In the SparkNote on Darwin's evolutionary
theory,
we looked at Darwin's proposed mechanism for evolution, natural selection. It
had five postulates:
- Individuals are variable.
- Some variations are passed down.
- More offspring are produced than can survive;
- Survival and reproduction are not random.
- The history of earth is long.
This last point was proven by geologists and astronomers. After the creation of
the modern synthesis theory, the remaining four postulates were modified to
include new information about genetics.
Individuals are Variable
Darwin knew that individuals were variable, that is, each individual in a
population carried a unique set of traits. What he did not know is what
produced this variability, namely genetic differences. Variation in the genes
of individuals arises from several sources. Mutation, the alteration of
existing genes to form new alleles, can arise from copying errors during
DNA replication,
DNA damage, and repair
or recombination during cell
division. Varation
also arises from sexual reproduction, wherein new combinations of DNA are
created through the independent assortment of genes.
Some Variations are Passed Down
This statement was a truly unique portion of Darwin's theory. In 1856, he did
not know about DNA. He did not know about recombination events.
He did not even know about genes. He merely understood that for selection to
occur, variations must be transmittable from parent to offspring. We now know,
that variation is caused by differences in genes and genes are passed on to
offspring. More importantly, different genes are passed on to offspring
independently of each other (independent assortment) and intact.
More offspring are produced than can survive
In most generations, more offspring are born than can survive to reproductive
age given selection pressures such as predation and limited food supply. For
example, many fish lay hundreds or even thousands of eggs at once, yet most of
the young will be eaten or will starve before they can produce young of their
own and pass down their genes (and the genes of their parents).
Reproduction and survival are not random
This portion of Darwin's theory is what we know as "survival of the fittest."
Since more offspring are produced than can survive, some must die. Those that
survive are those that have the greatest fitness. A trait that increases an
individual's fitness is called an adaptation. Those individuals with the
genes that convey traits that are best adapted to the environment in which the
organism lives (those with high fitness) are more likely to survive and
reproduce than those that are less adapted to their environment (those with low
fitness).
On the genetic level, a specific allele for a trait can produce an
adaptation and convey greater fitness. An individual with greater fitness is
more likely to reproduce and pass this allele on to the next generation. Since
more fit individuals produce more offspring, the percentage of individuals in
the next generation with the fit allele--the allelic frequency of that
allele--will increase. As this process repeats itself over many generations,
evolution occurs, since the beneficial allele comes to exist within most of the
population.
