The Causes of Variation in Living Organisms


Variation is the differences that exist between individuals. There are two types of variation. One is interspecific. This is the variation that exists between different species. For example, horses vary from ducks, which vary from mice. There is then intraspecific variation. This is the differences that occur within a species. For example, the number of eyes on a peacocks feathers of the length of a giraffe’s neck.

 Although individuals of the same species may appear similar, no two individuals are exactly alike. All the members of a species have the same genes. But individuals within a species can have different versions of those genes, which are called alleles. The alleles of an organism make up its genotype. Different genotypes result in variation in the phenotype, the characteristics displayed by an organism. Examples of variation in humans caused by genetic factors include eye colour (blue, brown, green etc) and blood type. Animals get their genes from their parents as a result of sexual reproduction, this means genetic variation is inherited. 

  Mutation is one way which increases the genetic variation within a species. It is the change of bases within the DNA sequence – either by deletion or substitution. Deletion causes frame shift and can alter the entire sequence. Substitution alters just one base and only affects one codon. They arise spontaneously during DNA replication due to certain rate frequencies (typically 1 per 100,000 genes per generation). Mutations are normally very rare, which is why members of a species all look alike and can interbreed. However the rate of mutations is increased by chemicals or by radiation. These are called mutagens, and include high energy ionising radiation such as x-rays and ultraviolet rays. These ionise the bases so that they don't form the correct base pairs. Some viruses can change the base sequence in DNA causing genetic disease and cancer. Other environmental influences such as smoking can increase the risk of cancer and other disease similarly. 

  It causes a change in either one or several amino acids coded for.  As such, the overall protein structure is change, perhaps to a non-functional state. For example, sufferers with PKU are unable to produce a functioning enzyme to break down the amino acid phenylalanine, leading to brain damage if patients do not follow strict dietary instructions in their early years. This is because the tertiary structure of their enzyme has a sequence of amino acids which do not fold into a tertiary structure complementary in shape to that of the substrate. This means no enzyme-substrate complexes can form. 

 Mutations in genes in gametes are passed on to offspring due to the gametes producing half of the offspring’s genetics. Changes within a single base pair can cause physiological differences that may either benefit or inhibit the offspring. These are known as hereditary mutations. Some mutations can cause physiological changes which allow the organism to survive better. This allows them more opportunity to reproduce and pass on their genes (including the mutated one). It is


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