What Darwin’s biogeographic observations
suggested
The other trigger to Darwin’s thinking
What must happen in speciation
Even the most severe and extreme of modern
creationists (call them “intelligent design theorists” if you wish) are
prepared to accept that small-scale evolutionary changes, up to and including
the origin of new species, are possible. Their acceptance of “microevolution”
is not surprising, since genetic technology has brought out the obvious,
continuous, and ubiquitous nature of gene-change. But it was not always this
way: through most historic periods in which nature was subject to analysis,
especially with a Christian mindset, it was viewed as fixed and unchanging – no
one was observing the arising of new species from any source, whether from existing
species, or from some sudden supernatural origin.
A major argument against the idea of new species
appearing was that it implied an incompleteness in the original creation
event(s); why were these putative new species not made along with all the
others, at the beginning? Did not Adam name all the organisms at that time,
implying that the set was complete? Few people were willing to suggest that
there had been separate waves of creation at different times (inference:
earlier sets were failures, or inadequate), and even fewer were willing to say
that there was ongoing and more or less continuous creation of new species all
the time (inference: few or no existing types were adequate at any
point). Species-creation had occurred in a few days at the beginning of time,
and no less an authority than the Creator Himself had pronounced that it was
good – suggesting that there was no need for further alteration.
The notion that new types of organisms arose from
time to time began to be taken seriously in the 19th century, when
people excavating for fossils noticed that certain types of organisms could be
found just so deep, and no deeper, in the record. Land-living organisms like
mammals and trees could be found fossilized only in layers on top of layers
containing uniquely marine species (though some marine species persisted into
the upper layers), and deeper even than the earliest marine organisms there
were no fossils of any kind to be seen. Although there was no way of dating the
layers explicitly, the idea that deeper layers were the oldest was already well
accepted, so there seemed to be some sort of change occurring over time, with
new types appearing… a view consistent with the production of new species. It
was also becoming clear to plant and animal breeders around that time that
reproductively incompatible strains of organisms could sometimes be created
from pre-existing types, and that modified organisms could be created that were
dramatically different in appearance even if they were indeed of the same
species. For example, consider all the breeds of dog, which vary in weight by a
factor of 100 but can still interbreed – many naturally-occurring and definitely
non-interbreeding types, like wild dogs and foxes, are much closer in
size and appearance than two breeds of dog, and this made the idea of species-to-species
changes more plausible.
When Charles Darwin was on his round-the-world
expedition as a naturalist on the Beagle in the 1830s, he famously
stopped in the Galapagos Islands,
in the eastern Pacific some 1,000km west of Ecuador. Having already visited the
South American mainland and collected specimens there, he was struck by two
things in the islands: (1) how similar the plants and animals were to those
he had seen on the mainland, and (2) the fact that the island species were
clearly not identical to the mainland ones. Of course the first
observation probably doesn’t come as a surprise to you, as you might expect the
islands and the adjacent mainland to have similar species, but the habitats in
the two areas are quite different – wet-tropical on the mainland, with high
mountains nearby, as opposed to dry and maritime, with volcanic lava soil on
the islands. Certainly in Darwin’s time no one expected that there should be
species-similarity in the face of habitat difference.
Darwin also couldn’t help noticing that the islands
were very young: some islands were well-vegetated, but several were essentially
bare-rock, bleak and inhospitable, and some were recently volcanically active.
Other volcanic islands were known to have arisen, and some studies had been
made of their colonization by organisms, even at that time. (We know now that
this island chain is no older than about seven million years.) It was his peculiar
cleverness and ability to combine information that led Darwin to realize that
he could account for the similar-yet-different organisms and the newness
of the islands with a single theory – that early South American colonists had
flown (birds, insects), or drifted (vegetation, lizards on floating trees), or
floated (salt-tolerant seeds), or been blown by winds or storms (small seeds,
spores) to the islands, then once isolated there in conditions different from
those they had faced before, they changed to better match the new conditions.
Other theorists like Lamarck had suggested that organisms changed to suit local
conditions, but the mechanisms claimed to produce the effect were more like
learning than what we today think of as evolutionary change; Darwin eventually
found a better mechanism.
[Oddly it was only several years after he returned
to England that Darwin noticed the dramatic differences among islands,
for example in the finches and their beak morphology – when collecting, he had
thrown all the finch specimens into a box labelled “finches, Galapagos”! Later
work, extending over 150 years and ongoing today, has developed the interisland
differences into a very detailed set of case studies, using genetic methods as
well as ecological data to tease out the history of diversification.]
Of course it’s difficult for us today to be sure
about why it was Darwin, rather than someone else, who developed the mechanism
we refer to as “natural selection”, and used it to account for speciation.
Probably one reason was that he maintained a very wide and diverse set of correspondents
while working for twenty years on his “big book”, the vast volume in which he
planned to lay out, beyond any doubt, the grand theory he envisioned (and the “short
version” of which, eventually, was published in 1859 as On the Origin of
Species). Scientists of Darwin’s time – invariably men, of the “upper”
classes in society – often wrote each other letters and traded specimens, and
they also kept in touch with collectors working all over the world (this is how
Darwin came into contact and later collaboration with Alfred Russel Wallace),
but Darwin went further: he gained valuable insights from farmers and
pigeon-breeders, people of “lower” classes, with whom many scientists would
have had no interest speaking. He found that breeders had been able to create
new types of plants and animals by allowing only certain individuals to reproduce
in each generation (and neutering others), and he knew that at first a breeder wishing
to produce a certain characteristic had to be satisfied with rudimentary traits,
and only later could expect fully developed ones. These processes of selection
imposed by people Darwin termed artificial selection, since they were intended to serve
human ends of commerce or aesthetics. He realized that if only certain individual
organisms in a population (what we would call today particular genotypes)
were successful in a given environment, the composition of a population could
change as a result of the differential success of the types, a process
analogous to artificial selection but in this case occasioned by advantages
accruing to the organisms themselves, so he decided to call it natural selection.
