Human Ecology 1 & 2 (updated)

What is Ecology?

Ecology is the study of the relationships between organisms and their environment — including physical, biological, and social environments — and the factors that determine the numbers and distribution of organisms.

Why is ecology important?

Ecology explains the day-to-day workings of natural selection, showing how environmental pressures shape species’ survival and reproduction.

What is Human Ecology?

Human ecology is the study of interrelationships between humans and their environment, viewed in an evolutionary context as selective pressures shaping biology, behaviour, and culture.

What are the three main types of ecology in human evolution?

• Biological ecology: How the environment shapes our biology (mechanistic focus).

• Evolutionary ecology: Evolution of behaviour and morphology in response to environmental change over time.

• Cultural ecology: Environmental shaping of socially transmitted information such as social structure, politics, and economy, relevant to organisms with social learning.

What is Natural Selection?

Natural selection (or individual selection) is the change in frequency of genotypes within a population from one generation to the next, due to differences in phenotypes’ ability to produce surviving offspring.
It acts on individuals, not species.

What is the key question in natural selection?

How did individuals with a specific behaviour or trait survive and reproduce better than others in the same population, time, and environment?

What is Adaptation (across generations)?

• As a process (verb): Successful interaction between a population and its environment.

• As a result (noun): Cultural or biological traits offering advantage in a given environment, increasing average fitness over generations.

How does natural selection cause adaptation?

Natural selection operates from one generation to the next, increasing the average fitness of a population through the retention of advantageous traits.

What is Genetic Adaptation?

A change in gene frequencies that improves survival and reproduction under specific environmental conditions — arising through competition for limiting resources and differential success in converting resources into biomass and offspring.

What happens when resources are limiting?

Individuals better able to acquire and use limited resources survive, grow, and reproduce more — leading to natural selection for those traits.

What must an organism do to persist over evolutionary time?

It must survive and reproduce, requiring access to food, water, shelter, and mates, while coping with hostile and changing environments, including competition with conspecifics (members of the same species).

What is the interface between ecology and evolution?

Natural selection is the differential replication of genotypes by phenotypes under environmental conditions.
Populations therefore change over time as ecological conditions influence reproductive success.

What happens in changing environments?

Variation in resource use and competition intensity leads to evolutionary change each generation, driving adaptive responses.

What are the three main types of responses to the environment?

• Acclimation: Short-term (minutes to hours) changes within an individual’s lifetime.

• Acclimatization: Medium-term (days to weeks) reversible physiological changes.

• Adaptation: Long-term genetic changes across generations.

What does the ability to acclimate and acclimatize represent?

Itself a genetic adaptation evolved through natural selection in environments that were variable on short time scales.

What is Acclimation?

• Timescale: Minutes to hours.

• Examples: Adrenaline response to threat; testosterone surge in confrontation; sweating for cooling.

• Nature: Physiological, reversible, and often species-wide (sometimes population-specific).

• Mechanism: Genetically based ability for short-term phenotypic change.

What is Acclimatization?

• Timescale: Days to weeks.

• Function: Maintains homeostasis (physiological stability).

• Mechanism: Reversible, phenotypic plasticity.

• Example: Skin tanning reduces UV damage.

• Genetic basis: Genetically determined capacity for reversible environmental response.

What are the main types of Genetic Adaptation?

• Species-wide traits: Very old traits shared across all humans (e.g., stereoscopic vision, bipedality, reduced body hair).

• Population-specific traits: Recent traits since the dispersal out of Africa, geographically clustered (e.g., body shape, lactose tolerance, sickle-cell trait, oxygen saturation differences).

What are examples of Species-wide Traits?

Trait	Selective Pressure / Advantage
Stereoscopic vision	Forest primates: depth perception to reduce falls, now aids distance judgment.
Bipedalism	Efficiency of terrestrial movement, freeing hands.
Reduction in body hair	Improved thermoregulation, likely coinciding with darker skin pigmentation.
Large brain	Foraging complexity, social group management, and problem-solving.

How do scientists study brain–environment relationships?

By observing patterns of association across animal groups rather than direct experiments, comparing whether similar correlations (e.g., social group size and brain size) appear repeatedly.

What were major temperature changes from 150 kya to the present?

Transition from Pleistocene to Holocene epochs:

• H. neanderthalensis extinction

• H. sapiens “Out of Africa” dispersal

• Increasing geographic differentiation among populations due to environmental variation.

What is Human Dispersal and its ecological context?

