The Evidence for Evolution
In order to demonstrate Evolution in the sense of Darwin, three points need to be shown:
1. Species can go extinct (branches in Darwin’s tree can end).
2. Species can change over time (the essence of any form of evolution).
3. Species trace back to a common ancestor (the essence of Darwinian evolution).
There are three main lines of evidence for these three points, all used by Darwin:
1) Fossil record, 2) Embryology & Anatomy, and 3) Biogeography.
Let us now come to the arguments for the above points.
I. Extinction of Species
Clearly, the main source of evidence for the fact that species can go extinct comes from the
fossil record. Actually, Darwin did not have to show this part because George Cuvier, who
was the founder of scientific paleontology and of comparative anatomy, had already given
convincing evidence. Cuvier established that many fossils clearly did not belong to any extant
species known at the time. There was some speculation about whether these animals and
plants could have been overlooked or taken refuge in remote parts of the world, but after
fossils of numerous gigantic forms had been found that could hardly be overlooked extinction
was commonly accepted.
However, Darwinism brought a new view on why species go extinct. Cuvier and his followers
believed in external catastrophic events as the course. Some (not the scientifically minded
Cuvier) assumed that the Biblical Flood could have been the last major extinction event.
Darwin’s view of a struggle for survival offers a different explanation: there is no external
catastrophe needed, population growth and competition are “bad enough”.Interestingly, the
catastrophe theory has experienced a sort of revival in recent years, however mainly for a few
mass extinction events (Permian extinction, dinosaurs). Of course, today we have the most
direct evidence of extinction as we observe the extinction of many species in our lifetime as a
result of the “human catastrophe”.
II. Change of Species
That species change over time was already Lamarck’s point and he gave some good
arguments for it. However, since nobody believed in Lamarck (partly because of Cuvier’s
fierce opposition), Darwin still had to convince his contemporaries. Evidence comes from
directly observable changes and, again, from the fossil record.
Sometimes, the change is fast enough to be observed in human history or even a lifetime.
Human domestication is an obvious example that was discussed by both, Lamarck and
Darwin. The phenotypic change that can be produced by breeding is staggering, a for
example the variety of dogs show. Darwin collected pigeons and demonstrated that according
to the taxonomic standards of his time they were sufficiently different to make “three good
genera and about fifteen species”.
Striking evidence comes, again, from the fossil record. There are often striking similarities in
the fossils found in a certain region with the living species in the same area (e.g., Australian
fossil mammals are mostly marsupial, living and extinct sloths are only found in Central and
South America, etc). The more ancient a form is, the more it differs from the living forms. The
pattern is so widespread that it had been known as the “law of succession” even before
Darwin, who took this “this wonderful relationship in the same continent between the dead
and the living” as important evidence for evolution.
The same holds true for differences among fossils: The strata where fossils are found are
connected by a chain: each is more similar to the adjacent. The fossil record also shows the
origin of major new taxa. E.g., reptiles _ mammals.
Paleontological characters to distinguishing reptiles and mammals:
Character
jaw
teeth
brain case
nostril openings in skull
locomotion
Reptiles
7 bones
simple, undifferentiated
small
2
waddling
Mammals
1 bone
complex, differentiated
large
1
forward, erect
In old strata, only reptilian characters found. In new strata, only mammalian characters found.
But no sudden jump: In intermediate strata (in the late Permian, 250 – 300 million years ago),
all kinds of intermediates are found; the characters change gradually and one by one.
Phenotypic plasticity
There is one remark of caution needed: Not all change of the
phenotype through time is automatically evolution. The change needs to be genetic (i.e.
heritable). Since the phenotype is also affected by the environment, phenotypic changes can
also result from environmental changes. In fact, this is often the case: In good years, plants
grow to a large size, in bad years they stay small. This phenomenon is called phenotypic (or
developmental) plasticity. In many cases, however, a pure environmental effect can be ruled
out. E.g. because the environment did not change (e.g. among the generations of a breeding
stock), or because environmental shocks applied today do not cause similar changes.
