Biology Reference
In-Depth Information
to the work of Mendel. Unlike Darwin, we know today that inherited adaptations
are encoded in genes, which are made of DNA. It follows that the genetic composi-
tion of the population changes with time - this is what we mean by the essence of
evolution today.
I shall now briefly discuss each postulate in turn, and then summarise studies
over the last thirty five years of finches in the Galapagos Islands as an example of a
natural population where these postulates have been found to hold.
Postulate 1:
Individuals in a population of a given species are variable.
We are all aware that the individuals in any group of people differ from one
another in many respects such as height, hair, eye and skin colour, facial appearance,
athletic ability, temperament, personality and intelligence. Individual variation is in
fact a universal feature of all species, and occurs at all levels from morphology to
DNA sequence.
Postulate 2:
Some of this variation is heritable.
Not all the differences between individuals are inherited, but it is the variations
in DNA sequence between different individuals that produce all those differences
that are inherited. Differences that are not inherited can be caused by different
behaviours. For example, the different eye colours of a blue-eyed athlete and a
brown-eyed sedentary person are due to inherited variations in their DNA, but their
different muscle development reflects their attitude to exercise and is not inherited.
Thus a blacksmith who develops strong arm muscles will not pass this character
directly to his children, but he may encourage them to follow his trade, and his
children will then develop muscular arms.
Postulate 3:
In every generation some individuals are more successful at surviving
and reproducing than others.
Your knowledge of people and their history will confirm the correctness of this
postulate for humans, and there is now abundant evidence from observations of other
species that is applies to them as well. The average Atlantic female cod fish produces
about two million eggs in each breeding season. About 99% of the hatchlings from
these eggs are eaten in their first month of life, while 90% of the remainder do
not survive beyond their first year of life. On average, each female cod produce
only two offspring that survive to reproduce. If she produced less than two such
offspring, the population would die out, because it takes two fish to produce eggs.
If she produced more than two on average, the population would rise to infinity or
until the food supply was exhausted. This production of more individuals than can
survive to reproduce is a universal feature of all species because the world does not
contain enough resources to support all the new individuals that are born. In his
topic, Darwin presents a similar calculation for elephants, which reached the same
conclusion.
Postulate 4:
Survival and reproduction are not random but depend upon individual
variation. This situation is summarised by the phrase “survival of the fittest”.
This phrase was proposed by the English philosopher, Herbert Spencer, in 1864
after he had read
On The Origin of Species,
and was subsequently used by Darwin.
The word “fittest” here does not mean the individual who is athletically superior,
as it does in normal language. Instead it means that people who are better adapted
