Mutations are not always deleterious. Mutations comes in different ways. They are, Inversion, translocation, duplication and deletion. Inversion refers to a gene sequence flipping. For example, from gene sequence AB to BA. Translocation refers to gene sequences upstream or downstream of the target sequence being removed and inserted into target sequence. Duplication refers to a particular gene or gene sequence doubling in adjacent to target gene or sequence. Deletion refers to removal of a certain gene or gene sequence permanently. Therefore, mutations are not always deleterious.
Mutations creates variations in gene sequences and gene types within a species. This creates a large gene pool which is necessary to ensure the continuity of the species. This is explained especially during changes in environment, where certain gene variations would not survive. However, due to mutations, certain variations could adapt better to the changes and survive, ensuring the continuity of the species. An example is pathogenic bacteria. When bacteria enters it's host, it will start to feed off the host's nutrients and undergo uncountable cell division to for reproduction. During this cell division, mutations may occur by chance, allowing some bacteria to have new genes. The change in environment occurs when the host takes antibiotics. The antibiotics starts to eradicate any bacteria it meets. However, due to mutations, a small amount of bacteria managed to survive due to them being able to adapt and achieve antibiotics resistance while the rest of those without the mutated gene dies off. This mutation ensures the continuity of it's species by evolving.
Hope this answers your question.