III. The Common Ancestor
Darwin’s novel element for the theory of evolution was his claim that all species trace back to
a common ancestor. The main evidence for this claim comes from striking similarities in the
morphology and organs among living species. Today, after Darwin, we distinguish two kinds
of these similarities:
Homologies refer to characters that are shared across species due to their common descent.
Homologies appear at any level of organismal organization, from molecular and genetic
(e.g. highly conserved developmental genes) to morphologic and behavioral (e.g. the
pentadactyl (five-digit) ground state of the tetrapod limb).
Analogies are similarities that are not due to common ancestry, but result from convergent
evolution if similar environmental conditions favor similar adaptive solutions (e.g. wings of
birds and bats or the placental and marsupial carnivores).
Of course, these similarities had been recognized since ancient times. At Darwin’s time there
was a heated debate about the cause for these similarities, which was indeed considered the
most important scientific question of the time. The two schools before Darwin were:
1. Functionalism The functionalists were led by (once more …) George Cuvier. Their claim
was that all species were optimally designed by the Creator. Their anatomy therefore
follows from the special problems they face in their everyday lives: Form follows function.
In today’s language they thus thought that all similarities are analogies.
2. Formalism The formalists were led by Geoffroy St-Hilaire, who was a friend of Lamarck.
They believed that the morphology of all species was based on common body plans
(Baupläne). The most famous of these body plans were the Owen’s Archetype and
Goethe’s Urpflanze. Although many changes are possible on top of these plans, every
species still carries many “reminders” of it. They thus explicitly did not believe in analogies,
but in “homologies” in the sense of common body plans.
The case against functionalism
Functionalism is a good theory to explain the “perfect” design (perfect fit between an organism and its environment) that is observed in many traits. In order to make the case against
functionalism, Darwin was therefore more interested in certain maladaptations and vestigial
organs that do not allow for a functional explanation and would not be expected under
creation by an omniscient designer.
Maladaptations are often legacies of evolutionary history. A nice example of maladaptive
anatomical design (not discussed by Darwin) is the vertebrate eye:
The vertebrate eye is often held up as evidence against evolution: how could natural
selection ever produce such a perfect structure? But: It is not perfect. There is a
serious design flaw that no engineer would allow: The retina is upside down. As a
consequence, light must pass through four different kinds of neurons and capillaries
before finding light sensitve cells. Since the optic nerve fibers are on the inside of the
eye, the nerve bundle must dive through the retina. Since there can be no light
sensitive cells at this point, the eye has a blind spot.
Vestigial organs are apparently useless structures left in adult. Examples include:
1) Whales have a pelvis, but no hind limbs. It just sits there with no apparent function
2) Boas and pythons have vestigial legs, Ilium (hip) and femur (thigh)
3) Cave organisms lose eyes to varying extents. Some have lenses, but no optic nerves,
some have nerves, but no lenses.
4) Humans:
• Middle ear bones: remnants of reptilian jaw.
• Appendix: Used by guinea pig, horse for bacterial digestion.
• Muscles to move ears; muscles to move tail.
Similar evidence comes from embryology. Darwin believed that embryology provided the best
evidence against special creation: under Creation, we expect organism to take a direct route
from embryo to the adult. Instead, the embryo goes through a variety of stages, many of
which show homologies between different species, even if they are no longer seen in the fully
developed animal.
E.g. bird limbs: Although birds are tetrapods and tetrapods all have five-digit limbs, an
adult bird appears to have a three-digit limb in its wings. In birds' embryos, however,
one finds that this limb develops from a five-digit precursor. Another example is teeth in
some toothless whales. Some toothless whales develop teeth as embryos that are
absorbed later in embryonic development. In development, mammals go through a
series of developmental steps that mimic the adult stages of other taxa. Specifically,
development proceeds from “primitive” adult forms to “advanced” adult forms. This is
the famous pattern captured in Haeckel's biogenetic law (or theory of recapitulation):
“Ontogeny recapitulates phylogeny.”
Embryology thus provides a source of evidence that is very hard to reconcile with a
functionalist view: Why should a toothless whale develop teeth that are later absorbed? Why
should organisms that are very different as adults have so many similarities as embryos? If
life forms developed independently, one would think that their embryonic development would
be distinct and reflect what the organism will look like when it is fully developed.
