Evolution
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Any gradual directional change; now most commonly used to refer to the
cumulative changes in the characteristics of populations of organisms from
generation to generation. Evolution occurs by the fixation of changes
(mutations) in the structure of the genetic material, and the passing on
of these changes from ancestor to descendant. It is well demonstrated over
geological time by the sequence of organisms preserved in the fossil record.
There are two opposing schools of thought regarding the pattern and tempo
of evolution.
The gradualist school is based on a model of evolution in
which species change gradually through time by slow directional change within
a lineage, producing a long graded series of differing forms. The punctuated
equilibria school is based on a model in which species are relatively stable
and long-lived in geological time, and that new species appear during outbursts
of rapid speciation, followed by the differential success of certain of the
newly formed species.
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while some people feel uncomfortable with the idea of evolution
for religious reasons, every single observation about the nature
of life on earth is consistent with evolution having
happened. The opponents attempt to gain comfort from the way in which
evalutianists sometimes disagree about some of the finer points about how
evalutian happened, but they can only do so because they have lost sight
of one key fact: every biologist in the world agrees that evalutian took
place, but most biologists are willing to speculate on how evalutian took
place, and to disagree with other biologists' speculations. we know it happened,
we just don't know exactly how.
The fossil evidence
This is often depicted incorrectly as the only evidence for evolution.
and it is certainly
convincing, but it is only the start. Fossil horses may be found with several
toes, while modern horses have a single toe in their hoof, missing links
can be found, like the bird reptile Archacoptryx, the
amphibian Ichthyostega, the lobefish, the early members of the genus
Homo, which walked upright, yet had smaller brains and limited cultures,
or the seed fern.
The most important evidence from fossils comes when the order of their formation
is examined. The science of geology has developed to the extent that we can
order the rocks of the world quite accurately from their fossils. Whenever
two fossils appear in the same sequence of rocks, the lower rock always has
the more primitive of the two fossils. The estimated dates of rocks that
were made in the past century can be independently checked by a variety of
dating methods, and while the scales may change as we realize the earth is
older than we ever imagined, the order of the rocks, and the order of the
fossils has always stayed the same.
Biogeography
The distribution of animal and plant groups around the world is only explainable
if you assume that similar species have evolved from a common ancestor. Before
we understood plate tectonics, a number of the Gondwanan distributions were
confusing, but now we can understand exactly why a marsupiial will be found
only in the Americas and Australia, which were once both joined to Antarctica
as part of Gondwanaland. It also explains why it is possible to find the
teeth of extinct ancestors of the platypus of Australia in South America.
Case studies
There are a number of case studies which, while not demonstrating the formation
of new species, show remarkable selection. Moths in an industrial part of
Britain changing their colour as pollution darkens the bark of trees in their
habitat, favouring a dark-winged form over a light-winged form, and many
case studies, some of them described by Darwin himself, of artificial selection
by farmers and animal breeders.
These studies do not show complete evolution, but they show how the appearance
of a species can be hugely changed, given some form of selection. If these
changes can happen in an observable period, runs the argument, think what
might happen in a longer period. while this is not good evidence for
evolution, it is a useful corroboration.
Convergent evolution
One of the basic arguments of evolution is that natural selection will shape
the species so that it is well-adapted to fit into its environment. When
similar environments exist in different parts of the world, similar-looking
species will evolve to fill an available niche. The marsupial 'moles', 'cats'
and 'mice' of Australia are all seen as good examples of similar conditions
producing similar species. The American possums are similar in appearance
to the Australian possums, even though the Australian possums are more closely
related to kangaroos, koalas or wombats, based on blood biochemistry.
Once again, this is not proof of evolution, but it is a clear
indication of the power of natural selection.
Adaptive radiation
In a new environment, such as a colonized island, new species are able to
develop from a single pair of colonists, as some members specialize. The
classic example here is found in the finches that flarwin observed and studied
in the Galapagos Islands, known ever since as
Darwin's
finches.