Members of a species or population which found themselves either isolated in a
new area, or still in the original area but experiencing some change in the
local conditions, might be required to change (or to die out), and new species
could arise if only enough change could accumulate before too many of the
individuals died.
A remarkable note about the robustness of natural
selection as an explanation for speciation is that it was taken to be a
workable theory even though there were two key things that
were unknown in Darwin’s time: (a) how inheritance worked, and (b)
how much time might be required for new species to appear.
Everyone agreed, even long before Darwin’s time,
that some traits were inherited with varying degrees of fidelity, that some
traits “ran in families”, and that there were predictable outcomes from
breeding certain individuals, but none of this was codified or understood quantitatively.
[Mendel was working on his pea experiments during Darwin’s lifetime, but
unfortunately none of his data ever came Darwin’s way.] Obviously speciation by
Darwin’s mechanism would work better with some modes of inheritance than others,
and it would have been easier for Darwin to make a case for his theory of speciation
if the role of interrupted gene-flow had been clear, but even the small amount understood
about inheritance made the theory plausible. Everything we have learned about
inheritance since Darwin – Mendel’s work, the early fruit-fly research of T.H.
Morgan, establishment of the structure and function of DNA by Watson and Crick
and co-workers, and the full range of modern techniques – vindicates the early
acceptance of the theory.
As to the time required for speciation, when Darwin
was working, most scientists believed (rightly, on the basis of the data they
had) that the age of the Earth was only 200-300 million years. Darwin had been
concerned throughout his research that any mechanism for speciation would have
to work quickly, perhaps not as fast as the consciously-directed artificial
selection, but fast enough to make the biodiversity known to exist seem possible.
We know today that a few hundred million years would not have been enough time
for everything to have evolved, and until the mid-20th century –
when the age of the Earth was pushed back with certainty to 4.5 billion years,
some 15-20 times longer than Darwin’s available scale – the time needed for
evolution was indeed a sticking-point. Even then, the mechanism was so simple
and so compelling that most scientists were prepared to accept it.
In order for a new species to come into existence,
it is necessary for a new
separate gene-pool to be formed. The only way that distinct
separate gene-pools can arise is through lack of gene-transfer, or as it is
often termed a barrier to gene-flow, between the incipient pools. This can
happen in some cases when a small number of founders colonizes a new
site, and no further input of individuals from the original population follows
them. The founders are likely to contain a set of genes unlike the total
present in the original population, and then finding themselves in new
conditions they are likely to continue to change in ways making them more
distinct – the result will often be swift genetic incompatability of the
groups. This is a form of peripatric
speciation (“near the original area”).
It also often happens that a population may become
subdivided, but into two or more large subunits rather than a main mass
staying put and a small group of colonizers going elsewhere. Subdivision could
occur, for instance, if a river changed its course and separated parts of a formerly
continuous range. Even if conditions in each area remained similar to the
original conditions, there could still be random genetic changes in each
area occurring independently of changes in the other areas, resulting in a
differentiation by a form of genetic
drift. [If the same changes had occurred in the continuous
range, they would have become diluted throughout the complete large population,
or eliminated as rare mutants often are in a large population.] If
conditions in the newly divided areas were to differ, then in addition to
random drift changes there would likely be fitness-determining changes
favouring different genotypes in each area, and engendering even more rapid
diversification. Both cases suggested here are examples of allopatric speciation (“happening
in separate areas”, of which peripatric speciation, mentioned previously, is a
subtype).
In plants, which have a marked tendency to
spontaneously double their chromosome count, it is possible for individuals
which undergo the doubling to become reproductively incompatible with their un-doubled
neighbours instantly – the chromosome-pairs cannot successfully combine
in potential hybrid individuals, leading to isolation due to a lack of introgression.
This formation of a new isolated population can happen even in the midst of the
parent population – a form of sympatric
speciation (“happening in the same area, together”), a process
which is otherwise difficult to achieve because barriers to gene flow are not
likely to arise spontaneously. There is considerable dispute
about whether sympatric speciation accounts for animal speciation at all.
One other way speciation can occur is by the partial
functional isolation of marginal individuals in a population. Consider what
conditions are like at or very near the extreme edge of a species’ range: only
the toughest (or maybe the luckiest) genotypes can survive, and they are existing
right up against the theoretical niche limits for their species. When they
breed, they are likely to breed almost entirely with their near-neighbours
experiencing the same conditions, and this will tend to “concentrate” the
marginal-success genes in those areas. (Individuals from the typical, central
genotypes of the population won’t go to the edge to find mates, and therefore
won’t dilute the edge genotypes with others.) If the conditions in the marginal
area are consistently awful for long enough, the marginal sets of individuals
may become genetically quite distinct from the rest of the population, even
though they are theoretically still continuously distributed alongside them
– this mode of speciation is termed parapatric speciation (“beside the original
area”), or sometimes peripheral
isolate speciation. Because marginal conditions place extremely
strong selection pressures on organisms, this may be an important kind of
speciation, but it can be difficult to demonstrate.