Download Basis of Darwinism

Basis of Darwinism
1. Organisms vary
2. Some of that variation is inherited
3. All organisms produce more offspring than can
survive
4. On the average, those that survive will be the ones
better adapted or suited to local environments
(natural selection)
N.B. As environments change, different characteristics
will become advantageous
How did Darwin develop his principles? Were they
an incredible burst of creativity?
Answer: Not entirely. The first two (organisms vary,
some of that variation is inherited) certainly weren’t
original with Darwin, but based on his own work.
Darwin studied artificial selection in various species.
The fact selection works says that some of the
variation is inherited. There is incredible variety in the
pigeons coming from human selection among the
varying characters of pigeons. (N.B. natural variation
in pigeons) Selection produced bald-headed pigeons,
pigeons with fan tails, and hundreds of others. Darwin
wrote a whole book about artificial selection.
Statement 3 - all organisms produce more offspring
than can survive Darwin only understood this after reading Malthus’
An Essay on the Principle of Population. In it, based
on data from the American colonies, Malthus said
that populations increase exponentially, but that
resources can increase only in a linear fashion. Thus,
inevitably, a population outgrows the resources it
needs. For the human population, Malthus, a religious
man, felt that overpopulation would lead to “misery
and vice”. Darwin turned this into the 4th principle -
Statement 4 - On the average, those that survive will
be the ones better adapted or suited to local
environments (natural selection).
This has been the most difficult of the statements to
verify experimentally. However, there have been
some recent successes.
How can experiments test the tenets of evolution?
First, the general pattern of the hypotheco-deductive
(“the scientific) method:
Steps in the scientific method:
a) make an observation that is interesting/
unexpected
b) propose an explanation for the observation this is called a hypothesis. It should make
testable predictions, i.e. the hypothesis should
be falsifiable.
c) test the hypothesis experimentally. If results
fit the hypothesis, don’t stop - do other
experiments.
d) If many experiments that could potentially
falsify the hypothesis don’t, then accept the
hypothesis. Otherwise modify or reject it.
The hypothesis of evolution by natural selection is
testable: → to more specific deductions
DEDUCTION 1. Natural selection can operate if
more offspring are born than can survive
DEDUCTION 2a. If the hypothesis is true, species
of the remote past must be different from those
alive now.
- fossils in sedimentary rocks -- few of the
species alive now occur as fossils in the remote
past
DEDUCTION 2b. The older the sedimentary strata,
the less chance of finding contemporary species
as fossils
- this issue was first tested by Sir Charles Lyell
(1797-1875) who had collected fossil shells
from the Tertiary period
- some species are remarkably persistent, e.g. the
horseshoe crab
current
fossil
EPOCH
Fossil
species
Alive
today
% of fossil
species still
alive
15 MYA
(recent
Pliocene)
226
216
96
older Pliocene
569
238
42
Miocene
40-50 MYA
(Eocene)
1021
1238
176
42
17
3
* if all species had been created in 4 days, you’d
expect all rocks to have the same array of species
DEDUCTION 3. - The age of the earth must be very
great, possibly many millions of years (contrary
to biblical dating)
- first estimates based on the thickness of strata,
assuming rate of deposition was constant and
layers had steadily built up over time (but
depostion varies with rainfall, slope, material,
etc.)
- 1940s: age of rock estimated in terms of the rate
of radioactive decay of materials in the rock
- C12:C14 If we know the ratio of C12 : C14 when
the organism was alive, and we know how
much C14 is in it now, and we know the rate of
decay of C14 (its ‘half-life’=5730 years), then
we can calculate how many years since the
organism died.
- Carbon dating is especially useful for dating
organisms K/Ar is used for older rocks
Aging rocks permits scientists to view earth’s history
as a sequence of layers, as is evident if you were to
travel down the Grand Canyon:
DEDUCTION 4. - If the members of a taxonomic unit,
e.g. phylum Chordata, share a common ancestry.
