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Why Evolution is True
Jerry A Coyne
Professor in the Dept of Ecology and Evolution
University of Chicago
“On The Origin of Species” by Charles Darwin was
published in 1859. It introduced the theory of evolution by
natural selection. Although hotly disputed at the time, the
weight of evidence found in its favour thereafter
subsequently persuaded the scientific community of its
validity, a mere 10 years after its publication. So why
does it still provoke public controversy in contrast to any
other widely-held scientific theory e.g. the theory of
gravity, the germ theory of causation of some diseases.
Evolution teaches that we are related to all other living
creatures and are as much a product of blind and
impersonal evolutionary forces as all other living things.
This doesn’t sit well beside the teachings of certain
religions that God created all living things and that
humans are a special case, created in his image.
Professor Coyne shows how the relatively new idea of
“intelligent design” is just creationism re-badged. The
central fight is between creationism and evolution. In the
UK, a 2006 poll by the BBC asked 2000 people to
describe their views on how life is formed and developed.
48% accepted evolution, 39% went for creationism or
intelligent design and 13% didn’t know. The idea of the
book is to document the weight of evidence in favour of
evolution.
The author seems to think that people don’t accept
evolution because it is described as “just a theory” so he
goes to great lengths to show that “theory” has a different
meaning in the scientific world than it does in everyday
parlance. A scientific theory is a statement of what are
held to be the general laws, principles, or causes of
something known or observed. It must be testable and
make verifiable predictions. This is all well and good but
he insists on saying that evolution is true (the title of the
book and repeated in the text many times) so when he
then comes out with - despite thousands of observations
supporting Darwin, new data might show it to be wrong –
scientists must be open to this possibility – he paves the
way for non-scientists to be confused and creationists to
crow. The title “evolution is true” is misleading because
he is talking about a scientific “truth” rather than using the
more common definition of truth as absolute. He should
have defined “true” as well as “theory”. I think you can say
– the theory of evolution by natural selection fits all the
observations we have made so far. Nothing we have
found out scientifically disproves the theory. Creationism
has no such weight of evidence behind it.
So what is the modern theory of evolution?
Life on Earth evolved gradually beginning with one
primitive species – perhaps a self-replicating molecule –
that lived more than 3.5 billion years ago; it then branched
out over time, throwing off many new and diverse species
and the mechanism for most, if not all, of evolutionary
change is natural selection.
This consists of six components: evolution, gradualism,
speciation, common ancestry, natural selection and
nonselective mechanisms of evolutionary change.
1. Evolution – genetic changes over time due to
mutations in the DNA (Darwin didn’t know about
DNA but its discovery explained the mechanism
2.
3.
4.
5.
6.
whereby evolution operates.
Species don’t all
evolve at the same rate.
Gradualism – it takes many generations to produce
a substantial evolutionary change.
Speciation – splitting of one species into two
Common ancestry
Natural selection – a tinkerer rather than a master
engineer
Genetic drift – caused by different families having
different numbers of progeny (i.e. evolution other
than by natural selection)
We can test these tenets of evolutionary theory in several
ways – we should find
 evidence of evolutionary change in the fossil
record
 evidence of one species dividing into two in the
fossil record
 examples of species that link together the major
groups thought to have common ancestry e.g.
birds with reptiles, fish with amphibians
 genetic variation for many traits
 examples of imperfect design
 natural selection acting in the wild
Chapter 2 goes through a fair amount of fossil evidence
showing the evidence for evolution through this medium
and how we know how old fossils are – including
radioisotope dating and other techniques.
Chapter 3 documents the vestigial organs and why their
presence is good evidence for evolution. Vestigial organs
no longer provide the function for which they evolved – for
example, ostriches have wings because they evolved from
birds that could – the ostrich uses its wings to balance
itself and see off enemies, not to fly. Humans have an
appendix (vestigial from when our ancestors ate leaves), a
coccyx (vestigial tail) or goose bumps (vestigial muscles at
the base of each hair which evolved to make fur stand on
end for insulation if its cold or to make the animal look
bigger to frighten off its enemies).
Atavisms are
anomalies that occur in a particular individual that look like
an ancestral trait – e.g. a horse born with extra toes, a
human baby with a tail. They are thought to be the reexpression of genes that were functional in ancestors but
silenced by natural selection. The information is still in the
genes so that when something goes awry in normal
development it allows the ancestral trait to be expressed.
Atavisms and vestigial traits show us that when a trait is
no longer use, or becomes reduced, the genes that make
it don’t instantly disappear from the genome: evolution
stops their action by inactivating them, not by snipping
them out of the DNA. From this we can make a prediction.
We expect to find, in the genomes of many species,
silenced or “dead” genes: genes that once were very
useful but are no longer intact or expressed. In other
words, there should be vestigial genes. In contrast, the
idea that all species were created from scratch predicts
that no such genes would exist, since there would be no
common ancestors in which those genes were active.
The function of a gene is to make a protein – a protein
whose sequence of amino acids is determined by the
sequence of nucleotide bases that make up the DNA.
Humans have about 30,000 genes and 2000 or so are
pseudogenes – genes that don’t function. These dead
genes always point to a gene that was active in one of our
ancestors. Evolution can encompass these facts but they
don’t make any sense if you think that life was created by
intelligent design.
All vertebrates begin development as an embryo in the
same way looking rather like an embryonic fish. Darwin
realised that this was due to the common parent-form of
each great class of animals.
Why does the human embryo appear to go through many
stages of its evolution from primitive life forms on its way
to becoming a fetus? The probable answer is that
development is a very conservative process. Many
structures that appear later require biochemical cues from
structures formed earlier so if you try to tinker with
embryonic development to streamline the process, you
might produce all sorts of adverse side effects. So the
embryo doesn’t actually replay all the adult forms of its
evolutionary history but it does go through many of the
embryonic forms of its evolutionary history. Other stages
of human development harking back to our ancestors –
the downy hair of the fetus shed about a month before
birth – the lanugo – and the grasping reflex of young
babies which they share with newborn monkeys and apes
but babies lose the ability after a few months.
Examples of poor design – asymmetrical nature of flat
fish, the recurrent laryngeal nerve, the urethra exiting
through the penis in the male, the birth canal exiting
through the pelvis in the female. These are all explained
neatly by evolution – an intelligent designer would have
had to be having a laugh or trying to fool people into
believing in evolution if he designed these features!
Chapter 4
The geography of life – this documents the evidence for
evolution from biogeography or the distribution of species
throughout the world. Small remote islands with no native
species of amphibian, reptile or mammal are home to a
larger collection of rare, exotic species than that found
anyhere else in the world. The evidence points to these
animals landing on the island thousands of years before
and evolving in unusual ways because of the very different
environment they found themselves in. Jerry Coyne says
that he has never seen a creationist book, article or
lecture that has tried to refute the biogeographic evidence
for evolution.
On page 97, he talks about the molecular clock for
evolutionary change and says that, “.. as species diverge
from their common ancestors, their DNA sequences
change in roughly a straight-line fashion with time.” This
is poorly worded – I had to look up Wikipedia to
understand what he was saying and it seems at first sight
to be at odds with the view that species may not evolve at
all for thousands of years – as he said on page 4. I think I
understand but he should have made this clearer.
If creation was true, why would the creator produce on
different continents but very similar environments,
fundamentally different plants or animals that look and act
very much alike e.g. cacti and euphorbs? If transplanted
into the other’s environment, they both do well. Another
example is (the placental) rabbits in Europe and the native
Australian marsupial, the bilby, which look similar – this is
a case of convergent evolution.