Wednesday, May 09, 2012

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.

‘This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.’  (Variations not injurious or beneficial would randomly fluctuate within a population.)

Imagine a countryside populated by all sorts of creatures, mice and bees and birds and hawks and flowers and trees and the like.  Imagine that there is a sudden change in the environment - say, an unusual lack of water.  What would be the effect of this environmental change?  Well, the most immediate and noticable effect would be changes in the numbers of individual organisms within each species.  Certain flowers might go extinct, while some species might have greatly reduced numbers, and some species would continue to do just as well as before or even increase in number.  All of these changes brought on by environmental change would in turn trigger new changes - a decrease in mice could decrease the hawk populations, while a certain grass might increase with the sudden lack of competition.  Species from more arid places might then invade, disrupting things even further.  Ecosystems remain the same only so long as the environment remains the same - even slight disruptions can have large effects.  

Although Darwin does not say this, he suggests that, since environmental change does occur, and since the relationships between organisms are so easily unsettled, it would be ridiculous for a Creator to produce fixed species that could never change to meet new challenges.  Natural selection, for Darwin, was an important means by which life could thrive, even if species themselves were lost.

Dandelions are not native to Canada, 
but have successfully invaded.
Darwin was impressed by invasive species.  They proved to Darwin that God had not created organisms that were perfectly adapted to their own environment.  Perfectly-adapted organisms would never give in to invasive species.  But we see the effect of invasive species all of the time - look here in Canada at the dandelion, starling, zebra mussel, purple loosestrife and earwig (just to name a few), all of which came from Europe and Asia and all of which have done quite well in Canada at the expense of native species.

‘And as foreigners have thus everywhere beaten some of the natives, we may safely conclude that the natives might have been modified with advantage, so as to have better resisted such intruders.’  (Note the British colonialism in this quote!)

Invasive species are so successful because God did not shape species.  Native species evolved to meet particular environmental conditions, and those conditions did not include species that at the time existed in a different geographic region.

In order to meet the threat of invasive species, native species would have to migrate or evolve, or face extinction.

The power of natural selection

For Darwin natural selection was a slower but more powerful process than artificial selection.  Whereas man can only select for variations that he can see, nature can select for both internal and external variations simultaneously, favouring even minute differences between individuals.  Although change must be slow, it is this greater power that allows nature to produce different species.  This, for Darwin, explained why farmers, despite thousands of years of artificial selection, rarely if ever produced new species.

‘Can we wonder, then, that nature’s productions should be far ‘truer’ in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?’

Indeed, argued Darwin, features that to us seem to be unimportant can be essential for the survival of a species.  Thus smooth-skinned fruits are more prone to attacks by a North American beetle than are those covered in down; purple plums are more vulnerable to a particular disease than are yellow plums; white pigeons are more visible to hawks than are other varieties.  Farmers would have no idea to select for those traits, but nature does, and therefore shapes raw variation to produce varieties and species well adapted to their environments.

Furthermore, natural selection can work beneficial changes at any age or life stage of a species.  But natural selection is also, in a sense, limited: it cannot effect changes in one species that benefit another species, without also benefiting the first species.  Thus a plant will not naturally produce fruit simply so that another organism can eat it.  The plant must gain a benefit from producing the fruit as well, and will either evolve a way to defend itself from the frugivore, or is taking advantage of the frugivore, perhaps as a means of spreading its seed.

Sexual selection

Female (left) and male (right) peacocks spiders.  The
differences between the sexes are due to sexual selection.
The male peacock spider raises his
colourful abdomen to attract his mate.

Darwin recognized what he considered to be a separate form of selection from natural or artificial selection: sexual selection.  Whereas Darwin considered natural selection to be the struggle for existence between members of the same species, sexual selection was the struggle ‘between the males for possession of the females.’  (Note this Victorian-era gender bias).  The result was not death to the unsuccessful competitor, but rather fewer offspring.  ‘Sexual selection is, therefore, less rigorous than natural selection.’

Whereas natural selection should produce males and females that are generally similar to one another in appearance, so long as they have the same ‘habits of life,’ sexual selection produces males and females that differ from one another in striking ways.


After a section on hypothetical examples of natural selection (in which Darwin proposes how plants and their pollinators could co-evolve) Darwin discusses an interesting tangent: that hermaphroditic organisms (organisms with both male and female parts) almost always tend to have their reproductive parts exposed.  Hermaphrodites shouldn’t have to worry about finding mates, since they produce both eggs and sperm/pollen, yet in plants there seem to be extreme adaptations that prevent self-fertilization and favour intercrossing with other individuals from the same species.  Even in hermaphroditic animals, which do often produce clones, the reproductive organs are open to receiving sperm or eggs from other individuals.  Darwin therefore makes the important conclusion that sex between different individuals, at least occasionally, is a rule within most species.  The reason, according to Darwin, is that sex produces a greater number of variations than does self-crossing.  Since variation is the foundation of natural selection, this prescient observation suggested that sex was itself an adaptation for evolution.

Circumstances favourable to natural selection

In this section of the chapter Darwin brings out some observations that would drive much of speciation research in the years to come.

      1.  Range size, dispersal, and extent of sexual reproduction (as opposed to asexuality or self-crossing) will influence the production of new varieties/species.  If a species has a large range and disperses far, there will likely be different environments within that range that subpopulations could adapt to, but intercrossing with individuals from the other end of the range will disrupt the formation of distinct varieties.  If individuals remain localized, or primarily self-cross, then the formation of varieties within a large geographic area is more likely.

      2.  Isolation is therefore of primary importance in the formation of varieties/species.  ‘Intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the same variety, true and uniform in character.’  If a population is isolated from other populations in a unique habitat, intercrossing will occur between individuals that exist within the same environment; selection can work to adapt the population to this environment.  If the population is not isolated, then individuals from other environments will come in to intercross, homogenizing the population across environments.

      3.  Isolation need not be geographic.  If two varieties inhabit the same area, they may never intercross if they have behaviours that cause them to prefer breeding with their own variety, or if they inhabit different microhabitats (for instance, different parts of the tree) or if they breed at different times of the year.

      4.   However, the isolated region cannot be too small, or there will be too few individuals inhabiting it; and too few individuals will decrease the chance of the production of favourable variations.  (Note: Darwin’s logic here is flawed.  The number of individuals that can survive is not as important as the number of individuals that can be produced.  Let’s say an island can only support thirty adults of a species, but each breeding results in 2 000 fertilized eggs.  There will be a great deal of variation produced, and there will be strong competition and thus strong selection among those surviving juveniles, such that the surviving adults should be better competitors and survivors than previous generations).

      5.   Counterintuitively, Darwin did not believe that species production was faster on small isolated islands.  He felt that the large mainland was more conducive to rapid adaptation, as it had more intense competition due in part to its greater species’ diversity.  (Note that continental drift was unknown at this time; Darwin, like others, believed that the land itself rose and fell over time.  When it fell, the sea would intrude, producing temporary continental islands that facilitated continental speciation).  This explained for Darwin why continental organisms seemed to easily invade islands – they were adapted for competition, while the island organisms were not. 

‘Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature’s power of selection.’

The second half of the chapter deals with Darwin’s principle of divergence, which requires its own post.

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