The structures of the proteins can be affected by changes in the environment or by mutations. In this case, is probably that the cause is a mutation, because it occurs in one specific type of spider. In some cases, although the structure of the protein changes, it has the same function, but less activity, or more in other cases. In this case, it's possible that due to the change in this protein the spider produces a different type of venom. However, it is also possible that this spider doesn't produce venom at all. If we want to know with certainty how this structural change affected the protein function we must know what kind of mutation happened.