Mr. Kousen is ....  Water Man

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Variation:

Most natural populations of organisms consist of individuals that are slightly different from each other.  Just as no two human beings are the same, each member of a species of plant or animal have slight genetic differences.

Selection:

Since there is genetic variation in a population the individuals in that population may respond differently to environmental factors.  Some individuals may thrive under a particular environmental factor, while other individuals may fail to thrive.  Individuals that fail to thrive may leave fewer progeny, while those that thrive may leave a larger number of progeny.

Evolution:

Through time the number of individuals, and the genes that they contain, in a population that are able to thrive in certain environmental conditions may increase.  Given enough time and continued environmental factors, the genetic constitution, and
even the outward appearance, of individuals in the population may be significantly altered.

Sources of Variation:

The greatest source of genetic variation in natural populations comes from the independent assortment and recombination of genes during sexual reproduction. Another source of variation is mutation.  Various environmental factors may cause the genetic material contained in the cells (the DNA) to change, which may alter the function and/or structure of the cells.  Some of the environmental factors which may cause mutation are radiation, chemicals, abrupt changes in temperature, physical force, or other factors which significantly alter the environment ofthe cell.

There are many different kinds of mutations.

Point mutations are changes in a single nucleotide (subunit) of the DNA molecule.  One nucleotide may be replaced by another nucleotide, or one nucleotide may be missing altogether.  Sometimes point mutations have little effect on the cell, while at other times point mutations can be lethal.

Deletions are the result of a loss of a section of the genetic material (chromosome) of the cell.  If the deleted segment is small or it does not contain critical genes, the effect on the cell may be small.  On the other hand, if the deleted segment is large or it contains genes that are critical for normal cell function, the effect on the cell can be large, maybe even lethal.

Insertions are the result of an addition of a section of the genetic material (chromosome) of the cell.  Since there is no loss of genetic material, insertions generally do not have as adverse of an effect as do other mutations.  However, the added genetic material may render inoperative the genetic material that is already in the cell.  If the inoperative genetic material is essential for normal cell operation, the cell may be adversely affected.

Inversions are the result of a rotation of a section of the genetic material (chromosome) of the cell.  The inverted section of DNA simply has been rotated 180 degrees with reference to its previous position in the chromosome.  This change may have little or no effect, or it may have serious implications for normal cell functioning.

Translocations are the result of moving a section of the genetic material (chromosome) of the cell and attaching it to another chromosome.  Again, since generally there is no loss of genetic information the effect on the cell may be minimal, but due to
the positioning of the added genetic material, some genes may be rendered inoperative.

Nondisjunction is the process whereby the chromosomes of dividing cells do not separate during anaphase of mitosis or during anaphase II of meiosis.  If some of the chromosomes exhibit nondisjunction while the others separate normally, the result is usually aborted cells.  If all of the chromosomes exhibit nondisjunction, there is the possibility of producing an unreduced gamete.  In animals this almost always results in aborted cells.

In plants, however, unreduced gametes may be tolerated.  If unreduced gametes are involved in fertilization, the plants growing from these seeds may have abnormally high numbers of chromosomes.  This condition is called polyploidy.  Polyploidy has
been an extremely useful tool to plant breeders in producing better crops.

There are several different kinds of polyploidy.

A segmental polyploid is when only one or some of the chromosomes in a genome (a set of genes) are present in abnormally high numbers.  For instance, chromosome number one may have three copies present instead of the normal two copies in a diploid cell. A complete polyploid is when all of the chromosomes in a genome (a set of genes) are present in abnormally high numbers.  For instance, all of the ten chromosomes of the soybean genome may have four copies in the cells instead of the normal two copies.

An autopolyploid is when all of the chromosomes of the polyploid come from the same species.  The most common way that this happens is when a plant that is normally self fertilized produces unreduced gametes.

An allopolyploid is when the chromosomes of the polyploid come from different species.  The most common way that this happens is when a plant that is normally cross fertilized produces unreduced gametes.

There are two kinds of selection:

Natural selection operates on populations without human intervention.  Wild plant populations may be subjected to some environmental factor which will cause some of their individuals to survive and leave a greater number of offspring in succeeding
generations than other members of the population.

Artificial selection operates on populations by means of human intervention.  Humans manipulate the individuals in the population and/or they manipulate the environmental factors to which the population is subjected.