The evolution of character

Biological evolution functions on very basic principles.

Seemingly random mutations introduce a variety of traits into the gene pool of a species.
The species traits drift and change ever so slightly over time
The surrounding environment "selects" from amoungst these slight changes for those traits that are most fit or beneficial
The benefited individuals out-compete their rivals and increase more rapidly in number
The less "fit" are replaced over time by the more "fit" and faster reproducing individuals
The entire species changes gradually over time and with sufficient time divergent successful branches may diverge sufficiently to become new species.
New species interact with older species and also may alter many of the interacting aspects within their environment

In short the environment changes species and species in turn change the environment

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The principles iterated through in this process are not unique to biological evolution.

Its seems that evolutionary principles are one of the most effective ways to adapt to an ever changing environment which also in turn further adapt that environment.
Lets take a look at these principles in action when it comes to the evolution of each of our characters.

Our mind is awash with seemingly random thoughts which may or may not lead to feelings.
Our minds may simply drift allowing for plenty of variety in these thoughts
We actively select certain thoughts and feelings to focus on, pruning out others, one simply need to think of dreams to see the variety that thrives when no pruning is done within the environment of the mind.
The more we focus on certain thoughts, themes begin to develop and the less persistent other unassociated thoughts fall by the wayside
Fully developed thought themes lead to actions and reactions
These actions/reactions reinforce the thoughts that led to them
Over time patterns of thoughts and actions develop into habits and become ingrained

In short the thoughts we select to entertain mold our actions and or actions mold our long term character.
source: @gavvet