Unit 3 Notes: Life Changing Through Time

Unit 3: Life Changing Through Time

• Unit 3 builds upon concepts from Unit 1 (Nature of Science) and Unit 2 (Genetics).

  • Science aims to explain the natural world.

  • Genetics explains mechanisms causing observable patterns.

• Unit 3 explores how life changes over time, addressing common misconceptions about evolution.

Learning Objectives

• Explain how mutation, recombination, and independent assortment constantly generate heritable variation within species.

• Identify two reasons why humans underestimate variation in natural populations and the misconceptions this promotes.

• Provide examples of strong associations between variation within a species and its environment.

The Variation Factory

• The "variation factory" continuously produces new variants through:

  • Mutation: The fuel, introducing new genetic changes.

    • Nearly all individuals have unique mutations not present in their parents.

    • Mutations can be beneficial, detrimental, or neutral.

  • Recombination: Shuffles mutations to different chromosome copies.

  • Independent Assortment: Packages chromosomes into gametes.

    • Each gamete contains different genetic combinations.

    • Billions of variations exist within gametes.

Measuring Variation

• Human genomes exhibit a significant number of variant sites per person.

  • Data shows between 3,800,000 to 5,000,000 genetic variants per individual.

• A substantial amount of DNA variation contributes to protein variation.

  • High-density lipoprotein (HDL) exhibits diverse structural variants.

• Protein variants lead to variations in physical features among individuals of the same species.

  • Example: Short crabs with diverse physical appearances due to protein and DNA differences.

Underestimation of Variation

• Most people have only observed a tiny fraction of the true variation within species, including humans.

  • Humans underestimate variation due to limited exposure to the full range of individuals within a species.

  • Perceptions of biological variation are biased because they are based on a small sample.

• Failure to acknowledge variation leads to cognitive biases.

Cognitive Biases and Variation

• Example: Tigers

  • Most people's mental image of a tiger is based on limited observations (e.g., zoo, TV).

  • The documented variation among tigers is much greater than commonly perceived.

  • Cognitive biases prevent people from understanding the true range of variation.

• The true range of variation within most species is often undocumented.

• Examples of variation in other species:

  • Snails: Same species with very different appearances.

  • Pigeons: Exhibit a wide range of variations not commonly recognized.

  • Ladybugs: Display diverse patterns due to the variation factory.

  • Plants: Exhibit one species that differs a lot.

• Even biologists are subject to cognitive biases regarding variation.

• Metacognition is essential to recognize and address these biases.

Human Variation

• Most people have seen less than 1% of all humans on Earth.

  • Seeing 1% of humans would equate to approximately 57,000,000 people.

• Representations of humans sent to outer space often reflect idealized and inaccurate versions of human variation.

Significance of Understanding Variation

• Recognizing the magnitude of variation is crucial for understanding evolution.

• Cognitive biases can lead to pathologizing naturally occurring variation.

• It is important to view variation as a natural and workable aspect of nature.

• Everyday thinking often considers outliers as noise or abnormalities, whereas biologists view them as essential for understanding variation.

• Variation should not be categorized as "good" or "bad" from a biological perspective.

• Outliers and noise are sources of new combinations that nature can utilize.

• The variation factory seems to produce different patterns in different species through the similar genetic processes.

Repeated Patterns of Variation

• Albinism: A repeated pattern across species (squirrels, crocodiles, snakes, humans).

  • Caused by similar genetic processes (mutations affecting melanin production).

• Different patterns of variation can arise from the same genetic processes.

  • However, variants can be sorted in different ways in different location.

• Example: Ladybugs with different coloration patterns in different locations, because the variants have been sorted differently.

• Association studies document patterns and relationships between variables.

  • Example: Association between plant density and ladybug coloration.

Human Examples of Variation and Association

• Lactase Persistence: Ability to digest milk as adults varies geographically.

  • High percentages in Western Africa, Arabian Peninsula, and Northern Europe.

  • Low percentages elsewhere.

  • This heritable pattern is not due to different mutational processes among humans.

• Insect coloration patterns:

  • Different patterns associated in different parts of its range, just like humans have different milk digestion patterns.

• Snake coloration patterns:

  • Different populations of the same species exhibit extremely different coloration.

• Skin Color: Varies across human populations.

  • Heritable variant, but no difference in genetic processes among human populations.

• All these cases have a lot of similarities and a lot of differences.

Building a Causal Model

• Biologists aim to develop unified explanatory models for these phenomena.

• Consider what is the same and what is different across cases.

• General pattern: Associations between heritable phenotypic variation and geographic location.

Key Points

• The variation factory operates constantly in sexually reproducing organisms.

• There is much more variation within species than we typically perceive.

• This variation is not randomly distributed; it exhibits associations with environmental conditions.

Questions to Consider

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

  • Yes, humans share the same genetic mechanisms.

• Do the same processes produce different outcomes?

• Are the same outcomes (patterns) due to the same processes?

Unit 3: Life Changing Through Time

• Unit 3 builds upon concepts from Unit 1 (Nature of Science) and Unit 2 (Genetics).

  • Science aims to explain the natural world by identifying patterns and underlying mechanisms.

  • Genetics explains the specific mechanisms that cause observable patterns of inheritance and variation.

Learning Objectives

• Explain how mutation, recombination, and independent assortment continuously generate heritable variation within species. Understand the roles of each process in creating genetic diversity.

• Identify two reasons why humans underestimate variation in natural populations and the misconceptions this promotes. Recognize how limited exposure and cognitive biases affect perception.

• Provide examples of strong associations between variation within a species and its environment. Understand how environmental factors can influence the distribution of genetic traits.

