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Gradualism and Punctuated Equilibrium
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. 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.
Gradualism
Beginning in the early 1970's, this model was challenged by
Stephen J. Gould, Niles Eldredge, and a few 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).
Punctuated equilibrium:
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. In such stressful situations,
populations would be expected to initially diminish and become isolated. Genetic drift would then
potentially speed up the rate of evolution. If by chance nature favored successful adaptations, the
population would again increase in numbers as a radically changed species. Conversely, if it favored
maladaptive variations, the population would decrease in numbers further and possibly even become
extinct.
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.
Short-lived species with quick generation replacement times usually evolve at a faster rate than
do large, long-lived species. This is because new genetic variations normally appear each generation as
a consequence of mutation in sex cells. Those variations may be selected for or against depending on
the environment at the time. As a consequence, quicker reproductive cycles generally result in speeded
up species divergence. It is not surprising that there are far more species of insects and microscopic
organisms than species of large trees and big animals such as elephants, horses, and humans.
Tropical species also generally evolve at a faster rate than do those from colder temperate
climates. Subsequently, tropical forests are more diverse ecosystems than forests in colder
regions. This is probably because warm environments promote shorter generation times and higher
mutation rates.
A relatively new but extremely important factor in affecting rates of evolution has been
people. There are now nearly 7 billion of us, and our numbers are growing rapidly. We have already
severely changed most environments on our planet to suit our needs. In addition, we are the super
predator around the globe, bringing many species to the brink of extinction and beyond. As a
consequence, humans have dramatically altered natural selection. The surviving animal and plant
species have responded to this pressure in a variety of ways. For instance, fish species that are heavily
exploited by people now usually have smaller bodies as adults and begin to reproduce at an earlier
age. It is also likely that because humans increasingly live in urban environments and rely on ever more
technology, the evolution of our species has accelerated and changed in ways that are yet to be
discovered.
It is 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.
Vocab:
Synthetic theory of evolution - the 20th century conception of evolution caused by a number of
complex and often interacting processes. This is essentially a combination of Darwin's natural selection,
Mendel's basic genetics, and theories of population genetics and molecular biology.
Regulator gene - also called homeotic gene is a gene that can initiate or block the functions of other
genes. Shortly after conception, regulator genes work as master switches orchestrating the timely
development of our body parts.
Microevolution - evolutionary change within a species or small group of organisms, especially over a
short period.
Macroevolution- major evolutionary transition from one type of organism to another occurring at the
level of species and higher taxa.