Variation and Location Pattern Notes

Variation and Location Pattern in Process

Learning Objectives:

• Draw a model that causally explains how a variation factory contributes to global biological patterns (e.g., human ability to digest milk).

• Explain how different heritable variants impact health, survival, and reproduction.

• Describe how different environments can promote or inhibit survival and reproduction.

• Integrate multiple processes to build a causal model explaining geographic patterns.

Association Between Variation and Location

• Patterns of variation within a species are associated with the location of specific populations on Earth.

Definitions:

• Species: A group of potentially or actually interbreeding organisms capable of producing fertile offspring.

• Population: A group of individuals of the same species living in the same area.

• Most species comprise many populations with individuals moving between them.

Variation in Species:

• Biologists consider the range and frequency of variants within a species.

• Example: Color variation in a species ranging from white to black, with varying frequencies of each phenotype.

Variation in Populations:

• Biologists consider the range and frequency of variants within and among populations.

• Example: Population one has the same range of color variation as the species, but with fewer black phenotypes. Population two is missing gray variants, with most individuals having a black phenotype.

Key Points:

• A single population may harbor only a subset of the total range of variation in the species.

• Populations can differ in the frequencies of particular variants.

Causal Models

• Question: Is the variation factory the only cause, or should we add other factors to our causal models?

• Explore this question using mice as an example.

Important Questions:

• Do the same processes (mutation, independent assortment, recombination) occur in different locations?

• Can the same processes produce different outcomes?

• Are the same outcomes due to the same processes?

The Role of Environment

• The environment sorts the variations that are beneficial, without intention or force.

• It dictates which variants make an organism more likely to survive and reproduce.

• "Environmental pressures," "evolutionary pressures," and "selective pressures" are misleading terms as they imply a force.

Lactase Persistence Example:

• Pattern: High percentage of adults who can digest milk in some regions (e.g., Western Africa), low percentage in others (e.g., Southeast Asia).

• Historical Context: Cattle raising in Western Africa provided adults access to dairy over generations.

• Causal Explanation:

  • The dairy-digesting phenotype (blue dot) was initially uncommon but present in both populations due to random mutation in the lactase gene.

  • In Eastern Africa, this phenotype provided a survival and reproductive advantage due to access to a new food source (dairy).

  • In Southeast Asia, it had no significant impact on survival or reproduction.

  • Environmental conditions, like the presence of cattle, impacted the frequency of variants over time.

Question and Answer:

• Question: Why are the variants from today different from generation one in Eastern Africa?

• Correct Explanation: The differential survival and reproduction of variants based on environmental conditions (production, transmission, and sorting of variation) accounts for differences in variation.

Processes Explaining Variation

• Genetic Processes: Mutations, independent assortment, and recombination provide heritable variation.

• Sorting of Variation: Differential survival and reproduction of variants depending on environments.

• Natural Selection: Variable variation transmitted and sorted differently based on different environments;

Common Ways Variation is Sorted:

• Original population shows a distribution of fur colors in mice, with intermediate colors being most common.

Types of Selection:

• Stabilizing Selection: Decreases variance by eliminating extremes, keeping the mean the same.

• Directional Selection: Shifts the mean value of a character; for example, darker variants become more common.

• Disruptive Selection: Increases variance by making extreme variants more common and intermediate variants less common.

Real-World Example: Coronavirus Variants

• The New York City health department tracks coronavirus variants over time, showing how distributions of variation change.

• Certain variants increase or decrease in prevalence, demonstrating the sorting of variation.

• This has implications for epidemiology, medicine, and drug design.

Integrating Multiple Processes

• The goal is to build a causal model explaining patterns in various life forms (humans, coronavirus, mice) by integrating:

  • Variation factory producing variation.

  • Environment sorting processes (directional, stabilizing, and disruptive selection).

Variation and Location Pattern in Process

Learning Objectives:

• Draw a model that causally explains how a variation factory contributes to global biological patterns (e.g., human ability to digest milk).

• Explain how different heritable variants impact health, survival, and reproduction.

• Describe how different environments can promote or inhibit survival and reproduction, considering factors like resource availability, climate, and interactions with other species.

• Integrate multiple processes to build a causal model explaining geographic patterns, including genetic variation, environmental factors, and selection pressures.

Association Between Variation and Location

• Patterns of variation within a species are associated with the location of specific populations on Earth, reflecting adaptations to local environmental conditions and historical events.

Definitions:

• Species: A group of potentially or actually interbreeding organisms capable of producing fertile offspring, maintaining genetic exchange.

• Population: A group of individuals of the same species living in the same area, characterized by interbreeding and sharing a gene pool.

