When a population is evolved, turtles are selected randomly for reproduction. The random selection is not completely random, however. It is fitness based random selection. This means that the randomness of the selection is biased to select more fit turtles more often then less fit turtles. Each turtle in the population has a fitness that is calculated based on its drawing. You control this fitness based on the multipliers that you set when you create a new population. These multipliers are described on the Library View page describing adding a new population.
Liking and Not Liking turtles is your way of directly guiding the evolution of your populations. When you like a turtle, its fitness is the value of the turtle's drawing fitness that is calculated when the turtle draws its picture. This fitness is based on things like how much the turtle moves, how much the turtle turns, how often the turtle changes color, and how often the turtle repeats a set of instructions. When you indicate that you do not like a turtle, its fitness is reduced to a number that is almost zero. Thus, when you indicate that you do not like a turtle, the probability that it will be selected to reproduce is very unlikely.
As mentioned above, when turtles are selected for reproduction, that selection process is influenced by turtle fitness. The process involves selecting a random number between two values. The two values are zero and the sum of all of the turtles' fitness values. Once the random number is generated, it is used to select the turtle. The following figure shows this graphically. The colored regions show turtles. The width of a colored region represents the fitness of the turtle.
As you can see, the wider a turtle's fitness region is, the more likely the random number generated will fall within the turtle's region and the more likely it is to be selected.
Occasionally, a turtle will evolve that has an extremely high fitness value. When a turtle has a fitness value that is very high, its probability of being selected for reproduction is also very high. When the other turtles in the population have relatively low fitness values, the highly fit turtle can be selected many times to reproduce. You will notice this when a turtle begins to appear many times in your population; almost as if it were being replicated into your population. When a very fit turtle begins to appear many times in your population, it can crowd out other turtles. When you consider the figure above, which illustrates mate selection, you can see that if a population contains many identical turtles with high fitness values, the mate selection process will almost exclusively choose those turtles for reproduction. This further crowds out other turtles and allows the highly fit turtles to dominate the population. This is not a desirable state when it comes to evolving new interesting turtles.
When you notice that one turtle is beginning to dominate your population, you can correct this situation using your ability to not like the dominant turtles. Simply not like all of the dominant turtles except one or two. This will reduce their crowding out of the other less fit turtles, and increase the chances of the other turtles being selected for reproduction. It should only take a couple of generations before other more fit turtles begin to evolve and reduce the domination of the previously highly fit turtle.
Population diversity refers to the diversity of the genetic information available within a population. In Evolved Art, genetic information is the programs of the turtles, more specifically, the instructions that those programs contain. If the population contains no programs with the instruction to change color, then the turtles do not know how to change their color when they draw. Once the instruction to change color is lost from all of the programs, there is no way for the turtles to get this capability back, except through mutation. Mutation is a random change to a turtle program. If this random mutation causes a program to gain the change color instruction, then that ability is reacquired by the population.
Thus, it is important to maintain population diversity as you evolve your population over time. Maintaining population diversity is not something that you have direct control over. The primary means of maintaining diversity is to avoid not liking too many turtles in any given evolution. Another way is to avoid having too many turtles that look exactly the same, since similar turtles have similar code.
One way to recognize when population diversity is declining is when the population seems to stagnate. If the turtles seem to always look the same when you evolve your population, you may be experiencing diversity decline. The primary means of increasing population diversity is to replace the code of turtles in the population with code from outside the population. For example, you can import a turtle from one of your gene pools, or from a turtle provided to you from another Evolved Art user. Another way is to use the Generate Random Turtle action with the Replace button in the Turtle View.