Transcript Unnumbered Figure - Shippensburg University of Pennsylvania

The “Theory” of Evolution can explain both short -term and
long term adaptation, including speciation and the radiation of
species to produce all of the biological diversity we see today.
Curiosity about the origins of present biological diversity is ancient. Attempts to
explain the biological diversity we see around us have ranged from the
mythological and religious to the scientific.
Attempts to use a scientific approach were instigated by observations of (among
others):
• The fossil record.
• Comparative anatomy.
• Comparative embryology and developmental studies.
• Geological discoveries that indicated the earth was much older than various
creation stories allowed for.
Figure 13.00a
Figure 13.11
Figure 13.9
Figure 13.12
Jean-Baptist Lamark
An accomplished French biologist - one of the
first to use the term. Contributed to the cell
theory; remembered mainly for his “theory of
acquired characters,”or transmutation, that
included a complexifying force that drove
organisms to evolve into more complicated
forms, and in which the use or disuse of
characteristics led to differences in their
inheritance (i.e. giraffes evolved long necks
because they needed to, not because of the
natural selection of some mutations over
others).
Lamark’s ideas pre-dated those of Darwin.
Thomas Malthus
British mathematician and economist
who in “An Essay on the Principle of
Population” (1798) theorized that
human population growth would one
day outstrip the food supply.
Darwin’s more generalized version
of this train of thought states that
resources in nature are always limited
and therefore must be competed for
by members of a population (and
among populations of different
species as well) creating a selection
pressure that will favor the survival
of some individuals over others.
Because his work on the topic was so comprehensive,
Charles Darwin is given most of the credit (or blame) for
originating the theory of evolution.
Darwin’s curiosity about nature, his ability to observe clearly and think creatively,
and the experiences he had as part of the HMS Beagle’s expedition to S. America
allowed him to create a theory of natural selection, or descent with modification,
that has stood the test of time.
A Theory of Evolution “timeline” showing the relationship of
Darwin’s work to that of other
influential thinkers of the time,
and other world events.
The Voyage of the H.M.S.Beagle
(to map the coast of S. America to produce better charts for Her Majesty’s
Navy )
Darwin’s finches
Finches on the Galapagos islands had diverged from each other recently enough
that their “relatedness” was more-or-less obvious.
Life on the Galapagos was “recent” in the sense that the islands had only been
thrust up out of the sea for a few million years, which allowed evolution in progress
to be seen more clearly.
Organisms like the Galapagos marine iguana are
found nowhere else on earth
Alfred Wallace, who as a
young man wrote a letter
to Darwin that prodded
him into finally publishing
his theory of evolution.
Wallace was studying Beetles in
Malaysia and came to essentially
the same conclusions about
natural selection as Darwin had.
In order to keep his “priority” on
the subject yet not slight Wallace,
Darwin published a joint paper on
the topic with Wallace before
publishing his full length book.
Wallace went on to become a
distinguished entomologist and
naturalist in his own right.
Title page of the evil book that
shook Victorian society to its
core, and is still provocative
today.
As he expected, Darwin was
vilified for his views by Victorian
society which was (superficially at
least) very religious in orientation.
The Evolution - Creation debate
persists to this day with Darwin still
being viewed as an evil person by
many of his Creationist and
“Creation Science” minded
detractors.
A Rogues Gallery of “evolutionist free thinkers” and their adversaries
(all except Lyell were portrayed in the film seen in class)
Capt. Robert Fitzroy
Geologist Charles Lyell
Lord Bishop Samuel Wilberforce
Anatomist Richard Owen
Naturalist & Professor Thomas Huxley
(“Darwin’s Bulldog”)
Charles Lyell and the Question of Time
Charels Lyell, although older, was a historical
contemporary and friend of Darwin who was not
featured in the “Dangerous Idea” film. Lyell is
given most of the credit for the theory of
uniformitarianism, the understanding that the
geologic features of the earth, as we now see it, are
the result of slow natural forces acting over vast
periods of time, and that these forces probably
operated in ancient times the same way as they do
now. Accordingly, the earth must necessarily be
much older than any interpretation of the
Biblical creation story would lead one to believe.