• Ice ages facilitated dispersal due to lower sea levels and climate shifts.

• Major dispersal of Homo sapiens ~70 kya from tropical/subtropical Africa into diverse environments.

• Enabled rapid adaptation to ecological variability.

What was the environment during the Last Glacial Period (120–20 kya)?

• Lower sea levels → more land exposed.

• Colder temperatures, extensive ice sheets, tundra, and deserts.

• Created mosaic landscapes of forests, savannahs, and scrublands — selective pressure for versatility.

How can phenotypic traits be understood in evolutionary ecology?

By analysing at different scales:

Level	Example	Adaptive Aspect
Species-wide	Bipedalism, large brain	Ancient genetic adaptation
Population	Skin colour, body form	Genetic variation by environment
Individual	Skin tanning, adrenaline response	Acclimatization / acclimation

What are Population-specific Adaptations?

Traits arising after human dispersal, shaped by local environments:

• Body shape: Adapted to temperature (Bergmann’s and Allen’s rules).

• Skin colour: Varies with latitude and sunlight (balance between UV protection and vitamin synthesis).

• Lactose tolerance: Linked to dairy domestication and cultural dietary practices.

What do genetic studies reveal about human variation?

• Most variation occurs within populations, not between them.

• Humans are more genetically similar to each other globally than chimpanzee subpopulations are within Africa.

• Increased migration reduces geographic differentiation.

What were the major transitions in human use of environments?

• Hunter-gatherer phase: Species-wide adaptations within Africa (100–70 kya).

• Post-dispersal: Serial population adaptations to new environments (e.g., cold, altitude, diet).

• Agricultural revolution (~10 kya): Increased population densities and resource use.

• Industrial and modern periods: Exponential energy use from animal to fossil-fuel power.

How has energy use affected population growth?

As energy use increased (e.g., animal power → agriculture → fossil fuels), population density and competition rose.
The current human population (~7.8 billion) reflects a growth rate (r) ≈ 0.013 per year historically modulated by events like the Black Plague.

What is the history of human responses to environments?

• Before 70 kya: Primarily species-wide adaptations within Africa.

• After 70 kya: Sequential adaptations to regional conditions (cold, altitude, UV).

• Today: High reliance on acclimatization and acclimation as plastic (reversible) responses, combined with cultural adaptation.

What is the first law of thermodynamics and how does it relate to ecology?

• The first law (conservation of energy) states that energy can neither be created nor destroyed, only transformed.

• In ecology, biomass represents “standing stocks” of energy.

• Energy enters ecosystems through solar capture via photosynthesis, and then flows through trophic levels.

What is the second law of thermodynamics and how does it apply to ecosystems?

• The second law states that every energy conversion increases entropy (disorder) and decreases free energy.

• Maintaining organization (life, biomass) requires energy input.

• Energy is lost as heat at every trophic conversion (sun → plants → herbivores → carnivores).

• It costs energy to maintain biomass organization — metabolism converts energy to heat.

How efficient are energy transformations in trophic systems?

• Maximum transformation efficiency ≈ 10% across trophic levels.

• Each step converts only about one-tenth of available energy into biomass.

• Eating lower on the trophic chain (e.g., plants) minimizes energy loss.

• Illustrated as:

  • 10% of solar input → plants

  • 10% of plant → herbivore

  • 10% of herbivore → carnivore

  • → cumulative loss ≈ 0.0001 of solar input.

What is Liebig’s Law of the Minimum (1840)?

• The distribution of a species is controlled by the environmental factor for which it has the lowest range of adaptability or control.

• The scarcest resource limits growth, reproduction, and survival.

• Example: Without water, humans cannot survive, regardless of food availability.

• Related principle: As levels of a limiting resource fall, population declines due to death or slowed reproduction.

What is the hierarchy of limiting factors in nutrition?

Depends on environment, life stage, activity, and reproduction.
Examples:

• Basic energy need: 1600–3000 kcal/day.

• Macronutrients:

  • Fats = 9 kcal/g

  • Carbohydrates = 4 kcal/g

  • Proteins = 4 kcal/g (most costly to digest)

• Micronutrients: Vitamins (A, B complex, folate) and minerals (Ca²⁺, Fe, I).

• Water: Essential; death after ~3 days without water.

What is the Law of Tolerance (Shelford, 1913)?

• No organism can live under every condition; each has upper and lower tolerance limits for environmental factors (e.g., temperature, oxygen).

• Geographic distribution is determined by these limits.

