Put simply the 2nd Law of Thermodynamics states the Entropy of a closed system will always increase, and a local decrease in entropy in an open system always results in a larger increase in external entropy.
Now when entropy is examined statistically it can be considered a measure of randomness. The more random a system is, the more disordered it is. The formula for statistical entropy is:
S is entropy.
k is the Boltzmann Constant = 
is the number of
equivalent equally probable configurations. This is a direct
measurement of disorder.
Random or disordered systems have such a significantly higher number of equivalent equally probable configurations, that they can basically be considered inevitable. Now entropy is not the same as disorder, but entropy is logarithmically related to disorder. Entropy can be considered a measurement of disorder in the way that the Richter Scale is a measurement of Earthquakes or decibels are a measurement of sound.
Reference| J Philip Bromberg,
Physical Chemistry, 1984, pg. 690 |
The theory of evolution requires some extremely large increases in order and by that an extremely large decreases in entropy. Order from chaos requires energy to be applied to the system in an organized manner. The problem for Evolution is that it lacks a mechanism for applying energy in an organized manner, this is particularly the case with regard to the origin of life.