5 topics with brief reviews: definitions, key facts, and class questions.
Natural selection is the main guiding factor of evolution, but it operates differently depending on environmental conditions. In this section, we examine its three classic forms: stabilizing, directional, and disruptive (diversifying) selection.
We will look in detail at the mechanisms of each type: how the intermediate norm is preserved, why the phenotype shifts when the environment changes, and how a population splits into extreme variants. Special emphasis is placed on a clinically significant example—the maintenance of genetic polymorphism of the ABO blood group system in human populations under the pressure of infectious diseases.
In medical biology seminars and exams, you will be required to do more than just memorize definitions. Instructors most often ask for clear examples of each form of selection and an explanation of the adaptive significance of a trait. The ability to link environmental pressures with the distribution of ABO alleles is your key to mastering the topic and earning top grades.
How many forms of natural selection exist in total?
More than 30 different forms are known in nature, but classical theory distinguishes three main ones: directional, stabilizing, and disruptive.
Who is the author of the theory of stabilizing selection?
The theoretical basis of this type of natural selection was developed in detail by the prominent Soviet biologist I.I. Schmalhausen in 1946.
Who first formulated the principle of directional selection?
This crucial evolutionary mechanism was first formulated and described by Charles Darwin as part of his fundamental theory of evolution.
Against which forms is disruptive selection directed?
It is directed strictly against average, intermediate phenotype forms, favoring the survival of more than one extreme variant.
What is selective pressure in the context of the AB0 system?
This is an evolutionary process in which infectious diseases promote the survival of carriers of certain blood groups and the death of others, thereby altering allele frequencies in the population.
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