THE AGE OF THE EARTH AND THE RATE OF EVOLUTION
Evolution – not change in morphology over time due to change in environment
- not involving things which are not replicating beings
How old is the earth?
Darwin, consulting with geologists, thought it was about 300 million years. This was considered
a long time and possibly long enough for evolution to occur.
Religious authority assumed the Bible to provide the answer: James Ussher, Protestant
Archbishop of Armagh : “The evening of Sunday, October 23, 4004 BC”. He calculated this from
family history mentioned in the Bible (Bible does not actually specify).
William Thomson (Lord Kelvin) thought he could destroy Darwinian evolution by showing the
earth was too young. He was the most preeminent physicist of the time – Darwin was
intimidated.
Thomson calculated a young earth from physics
The earth started out as molten – round sphere of extremely hot solid.
Knowing its size and the current rate of heat loss from the surface, you can calculate the age
since the molten state.
- Heat gradient known from mines: one degree every 50 feet (gets hotter as you go deeper)
Assumes no unknown source of energy.
Came up with estimate of around 20-40 million years (24.1 million years old plus/minus
100,000 years)
Similar calculations can be done for the age of the sun, again suggesting a young (100 million
years) solar system.
So why was Thomson so far out?
The usual story is that he didn’t know about radioactivity, a missing source of heat.
Discovered by the Curies in 1900
When this was discovered not only could dating be done accurately (radioactive dating) but the
calculations needed to be revised.
Thomson was not happy but shut up.
Truth is a little more complex:
This is partly true: Radioactivity makes a huge effect on the age of the sun.
The major missing component of Thomson’s calculations, however, was the lack of convection
in the upper mantle. Hot rocks move upwards as they heat and down as they cool again.
Thomson assumed there was no movement in the mantle.
John Perry in 1895 estimates possibly 2 billion years allowing for this – but was ignored by
Thomson!
, The best current evidence:
Note the long period of little evolutionary change then the burst of activity – anatomically
modern humans are incredibly recent in evolutionary history.
So how fast can evolution go?
Fisher’s fundamental theorem:
Rate of evolution = variance in fitness
There has to a genetic component so variation in fitness (diverse gene pool). Little variation
means similar gene pool so slow change in the population. High variation in fitness means a
very rapid change in the population.
Conversely…
If you know how much change in allele frequencies there has been you can calculate s, the
selective coefficient (from JBS Haldane):
s= Selective coefficient – strength of selection.
If you know the frequency of the allele at the start (p0), frequency of the allele at the end (pn),
and number of generations allowed (n), you can work out the strength of selection.
Evolution – not change in morphology over time due to change in environment
- not involving things which are not replicating beings
How old is the earth?
Darwin, consulting with geologists, thought it was about 300 million years. This was considered
a long time and possibly long enough for evolution to occur.
Religious authority assumed the Bible to provide the answer: James Ussher, Protestant
Archbishop of Armagh : “The evening of Sunday, October 23, 4004 BC”. He calculated this from
family history mentioned in the Bible (Bible does not actually specify).
William Thomson (Lord Kelvin) thought he could destroy Darwinian evolution by showing the
earth was too young. He was the most preeminent physicist of the time – Darwin was
intimidated.
Thomson calculated a young earth from physics
The earth started out as molten – round sphere of extremely hot solid.
Knowing its size and the current rate of heat loss from the surface, you can calculate the age
since the molten state.
- Heat gradient known from mines: one degree every 50 feet (gets hotter as you go deeper)
Assumes no unknown source of energy.
Came up with estimate of around 20-40 million years (24.1 million years old plus/minus
100,000 years)
Similar calculations can be done for the age of the sun, again suggesting a young (100 million
years) solar system.
So why was Thomson so far out?
The usual story is that he didn’t know about radioactivity, a missing source of heat.
Discovered by the Curies in 1900
When this was discovered not only could dating be done accurately (radioactive dating) but the
calculations needed to be revised.
Thomson was not happy but shut up.
Truth is a little more complex:
This is partly true: Radioactivity makes a huge effect on the age of the sun.
The major missing component of Thomson’s calculations, however, was the lack of convection
in the upper mantle. Hot rocks move upwards as they heat and down as they cool again.
Thomson assumed there was no movement in the mantle.
John Perry in 1895 estimates possibly 2 billion years allowing for this – but was ignored by
Thomson!
, The best current evidence:
Note the long period of little evolutionary change then the burst of activity – anatomically
modern humans are incredibly recent in evolutionary history.
So how fast can evolution go?
Fisher’s fundamental theorem:
Rate of evolution = variance in fitness
There has to a genetic component so variation in fitness (diverse gene pool). Little variation
means similar gene pool so slow change in the population. High variation in fitness means a
very rapid change in the population.
Conversely…
If you know how much change in allele frequencies there has been you can calculate s, the
selective coefficient (from JBS Haldane):
s= Selective coefficient – strength of selection.
If you know the frequency of the allele at the start (p0), frequency of the allele at the end (pn),
and number of generations allowed (n), you can work out the strength of selection.
