely related species resemble varieties more than they do species, in that they have limited ranges.  This is inexplicable if God created species, but makes sense if they are similar because they are newly-formed species that have yet to diverge distinctly from one another.

Together, these facts only make sense if species evolved from varieties.  Not only so, says Darwin, but just as varieties cluster into species based on common descent, so to do species cluster into genera, and genera into ever larger groups, finally explaining the nested hierarchy proposed by Linnaeus in the 1700s.  ‘And thus, the forms of life throughout the universe become divided into groups subordinate to groups.’ 

But how does this work?  That is the question posed in the next two chapters.

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