Ecology April 6th

There are three different types of natural selection relevant to population changes:
- Directional Selection: Favors one extreme phenotype over the other, causing a shift in allele frequency.
- Stabilizing Selection: Favors intermediate phenotypes, reducing variation.
- Disruptive Selection: Favors extreme phenotypes, leading to increased variation.

Genetic drift refers to changes in allele frequencies within a population due to random sampling effects.
Key considerations related to genetic drift:
- Genetic drift does not necessarily involve natural selection, even if phenotypic changes occur in a population.
Founder Effect:
- Occurs when a small number of individuals establish a new population, potentially with a different allele frequency than the original population.
- Example: If dark-colored fish colonize a new pond, the resulting population may have a higher frequency of dark coloration due to their initial overrepresentation in the founder effect.
Population Bottleneck:
- Occurs when a population’s size is significantly reduced for at least one generation, potentially leading to a loss of genetic variation.
- Example: If only darker fish survive a severe reduction in population size, this can result in an increased frequency of the dark allele, not due to fitness advantages, but rather through the bottleneck effect.

Evolution through genetic drift does not depend on individual fitness.
Importance of genetic variation:
- For natural selection to occur, there must be genetic variation within a population; without it, adaptation cannot take place.
- If a population consists solely of clones (with identical alleles), natural selection cannot act upon them.

Gene Flow:
- Gene flow refers to the transfer of alleles or genes from one population to another through migration of individuals or gametes (e.g., pollen).
- Populations are composed of individuals that are more likely to mate with others within the same population than with those in different populations.
- Gene flow can counteract the effects of genetic drift and natural selection by introducing new genetic material into a population.
Factors affecting speciation:
- Speciation requires reduced gene flow between populations to enable divergence and local adaptation.
- This can occur spatially through geographical barriers or temporally through differences in mating times.

Local adaptation occurs when different populations evolve distinct traits that enhance survival in their specific environments.
- Example: A population of plants in the mountains may evolve to have shorter, more compact structures compared to taller plants at the coast, as adaptations to different environmental conditions.
Common Garden Experiment:
- A method used to test if differences observed are due to genetic adaptation or phenotypic plasticity.
- Plants or animals from different environments are grown in a controlled setting to observe growth and traits.
- If coastal plants maintain their tall structure when grown in mountain conditions, they exhibit phenotypic plasticity. If they adapt to become shorter, it supports local adaptation hypothesis.

Phenotypic Plasticity:
- The ability of a single genotype to express different phenotypes in response to varying environmental conditions.
Example of phenotypic plasticity:
- If plants from the mountainous regions are grown at coastal locations and become taller, this indicates plasticity rather than permanent genetic changes.

Definition of Species:
- A group of individuals that can interbreed and produce viable, fertile offspring; this definition is typically used in the biological species concept.
- Speciation occurs when populations become reproductively isolated due to barriers to gene flow.
Barriers that can lead to speciation:
- Geographical: Physical barriers like rivers or mountains that separate populations.
- Temporal: Mating at different times or seasons can prevent interbreeding.
Incipient Speciation:
- Refers to the initial stages of speciation, where populations show signs of divergence without complete reproductive isolation.

Life history strategies can be categorized into two main types: r-selected and K-selected species.
- r-selected species:
- Typically exhibit high reproductive rates, produce many offspring, and invest little effort in raising those offspring.
- Found in less crowded, disturbed habitats.
- K-selected species:
- Invest more time and resources in raising fewer offspring with a higher likelihood of survival.
- Found in stable, crowded environments where competition for resources is high.
Trade-offs between reproductive strategies exist, influenced by environmental conditions and resource availability.

The study of clutch size in birds has shown how different environmental pressures can influence reproductive strategies.
- Researchers manipulate clutch sizes to study optimal reproductive outcomes, focusing on factors like offspring survival and parental investment in food and care.
- Iteroparous species (which reproduce multiple times) may have different optimal clutch sizes compared to semelparous species (which reproduce only once).

Environmental conditions, such as resource availability or seasonal variations, greatly impact reproductive success and clutch size decisions.
The ideal reproductive strategy will adjust based on the context, aiming to maximize offspring survival while maintaining parental health and future reproductive opportunities.

The continual interactions between genetic drift, selection, gene flow, and environmental factors lead to dynamic changes in populations, species diversity, and adaptation within ecosystems.
Understanding these processes is essential for effective conservation strategies, especially in the face of habitat alteration and climate change.