This understanding has since received massive
amounts of support from modern techniques,
radioisotope dating, evidence for plate tectonics and
so on.
Lyell was receptive to Darwin’s ideas on evolution and encouraged him to publish them,
although initially he seems to have not fully appreciated how much Darwin’s ideas differed
from those of Lamark, he eventally supported the theory of Natural Selection in full.
So the deposition of all of
these sedimentary layers of
rock took millions of years,
not to mention the time it took
the Colorado River to erode its
way down through them.
Similar empirical observations
and reasoning used by Lyell as
he studied the structure of Mt.
Etna on Sicily led to his
original theory of
uniformitarianism and its
implication that the earth must
be, at minimum, millions of
years old.
Because these processes probably worked at the same rate
historically as they do now, the earth must be VERY old.
Plate Tectonics and Paleogeography
Modern geology has allowed us to reconstruct what the earth was like in ages past.
From: http://www2.nau.edu/rcb7/globehighres.html
Plate Tectonics and Paleogeography - 2
This picture shows the
location of PA in the
Devonian period, 417 to 354
Million Years Ago.
Devonian age strata are
exposed throughout eastern
PA.
You are here
equator
From:
http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html
So how does evolution
work? Darwin’s genius
does not reside merely in
his understanding that
evolution occurred, others
before him had come to the
same conclusion.
His seminal contribution
was the mechanism that he
proposed to be responsible
for it:
Natural Selection
Only ”survivors” are able to pass on their genes (which differ due to mutational
events during DNA replication) to their offspring; so eventually all the members of a
population have the altered genes, and we say the population has adapted. Many
adaptations added over long periods of time accumulate within an isolated
population to produce reproductive isolation and speciation.
The type of mutations necessary for this scenario to play out must involve prior
gene duplication to allow for new traits to be created without disabling essential old
traits. This is seen also in that, as species have evolved over time, the trend has
been toward more complex organisms with more DNA (on average ) per cell.
“Descent with modification” has therefore favored the development of more
complex organisms over time.
Individual genetic variation is the result of mutation and provides the
spectrum of traits that can be selected among by natural selection.
More intra-species variation
Examples of short-term adaptation produced by natural
selection are easy to find, especially with species like insects
and bacteria that have short generation times.
Selection and
adaptation down on the
farm
Adaptive camouflage in different mantis species from different locations also
shows natural selection at work.
Geographic isolation allows populations to evolve that occupy the same
niches found elsewhere but in different “forms” = species. Marsupials
were able to persist in Australia long after they had been out-competed by
placental mammals elsewhere.
The result of accumulated adaptations is speciation = microevolution
Speciation Mechanisms
Changes in individuals within a
population that prevent
successful hybridization with
individuals of similar but
different populations result in
new species.
Usually population isolation
over long periods is necessary
for these to occur.
Populations that can no longer
inter-breed are by definition
different species.
Given vast tracts of time this process of cumulative adaptation -> > -> speciation leads to macroevolution with a common ancestor
producing many species that in turn arise and become extinct over
time.
Modern gene (DNA) sequencing
techniques clearly show
similarities in gene structure that
are greater between species that
one would judge to be more
closely related based on visual
clues.
Evidently what enables the
roughly 2% difference between
chimp and human DNA to have
such a significant effect is that
much of the difference is in genes
that control development - genes
that turn other genes on and off as
an individual fetus develops
before and after birth.
THE “C VALUE PARADOX”
In general, more complex organisms have more DNA per cell because of the accumulation
over time of gene duplications and new genes that have resulted from recombination of
fragments of some of these extra genes. However, exceptions to this general trend exist.
This is termed THE “C VALUE PARADOX”
How can the evolution of increasing complexity be
explained?
If a needed gene were to somehow mutate into another that produced a new
protein with a valuable function, it would be of Zero value if the first needed
gene product were no longer being produced as a result. The organism would
probably die (and that’s not evolving).
