2026 Honors Biology Evolution Study Guide

Evidence for the Theory of Evolution and Fossil Dating

• Conceptual Overview of Scientific Support: Scientific information supports the theory of evolution by demonstrating how species change over time through observable data.

• Fossil Dating Techniques:

  • Relative Dating: This method involves comparing rock layers to determine the chronological order of fossils without determining their exact age.

    • Older vs. Youngest Layers: According to the principle of superposition, in an undisturbed sequence of rocks, the layer at the bottom (e.g., Layer M) contains the oldest fossils, while the layer at the top (e.g., Layer A) contains the youngest fossils.

    • Drawbacks: One significant limitation of relative dating is that it provides only a sequence of events and does not provide an actual age in years.

  • Absolute (Radiometric) Dating: This method provides a specific numerical age for a fossil or rock layer by measuring the decay of radioactive isotopes.

    • Carbon-14 ($C-14$) Properties: $C-14$ has a half-life of $5700\,\text{years}$. Every $5700\,\text{years}$, half of the remaining $C-14$ decays into Nitrogen-14 ($N-14$).

• Mathematical Applications of Absolute Dating:

  • Scenario 1 (Determining Half-Lives): If a fossil is found with $1/8$ of its original $C-14$, the number of half-lives passed is calculated as follows:

    • $1 \rightarrow 1/2$ (1 half-life)

    • $1/2 \rightarrow 1/4$ (2 half-lives)

    • $1/4 \rightarrow 1/8$ (3 half-lives)

    • Total: $3$ half-lives.

  • Scenario 2 (Calculating Age via Fractions): If a fossil contains $1/16$ of its original $C-14$:

    • $1 \rightarrow 1/2 \rightarrow 1/4 \rightarrow 1/8 \rightarrow 1/16$ (4 half-lives).

    • Age Calculation: $4 \times 5700\,\text{years} = 22,800\,\text{years}$.

  • Scenario 3 (Calculating Remaining Mass): If a rock starts with $200\,g$ of $C-14$ and is $11,400\,\text{years}$ old:

    • Number of half-lives: $\frac{11,400}{5700} = 2\,\text{half-lives}$.

    • Mass degradation: $200\,g \rightarrow 100\,g$ (1st half-life) $\rightarrow 50\,g$ (2nd half-life).

    • Remaining mass: $50\,g$.

Comparative Anatomy: Homologous, Analogous, and Vestigial Structures

• Homologous Structures:

  • Definition: Structures in different organisms that share a common ancestral origin, even if they serve different functions in the present day.

  • Evidence for Evolution: They demonstrate that different species evolved from a common ancestor, inheriting a basic structural blueprint that was modified over time.

  • Functionality: Homologous structures do NOT always have the same function. For example, the forelimbs of humans, bats, and whales are homologous but are used for grasping, flying, and swimming, respectively.

• Analogous Structures:

  • Definition: Structures that perform a similar function but do not share a common evolutionary origin.

  • Examples: Wings of a bird and wings of a butterfly; both are used for flight but evolved independently.

  • Lack of Evolutionary Evidence: Analogous structures are NOT evidence for evolution from a common ancestor because they represent convergent evolution—independent solutions to similar environmental pressures rather than shared heritage.

• Vestigial Structures:

  • Definition: Remnants of organs or structures that had a function in an early ancestor but no longer serve a purpose in the modern organism.

  • Case Study: Cavefish vs. Minnow:

    • Minnows live near the surface and have functioning external eyes connected to optic nerves.

    • Cavefish live in dark caves and have no external eyes, yet they still possess optic nerves.

    • Vestigial Organ: The optic nerve in the cavefish is the vestigial structure because it is a non-functioning remnant of a structure that would operate an eye.

    • Ancestor Connection: The presence of these nerves suggests the cavefish and minnow shared a common ancestor with functioning eyes.

  • Case Study: Whale Pelvis and Leg Bones: Whales possess a pelvis and leg bones despite not using legs for locomotion. This suggests whales are related to land-dwelling mammals that used these bones for walking.

Comparative Biochemistry and Embryology

• Comparative Biochemistry (Cytochrome C):

  • Mechanism: Cytochrome C is a protein involved in cellular respiration in all eukaryotes. By comparing the amino acid sequence of this protein, scientists determine evolutionary distance.

  • Data Analysis (Differences relative to humans):

    • Chimpanzees: $0$ differences (Most closely related).

    • Rhesus monkeys: $1$ difference.

    • Dogs: $13$ differences.

    • Chickens: $18$ differences.

    • Rattlesnakes: $20$ differences.

    • Yeasts: $56$ differences (Least closely related).

  • DNA Correlation: Similar amino acid sequences in proteins imply high similarity in DNA sequences, as DNA codes for the production of proteins.

• Comparative Embryology:

  • Overview: Study of how embryos of different species develop. If embryos look similar during early stages, it suggests a common ancestor.

  • Relationship Example: Organisms like salamanders show closer embryonic resemblance to other amphibians or certain vertebrates than to more distant taxa.

Natural Selection as a Mechanism for Evolution

• Four Main Principles of Natural Selection:

  1. Variation: There must be differences in traits within a population (e.g., variations in bird leg lengths).

  2. Overproduction (Too much reproduction): Organisms produce more offspring than the environment can support, leading to a struggle for existence where not everyone survives.

