Exam 1
Proximate vs. Ultimate Disease Causation
Proximate Disease Explanations:
Focus on direct symptomatology, mechanistic biological pathways, and immediate "what" and "how" factors causing a condition.
Identify near-term physiological causes, such as pathogen invasion or acute biochemical dysfunction.
Provide essential medical value for immediate clinical interventions, though incomplete for explaining systemic population disparities or evolutionary disease vulnerability.
Ultimate Disease Explanations:
Address the broader evolutionary, biological, and historical "why" behind overall disease vulnerability.
Examine historical trajectories and environmental, social, political, economic, and cultural pressures shaping human biology across long timescales (spanning tens of thousands, hundreds of thousands, or billions of years).
Biocultural Perspectives: Integrate human biological evolution with political, economic, social, and cultural factors to explain differential disease susceptibility across distinct populations.
Embodiment of Lived Experience:
Refers to the process by which social, environmental, and personal lived experiences are physically incorporated into human physiology, altering biological vulnerability.
Stress and Immune Function: Sustained psychological or environmental stress exerts a direct negative impact on the human immune system.
Academic Stress Example: Students frequently experience acute infectious illnesses (such as common colds or COVID-19) immediately following final examinations because sustained high stress impairs the body's immune response, preventing effective defense upon pathogen exposure.
Evolutionary Trait Paradox & Pregnancy Sickness:
Pregnancy sickness (morning sickness) is experienced by approximately to () of pregnant individuals.
The Paradox: On its face, nausea and reduced nutrient uptake during pregnancy appear evolutionary disadvantageous to fetal growth and maternal health.
Functional Hypothesis: Pregnancy sickness acts as an evolved functional trait supporting survival and reproduction. It induces aversion to dietary sources historically prone to bacterial contamination or toxins (e.g., meat and spoiled food in historical periods prior to refrigeration and UV food processing), thereby protecting the embryo and maternal health.
Environmental Inertia and Evolutionary Medicine
Environmental Mismatch & Inertia:
Human histopathological traits and genetic variations evolved over millions of years under specific ancestral environmental pressures.
Physiology adapted to past ancestral environments can result in clinical vulnerabilities when placed in rapid modern environmental shifts.
Genetic Determinants, Cancer, and Evolutionary Models:
BRCA Gene Mutation: Inherited genetic variants such as the mutation linked to elevated breast cancer risk represent genetic disadvantages rather than beneficial evolutionary adaptations.
Unlike pregnancy sickness, breast cancer offers no functional evolutionary advantage.
Evolutionary medicine explains conditions like breast cancer through systemic biological trajectories (such as historical shifts in lifetime reproductive cycles and hormone exposures) rather than classifying cancer itself as an adaptive trait.
Mechanisms of Natural Selection and Adaptation:
Natural Selection: The process by which traits offering a superior fit to a given environment increase in frequency within a population because organisms carrying those traits survive longer and reproduce more successfully.
Fitness: Defined by an organism's survival and reproductive success relative to its specific environmental context.
Adaptations:
Characteristics fit for a very specific environment.
Adaptations are never absolute, universal, or permanent traits.
Camouflage Example: A camouflaged organism effectively evades predators and achieves higher reproduction in its native habitat. If placed into a completely different environment where its color pattern no longer matches the background, the camouflage ceases to function as an adaptation.
Historical Trajectories in Evolutionary Thought
Nature of Scientific Theory:
A scientific theory is not a guess; it is an empirically grounded, rigorously tested explanation of natural phenomena.
Evolutionary theory explains the mechanisms by which biological populations change over time through genetic variation and natural selection.
Aristotelian View (Fixed and Immutable World):
Asserted that the Earth is completely full and static, maintaining a fixed quantity of species without addition or extinction.
Held that species are immutable (unchanging) and that all individuals within a species (such as foxes in a garden) are identical across all contexts and times.
Jean-Baptiste Lamarck ( Century):
Observed that the natural world is characterized by continuous dynamic change rather than static states.
Proposed the mechanism of inheritance of acquired characteristics driven by environmental demands:
An environmental pressure forces an organism to exert physical effort during its individual lifetime.
The organism acquires modified physical structures in response to this demand.
Lamarckian Rabbit Example: A rabbit pursued by a coyote faces environmental pressure to run faster. The rabbit exerts massive physical effort into its quadriceps muscles during its lifetime to become faster, and subsequently passes these modified muscle structures directly to its offspring.
Modern Epigenetic Caveat: While direct physical exertion does not rewrite genetic sequences, environmental interactions can alter gene expression (epigenetics), which can exert transgenerational influences.
