The difficulties of defining species

- species aren't fixed forever, but change and evolve over time. Some idviduals may develop into new species after some time

- within a species there can be a considerable variation among individuals (dogs and artificial selection)

- many species are extinct and most of these have no fossil record

- some soecies rarely, if ever reproduce sexually

- members of different groups of the same species may be isoltated and so never meet and therefore never interbreed

- groups of organisms that are isolated from one another may be classified as different species 

- some species are sterile 

The DNA molecule is adapted to carry out its functions in a number of ways:

- it's very stable and can pass from generation to generation without change 

- its two separate strands are joined only with hydrogen bonds which allow them to separate during DNA replication and protein synthesis

- it's a extremely large molecule and therefore carries an immense amount of genetic information

- by having the base pairs within the helical cylinder of the deoxyribose-phosphate backbone, the genetic information is to some extent protected from being corrupted by outside chemical and physical forces 

The function of the remarkable molecule that is DNA depends on the sequence of base pairs that it possesses. This sequence is important to everything it does.