Synthesis
Genetics and natural selection
combine
Norton and Hardy
• Statistical calculations begin to show that an
advantageous gene can spread quite quickly through a
population– even if it only provides a small advantage.
• Hardy-Weinberg equilibrium expresses how the
proportion of various genotypes (in a population ‘at
equlibrium’ i.e. with no net selection acting on it) is
determined by the frequencies of the alternative alleles
for a particular gene: Where p is the proportion of alleles
of form A, and q is the proportion of alleles of form a, the
proportion of genotypes in the next generation (given
random mating) is:
• p2 (of type AA): 2pq (of type Aa): q2 (of type aa).
Calculating genetic change
• Consider Norton’s table (p. 184).
• Let’s think about the numbers here and why they
vary as they do.
• First think of the difference that the different
strengths of the selection effect makes.
• Then think of the difference that dominance vs.
recessiveness of the new gene makes.
• Take-home: Natural selection can shift the
frequencies of genes in a population fairly
quickly.
Ronald Fisher
• Fisher was a statistician
with a serious interest in
evolution.
• His statistical analysis of
gene selection showed
that evolution could
proceed gradually using
small mutations instead
of suddenly forming new
species by a single
mutational ‘jump’.
The Genetical Theory of Natural
Selection
• Published in 1930, this book explained in detail
how gradual evolution could be accomplished
under Mendelian genetics.
• Chance survival or loss of genes is important
only for very small populations.
• Mutations were sufficiently rare that only with the
help of selection could they be expected to add
up and create substantial evolutionary change in
a population.
• Even directed mutations can’t resist the impact
of selection forces: orthogenesis is ruled out.
The upshot
• Contra de Vries and Bateson (and the early Thomas
Morgan) the only defensible theory of evolution, given
Mendelian genetics, is evolution by natural selection.
• British biologist J.B.S. Haldane reached the same
conclusions independently, on the basis of his own
mathematical analysis of genetics and natural selection.
• Statistical analysis of the changing population showed
that the dark form of the peppered moth (Biston
betularia) had enjoyed a 30% selective advantage over
the normal form. The intensity of selection in wild
populations is sometimes quite high!
Polymorphism
• Populations don’t always tend to ‘fix’ one allele
at each position.
• Sometimes the heterozygotes (those with both
forms of the allele) actually have an advantage–
in this case, selection will tend to preserve both
alleles, at proportions that balance the
disadvantages of each homozygotic form and
the advantage of the heterozygotes.
• Sometimes combinations of genes contribute to
selective advantages and disadvantages– this
can also lead selection to maintain
polymorphism in a population.
William Castle
• Ovary transplants (once more, inheritance of acquired
characteristics fails the test).
• Genes interact (hooded gene in rats interacts with genes
biasing the pattern towards all-black and all-white coats).
• So genotype does not map ‘1 to 1’ into phenotype.
• This pleiotropy unites each organism into a distinct
individual through the subtle effects of the particular
gene combinations it has.
• It also ensures that there is a substantial pool of genetic
variation for selection to operate on.
Sewall Wright
• Student of Castle.
• Involved in these
experiments.
• Also mathematically
gifted, like Fisher and
Haldane.
• But involved in
agricultural breeding
work as well.
A strategy for breeding
• Wright was interested in exploring rare combinations of
genes for their potential in breeding new and desirable
traits.
• His strategy was to use inbreeding to concentrate
particular combinations of pure (homozygotic) traits.
• Then crossbreeding the best of these animals could
produce desirable combinations of traits that weren’t
visible in the original population because the
combinations were so unlikely under random mating.
• This kind of breeding has contributed to new crops and
new food animals (such as dairy cattle) that are far more
productive than their ancestors were.
In nature
• The same could occur in nature, in a large
population made up of mostly-isolated
subpopulations.
• The variation and different combinations of
genes that arise in different subpopulations
would allow for the rapid accumulation of
advantages in one population or another, which
could then spread throughout the larger
population over time.
• Here Wright differed from Fisher and Haldane,
who thought of variation at a more individual
level.
Reconciliation
• This work offered a reconcilliation between the
geneticists and the naturalists.
• Naturalists focused on continuous variation in natural
populations and how selection would act on that
variation over time.
• Geneticists focused on discrete (even dramatic)
differences and their origins as ‘mutations’.
• But small mutations, as Fisher, Haldane and Wright
showed, could spread through a population by natural
selection, producing the continuous variation and slow
change the naturalists believed in using the mechanisms
that the geneticists accepted.
Isolation and speciation
• Restrictions on interbreeding lead to local ‘races’
or subspecies.
• The line between subspecies and species was
and remains a vague one.
• Darwin distinguished evolution as change in a
single lineage over time and evolution in the
‘branching’ sense, in which a single lineage
separates into two (or more).
• This, he thought, would normally require some
degree of isolation.
Wagner
• Distinct but close species are often separated by
a geographical barrier (mountain range, or even
a ridge separating one valley from another, or a
river, or…)
• Believed this was the key to speciation.