That should be reflected in their structure
(“Homology” = similarity of structure based on
common descent. C.f. ‘Analogy’)
- e.g., the anterior paired appendages of
vertebrates:
- pectoral fins of fish, whales, dolphins
- wings of birds, bats, pterodactyls
- hoofed forelegs of horses and cattle
- tool-using arm of humans
DEDUCTION 5. If the members of a taxonomic unit
share a common ancestry, that should be
reflected in their embryonic development.
Ernst Haeckel (1834-1919) said “Ontogeny
recapitulates Phylogeny”
e.g. in the basic chordate body plan
1. Possession of a notochord
2. Gill slits or pouches in the pharynx
3. Dorsal hollow nerve tube
- human adult has only one of these (the dorsal
nerve tube which becomes brain and spinal cord)
- AS EMBYOS we have all three!
DEDUCTION 6. If there is unity of life based upon
evolution and common ancestry, that should be
reflected in the molecular processes of
organisms.
There is considerable evidence of common
metabolic heritage
-1. Amino acids. When produced in the lab, there
are two stereo-isomers
D-form (right handed)
L-form (left handed)
Life is all left-handed amino acids (with the
exception of cell walls of some bacteria)
2. There are only 23 amino acids in biological
systems
3. There are very similar sequences of amino
acids in proteins of very different organisms
- e.g., of 104 amino acid positions in the
sequence for cytochrome c, yeast and horse
are identical for 64
- e.g., insulin in pigs and humans is very
similar
4. Genetic programs are common: there are only
2 nucleic acids in all life forms: DNA (or in
some viruses RNA) provides the basic code.
- each have only 4 bases (ATGC in DNA and
AUGC in RNA)
5. ATP is the energy currency in all organisms
e.g. microcomplement fixation method
- compare antibodies produced by test organisms
(rabbit/rat) versus antigens from different taxa
(using specific proteins in blood serum)
- ‘immunologic distance’ data show conservatism,
e.g. primates:
gibbon
siamang
orangutan
human
chimp
gorilla
Old world monkeys (mandrills & macacs)
Does the evidence suggest a slow, steady pace for
evolutionary change?
Not always. The fossil record seems to show “jumps”
at many points. Sometimes called “macroevolution”,
a misnomer, it seems that the fossil record shows long
periods of stasis in the morphology of a species, then
rapid change as a new species appears.
There is an explanation, and it isn’t that the pace of
selection changes.
Until recently studies have been observational, rather
than experimental. An example of a modern
experiment documenting evolution:
Studies of guppy life histories in ponds in Central
America
Observations:
Pike-cichlids predate larger, mature guppies in
ponds where the pike cichlids occur.
Killifish predate juvenile guppies (but less
intensely) in ponds where they are found.
Have the different predators driven evolution
differently in ponds where the differing predators
occur? Do the guppies differ?
The life histories of guppies differ in ponds with
pike-cichlids and in ponds with killifish.
Guppies in ponds with pike-cichlids mature
earlier, are smaller at maturity, and produce larger
broods than those living with killifish.
Good start! But is it the predators that drive the
difference in life history?
Hypothesis:
Feeding preferences of predators select for the
size and age of maturity in guppies.
Test:
Transplant guppies from an area with pikecichlids to one with killifish (but no guppies prior
to transplantation). This is the experimental group.
Remember: we need to use a number of ponds to
ensure sufficient replication.
We also need to keep track of a number of ponds
with guppies and pike-cichlids both present, using
those guppies as controls.
Prediction:
Transplanted guppies should (after generations
of selection) mature later, at larger size, and
produce smaller broods
Results:
Compared to controls (guppies not transplanted)
over 11 years (or 30-60 generations) maturation
of transplanted guppies occurred significantly
later, at significantly larger size, and they
produced significantly smaller broods. The life
histories of the transplanted guppies had “evolved”
exactly as the hypothesis had predicted.
It is very difficult to design and execute experiments
that can test the hypothesis of evolution. This
experiment, done by David Reznick and John Endler,
is a very good test. It supports evolution as Darwin
presented it. Does it “prove” evolution? Of course
not. There have been other tests, in sufficient
numbers that we have elevated evolution to the status
of a theory, and recognize it as the cornerstone of
modern biology.