The Theory of Evolution

This theory really fascinate me! Why? It has been a very controversial subject ever since Charles Darwin proposed it to the Science World.

Are we really descent from lowly creature that evolved millions of years ago? Are we created by supernatural forces a few thousand years ago?

What is exactly a Theory of Evolution?
Darwin defined this term as "descent with modification." It is the change in a lineage of populations between generations. In general terms, biological evolution is the process of change by which new species develop from preexisting species over time; in genetic terms, evolution can be defined as any change in the frequency of alleles in populations of organisms from generation to generation.

When we talk about evolution there are two things that we should consider seriously; Natural Selection and Mutation.

Natural selection: The differential survival and reproduction of classes of organisms that differ from one another in one or more usually heritable characteristics. Through this process, the forms of organisms in a population that are best adapted to their local environment increase in frequency relative to less well-adapted forms over a number of generations. This difference in survival and reproduction is not due to chance.

Mutations: Changes in the genome (genetic constitution). There are quite a number of ways in which mutations can happen. They also differ in the way that they impact evolution.

Mutations which occur when the genome is copied during reproduction are known as vertical transfer mutations. They are called vertical transfer mutations because they are transferred from ancestor to descendent along vertical lines of descent. In the original work on population genetics it was assumed that all mutations were vertical transfer mutations.

Horizontal transfer mutations occur when DNA is moved from one organism to another. Horizontal transfer can be a major source of evolutionary novelty. It is important because new genes can be propagated much more rapidly by horizontal transfer than by vertical transfer. If evolution is depicted by the tree, vertical genetic movement is the transmission of genes down branches; horizontal genetic movement is the transmission of genes between the branches.
Intra-organism transfer mutations occur when genes or parts of genes move around within an organism.

Strictly speaking, hybrids (mating across species) are not mutants. In many groups of species, particularly among plants, genes are transferred from to one species to another via hybrids.

Types of mutations:

1. Point mutations
   The most common type of copying error is the point mutation. In this form of mutation the nucleotide at a site is replaced by a different nucleotide. When people talk about mutation rates they are usually talking about rates of point mutations.
   Effects of point mutations: Point mutations in junk DNA are common but have no effect. Sometimes point mutations in regulatory regions have no effect and sometimes they alter the expression of some genes.


2. Additions and deletions
   During copying a segment of DNA may be deleted or a new segment may be inserted. Typically this happens as a result of chromosome breakage or realignment. (See below.) Additions and deletions can also be produced by certain types of horizontal transfer.
   Effects of additions and deletions: If the length of the new or deleted segment is not a multiple of three the translation will be garbled after the point at which the insertion/deletion occurred because the frame reading is now misaligned. This is known as a frameshift mutation. In some genes there are segments that may be duplicated as a block. This is known as tandem duplication.


3. Chromosomal duplication
   Sometimes one or more chromosomes are duplicated during reproduction; the offspring get extra copies of those chromosomes.
   Effects of chromosomal duplication: Duplicating only one chromosome is generally disadvantageous; an example in human beings is Down's syndrome. Having multiple copies of all of the chromosomes is known as polyploidy. Polyploidy is rare in fungi and animals (although it does occur) and is common in plants. It has been estimated that 20-50% of all plant species arise as the result of polyploidy.
   Gene duplication is very common; it is important because it provides a way to evolve new capabilities while retaining the old capabilities. All intermediate stages can be found in nature, from a single gene with alternate alleles to nearly identical duplicated genes with slightly different functional alleles to gene families of evolutionarily related genes with different functionalities.


4. Chromosomal breakage and realignment
   During reproduction a chromosome may break into two pieces or two chromosomes may be joined together. A section may be moved from one part of the chromosome to another or may be flipped in orientation (inverted). This is the mechanism by which deletions, duplications and transpositions my occur.
   Effects of chromosomal breakage and realignment: Quite often these types of changes do not affect the viability of the organism (the genes are still there; they're just in different places) but, in sexually reproducing species, they may make it less likely for the organism to produce viable, fertile offspring.


5. Retroviruses
   Certain viruses have the ability to insert a copy of themselves into the genome of a host. The chemical that make this possible (reverse transcriptase) is widely used in genetic engineering.
   Effects of retroviruses: Usually this is a way for the virus to get the host to do the work of reproducing the virus. Sometimes, however, the inserted gene mutates and becomes a permanent part of the host organism's genome. Depending on the position of the viral DNA in the host genome, genes may be disrupted or their expression altered. When insertions occur in the germline of multicellular organisms, they can be passed on vertically.


6. Plasmids
   Plasmids are little pieces of circular DNA that are passed from bacterium to bacterium. Plasmids can be transferred across species lines.
   Effects of plasmid transfer: Plasmid transfer is an important way of spreading useful genes such as those which confer resistance to antibiotics. Plasmid transfer is an example of horizontal transfer.


7. Bacterial DNA exchange
   Bacteria can exchange DNA directly. They often do this in response to environmental stress.
   Effects of bacterial DNA exchange: Exchange is often fatal to one or both of the bacteria involved. Sometimes, however, one or both of the partners acquires genes which are essential for the current environment.


8. Higher level transfer
   Some parasites can pick up genetic material from one organism and carry it to the next. This has been observed in fruit flies in the wild.
   Effects of higher level transfer: When this happens novel alleles can spread much more rapidly through a species than they would for ordinary gene flow.


9. Symbiotic transfer
   When two organisms exist in a close symbiotic relationship one may "steal" genes from the other. The most notable example of this are mitochondria. In most organisms with mitochondria most of the original mitochondrial genes have moved from the mitochondria to the nuclear genome.
   Effects of symbiotic transfer: A major effect is that the symbiotic relationship changes from being optional to be obligatory.


10. Transposons
    Transposons are genes that can move from one place in the genome to another.
    Effects of transposons: Depending on the position of insertion, transposons can disrupt or alter the expression of host genes. In some species most mutations due to transposon insertion. For example, in Drosophila, 50-85% of mutations are due to transposon insertions.

In The Theory of Evolution, the mutation that suppose to have occurred are the beneficial or favorable mutation that are hereditary and together with forces of natural selection, our world is full of diverse and complex fauna and flora that evolved from simple common ancestor.