First, let me make it clear: The theory of evolution does NOT state that complex entities like a living cell came about fully functional out of nothing.
It is NOT stating that the complexity of life came about by pure chance.
It is NOT
stating that the millions of cells in the human body clustred with great specificity at random.
It is NOT stating that the millions of molecules in a living cell clustred with great specificity at random.
It is NOT even stating that the hundreds
of amino acids in a protein clustred with great specificity at random.
Web pages on evolution are abundant. But most often they combine a description of the theory of evolution with evidence for evolution, and often the two are intermingled so
that it becomes a little blurred what is what .
Here I just present the theory.
Look here for a more detailed description and evidence on evolution.
Look here for information on Origin-of-Life theories.
Look here for detailed information on genetics. And here for information on the appearence of new genes.
Above is a document with my comments to AiGs introduction to the nature of evolution.
The theory of
evolution states that all living organisms descended from a single common ancestor.
The central statement being: ‘Descent with modification’.
Through time, genetic changes then have resulted in the plethora of organisms we see today.
The result of this process is that organisms fall into distinct groups that share a recent common ancestor that they do not share with any other organism. Such a group is called a monophyletic group. The result of evolution is that all of life can be divided
into such monophyletic groups.
Example: According to evolution Humans, Chimps, Gorillas and Orangutans form the family ‘Hominidae’ (Great apes). Hominidae and the family ‘Hylobatidae’ (Gibbons) form the superfamily ‘Hominoidae’
(Apes). Together with several groups of monkeys, the superfamily Hominoidae form the infraorder ‘Simiiformes’ (Apes and Monkeys). Together with several groups of lemurs and others, Simiiformes form the order ‘Primates’. Together with
several other orders (such as Rodents, Ungulates, Marsupials and others), Primates form the Class ‘Mammalia’ (Mammals). Together with Birds, Turtles, Bony fish and other groups, the Mammals form the Phylum ‘Chordata’. Together with
Insecs, various worms, Jellyfish and others, Chordata form the Kingdom Metazoa (Animals). Together with Plants, Fungi and several groups of single-celled organisms, Metazoa form the Superkingdom ‘Eukaryota’. Eukaryota, Bacteria and so-called ‘Archaea’
taken together are all of life (Viruses are normally not considered to be living organisms).
The way organisms are related is called their 'Phylogeny'. If you are interested in more details of the phylogeny outlined above, look here. The naming of organisms is called 'Taxonomy'. The Taxonomy should reflect the Phylogeny in such a way that all Taxonomic groups are 'Monophyletic'. A monophyletic group is a group of all organisms that are each others closets relatives.
E.g. there is no group called 'Reptiles' because such a group (Crocodiles, Turtles, Lizards and Snakes) is not monophyletic. A group including these animals also should include Birds in order to be monophyletic. Also there is no group of 'Great apes' that
do not include Humans, as Humans are closer related to Chimps than to Gorillas and Orangutans.
According to evolution, and as a consequence of common descent, this system of ‘groups within groups’ (or Chinese box) can be employed to all
Horizontal gene transfer
There is an important exeption to the pattern outlined above. In Bacteria and Arcaea genes can be transferred 'horizontally' in stead of'vertically'. That is, in stead of just being tranferred to the
next generation by cell-devision, a gene can be tranferred to another bacterial cell. This will result in a kind of 'Lawn og Life' in stead of 'Tree of Life'. There is (at least to my knowledge) no comfirmed evidence of such a process in multicellular organisms,
but other processes (such as species hybridization) can give a similar result.
Retroviruses (HIV is a retrovirus) have the ability to transfer its genome to the host. In fact that is part of its normal (life-)cycle. In theory a gene from an infected
individual could be tranferred horizontally to another individual, carried along on the genome of such a virus. If this ever happens, it is an extremely rare event.
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.
there has to a mechanism for building up complexity. The mechanism here is called ‘Natural selection’.
Another important mechanism is ‘Genetic drift’.
genetic 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 another.
Together these forms of mutations ensure that genetic information can multiply and vary endlessly among individuals.
For a linguistic analogy to DNA mutations, read the attached document.
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.
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 gene.
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 frequency.
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’.
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 a population.
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 genetic drift.
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 small 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.