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.
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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
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| Female (left) and male (right) peacocks spiders. The differences between the sexes are due to sexual selection. |
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| 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.
Intercrossing
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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