The evolutionary answer is that evolution is conservative - it makes use of what is there
before. From the point of view of a natural process with limited resources, developing
something new is much more difficult than modifying what already exists. The embryological
similarities are explainable by common ancestry. Whales develop teeth embryonically
because they evolved from ancestors that had teeth. Birds develop their three-digit limbs as
embryos from five-digit limbs because they evolved from five-digited ancestors. Such
development makes sense in the historic light of evolution.
The case against formalism
The main criticism of formalism, even prior to Darwin, has been twofold. First, it has problems
to explain adaptations. And second, most importantly, the whole concept of archetypes and
universal body plans (while relatively flexible) was more mystical than scientific. In his theory
of evolution as a branching process, Darwin turned the archetype into the ancestor. This way,
he was able to give the concept of homology (which was formally defined through body plans)
the scientific basis it needed. On top of this, he was able to explain a number of further
observations that neither of the former theories could account for. The most important
evidence comes from biogeography.
Biogeographical evidence
“While on board HMS (“Her Majesty’s Ship”) Beagle, as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of S. America, and in the geological relations
of the present to the past of the inhabitants of that continent. These facts seemed to me to
throw some light on the origin of species - that mystery of mysteries”
What struck Darwin was the fact that species in South America were more similar among
each other than to Old Word species, even if they lived in environments that were much more
similar across the continents (e.g., in contrast to most Old World monkeys, New World
monkeys have a tail). A particularly striking pattern is the distribution of organisms on volcanic
islands that have never been part of a continent, such as the Galapagos Islands.
1) Oceanic islands have very few “kinds” of organisms. E.g., Galapagos archipelago has very
few species of birds (in comparison with the hundreds of species in the continent), New
Zealand has no mammals (except those introduced by man, e.g., goat, cow).
2) Particular groups of organisms are present in spectacular numbers. E.g., although few
species of birds are present on Galapagos (26), 21 of these are endemic (occur there and
nowhere else) and 14 of these 21 are finches. This is a very unusual distribution, not
known anywhere else (unbalanced faunas).
3) If we take a look where the most similar animals and plants to the endemic species on
island are, the answer is: On the nearest Mainland (Galapagos and South America, over
750 km apart). This is not explained by similarity of environments: The Galapagos Islands
(volcanic rock) look very different from South America coast (lush jungle).
4) If organisms found on volcanic islands are compared to the ones that are missing, there is
a very striking difference
Found
Plants
Insects
Birds
Missing (Some exceptions of course, e.g. tortoises)
amphibians
reptiles
freshwater fish
Mammals (except bats)
Darwin wonders: Why would the Creator decide to put amphibians, reptiles, and mammals
everywhere except on oceanic islands? He asks if there might be an evolutionary explanation. He made the point that the main difference between the two lists is one between good
and bad dispersers. It is easy to blow bats across water masses in storms, but hard to blow
elephants, wolves, or even mice. For plants, Darwin pointed out that seeds and fruits often
swept out to sea and did some experiments himself to demonstrate that the seeds of many
species are still able to germinate even after being kept in salt water for a long time.
A single common ancestor to all of life?
Darwin himself did not claim that all living species would necessarily trace back to a single
common ancestor. He suggested it could have been several independent ancestors, e.g. one
for each of the four main branches of animals that had been identified at this time. Today,
however, it is commonly assumed that all life indeed had a single origin. The strongest
evidence for this claim is the universal genetic code: It is possible to take isolated hemoglobin
mRNA from a rabbit, inject it into E. coli (which doesn’t have the gene for hemoglobin and
thus doesn’t make it), and have the E. coli make hemoglobin. Why is the code universal?
There explanations are possible: Chemical constraints or historic contingency.
The universality could result from chemical constraints (i.e. each triplet has some chemical
affinity with its amino acid). However, there has been no evidence for such a constraint and a
number of facts make this theory unlikely (e.g. the anticodon and amino acid attaching sites
are on opposite ends of the tRNA). Most people think that the original choice of the code was
just an historic accident. Current theoretical work suggests that code evolved early on in order
to reduce the number of harmful amino acid substitutions that occur by mutation. Once it had
evolved, however, it would be strongly maintained and any deviation from the code lethal.