Selection and its types

The term selection (= selection) in population genetics characterizes a situation that does not meet the limiting conditions for deriving the basic model (i.e. the average fertility of individuals is the same, independent of their genotype , so there is no selection).

It is among the classical evolutionary mechanisms of Darwinism; there is always a reduction in the number of offspring .
To measure the intensity of selection, we use the average number of offspring from a parent of a certain genotype; relative number is used, not absolute, where this relative value can be based on a ratio, for example, to the average number of offspring of all phenotypes or on a ratio to the number of offspring of the most fertile phenotype .
From this we can derive: (i = max) i.e. the average number of offspring of genotype i / the average number of offspring of the most fertile genotype, where wi = absolute average number of offspring of genotype $w<i={\frac {w'_{i}}{w}}$ it:
Relative reproductive capacity (also adaptive value) of a given genotype = $w'_{i}$ .
Election coefficient , which characterizes the intensity of selection: $s=1-w_{i}$ .

Selection against homozygotes[edit | edit source]

The starting population is in Castle–Hardy–Weinberg equilibrium .

When selection of intensity s i against recessive homozygotes aa (i = aa) begins, it will:

frequency before selection : for (for , for , ); $aa=q^{2}$ (pro $AA=p^{2}$ , pro $Aa=2pq$ , $\sum =1$ );
selection intensity $s_{i}$ : pro aa = s (pro AA = 0, pro Aa = 0);
relative reproductive capacity $w_{i}$ : pro aa = 1 − s (pro AA = 1, pro Aa = 1);
freequency after selection $aa=q^{2}(1-s)$ (pro $AA=p^{2}$ , pro $Aa=2pq$ , $\sum =1-q^{2}s$ ).

We calculate the gene frequency in the generation after selection as: q´ = ${\frac {q(1-qs)}{1-q^{2}s}}$ .

The size of the change is characterized by the selection difference (difference): Δq = q´ − q = $-{\frac {pq^{2}s}{1-q^{2}s}}$ .
∆q allows us to find out whether the investigated polymorphism is stable or transient, or how fast selection-induced changes take place.

In the case of stable polymorphism , ∆q = 0; this happens when the numerator of the fraction in the equation is 0, which can happen when:

p = 0, i.e. the population consists only of homozygotes aa;
$q^{2}=0$ (q = 0), the population consists only of AA homozygotes;
s = 0, the considered selection does not occur;

These three conditions = the so-called trivial (general, basic) conditions.
Under non-trivial conditions, gene frequencies change (allele a decreases).
The magnitude of this decline is directly proportional to the intensity of selection and the square of the gene frequency.
If the gene frequency is high at the beginning, its decrease is greater.
At smaller values of q, the decline slows down (biologically explainable by the fact that at low values of q, most a alleles in the genotype are heterozygotes, against which the given type of selection does not act).
The dynamics of these gene frequency changes for the values of the selection coefficient have a sinusoidal character in various ways in the graph (dependency on time measured in generations).

Selection against both types of homozygotes[edit | edit source]

Basic types of selection

The so-called heterozygote preference (also referred to as super dominance or heterosis) ( and i ) leads to an equilibrium state where . ( $s_{Aa}=0$ a $s_{AA}>0$ i $s_{aa}>0$ ) $q_{rovn.}={\frac {s_{AA}}{s_{aa}+s_{AA}}}$ .

Thus, equilibrium values of paq depend only on selection coefficients, not on allele frequencies.

E.g. hemoglobinopathies in the homozygous constitution cause anaemia .
A selection factor that affects dominant homozygotes is malaria infection , to which heterozygotes are partially resistant.
An extreme case is a balanced lethal system , where and the population then consists only of heterozygotes (e.g. the T locus in the mouse, which is involved in the development caudal part of the body) $s_{AA}=s_{aa}=1$ a
Some dominant (T) and recessive (t) alleles have a lethal effect in the homozygous constitution , usually due to severe disturbances during ontogeny ; whereas the mechanism of damage is different for dominant and recessive homozygotes, heterozygous individuals are viable.

Selection against heterozygotes[edit | edit source]

Selection

E.g. fetal erythroblastosis .
Except for the situation where p = q = 0.5, equilibrium can only be reached under trivial conditions.
Since the non-trivial equilibrium p = q = 0.5 is easily violated, practically always under this type of selection there is a situation where the gene frequency of the allele with a higher frequency increases.

Types of selection

1. Normalizing selection

is applied in maintaining the current state of the population by excluding deviations from the norm; eg hereditary diseases.

2. Balancing selection

maintains a certain degree of polymorphism in the population; e.g. heterozygote preference ( sickle cell disease ).

3. Directional selection

is applied especially when the external conditions change, its action selects the best adapted phenotype.

This is a typical effect of natural selection in the sense of classical Darwinism.
E.g. industrial melanism of some insects.

Fisher's fundamental theorem of natural selection is: "The rate of rise in (relative) fertility of any organism at any time is equal to the genetic variance of (relative) fertility at that time". Better: "The greater the genetic variability that can be acted upon by selection toward higher fitness, the greater the progress in fitness." or "The rate of change of a trait depends exclusively on the additive genetic variance in the fitness of that trait." (1930)

Links[edit | edit source]

Source[edit | edit source]

STEFÁNEK, Jiří. Medicine, diseases, studies at the 1st Faculty of Medicine, UK [online]. [feeling. 2/11/2010]. < https://www.stefajir.cz/ >.

Category:Biologiy Category :Genetics