• Tried to make this into a general theory of
evolution separate from natural selection.
• But not all isolation is geographical– ecological
patterns could also separate two lineages.
Gulick
• Separation by degrees in land snails.
• A big lesson: the same phenomenon
here, laid out spatially, as we encounter
with all species when we think in terms of
long periods of time:
• Each small unit of change is not enough to
make a new species– but as we follow
from one group to the next, we get new
species and eventually even new genera.
Sumner and subspecies of mice
• Genetic/ breeding studies on different subspecies (lighter
and darker) of peromyscus mice showed the standard
pattern of ‘Mendelian segregation’: In the first
generation, colours were intermediate, but the full range
reappeared in the second generation, with intermediates
indicating several genes were involved in the
differences.
• The differences were not driven by climate– the shift
from one to the other form took place in as little as 10
miles.
• Environment (light sand beaches vs. darker forest floors)
provided a natural explanation of the differences, in
terms of natural selection.
Dobzhansky: the synthesis applied
to natural field studies
• Theodosius Dobzhansky left Russia to work with
Thomas Morgan at Columbia, and then followed Morgan
to Stanford.
• He was the first biologist to integrate detailed field
studies of variation in species with genetics and natural
selection.
• One example of natural selection he studied was the
development of pesticide resistance in scale insects.
• Resistance had originated in small areas, and then
spread to other areas– just as if it were due to a mutation
in one population that then spread as the insects that
carried it survived and spread while others died.
• For Dobzhansky, evolution just is change in the
frequencies of genes in a population over time.
The idea of a species
• When they inhabit the same territory, species
are clearly, sharply distinct.
• But when we trace them across different
regions, we find differences– and the differences
tend to increase the further we go.
• In fact, there are things called ring species, in
which a group of interbreeding populations
stretch out across a ring of territory, but the
populations at opposite ends of the ring, which
overlap territorially, are distinct: They don’t
interbreed at all.
• Northern gulls are one example.
Northern Gulls
Warblers
So what is a species?
• Aristotle thought in terms of essences: A species was all
the living things that shared an essence.
• For Aristotle, these essences are preserved (passed on
unchanged) in reproduction– so they can’t change at all.
• Thinking in this way makes the ‘type’ more real and
permanent– it’s because they have the type’s
unchanging essence that individuals belong to a
particular species.
• But when we look at the world, what we see first isn’t
types– it’s individual organisms, all of which vary to one
degree or another.
Populations
• Instead of thinking in terms of types, biologists
like Darwin and Wallace began to think in terms
of populations– groups of very similar organisms
that reproduce, sharing their genes through the
generations.
• This makes the individual organism the starting
point, instead of the type.
• And it raises questions about essences: Is there
really some basic set of characteristics that
constitute a type for each species? Ring
species make this question particularly hard to
answer.
Mayr: The synthesis applied to
taxonomy
• Mayr, like Dobzhansky, came to think of species in a
more population-based way.
• What unites a population over time is interbreeding.
• So Mayr followed Dobzhansky in setting the line
between species at the point where two populations
would not interbreed if they shared the same territory.
• This can be due to sterility, behavioural differences,
differences in mating preferences, or a combination of
these.
• This is often called the biological species concept.
• Mayr was able to trace the development of species of
tropical birds using this criterion.
Lack
• Lack applied the same ideas to Darwin’s finches
on the Galapagos.
• Studying them in greater detail, Lack showed
that there were 14 species of finch on the
Galapagos.
• He also established how they had spread from
island to island and gradually developed into
distinct species that sometimes now lived on the
same island.
• Lack also showed that the differences in beak
shape in the finches were adaptations to
different diets.
Simpson: the synthesis applied to
paleontology
• Paleontology was the last field of biology to
adopt the synthesis.
• Paleontologists like Cope had long focused on
‘evolutionary trends’ which they could see in the
fossil record.
• Thinking in terms of trends made natural
selection less convincing to them– orthogenesis
remained popular despite the arguments of
Fisher, Haldane and Wright.
• Simpson’s mentor, Osborn, was student of
Cope’s and loyal to the traditional view.
Tempo and Mode in Evolution
• Simpson brought the synthesis to bear on
paleontological evidence in this book (1944).
• He used fossils to estimate the rate of evolutionary
change, and compared these rates to those that had
been observed in nature by people like Dobzhansky.
• Change proceeded at varying rates in the fossil record,
from quite quickly to very slowly.
• Simpson also emphasized the ‘bushy’ quality of
evolution– instead of following a single lineage to trace
the history of a present-day group, he tried to look at the
history of the whole family of groups, living and extinct,
that derived from a common ancestor.
Horses over time
Natural selection
• The pattern was not one of linear trends–
evolution had moved at different rates, and in
different directions from time to time and place to
place.
• This made perfect sense from the point of view
of natural selection.
• Naturalists, taxonomists and paleontologists
were now all on board– the modern synthesis
had been fully integrated into the main fields of
biology.