• Most species comprise many populations with individuals moving between them, leading to gene flow and shared genetic ancestry.

Variation in Species:

• Biologists consider the range and frequency of variants within a species to understand its evolutionary potential and adaptability.

• Example: Color variation in a species ranging from white to black, with varying frequencies of each phenotype, reflecting genetic diversity and adaptation to different environments.

Variation in Populations:

• Biologists consider the range and frequency of variants within and among populations to assess genetic diversity and adaptation to local conditions.

• Example: Population one has the same range of color variation as the species, but with fewer black phenotypes. Population two is missing gray variants, with most individuals having a black phenotype, indicating genetic drift or local selection pressures.

Key Points:

• A single population may harbor only a subset of the total range of variation in the species due to founder effects, genetic drift, or local adaptation.

• Populations can differ in the frequencies of particular variants due to natural selection, genetic drift, gene flow, and mutation.

Causal Models

• Question: Is the variation factory the only cause, or should we add other factors to our causal models to fully explain observed patterns of variation?

• Explore this question using mice as an example, analyzing how genetic and environmental factors interact to shape phenotypic traits.

Important Questions:

• Do the same processes (mutation, independent assortment, recombination) occur in different locations, and if so, at the same rates?

• Can the same processes produce different outcomes depending on the genetic background and environmental context?

• Are the same outcomes due to the same processes, or can different combinations of genetic and environmental factors lead to similar phenotypic results?

The Role of Environment

• The environment sorts the variations that are beneficial, without intention or force, by favoring traits that enhance survival and reproduction in specific conditions.

• It dictates which variants make an organism more likely to survive and reproduce, leading to adaptation to local environments.

• "Environmental pressures," "evolutionary pressures," and "selective pressures" are misleading terms as they imply a force; selection is a consequence of differential survival and reproduction.

Lactase Persistence Example:

• Pattern: High percentage of adults who can digest milk in some regions (e.g., Western Africa), low percentage in others (e.g., Southeast Asia), correlating with the history of dairy farming.

• Historical Context: Cattle raising in Western Africa provided adults access to dairy over generations, favoring the persistence of lactase production into adulthood.

• Causal Explanation:

  • The dairy-digesting phenotype (blue dot) was initially uncommon but present in both populations due to random mutation in the lactase gene.

  • In Eastern Africa, this phenotype provided a survival and reproductive advantage due to access to a new food source (dairy), leading to increased fitness and prevalence.

  • In Southeast Asia, it had no significant impact on survival or reproduction, resulting in a lower frequency of the lactase persistence allele.

  • Environmental conditions, like the presence of cattle, impacted the frequency of variants over time through natural selection, shaping the genetic structure of populations.

Question and Answer:

• Question: Why are the variants from today different from generation one in Eastern Africa, despite starting with similar genetic variation?

• Correct Explanation: The differential survival and reproduction of variants based on environmental conditions (production, transmission, and sorting of variation) accounts for differences in variation, leading to adaptation to local environments.

Processes Explaining Variation

• Genetic Processes: Mutations, independent assortment, and recombination provide heritable variation, generating the raw material for evolutionary change.

• Sorting of Variation: Differential survival and reproduction of variants depending on environments, leading to adaptation and divergence among populations.

• Natural Selection: Variable variation transmitted and sorted differently based on different environments; Natural selection acts on this variation, favoring traits that enhance survival and reproduction in specific environments.

Common Ways Variation is Sorted:

• Original population shows a distribution of fur colors in mice, with intermediate colors being most common; Environmental factors can then shift this distribution through different modes of selection.

Types of Selection:

• Stabilizing Selection: Decreases variance by eliminating extremes, keeping the mean the same, favoring intermediate phenotypes in stable environments.

• Directional Selection: Shifts the mean value of a character; for example, darker variants become more common, leading to adaptation to changing environmental conditions.

• Disruptive Selection: Increases variance by making extreme variants more common and intermediate variants less common, promoting diversification and potentially leading to speciation.

Real-World Example: Coronavirus Variants

• The New York City health department tracks coronavirus variants over time, showing how distributions of variation change, reflecting the ongoing evolution of the virus.

• Certain variants increase or decrease in prevalence, demonstrating the sorting of variation, driven by factors such as transmissibility and immune evasion.

• This has implications for epidemiology, medicine, and drug design, informing public health strategies and vaccine development.

Integrating Multiple Processes

• The goal is to build a causal model explaining patterns in various life forms (humans, coronavirus, mice) by integrating:

  • Variation factory producing variation through mutation, recombination, and gene flow.

  • Environment sorting processes (directional, stabilizing, and disruptive selection) driven by environmental factors and biotic interactions.