All three types of selection are trends across generations in which the average frequency of a trait in a population changes. Population ecologists use ALLELE FREQUENCIES (the proportions of VARIABLE alleles at a given gene locus measured in a population) and measure change across generations.
This is easiest visualized using normal graphs – most individuals in the population will be average (or MEAN) and become fewer at the extremes.
For example here is a graph of human IQ. 100 is the mean. As it goes higher or lower there are fewer individuals with those IQ’s.
The mean frequency moves up or down. An example of this might be giraffe neck-length where the mean neck size got longer and longer over generations to reach the delicious dinner leaves at the tops of trees. Like this…
The frequencies in the population shift towards the mean while fewer occur at the extremes. An example of this might be the number of eggs a bird lays each year. Too many eggs is a waste of biological energy as some chicks will starve. Too few and well they could have had more progeny. 🙂 So to lay the average number of chicks that could be raised is preferred. Like this…
The frequencies in the population shift towards the extremes while fewer and fewer occur at the mean. An example of this might be frog size. To avoid predators tiny frogs can hide and huge frogs are too big to swallow whereas average-sized frogs make a mighty tasty lunch for a bird. Like this…
Disruptive Selection may even lead to given enough time!!!

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