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|
| Gradualism |
Throughout
most of the 20th century,
researchers developing the synthetic theory of evolution primarily focused on
microevolution
,
which is slight genetic change over a few generations in a population. Until
the 1970's, it was generally thought that these changes from generation to generation indicated that past
species evolved gradually into other species over millions of years.
This model of
long term gradual change is usually referred to as
gradualism or phyletic gradualism
. It is essentially the 19th century
Darwinian idea that species evolve slowly at a more or less steady rate. A natural
consequence of this sort of macroevolution
would be the slow progressive change of
one species into the next in a line, as shown by the graph on the right.
 |
|
| Punctuated equilibrium |
Beginning
in the early 1970's, this model
was challenged by Stephen J.
Gould, Niles Eldredge, and other leading paleontologists
. They
asserted that there is
sufficient fossil evidence to
show that some species remained essentially the same for
millions of years and then underwent short periods of very rapid,
major change.
Gould suggested that a more
accurate model in such species lines would be punctuated equilibrium
(illustrated
by the graph on the left).
 |
|
| Long periods of stability
and short episodes of change |
The punctuated, or rapid change periods, were presumably the result of major environmental changes in such things as predation pressure, food supply and climate. During these times, natural selection can favor varieties that were previously at a comparative disadvantage. The result can be an accelerated rate of change in gene pool frequencies in the direction of the varieties that become the most favored by the new environmental conditions. It would be expected that long severe droughts, major volcanic eruptions, and the beginning and ending of ice ages would be likely triggers for rapid evolution.
Random mutations provide variations that help a species survive. Mutations in regulator genes in particular can quickly result in radically new variations in the organization of the body and its important structures. As a consequence, changes in these genes can result in a greater likelihood that at least some individuals will have variations that will allow them to survive during times of extinction level events. In this situation, subsequent generations would be significantly changed from the generations before the period of severe natural selection. In other words, regulator genes probably play an important part in the rapid change phases of punctuated evolution.
It is now quite apparent that the evolutionary history of life on this planet is extremely complicated. Different species have evolved at different rates and those rates have changed through time in response to complex patterns of interaction with other species and other environmental factors. In addition, it is clear that most species lines have already become extinct as a result of their inability to adapt to changed conditions.
Origin of Species
Where do new species come
from? That is a key question that the biological sciences have been
asking for more than 200 years. Charles Darwin gave us part of the
answer in his explanation of natural selection. The
remainder came as a result of Gregor Mendel's experiments with basic genetic
inheritance and the 20th century discoveries of the other
natural processes that can cause evolution. We now know that
evolution can occur in two different patterns--adaptive radiation
into
multiple species results in cladogenesis
and
successive speciation
within a single evolutionary
line results in anagenesis
.
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 |
|
|
Adaptive radiation resulting in cladogenesis |
Successive speciation within a single species line resulting in anagenesis |
Adaptive radiation is the progressive diversification of a species into two or more species as groups adapt to different environments. Natural selection is usually the principle mechanism driving adaptive radiation. The initial step is the separation of a species into distinct breeding populations. This usually happens as a result of geographic or social isolation. Over time, the gene pools of the isolated populations diverge from each other by gradually acquiring different mutations and sometimes as a result of random genetic drift. When the populations are in dissimilar environments, environmental stresses are often not the same. As a result, nature selects for different traits existing within the gene pools of the populations. Over time, the populations genetically diverge enough so that they can no longer reproduce with each other. At this point, they have become separate species and usually continue to diverge in subsequent generations. In intermediate stages, the two newly or about to be separated species may be able to interbreed and produce children, but most of them are likely to be sterile. This is the case with the offspring of horses and donkeys--i.e., mules. Eventually, however, species genetically diverge so much that they are unable to produce any children. This is the case with sheep and cattle.
The evolution of species by successive speciation occurs within a single evolutionary line without the branching of adaptive radiation. This takes place when the members of a species consist of a single breeding population for many generations. Descendant generations experience continuous spontaneous mutations and new directions of natural selection as the environment changes. This results in progressive changes in the gene pool frequencies of the population. At any one time, all members of the population are the same species. However, as generations subsequently replace each other, the gene pool is transformed--i.e., it evolves. Eventually, the changes are great enough that if descendants could go back in time to mate with their distant ancestors, the genetic differences would prevent them from producing fertile offspring. In other words, they would be different species.
In the real world, the patterns of evolution can be very complex and changing. Both adaptive radiation and successive speciation can go on simultaneously.
Origin
of Life
It may seem strange that the question of the ultimate origin of life on earth was not discussed at the beginning of this tutorial. It was an intended omission. The focus has been on the processes by which living things change through time, not on how life first came about. These are separate issues. A consideration of ultimate origins bridges into the realm of religion for many people. Regardless of whether you believe that life began spontaneously as a result of natural processes or was due to divine intervention, it is sobering to realize that science is close to being able to create life out of non-living substances. In fact, most of the initial steps have already been taken. The video linked below shows just how close we are to creating living organisms.
Artificial Life--excerpt from the PBS series Nova Science Now (October 18, 2005)
This link takes you to an external website. To return here, you must click the "back" button
on your browser program. (length = 15 mins)