Charles Darwin and Natural Selection:
Spent years traveling aboard a ship along diverse coastlines, collecting extensive geological, meteorological, and biological specimens.
Spent years analyzing observational data, studying finches and other specimens, before publishing On the Origin of Species in .
Challenged creationist views that living species were individually created by a higher being as permanent, immutable forms.
Key Insights:
Recognized natural individual variation within populations (e.g., subtle variations in beak size, body size, and coloration among Galápagos finches).
Inferred that individual variability is heritable and central to differential survival and reproductive success.
Darwinian Deductions and Population Dynamics
Foundational Observations:
Overproduction of Offspring: Species (such as frogs, mice, insects, and finches) possess the biological capacity to produce vast quantities of offspring.
Population Stability: Natural population sizes tend to remain relatively stable over extended periods.
Resource Limits: Environmental resources (food, water, territory, mates) are finite and limited.
Heritable Variation: Individuals within a population exhibit physical and functional variations, much of which is inherited from parents.
Core Deductions:
Because more individuals are born than available environmental resources can support, organisms engage in a severe struggle for existence.
Survival and reproduction are non-random: individuals possessing advantageous variations suited to environmental demands survive and reproduce at higher rates.
Advantageous traits are passed to the next generation in higher proportions, causing beneficial traits to increase in population frequency over time.
Industrial Melanism in Peppered Moths
Peppered Moth (Biston betularia) Trajectory in Industrial England:
Pre-Industrialization: Light-colored (white) moths were camouflaged against light lichen-covered tree bark, whereas dark-colored moths were easily seen and eaten by birds. Light moths existed at high population frequencies, while dark moths were rare.
Industrialization Effects: Heavy coal burning deposited black soot onto tree trunks, darkening the bark.
Selective Pressure Shift: Dark-colored moths became camouflaged against soot-darkened bark, surviving at significantly higher rates. Light-colored moths were heavily preyed upon by birds.
Population Shift: The frequency of dark-colored moths increased dramatically in post-industrial populations, while light-colored moths underwent a steep decline.
Questions & Discussion
Student Discussion on Lecture Pace:
Observation: Students discussed the rapid acceleration of the lecture delivery speed, noting difficulty in keeping pace with handwritten or typed notes.
Clarification: Students observed that despite the rapid pace, class runs until , providing sufficient time to complete the remaining slide material and interactive activities.
Student Analysis of Peppered Moth Selection:
Inheritance: Dark-colored moths survived bird predation and transmitted dark coloration traits to their offspring, elevating dark moth survival rates across generations.
Nature of Competition:
Question: Does bird predation count as competition, or does competition strictly refer to intraspecific competition within the same species?
Conclusion: Predation serves as the primary selective force. As dark moth survival rates increased, the dark moth population density expanded, subsequently increasing intraspecific competition for local environmental resources among dark moths.
Differential Survival: Advantageous phenotypic survival reduced overall wing color variation in the population over time.
Galápagos Finch Long-Term Research
Dynamic Dot Selection Model:
Generation 1: Baseline population displays color variation (clear dots, light gray dots, and dark dots).
Generation 2: Genetic mutations introduce novel color variations into the population gene pool.
Generation 3: Environmental shift renders the lightest dot phenotype non-adaptive, resulting in failed survival and reproduction for light dots.
Subsequent Generations: Continued mutations combined with fluctuating environmental pressures continuously alter dot phenotype frequencies over generations.
Peter and Rosemary Grant's Long-Term Field Study:
Background: Initiated in by Peter Grant (emeritus professor at Princeton University) and Rosemary Grant on Galápagos islands such as South Island / Daphne Major.
The Drought Event:
A severe drought struck the island in , causing small seeds from annual plants to disappear.
The remaining food supply consisted exclusively of large, hard, drought-resistant seeds (caltrop/pills).
Finch mortality crashed the population dramatically due to starvation.
Morphological Selection on Beak Dimensions:
Finches with small, shallow beaks could not crack open large, hard seeds and died at high rates.
Finches with larger overall body size and larger, deeper beaks successfully cracked hard seeds, survived, and reproduced.
Heritable Trajectory: Average beak size in the surviving population increased significantly in offspring generations following the drought.
Context-Dependent Adaptation:
Long-term data through showed beak size fluctuating bi-directionally alongside wet and dry environmental cycles.
Large beak size is adaptive during drought conditions dominated by hard seeds, whereas smaller beak size becomes adaptive during wet periods abundant in small seeds.