We have been discussing for the past several lectures some patterns of
inheritance which result in the phenotypic expression of each
individual's specific combination of alleles or genotype
.
We have seen that through recombination, segregation of alleles, and
independent assortment of homologous chromosomes during meiosis, much variation
occurs among the individuals in populations. We have seen, too, that mutation is
a source of increasing variation within populations.
Some inheritance
patterns, such as multiple alleles of a single gene, and the continuous
variation resulting from polygenic inheritance, are seen only within the
framework of population genetics .
We have also
mentioned that the frequency of a gene (or specifically, an allele) affects its
appearance in populations. For example, in human blood types, B is a co-dominant
allele, though not common within most populations, so that O type and A type
phenotypes are much more abundant.
In the next few lectures we will look
more at some aspects of populations, the variations which appear within
populations, with reference to the way an individual "contributes" to the
population's genetic mix or gene pool , and how this gene pool
is changed from generation to generation, which is the study of
evolutionary biology .
As an introduction, let's begin
with a working biological definition of evolution , to avoid
the confusion and misconceptions that sometimes surround this
term.
Evolution is biologically defined as the change in the frequency of
a gene's (or allele's) appearance in a population's gene pool from generation to
generation (through time). More simply, as your texts states: Evolution is
inheritable change in organisms over time.
Note the following:
In 1858 two naturalists, Alfred Wallace and Charles Darwin presented a paper
to the Linnean Society of London on the origin of species by means of natural
selection. They proposed that new species originated from preexisting species
through descent with modification driven by natural selection. They based their
paper on their observations while traveling extensively throughout the world as
naturalists. They also had the advantage of knowing the works of others who had
presented ideas about the origin of species, some of which we shall
mention.
Their paper was received poorly. In part, because the prevalent
view was that all things are "as is" with each distinct life form remains
unchanged through time, and in part, because Darwin and Wallace were not able to
provide biological validation for their sources of variation. Genetics wasn't
yet known.
Throughout recorded history, humans have been trying to find natural
explanations for our observations of events around us, including explanations
for the vast diversity of organisms that inhabit this earth with us. Humans of
all cultures have always tried to group, or classify organisms,
in attempts to clarify what has been observed. One of the concerns in
classification was to determine what comprised a unique group of organisms, what
we now call a species, a question still debated to some extent
today. In earlier centuries, most considered visual characteristics for defining
groups or species; today, we mostly use reproductive isolation as the major
determinant of a species. (More on this subject later).
Plato, the early
Greek philosopher, proposed that each object on earth was a reflection of its
divine "ideal form". Aristotle (PlatoÕs pupil) had one of the more interesting
systems of classification among the earlier "biologists". He developed a "ladder
of nature" from simple to more complex, using standards that made sense at that
time.
In the 6th century BC, Anaximander proposed that organisms were
not created, but animals changed through time. He proposed that vertebrate
animals, for example, descended from fishes.
As centuries went by many schemes existed for classification, but the
prevailing thoughts were that each species, or type of organism was seen to have
a fixed place in the order of things, since the beginning of
time, or creation.
The problem became more difficult as European
biological exploration encompassed more of the world in the 17th and 18th
centuries, and more and more different habitats and types of organisms were
described.
By the 18th century, the prevailing thoughts on the
"cataloging" of living organisms were being questioned, along with the long-held
premise that each organism and indeed our earth had been created "as is" all at
one time. Why was this so?
Evidences for a natural, scientific,
system for explaining species diversity, and for questioning an "as is"
creation.
By the 1800's there were a number of
questions being asked, based on evidence from geology, fossil studies,
distribution or organisms on earth and anatomical studies of
organisms.
Fossil Evidence
Fossil
studies showed that many organisms exhibit sequential little changes as time
passes. Was it likely that these organisms were "recreated" each generation or
would it be possible that organisms changed through time.
Age of
the Earth
By the 1700's, evidence existed that the earth was
significantly older than the prevailing European thought, based on biblical
metaphor or a few thousand years.
