Natural Selection (Evolution)

Adaptation: a phenotypic trait acted upon by natural selection (N.S.)
- Current function: increases fitness
- Natural Selection: an evolutionary force that acts on individuals and their phenotypes
Alternatives to Natural Selection:
- Exaptation/Cooption: indirect advantages formed from traits developed for different functions
- Genetic Drift: a powerful evolutionary force representing random changes in allele frequencies
Genome Signatures of Selection:
- Linkage Disequilibrium: non-random association of alleles at different loci
- Ka/Ks Ratios: measure of the rate of non-synonymous substitutions to synonymous substitutions
- FST approaching 1: an indicator of highly diverged populations; variability within populations is low compared to between populations
Types of Selection:
- Directional Selection
- Disruptive Selection
- Stabilizing Selection
- Frequency-dependent Selection
Evolutionary Trajectories: tracked on phylogenetic trees
Standing Genetic Variation:
- Involves neutral variants and alleles which allow for a rapid response to changing environments
- In novel environments, evidence of selection can be observed (FST approaches 1)

For decades, many scientists operated under the “adaptationist program” which proposed that all traits were adaptations produced by natural selection.
Genetic Drift must be accounted for before concluding that a trait change is due to N.S.
- It serves as the “default” or reference state for testing natural selection.

Equations for Genetic Drift:
- Equation 1: Time to fixation, $T = 4N$ generations
- Equation 2: Probability of fixation, $p = rac{1}{2N}$
The addition of a selection coefficient $s$ shortens time to fixation significantly:
- For $N=10^6$ , $s=0$ : average time to fixation: 4 million generations; with $s=0.01$ : average time to fixation: 2900 generations
- For $N=10^4$ , $s=0$ : average time to fixation: 40,000 generations; with $s=0.01$ : average time: 1,900 generations
- Therefore, the substitution rate ( $K$ ) of mutations under positive selection surpasses that of neutral mutations.

Example:
- Hu et al. (2023) indicate that many human traits have arisen from genetic drift rather than natural selection.

Natural Selection:
- Results in adaptations and evolutionary modifications that enhance survival and reproductive success (fitness)
- Must be assessed within the specific environment since environmental conditions change over time
- Over time, geographical separation of populations causes speciation

Correlated evolutionary forces:
- Genetic Drift: random mutations, with most mutations being fixed or lost randomly
- Gene Flow: genetic information shared among individuals within the same species, helping maintain species integrity.
Natural Selection: promotes individuals with advantageous alleles (e.g., coloration) to have higher fitness in a specific setting.
Four Evolutionary Forces:
1. Genetic Drift
2. Gene Flow
3. Natural Selection
4. Mutation

Adaptations: traits that enhance comparative fitness
1. Genetic basis capable of being inherited
2. Individuals with adaptations exhibit increased fitness
3. Function of an adaptation must coincide with the origin of the trait
- A phylogenetic framework is needed to establish the origin of function and the trait at the same evolutionary node.

Stabilizing Selection:
- Typically favors medium traits, as in the example of cat tail sizes where small tails compromise balance and large tails are prone to drag or injury.
Disruptive Selection in Stickleback Fish:
- Two distinct groups from a once cohesive population evolve based on environmental pressures.
- Saltwater individuals grow larger, while freshwater individuals exhibit different traits due to differing environmental needs.
Directional Selection:
- Example in Diamondback Terrapin (Malaclemys terrapin):
- Selection pressures favoring higher bite forces correlate with prey types (e.g., hard-shelled mussels).
Interspecific Interactions:
- Barn Swallows exhibit resistance to mite infestation linked with tail length indicating a correlation between sexual selection and natural selection.

Fitness of genotypes can be influenced by their frequency within a population:
- Positive Frequency-dependent Selection: favors common traits (examples include Müllerian mimicry in unpalatable species)
- Negative Frequency-dependent Selection: favors rarer traits and leads to polymorphism within populations.

Müllerian Mimicry Example:
- Sympatric butterfly species exhibit positive frequency-dependent selection. Common traits within geographical populations correlate with fitness leading to predator avoidance.
Exemplifying Inverse Frequency-dependent Selection:
- Maintaining polymorphisms by oscillating fitness traits over time where one allele is rare while another is common, fortifying ecosystem diversity.

Adaptations enhance fitness through natural selection, whereas genetic drift can result in traits devoid of adaptive significance.
Various forms of selection such as stabilizing, disruptive, and directional play roles in shaping population traits.
Understanding frequency-dependent selection emphasizes the nuanced interactions between genotype prevalence and fitness outcomes, further compelling the study of evolutionary biology.