Continuing the Journey
Beyond the synthesis
Review
• The modern synthesis reached ‘totality’ with the
inclusion of developmental studies, taxonomy,
field studies of natural selection and genetics
and, finally, the application of data on variation
and selection to the fossil record.
• The result was an integrated science of biology,
with the synthetic theory of evolution at its core.
• Hence Dobzhansky’s famous remark, nothing in
biology makes sense except in the light of
evolution.
Further tests of the synthesis
• But biology (of course) doesn’t just stop there.
• The synthesis was well established by, say, 1950– but
there was still plenty of work to be done.
• Some of this work extended and refined our knowledge
of established fields in biology.
• But some of it created entirely new fields of study, some
of which are still emerging today (evo-devo, for
example).
• In sum, this work has contributed immensely to the detail
and to the range of evidence supporting evolution by
natural selection.
DNA and RNA
• The central new discovery was the structure of DNA in
1953, which Watson and Crick realized allowed this
molecule to store information and to reproduce itself.
• The chemical basis of ‘genes’ had finally been identified
in the form of nucleic acids, DNA and RNA.
• Most organisms use the (more stable) DNA to store
genetic information, but they translate DNA information
into RNA before building proteins according to the
instructions of the genetic code.
• The code itself took about 10 years to unravel– it is a
highly redundant code that relates triples of RNA bases
to particular amino acids (and sometimes stops and start
‘commands’ for transcription); the resulting sequence of
amino acids is the ‘target’ protein that gene codes for.
Molecular biology
• This discovery and the chemical techniques that
rapidly emerged as scientists studied DNA,
RNA, protein manufacture and how to determine
various properties of these molecules led to an
explosive growth in knowledge of biochemistry,
creating the field we call molecular biology.
• Among other things, this chemical
understanding allowed us to see how variation
arises in the genome, as various kinds of
copying errors and damage alter the DNA that
cells pass along to their progeny.
Techniques
• Over time methods for sequencing DNA,
sequencing proteins, amplifying small
samples of DNA into large quantities of
similar DNA, measuring which genes are
actively producing proteins in a cell at a
given time, inserting new genes into the
genome, amplifying the expression of
certain genes and more were developed.
The Tree, again
• Molecular data provides us with yet another way
to study the relations (similarities and
differences) between different organisms– that
is, another way to do taxonomy.
• Molecular taxonomy gives us the same tree of
relations we find in taxonomy, the fossil record
and development.
• The closer two organisms are to a common
ancestor, the more biochemically similar they
are.
More
• Further, there is something striking about the
DNA code itself.
• First, it is as arbitrary as language– there is
nothing about a particular sequence of bases
that makes it indicate one amino acid instead of
another.
• It’s only because of the apparatus for building
proteins that a particular sequence of bases
specified the particular amino acid that it does.
Change the apparatus and you could build the
same protein from a very different sequence.
All life is related
• But the code is the same across the entire biological
world (there are some very slight differences between
DNA code in our mitochondria, which still retain some
DNA of their own, and the code in the nuclei of our cells).
• This is both unnecessary, biochemically, and unlikely
unless life really does share an ancestor.
• Given a single ancestor, any substantial change in an
established code seems to be a recipe for disaster.
• Hence the strong conservation of this code (and of other
basic structures– cytochrome c for instance, and
hemoglobin too– proteins that vary, but in a very
constrained way since they are essential to life as we
know it).
Genetic comparisons
• Being able to sequence DNA (it looked impossible at first, but it
turned out to be possible with the help of restriction enzymes and
gel electrophoresis) led to direct genetic comparisons with different
species, including
• Our 98%+ genetic match (based first on DNA hybridization
measures) with chimpanzees. (Closer than horses and zebras!)
• This reinforced the immunological/antibody test by Sarich and
Wilson (214).
• DNA change can also be used as a rough sort of clock, since many
differences accumulate fairly randomly, at a fairly steady rate over
time.
• The result put the split between humans and chimpanzees at about
5-6 million years.
• Later work applied DNA sequencing directly, confirming and refining
these results.
Fossils again
• Diarthrognathus (and Probainognathus) and the reptilemammal transition.
• Australopithecines and human ancestry (teeth
comparisons linking them to us rather than to the apes;
also upright walkers, as hip morphology, occipital
condyles and the position of the foramen magnum
showed).
• Pithecanthropus erectus becomes Homo erectus (work
done by Mayr).
• Neck down, these are human (a bit robust…); heads are
different though– long, low skulls with 2/3 our cranial
capacity, heavy jaws & teeth, eyebrow ridges…A good
intermediate, both in character and time.
Other finds
• Robust australopithecines– the bushy tree of
hominid evolution…
• Homo habilis– a transitional form between
Australopithecines and Homo (smaller brain
size, more primitive skull morphology, but
indications of tool use). See also rudolfensis.
• Pushing back the dates (australopithecines go
back nearly 4 million years now; Lucy 3.2).
• Turkana boy: erectus ~ 1.6 million years ago. 6
feet tall at adulthood… a scary creature using
stone tools and hunting on the plains of Africa!
More field studies of natural
selection
• Galapagos finches (Geospiza fortis) on Daphne
major during a drought; detailed, multi-year
observations.
• 5% shift up in mean values for beak size due to
the drought and later a rainy period led to a 2%
decrease. One generation is enough for such
changes to be significant!
• Development and life-history strategy (age at
sexual maturity, brood size) also important
influences on success and can be selected
for/against: guppies and cichlids and mortality…
Altruism
• This is one of the themes of the next section of the
course.
• How can altruism be supported by natural selection?
Even warning colours don’t do the caterpillar that gets
eaten and spat out much good…
• Hamilton’s kin-selection as an answer (Haldane: I’ll lay
down my life for two brothers, or eight cousins…)
• Our kin share our genes- in acting altruistically towards
them, we can actually improve our genetic ‘fitness’ even
though it disadvantages us as individuals.
– Lion behaviour.
– Warning cries.
– Evolution shapes behaviour too!
Mass extinctions
• The KT boundary. The iridium anomaly and the Alvarez hypothesis.
70% of all species!
• The Permian-Triassic boundary. 95% of all species! Trap eruptions?
Formation of a single super-continent? Climate change?
• Others. (The ‘big five’– 233f)
• ‘Resetting’ the competition… imposing suddenly conditions that life
had not adapted to/ been selected for survival in.
• The lucky few carry on… somehow they were more resistant/ in the
right place or the right ‘niche’ to survive whatever triggered the
extinction.
• Gould’s hypothesis: once more, against the ‘chain of being’ and a
teleological view of evolution.