Common
Structures
Why did organisms, such as all vertebrates, have common
anatomical structures, even where there was no need for some of these
structures.
Distribution of Organisms
Why was the
distribution of organisms in the world so unusual? How could some organisms be
so common in one area and absent or rare in others with similar habitats
(assuming, as was done in the 18th century) that all organisms and the earth
were created at the same time?
Some proposed answers for these
observations ---
Catastrophism
Proposed by
Cuvier in the 1700's and reinforced by Agassiz in the early 1800's,
catastrophism hypothesized that species were created at one time, but a series
of catastrophes periodically destroyed many species. The survivors of these
catastrophes were found in the current world. However, the fossil record did not
contain remains of any of the species from the current world, although one would
think that at least some of the individuals of the surviving species would have
perished in the catastrophes. Agassiz proposed multiple creation and catastrophe
periods to answer that question, but to do so, he proposed that there had been
50 catastrophes and new creations.
Buffon and species changing
through time
In the late 1700's, a zoologist, Buffon, speculated
that perhaps there had been several "centers of creation" and that species were
"conceived by nature and produced by time". His hypothesis was not widely
accepted since he had no mechanism to explain how nature could do this, or how
there had been enough time on earth for the modifications to occur. Buffon used
the classification schemes of Linneaus to support his ideas on how
organisms related to each other.
Geological Time
Frame
Lyell and Hutton, geologists of the 17th and 18th century,
argued that geological events did not require catastrophes, but were natural
processes of weather, volcanism and earthquakes.
Lyell
had argued that the processes that formed the earth's surface "today" had also
been active in the past, proposing that the earth was millions of years old,
instead of the few thousands that was believed at the time by most. This meant
that the time needed for changes in living organisms could be put into a time
frame of many millions of years, long enough for evolution to have occurred.
Natural events, repeated over time, explained the findings in rock layers, a
proposal given the term
uniformitarianism.
Lamarck's
Contributions
About this same time, Lamarck proposed that all
organisms had been created in a simple state and were improved by gradual
changes into more complex structures. He was much taken by the gradual changes
shown in many organisms in the fossil records. The motivation for the
improvement, according to Lamarck, was a innate "drive for perfection". The need
to have a better structure resulted in some body force directed to fulfilling
that need which was then passed on to one's offspring. Lamarck was the first to
propose that one's environment was important to survival, although he erred in
proposing how changes occurred, and were subsequently passed on from one
generation to the next. (But he also preceded Mendel). Lamarck's work has often
been called evolution through the inheritance of acquired characteristics, but
that is not exactly what he really did.
Thomas
Malthus
Thomas Malthus, made an indirect contribution to the growing
question of how we get such variation among organisms when he proposed that all
populations reproduce in great numbers but survival is limited by available
resources. Individuals compete for those limited resources. As a result of
competition, only those best suited survive. Malthus wrote his essay in 1798,
and addressed his concerns to the rapidly increasing human
population.
Along came Darwin and Wallace
As
mentioned, both Darwin and Wallace made extensive journeys to collect species
from lesser known parts of the world. Both, on return, set out ideas which might
explain the origin of the diversity of organisms they observed. Their ideas were
presented at the Linnean Society of London in 1858, followed by Darwin's book
On the Origin of Species by Means of Natural Selection in
1859.
The theory of Natural Selection was developed to explain
adaptations of organisms to their environment, or surroundings. The major
statements of this theory are:
In more modern times, a mutation in cockroaches which resulted in a distaste for glucose resulted in populations resistant to a common roach treatment (a poison that contained glucose). It took only a few generations to accomplish this (thanks to the poison killing all roaches lacking the glucose distaste gene which ate the poison).
Antibiotic resistance in bacteria is another good example of contemporary
evolution in action. We shall discuss selection as a mechanism for the process
of evolution in more detail a bit later.
Virus evolution, including the
evolution of HIV is also studied.
Although Darwin and Wallace presented ample evidence of the variations in species, there was no way to document the mechanisms by which variations could be passed from generation to generation for about forty years.