The major source of variation is mutation, which is random, Natural Selection only "selects" these mutations. The other process such as genetic drift are also random.
My question was when you read this- how do you understand this?
Does random (mutation, genetic drift, etc..) in this context means that it just happened to have happened (chance) this way, and that it wasn't purposefully caused?
Nature, more precisely "the nature of living things", as far as I can conceive, does not work "randomly". It works according to the principles such as action- reaction, availability of resources, trial and error. If a gene mutates, there are always environmental suspects as much as its inner mechanism, or relationality with other genes. We simply do not exactly know how DNA codes interprete surrounding environment, but we know that cells are organised to evaluate light, noise, vibration, gravity, space, heat and every other physical phenomenon. If we can not precisely estimate weather condition in next 30 days, we are still not very near to explain how living things are mutating according to this or many other environmental chaos. Here "randomness" actually means "we don't know yet".
We make mutations in our laboratories and we can see that these codes are well open to be changed by external effects -in this example our scientist apply wind, heat or radiation on purpose. In nature, this effect occurs randomly(!). Actually, it does not; it occurs according to its inner mechanism and outer environment. Even a non-living system (take a Volcano) depends on surrounding pressure, heat accumulation and numerous other factors. We don't know exactly what they are, and we say that "volcanoes erupt randomly": According to our knowledge of course, not according to its existential mechanism. And imagine the effects of this volcano eruption over nearby living things.
This is a hypothetical question: If we eliminate all environmental surrounding effects, do cells still mutate? It is impossible to answer this question since the whole existential elements of any cell is organised according to a performance, a role, or a function within, for, around, in, nearby of an environment.
The gene code, that makes eye organ in various living things was not structured randomly; nor suddenly. Certain cells are specialized in light detection, which itself requires some time. Then they have become a free tool within the gene pools of the planet. The information of light sensitive cells were used in different species in different environments, each species (or a DNA project), which has issues with light, used these cells in their gene stuructures and eye has become the winner among many different projects of living world. Today every species which has eye use the same DNA code to make an eye on body. This is not random, this is a project which was developed by living things and coded for their generations throughout billions of years. Having a set of eye gene, ear gene, heart gene, etc. will make you what you are and will help you to survive in this environment.
Purpose: Using the light force through an environment recognition device. Trials and errors and many of us have eye(s). This is one way of looking it...
Purpose: Nothing. Light already constantly hits everything in front of it (living or non-living). "Randomly" some cells starts to react them differently than others. Because their chemistry change through light. Plants do not have eyes and they react light through photosynthesis, or with other ways, like some flowers blossom in day light, close themselves at night time, etc. Other cell types in animals "randomly" mutated and eye is evolved. This is another way of looking it.
Each of these approach, or some others will necessarily highlight only certain part of what is really going on. We will ask questions, make some research and create some predictions. Before we knew DNA, there were more "random" explanations for living things, today we have more explanations. And hopefully one day we will be able to explain more about mutations, too.