Curiosities
Even some of the stranger aspects of humanity are easily explained by looking
at the ways in which evolution can take place. In particular,
humans are much more similar to baby chimpanzees than they are to adult
chimpanzees. we mature less than the other primates, and it turns out that
we humans are paedomorphic apes: we have gained a
great deal by holding off, from remaining immature, from putting off what
any ape would regard as true adulthood.
Anatomy
The pentadactyl limb of the vertebrates is just one example of the similarities
that can be seen throughout the animal kingdom. The equivalent bones are
found in equivalent places, sometimes with a reduction, as in the case of
the horse's hoof, now based on a single 'toe', but that bone is clearly the
same as the equivalent bone in an elephant or a fossilized giant kangaroo.
The shape we are all in is a result of inheritance, and some features are
either neutral in evolution, or blind alleys that we are stuck with. Men
suffer inguinal hernias because their lower belly muscles are better designed
for walking on all fours, and baby kangaroos are forced to be born when they
are very small, simply because their mothers have a birth canal structure
which only allows a very small baby to pass through.
Most animals show vestiges of one form or another, traces of characters that
their ancestors held, but they have almost lost.
Humans
have a tiny caecum called the appendix, a remnant of something our plant-eating
ancestors needed, and we also have a remnant tail bone. Horses have remnants
of the toes that have disappeared from their feet, and some of the whales
and dolphins have tiny remnants of their pelvises and hind limbs, leftover
from the days when their ancestors lived on land.
Biochemistry
Time and time again, biochemists have studied the amino acids in key proteins,
like cytochrome c, and found that similar species have similar proteins,
while less similar (and presumably more distantly related) individuals have
more differences in the proteins. These studies also throw us a few surprises,
findings that may cause us to question some of our assumptions about
classification. They do not make us question evolution itself, because
classification is no more than a handy way of storing information.
For example, turtles are more similar to the birds in their cytochrome c
than they are to a rattlesnake. Biologists usually group the turtle and the
rattle-snake together as reptiles, because they are both cold-blooded, lay
eggs, and have scales, but birds lay eggs and have scaly legs. Maybe we should
take more notice of those features than we do of warm-bloodedness.
It is worth noting that biochemical evidence tells us useful things. For
example, the duck, chicken and turkey are all closer to each other than any
of them is to the turtle. The marsupials of Australia, including the Australian
possums, are all closer to each other than they are to the similar-looking
opossums of the Americas, which must have been evolving separately since
Gondwanaland broke up. Once again, most of what we find is entirely consistent
with evolution, and the rest is not inconsistent with evolution.
In recent times, with more and more genomes and DNA sequences being
worked out, we have clear evidence that the species which look similar turn
out to be similar in their DNA as well-a chimpanzee and a human have 98.4%
of their DNA in common. The pygmy chimpanzee and the common chimpanzee are
99.3% the same, while gorillas are about 97.7% the same as humans or either
of the chimpanzees.
Written by Peter Macinns
Fallacies
about evolution
(September 1997)
There are two
standard
fallacies about evolution: the first is that small is always replaced
by large as evolution proceeds, and the second is that every step in evolution
goes forward.
The second fallacy is easier to deal with, since if life forms always improved,
they would never go extinct. The first fallacy was given to us by a 19th
century American fossil- hunter, Edward Drinker Cope in a proposition eponymously
known as Cope's Rule: over time, the average body size within a genus of
animals will tend to enlarge. Now we have evidence that Cope's rule has been
flouted by a lowly mollusc. In a paper in
Nature this month, David Jablonski
reports that molluscs have shrunk in size through the aeons.
Jablonski studied almost 1100 species of clams, oysters, and snails from
the Gulf and Atlantic coastal Plain of North America that evolved within
191 genera over 16 million years. One-third of the genera did indeed get
bigger; but one-third got smaller. Smaller is better, it seems, when it is
advantageous to reproduce early and often.
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