My question is whether mutations which lead to evolution occur by any environmental force or it occur spontaneously? I have read in genetics that x-rays, ultraviolet rays and chemical mutagens are the causes of mutation but how are they responsible for evolutionary mutations?
Mutations occur spontaneously because all molecular processes are stochastic in nature: DNA replication is an imperfect process. However, certain external factors can increase the probability of a mutation to occur or even cause specific types of mutations.
Both spontaneous mutations and this kind of externally driven mutation contribute to the genetic variability that is the basis for evolution. It is sufficient that variation exist for evolution to be possible, no matter the origin of this variation. X-rays are probably not common in nature, but there is plenty UV on the surface of earth, and there probably exist chemical mutagens that are formed without human contribution.
Evolution is, by definition, not spontaneous.
Be wary of conflating "mutation" with "new or changed features".
Mutation is only one of many causes for feature change between generations; inheriting traits from parents is another. Changes in nutrition and education can cause population-wide changes in height and IQ, without being based on mutation. All these things can lead to new features without mutation being involved at all.
The term mutation covers a lot of things which can cause inheritable and non-inheritable visible changes in future generations.
A genetic mutation (as opposed to an in-utero developmental mutation, etc) is a change in genes. The mutation may be in a non-functional section of the genome, so may do nothing. Or it may cause cell division to fail and kill the child. Or it may do anything in between.
But it's not evolutionary. No mutation is, since mutations are one-off incidents.
Now, over time, favorable genetic mutations may be selected for, and built upon by other mechanisms of feature change, to the point where everyone has it. Again, this is not instant, and will take many generations to spread through everyone.
In a sense all mutations "lead to evolution". The genetic make up of the population has changed. Some mutations become "fixed", are persistent in the gene pool. You are thinking of evolution as progressive, it isn't; it is statistically adaptive.
Is melanin depletion (pale skin)"evolution"? In gloomy high latitude places it enhances vitamin D production. In hot low latitude places it makes melanomas more likely.