For evolution to work there
has to be one or more mechanisms.
First, there has to be mechanisms for creating genetic variation. All such
mechanisms go under the common name ‘mutations’, but there are several
different groups of mutations.
Second there has to a mechanism for building up complexity. The mechanism here
is called ‘Natural selection’.
Another important mechanism is ‘Genetic drift’.
material in living organisms is DNA, which is built as a string of so-called ‘nucleotides’. Look here for an introduction to genetics.
There are several distinct form of mutations. Here are the most important ones.
Point mutation: A single nucleotide is changed to another nucleotide.
Duplication: One string of nucleotides is duplicated into two identical copies.
Insertion: One or more nucleotides are inserted into the original string of nucleotides.
Translocation: A string of nucleotides is transported from one location to
Together these forms of mutations ensure that genetic information can multiply
and vary endlessly among individuals.
For a linguistic analogy to DNA mutations, look here.
For a more detailed explanation to how genetic information multiply, look here.
(It is a rather technical paper, but give it a try. If you are not familiar with genetics, refer to this Wikipedia article for technical details and terminology)
This is the main
mechanism in evolution as it is responsible for the building up of complexity.
First, it is important to realize that nothing in evolution is governed by any
form of conscious decisions.
To explain natural selection think of a group of interbreeding animals (or plants or bacteria or whatever), living
in a certain area. Such a group is called a population. Within the population,
mutations result in genetic variation. Some of this genetic variation will influence
the survival and reproducibility of the carrier.
As a result of this genetic variation, some of the animals have more offspring
than others. These lucky individuals pass on their genes to the next
generation, as do also the less lucky ones, just in smaller numbers. As a
result of this differential reproduction (and because the resources, and
therefor the number of individuals, are always limited) the frequency of the
alleles (alleles = different versions of the same gene) of the genes in question will change from one generation to the next.
The alleles resulting in more offspring will have a higher and higher frequency
in subsequent generations, ultimately being the only allele of that particular
This is a never-ending process. New mutations occur all the time, so the
population is in a constant process of changing frequencies of gene-alleles.
Natural selection is constantly weeding out mutations that result in less offspring
and at the same time those mutations that result in more offspring increase in
According to the theory of evolution, this never-ending process can result in
an increase in complexity.
An organism’s ability to have offspring is called its ‘Fitness’.
'The Selfish Gene'
This is the title of Richard Dawkins' book from 1976.
In the book Dawkins argue that natural selection always acts on individual genes, even when it seems to act on the organismal or group level. The idea is that it is actually not the reproduction of the individual that is important, but the reporoduction of the gene in question.
It can be examplified by those spiders where the male is killed and eaten by the female after he has fertilized her eggs. This behavior seems to contradict Natural Selection. How could such a gene be benificial for the male carrier. Well it isn't, but that doesn't matter. What matters is that this behavior leaves the eggs with a better chance to survive if the female is stronger than those females that do not eat the male. And if chances are that the male will be eaten by a predator before it has a chance to mate with another female, then the gene that forces the male to have a behavior that results in it being eaten by the female, will pread in the population, on the expence of the alternatives.
As outlined above, Natural
selection is the non-random survival of alternative alleles in the gene pool of
Genetic drift is the random contribution. Two genetically identical individuals
have the same chance of survival and reproduction, but in praxis, they do not necessarily
have the same number of offspring. In addition, an organism with high fitness
could die before it had a chance to reproduce. While an organism with low
fitness could survive to reproduce, simply by chance. This chance result in
The most illustrative example of genetic drift is to think of two alleles contributing
the same fitness to the organism carrying the allele. From generation to generation,
simple chance will result in variation in the frequencies of the two alleles.
The result will be that eventually - the time-span depend on the size of the population -
one allele will disappear leaving the other to dominate the population.
Genetic drift is always at
work. It is more important the smaller the difference between the fitness
provided by two alleles. And more important in smal populations. However, even if there is great difference between
fitness, an allele providing less fitness occasionally might outcompete the
alternative allele. In addition, it is important to notice, that when a
mutation result in a new allele that is not already present in the population,
there is a great risk that it will be lost in a few generations, due to genetic
drift, simply because it is so rare.