What is necessary is prior gene duplication. This sort of process makes
“extra” gene copies available that mother nature can ”play with” through
mutations that recombine gene pieces into new constructs.
DNA analysis (sequencing) clearly demonstrates that this, in fact, is how
evolution has proceeded at the molecular level.
This is a slow process and wouldn’t necessarily - in itself -lead to evolution.
However combined with the process of natural selection working over vast
reaches of time it has created the history of organismal diversity that we see
all around us today.
We are no different
A computer-generated “average”
human being as she would appear
if the various human facial
feature differences could be
somehow “blended”
The picture represents, in essence,
a sort of computer-generated
counter evolution.
Odds & ends
The mechanisms that have contributed to the tremendous natural
diversity we see today go beyond the simple form of natural
selection that we have explained to this point, and its fair to say that
we don’t totally understand them all. A sampling follows:
Types of natural selection results
The benefit, or lack thereof conferred by
a gene can depend on the environment
and the genetic background within which
it occurs.
Even genes that seem to represent
harmful mutations can be
maintained in populations if they
confer an advantage in certain
environments.
The gene for altered hemoglobin
that results in sickle cell disease
also confers resistance to infection
by the malaria parasite.
So the presence of the parasite
creates a positive selection pressure
for maintaining what would
otherwise be a harmful gene within
populations.
Genetic Drift: Chance alone causes allele frequency in a small population to change over
time. This is one of several mechanisms that favor gene pool changes over time that are not
powered by natural selection
The Bottleneck effect causes changes in allele frequency; in effect creating a new gene
pool from a selected sub-set of the parent population alleles. The Founder effect works in
a similar way; here a random sub-sample is split off to found a new population.
Gene pool allele frequencies resist change over time.
Hardy was a pure mathematician who formulated the “Law” after being asked by Punnett why it
was that dominant genes wouldn’t increase in a population over time. He evidently held applied
math in some contempt which comes through in the wording of his 1908 paper (Mendel was
“rediscovered” in 1900) on the subject:
To the Editor of Science: I am reluctant to intrude in a discussion concerning matters of which I
have no expert knowledge, and I should have expected the very simple point which I wish to make to
have been familiar to biologists. However, some remarks of Mr. Udny Yule, to which Mr. R. C.
Punnett has called my attention, suggest that it may still be worth making...
Suppose that Aa is a pair of Mendelian characters, A being dominant, and that in any given
generation the number of pure dominants (AA), heterozygotes (Aa), and pure recessives (aa) are as
p:2q:r. Finally, suppose that the numbers are fairly large, so that mating may be regarded as
random, that the sexes are evenly distributed among the three varieties, and that all are equally
fertile. A little mathematics of the multiplication-table type is enough to show that in the next
generation the numbers will be as (p+q)2:2(p+q)(q+r):(q+r)2, or as p1:2q1:r1, say.
The interesting question is — in what circumstances will this distribution be the same as that in the
generation before? It is easy to see that the condition for this is q 2 = pr. And since q12 = p1r1,
whatever the values of p, q, and r may be, the distribution will in any case continue unchanged after
the second generation (. . . Yeah, right)
Hardy and Weinberg never worked together
W. Weinberg
G. Hardy
Wilhelm Weinburg was a German physician who also had an
academic interest in genetics. Ironically he had lectured on his
“principle of genetic equilibrium” in 1908, the same year that
Hardy’s paper was published. The coincidence, however, wasn’t
discovered until 1943. Eventually both men were given credit for the
concept.
Hardy-Weinberg equlibrium conditions
The allele frequency equilibrium described by Hardy and Weinberg
only holds when:
•
The population is extremely large
•
Mating is random
•
Mutation is absent
•
Natural selection is not at work
•
Immigration and emigration are non-existent (no gene flow)
Figure 23.3a The Hardy-Weinberg theorem
Figure 23.3b The Hardy-Weinberg theorem
Extra Photo 13.03x2