let me give this a shot, i'll try to be as brief as possible...

atoms are the basic building blocks of this world and all we see in it. atoms are made of protons, neutrons, and electrons. all atoms have specific properties determined by the quantity of these three particles. these three particles have are very very (though not perfectly) predictable relationships with one another. the relationships are not perfectly predictable because we lack the means to make perfect predictions about them. these relationships fall under the realm of physics.

atoms form to make molecules, in some cases. the forming of molecules is also very predictable. the combination and arrangement of atoms in a molecule gives molecules unique characteristics which are predictable as well. molecules are explained via chemistry, which is a shorthand for physics when systems of atoms are involved.

in living things, molecules and sometimes even atoms form to make cells. cells have specific parts, the functions of which are easily understood. the way a cell behaves is also understood and explained through biology, a short-hand for chemistry when so many chemical substances are involved.

cells may join to form tissues, organs, and organisms. we can predict the processes occurring inside living organisms, and the mechanisms which cause said organism to behave as it does. anatomy and organismal biology are shorthands for cellular/molecular biology when so many cells are involved.

organisms interact with one another through predictable processes explained by ecology. (i think...) ecology is a shorthand for organismal biology when the number of organisms is increased from just the one.

the theory of evolution falls under organismal biology. the principle of natural selection falls under ecology. now, let's address your question for serious.

let's put a cat and a mouse in a locked room with nothing else. both are prime, healthy specimens. what happens does not require much imagination. the cat will eat the mouse. this is fundamentally explained by physics, at the atomic level.

now what i think you're saying is that natural selection is random not because the animals' behavior is random but because what happens is random, in relation to the animal. this is true at the macroscopic level but not true at the atomic level. however, at the macroscopic level, this is true in individual case scenarios but statistically not true, as it evens out. eventually all animals are subject to the same conditions (statistically) so whether it's random or not has no bearing.

if an organism can survive these mean conditions, its genes are preserved. if it cannot, its genes are not preserved. assuming the mean conditions stay the same, only the best organisms' genes will remain. assuming the mean conditions change, there will be a shift (evolution) in a species' genes. depending on what genes are combined in sexual reproduction, and what mutations occur, new and different species can come into existence. this explains simultaneously the wide variety of life on earth as well as the fact that they can split even more, and their genetic makeup can shift via evolution and natural selection. evolution is the after-effect of natural selection.