Selection

Definition: Natural selection refers to the process by which certain individuals possessing specific traits are more likely to survive and reproduce in a given environment.
Key Concept: "Survival of the fittest" indicates that not all individuals have an equal chance of survival based on their traits.

Definition: Breeding refers to the intentional selection of certain animals to reproduce based on desirable traits.
Contrast to Natural Selection: Breeding can occasionally contradict the principles of natural selection. For example, domestic turkeys have been selectively bred for larger breast meat, which has led to infertility in copulation.

Domestic Animal Breeders:
- Aim: To enhance and improve their breed through selective genetic changes.
Conservationists:
- Aim: To maintain the existing species or breed without genetic changes.

Definition: The Hardy-Weinberg Equilibrium is a principle that states that gene frequencies within a population remain stable over generations if certain conditions are met.
Assumptions for Stability:
1. Mating is random.
2. No selection occurs.
3. No mutations are present.
4. No migration occurs (closed population).
5. Population size is large enough to avoid genetic drift.

Definition: Assortative mating is a non-random mating pattern in which individuals choose mates based on specific traits.
Types of Assortative Mating:
- Positive Assortative Mating: Both mates have the same trait ("like to like" or homogamy).
- Negative Assortative Mating: Both mates have different traits (mating opposites).

Concept: In many species, a dominance hierarchy exists, where older or dominant males tend to mate with the majority of females.
Examples:
- Deer
- Wild Horses

Implication: Negative assortative mating encourages the mating of opposite homozygotes, which leads to the production of heterozygotes.
Example in Horses:
- Crossing Sorrell (DD) with Cremello (dd) produces Palomino (Dd).

Impact on Codominant Alleles: Positive assortative mating can affect the distribution of traits such as in roan cattle.
Mathematical Representation: For mating that is inter se, the contribution can be calculated as:
- $R^R imes R^R = p^2$
- $Rr imes Rr = 2pq$
- $r^r imes r^r = q^2$
- Therefore, for a population:
- Frequency of (RR) = $p^2 + 1/2(pq)$
- Frequency of (rr) = $q^2 + 1/2(pq)$
- Frequency of (Rr) = $pq$
- Traits can lead to a ratio of offspring like 1RR: 2Rr: 1rr for black and white offspring.

Definition: Fitness (denoted as ω or w) refers to the ability of an organism to contribute to the gene pool of the next generation, assessed by the proportion of offspring that can mate.
Fitness Range: Fitness values range from 0 to 1.
Coefficient of Selection (s):
- Definition: A measure of the loss of fitness, or the proportion of the population that cannot reproduce.
- Relationship: The coefficients satisfy the equation: $w + s = 1$ , which can be rearranged to give $w = 1 - s$ .

Context: In the Manx cat breed,
- Genotype MM (homozygous dominant) is lethal.
- Thus, $w = 0$ and $s = 1$ .
Breeding Behavior: If breeders cull 70% of normal-tail kittens, the genotype selected against is mm (homozygous recessive).
Fitness Values of the Normal Tail Trait:
- $w = 0.3$
- $s = 0.7$
Genotypes:
- MM = lethal
- Mm = Manx tail
- mm = normal tail

Owners and breeders influence most selection processes.
Selection Against Traits: Any trait that is undesired may either completely eliminate breeding for that trait or reduce the proportion of individuals that are bred.
Equations to Calculate Dominant Allele Frequency After Selection Pressure:
- Calculation must also consider when phenotypes are not fertile or viable due to selection pressures.
- Adjustment to Hardy-Weinberg equilibrium can be expressed as:
- $(w)(p^2) + (w)(2pq) + (w)(q^2) = 1$
- This shows that formulas are derived for varying selection scenarios.

Context: Selection is aimed at increasing the frequency of a recessive trait, example red coat color.
Calculations:
- $P_1 = P(1-s)$
- If selecting for recessive from dominant calculations yields $1-P$ .

Complete Dominance Favoring Recessive: Selection processes lead to a decrease in the dominant allele frequency $p$ ,
- Resulting in a potential fixation of the recessive allele.
No Dominance: A heterozygote shows traits exactly between two homozygotes leading to stable frequencies that may favor an allele’s fixation.
Incomplete Dominance: Differentiated from no dominance, but displays traits between homozygotes with varying fitness.

Key Trait: Fecundity gene (F allele) increases litter size.
- Genetic Contribution:
- Genotype FF: Litter Size = 5; $w = 1$
- Genotype Ff: Litter Size = 3; $w = rac{3}{5} = 0.6$
- Genotype ff: Litter Size = 2; $w = rac{2}{5} = 0.4$

Definition: Overdominance refers to the phenomenon where heterozygotes exhibit higher fitness compared to either homozygote.
Applications: This might include traits like birthweight where both extreme ends face higher mortality, while intermediate values yield optimal survival.

Phenotype Discussion: The homozygous recessive trait is linked to the disease. Heterozygous individuals (carriers) demonstrate resistance to malaria, hence a greater fitness than homozygous dominant individuals.

Complete Dominance Favoring Dominant and Recessive alleles.
Derived formulas encompass frequency calculations under selective pressures, expressed mathematically in various configurations:
- $AA = p^2$
- $Aa = 2pq$
- With implications for both mating patterns favoring recessive or dominant traits.

Understanding these concepts is essential for comprehending genetics and evolutionary biology. You will not be assessed on deriving the equations themselves, but understanding their implications and applications is paramount in the field.