Rates Of Evolution
The rate of evolution is a measurement of the change in an evolutionary
lineage over time.
Method for measuring the rate of evolution
The method for measuring the rate of evolution can be illustrated by work done
by MacFadden on horse teeth: horse teeth are classic materials in the study of
evolution.
The rate of evolution is measured as follows:
Suppose that a character has been
measured at two times, t1 and t2 ; t1 and t2 are expressed as times before the
present in millions of years.The time interval between the two samples can be
written as: Dt = t1 - t2, which is 1 million years if t1 = 15.2 and t2 =14.2 The
average value of the character is defined as x1 in the earlier sample and x2 in
the later sample; we then take natural logarithms of x1 and x2 (the natural
logarithm is the log to base e where e = 2.718, and it is symbolized by ln ).
The evolutionary rate (r) then is:
r = (ln(x2) - ln(x1)) / Dt
The rate of evolution is measured in 'darwins'. Haldane (pictured opposite)
defined a 'darwin' as a unit to measure evolutionary rates; one darwin is a
change in the character by a factor of e in one million years.The formula
above for r gives the rate in darwins provided that the time interval is in millions
of years.
Is the rate of evolution determined by selection?
Rates of evolution in the fossil record have been measured for many characters,
in many species, at many different geological times. Are the rates of change in
the fossil record consistent with the mechanisms of evolutionary change studied
by population geneticists? While we cannot prove that selection is responsible
for changes in the fossil record, we can find out whether the results of research in
the two areas are consistent. If for example, the fossil evolution rates are significantly higher than rates observed in artificial selection experiments, it
would suggest that selection cannot be the only cause of evolution.
In fact, the rates of evolution observed in artificial selection experiments are far
higher than those measured in fossils. We can conclude that the known
mechanisms of population genetics, natural selection and random drift
(though it is questionable whether drift is important in morphological evolution)
can comfortably accommodate the fossil observations
Rates of evolution vary
It has been observed that there is an inverse relation between the rate of
evolution and the time interval over which it was measured: the observed
cases of rapid evolution have tended to be for shorter intervals than the cases of
slower evolution.
Here is an explanation:
If the direction of evolution fluctuates, the rate of evolution
measured over a short interval is inevitably higher than the rate measured over a
longer time interval because the short-term changes cancel out. For example, it
has been shown that the Galapagos finches’ beaks evolved to be larger in
times of food shortage and smaller in times of abundance (the finch Geospiza
fortis is shown opposite). Over time, these changes in size cancel each other out.
Punctuated equilibrium
Punctuated equilibrium is the theory put forward by Eldredge and Gould to
explain the fact that the fossil record does not show smooth evolutionary
transitions.
A common pattern is for a species to appear suddenly, to persist for a period,
and then to go extinct. A related species may then arise, but with little sign of any
intermediate forms between the ancestor and descendant.
According to the theory of punctuated equilibrium, evolution proceeds
relatively rapidly during speciation: between speciation events the population
remains relatively constant in a condition called stasis. Speciation occurs
through the peripheral isolate model when a subpopulation splits off and evolves rapidly into a new species.It has also been suggested by Gould that
punctuated equilibrium is due to macromutations.
The rate of evolution is a measurement of the change in an evolutionary
lineage over time.
Method for measuring the rate of evolution
The method for measuring the rate of evolution can be illustrated by work done
by MacFadden on horse teeth: horse teeth are classic materials in the study of
evolution.
The rate of evolution is measured as follows:
Suppose that a character has been
measured at two times, t1 and t2 ; t1 and t2 are expressed as times before the
present in millions of years.The time interval between the two samples can be
written as: Dt = t1 - t2, which is 1 million years if t1 = 15.2 and t2 =14.2 The
average value of the character is defined as x1 in the earlier sample and x2 in
the later sample; we then take natural logarithms of x1 and x2 (the natural
logarithm is the log to base e where e = 2.718, and it is symbolized by ln ).
The evolutionary rate (r) then is:
r = (ln(x2) - ln(x1)) / Dt
The rate of evolution is measured in 'darwins'. Haldane (pictured opposite)
defined a 'darwin' as a unit to measure evolutionary rates; one darwin is a
change in the character by a factor of e in one million years.The formula
above for r gives the rate in darwins provided that the time interval is in millions
of years.
Is the rate of evolution determined by selection?
Rates of evolution in the fossil record have been measured for many characters,
in many species, at many different geological times. Are the rates of change in
the fossil record consistent with the mechanisms of evolutionary change studied
by population geneticists? While we cannot prove that selection is responsible
for changes in the fossil record, we can find out whether the results of research in
the two areas are consistent. If for example, the fossil evolution rates are significantly higher than rates observed in artificial selection experiments, it
would suggest that selection cannot be the only cause of evolution.
In fact, the rates of evolution observed in artificial selection experiments are far
higher than those measured in fossils. We can conclude that the known
mechanisms of population genetics, natural selection and random drift
(though it is questionable whether drift is important in morphological evolution)
can comfortably accommodate the fossil observations
Rates of evolution vary
It has been observed that there is an inverse relation between the rate of
evolution and the time interval over which it was measured: the observed
cases of rapid evolution have tended to be for shorter intervals than the cases of
slower evolution.
Here is an explanation:
If the direction of evolution fluctuates, the rate of evolution
measured over a short interval is inevitably higher than the rate measured over a
longer time interval because the short-term changes cancel out. For example, it
has been shown that the Galapagos finches’ beaks evolved to be larger in
times of food shortage and smaller in times of abundance (the finch Geospiza
fortis is shown opposite). Over time, these changes in size cancel each other out.
Punctuated equilibrium
Punctuated equilibrium is the theory put forward by Eldredge and Gould to
explain the fact that the fossil record does not show smooth evolutionary
transitions.
A common pattern is for a species to appear suddenly, to persist for a period,
and then to go extinct. A related species may then arise, but with little sign of any
intermediate forms between the ancestor and descendant.
According to the theory of punctuated equilibrium, evolution proceeds
relatively rapidly during speciation: between speciation events the population
remains relatively constant in a condition called stasis. Speciation occurs
through the peripheral isolate model when a subpopulation splits off and evolves rapidly into a new species.It has also been suggested by Gould that
punctuated equilibrium is due to macromutations.
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