Without variation, there would be no evolution by natural selection in any species. Genetic variation refers to differences in genes that result in slightly different traits in individuals. This variation can come from sexual reproduction, crossing over during meiosis, or random mutations. Traits that result from genetic variation can be anything from shapes and sizes of beaks, coloring, body size, age of sexual maturation, etc. The entire concept of evolution by natural selection relies on the fact that some of these variations make it more likely for an individual to survive to reproduce, thus passing on those genes. Over time, these traits will become more common in a population.
Here's an example that includes an issue we are dealing with today. When bacteria reproduce, random mutations occur every now and then in their DNA. This is also true for all living things. Sometimes a mutation occurs that provides an individual bacterium with some resistance to antibiotics. If this bacteria is then exposed to antibiotics, it may survive when the others don't and then reproduce, passing on its trait for resistance. Because of the extreme selection pressure on the bacteria by the antibiotics, this resistance may become very common in the population in a short period of time. The population has evolved and has a new trait. The key here is that without the initial genetic variation provided by the random mutation, this evolution would not have happened.
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