  3. Adaptations: Certain traits make an individual better suited to survive and reproduce. These traits are then passed to the next generation.

  4. Descent with Modification: Over time, natural selection results in populations where more individuals possess desirable adaptations, changing the characteristics of the lineage.

• Core Concepts of Selection:

  • Main Source of Variation: Genetic mutations and recombination of genes during sexual reproduction.

  • The Determinant of Survival: The environment "chooses" which traits are helpful. Traits that provide a survival advantage in a specific environment are selected for.

  • Fitness: A measure of an organism's ability to survive and produce offspring relative to other members of the population. An organism is "fit" if it successfully passes its genes to the next generation.

• Historical Examples and Case Studies:

  • Peppered Moths:

    • Before $1845$, most moths in England were white because they blended into light-colored lichen on trees.

    • After $1845$, industrial pollution darkened the trees. Darker moths were better camouflaged and survived predation, leading to a population shift.

    • Process: This was not a conscious choice by the moths but the process of Natural Selection. As pollution decreases, the population will likely shift back toward white moths.

  • Rock Pocket Mice (New Mexico):

    • A volcanic eruption left dark lava patches on pale sand.

    • Population Data (1920–2000):

      • 1920: $400\,\text{tan}$, $35\,\text{black}$.

      • 1940: $150\,\text{tan}$, $100\,\text{black}$.

      • 1970: $83\,\text{tan}$, $306\,\text{black}$.

      • 2000: $20\,\text{tan}$, $395\,\text{black}$.

    • Analysis: Dark lava most likely covered the sand around $1940$, as that is when the black mouse population began to increase rapidly. The mice did not choose the change; rather, black mutants survived better on dark rock.

Graphical Analysis of Advantageous Traits

• Directional Selection: Occurs when natural selection favors one of the extreme variations of a trait (e.g., if only the dark color becomes beneficial, the entire graph shifts toward the dark phenotype).

• Stabilizing Selection: Occurs when the average/intermediate phenotype is favored, and extreme phenotypes are selected against (e.g., light brown mice survive better than pure white or dark brown mice).

• Disruptive Selection: Occurs when both extreme phenotypes are favored at the expense of middle/intermediate phenotypes (e.g., hummingbirds with very short bills for shallow flowers and very large bills for deep flowers thrive, while medium-billed birds cannot access either efficiently).

Allele Frequencies and Population Genetics

• Definitions:

  • Gene Pool: The total collection of all alleles (genes) in a population.

  • Allele Frequency: The proportion of a specific allele (e.g., $B$ or $b$) in a gene pool. It is calculated by dividing the instances of a specific allele by the total number of alleles for that gene.

• Allele Frequency Calculation Example (Rabbits):

  • Population in 2000:

    • Homozygous Dominant ($BB$): $10$ individuals ($20\,B$ alleles).

    • Heterozygous ($Bb$): $10$ individuals ($10\,B$, $10\,b$ alleles).

    • Homozygous Recessive ($bb$): $10$ individuals ($20\,b$ alleles).

    • Total Alleles: $60$.

    • Frequency: $B = 30/60 = 0.50$; $b = 30/60 = 0.50$.

  • Population in 2015:

    • Homozygous Dominant ($BB$): $20$ individuals ($40\,B$ alleles).

    • Heterozygous ($Bb$): $5$ individuals ($5\,B$, $5\,b$ alleles).

    • Homozygous Recessive ($bb$): $5$ individuals ($10\,b$ alleles).

    • Total Alleles: $60$.

    • Frequency: $B = 45/60 = 0.75$; $b = 15/60 = 0.25$.

  • Evolutionary Conclusion: Because the allele frequencies changed from $0.50$ to $0.75$ for $B$, evolution IS occurring in this population.

  • Variation: The year $2000$ had more variation because the allele frequencies were more evenly balanced ($0.50/0.50$) compared to the dominance of the $B$ allele in $2015$.

Mechanisms of Evolution and Speciation

• Hardy-Weinberg Principle: Describes a hypothetical population that is NOT evolving (Genetic Equilibrium). For this to occur, five conditions must be met:

  1. No mutations.

  2. No gene flow (no migration in or out).

  3. Random mating (no sexual selection).

  4. Extremely large population size (prevents genetic drift).

  5. No natural selection (everyone has equal survival/reproduction).

• Alternative Evolutionary Forces:

  • Genetic Drift: A change in the gene pool due to chance/random events. It affects small populations most significantly.

  • Gene Flow: The movement of alleles into or out of a population through migration.

  • Sexual Selection: Selection based on traits that improve an organism’s chance of finding a mate, which can alter the gene pool even if the traits aren’t ideal for survival.

• Speciation:

  • Definition: The emergence of a new species when one species separates into two.

  • Determination Point: Two groups are officially considered separate species when they can NO LONGER interbreed to produce fertile offspring.

  • Galapagos Finches: Speciation likely occurred as finches flew to different islands with different food sources. Over time, through geographic isolation and adaptation to specific seeds or insects (Natural Selection), they became reproductively isolated.