Mutation are two types somatic and germ cell mutation...as there is mutation in germ cell it will alter the normal behavior and produce variation !
random fertilization also causes variation as different setup of genes meets with another different...so chances of variation increases !
and third crossing over is in meiosis when chiasmata forms....so in this two homologous chromosome attach to each other and on separation ... some part exchange so leads to variation !
Explanation: The biological fitness of an organism is dependent on its ability to survive and reproduce in a given environment. A trait that increases ability to survive, but makes an individual sterile, decreases fitness because the organism cannot produce offspring to carry on the trait.
thank you
please mark as brainliest