The Variation Factory

• The "variation factory" continuously produces new variants through:

  • Mutation: The fuel, introducing new genetic changes.

  • Nearly all individuals have unique mutations not present in their parents, leading to individual genetic uniqueness.

  • Mutations can be beneficial, detrimental, or neutral, influencing adaptation and evolution.

  • Recombination: Shuffles mutations to different chromosome copies during meiosis, creating new combinations of alleles.

  • Independent Assortment: Packages chromosomes into gametes, ensuring each gamete contains different genetic combinations.

  • Each gamete contains different genetic combinations, leading to billions of variations within gametes.

Measuring Variation

• Human genomes exhibit a significant number of variant sites per person.

  • Data shows between 3,800,000 to 5,000,000 genetic variants per individual, highlighting the extent of human genetic diversity.

• A substantial amount of DNA variation contributes to protein variation.

  • High-density lipoprotein (HDL) exhibits diverse structural variants, impacting its function and association with cardiovascular health.

• Protein variants lead to variations in physical features among individuals of the same species.

  • Example: Short crabs with diverse physical appearances due to protein and DNA differences, illustrating how genetic variation manifests in observable traits.

Underestimation of Variation

• Most people have only observed a tiny fraction of the true variation within species, including humans.

  • Humans underestimate variation due to limited exposure to the full range of individuals within a species.

  • Perceptions of biological variation are biased because they are based on a small sample, leading to inaccurate generalizations.

• Failure to acknowledge variation leads to cognitive biases.

Cognitive Biases and Variation

• Example: Tigers

  • Most people's mental image of a tiger is based on limited observations (e.g., zoo, TV).

  • The documented variation among tigers is much greater than commonly perceived, including differences in size, coloration, and stripe patterns.

  • Cognitive biases prevent people from understanding the true range of variation, leading to simplified and often inaccurate mental models.

• The true range of variation within most species is often undocumented.

• Examples of variation in other species:

  • Snails: Same species with very different appearances.

  • Pigeons: Exhibit a wide range of variations not commonly recognized.

  • Ladybugs: Display diverse patterns due to the variation factory.

  • Plants: Exhibit one species that differs a lot.

• Even biologists are subject to cognitive biases regarding variation. Awareness of these biases is important to prevent misinterpretations.

• Metacognition is essential to recognize and address these biases. Biologists can use statistical and computational tools to accurately quantify and analyze variation patterns.

Human Variation

• Most people have seen less than 1% of all humans on Earth.

  • Seeing 1% of humans would equate to approximately 57,000,000 people, illustrating the vastness of human diversity.

• Representations of humans sent to outer space often reflect idealized and inaccurate versions of human variation. More inclusive and representative depictions are necessary to accurately represent humanity.

Significance of Understanding Variation

• Recognizing the magnitude of variation is crucial for understanding evolution and adaptation.

• Cognitive biases can lead to pathologizing naturally occurring variation, potentially leading to stigmatization or medicalization of normal traits.

• It is important to view variation as a natural and workable aspect of nature, recognizing its role in resilience and adaptation.

• Everyday thinking often considers outliers as noise or abnormalities, whereas biologists view them as essential for understanding variation and identifying novel traits.

• Variation should not be categorized as "good" or "bad" from a biological perspective; instead, it should be understood as a spectrum of possibilities.

• Outliers and noise are sources of new combinations that nature can utilize, providing the raw material for evolutionary change.

• The variation factory seems to produce different patterns in different species through the similar genetic processes, reflecting diverse evolutionary pathways.

Repeated Patterns of Variation

• Albinism: A repeated pattern across species (squirrels, crocodiles, snakes, humans).

  • Caused by similar genetic processes (mutations affecting melanin production), illustrating convergent evolution.

• Different patterns of variation can arise from the same genetic processes.

  • However, variants can be sorted in different ways in different locations, leading to distinct phenotypic distributions.

• Example: Ladybugs with different coloration patterns in different locations, because the variants have been sorted differently.

• Association studies document patterns and relationships between variables.

  • Example: Association between plant density and ladybug coloration, illustrating how environmental factors influence trait distribution.

Human Examples of Variation and Association

• Lactase Persistence: Ability to digest milk as adults varies geographically.

  • High percentages in Western Africa, Arabian Peninsula, and Northern Europe, reflecting historical patterns of dairy farming.

  • Low percentages elsewhere. Most adults lose the ability to digest lactose.

  • This heritable pattern is not due to different mutational processes among humans but rather to the selection for lactase persistence in populations with a long history of dairy consumption.

• Insect coloration patterns:

  • Different patterns associated in different parts of its range, just like humans have different milk digestion patterns, reflecting adaptation to local environmental conditions.

• Snake coloration patterns:

  • Different populations of the same species exhibit extremely different coloration, potentially driven by factors such as camouflage and thermal regulation.

• Skin Color: Varies across human populations.

  • Heritable variant, but no difference in genetic processes among human populations.

• All these cases have a lot of similarities and a lot of differences. Comparative analysis provides insight on evolutionary and biological processes.

Building a Causal Model

• Biologists aim to develop unified explanatory models for these phenomena.

• Consider what is the same and what is different across cases.

• General pattern: Associations between heritable phenotypic variation and geographic location, reflecting the interplay between genes and environment.

Key Points

• The variation factory operates constantly in sexually reproducing organisms.

• There is much more variation within species than we typically perceive.

• This variation is not randomly distributed; it exhibits associations with environmental conditions.

Questions to Consider

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

  • Yes, humans share the same genetic mechanisms, but the effects can vary based on environmental context and selective pressures.

• Do the same processes produce different outcomes? Yes, they can due