• Example: No oxygen → no human habitation.

• Biological adaptation: Body size, metabolic rate.

• Cultural adaptation: Clothing, housing — increases tolerance to cold or heat.

What is Carrying Capacity (K)?

• The maximum population size of a species that an environment can sustain indefinitely.

• Determined by resource availability and environmental constraints.

• Limiting factors change carrying capacity.

• Regulating factors affect birth and death rates as population density changes (e.g., disease spread like cholera or COVID-19).

Where does natural selection act in relation to tolerance limits?

• Selection acts near the edges of tolerance ranges or at environmental minimums, where survival and reproduction are challenged.

• Example: Starvation tolerance — selection favors individuals who can survive on less food, slow growth, or migrate to better conditions.

What are Abiotic Factors and how do they affect humans?

• Abiotic factors are non-living environmental variables (temperature, UV radiation, water, etc.) that influence adaptation.

• Many species-wide human adaptations evolved as responses to abiotic environments (e.g., temperature, UV exposure).

What is Bergmann’s Rule?

• Among homeothermic mammals, body size increases as ambient temperature decreases.

• Larger bodies conserve heat better due to lower surface-area-to-volume ratio.

• Smaller bodies are favored in hot climates for better heat dissipation.

• Example: Polar mammals are large; tropical mammals are smaller.

Why do larger bodies conserve heat more effectively?

• Heat loss occurs from the body surface.

• Larger bodies have less surface area relative to volume, reducing heat dissipation.

• Humans produce internal heat and must maintain a core temperature of ~37°C; equilibrium with environment (~29°C) requires minimizing heat loss in cold climates.

What does human body size variation show globally?

• Populations near the equator = smaller, slender bodies.

• Populations near the poles = larger, broader bodies.

• Reflects adaptation to ambient temperature and supports Bergmann’s Rule.

• Humans remain better adapted to tropical than to arctic climates — due to longer evolution in warm regions.

How do humans adapt morphologically to hot climates?

• Problem: Excessive internal heat buildup, small temperature gradient with the environment.

• Adaptations:

  • Larger surface area relative to volume.

  • Smaller trunks and elongated limbs to enhance heat loss.

  • Sweating and vasodilation increase evaporative and convective cooling.

What is Allen’s Rule?

• In mammals, appendage length decreases with decreasing mean temperature.

• Short limbs conserve heat in cold climates; long limbs enhance cooling in hot climates.

• Human data: Relative sitting height (trunk length) inversely correlates with leg length; longer limbs = adaptation to warmth.

What factors complicate the relationship between body form and climate?

• Adaptation pathways vary: dry vs humid heat, diet, and energy use.

• Recent migrations have disrupted genetic–environmental matching.

• Example: Some populations show reduced expression of growth hormone receptor, altering growth patterns independent of temperature.

How do UV radiation, folate, and vitamin D interact in human adaptation?

• High UV (tropics):

  • UV destroys folate, a nutrient critical for DNA synthesis and fetal development.

  • Melanin production increases (darker skin) to block UV and protect folate.

• Low UV (high latitudes):

  • Reduced sunlight → lower vitamin D synthesis.

  • Lighter skin evolved to permit more UV penetration for vitamin D production.

• This represents a trade-off between folate protection and vitamin D synthesis.

How has skin colour evolved geographically?

• Dark pigmentation evolved in tropical regions with high UV radiation.

• Light pigmentation evolved in populations migrating to high latitudes (~70–10 kya).

• Clinal variation: gradual change in skin tone correlated with latitude and UV intensity.

• Human migration across hemispheres (~60–12 kya) produced observable global gradients.

What is clinal variation in skin colour?

• Gradual, continuous variation of skin colour along geographic gradients of UV radiation.

• Reflects evolutionary adaptation to solar exposure rather than discrete racial groupings.

• Supported by genetic studies (Barsh 2003; Tishkoff 2019).

What are examples of cultural adaptations to UV and heat?

Loose, layered clothing reduces UV exposure and prevents dehydration in hot, arid environments.

• Clothing insulation can also prevent heat loss in cooler deserts and high solar-radiation regions.

• Demonstrates that humans rely on technological and behavioural strategies in addition to biological ones.

What is the summary of ecological adaptation patterns in humans?

• Human populations show clinal adaptive variation in:

  • Body size (Bergmann’s rule).

  • Extremity length (Allen’s rule).

  • Skin pigmentation (UV exposure).

• These traits evolved since dispersal out of Africa (~70 kya) as responses to abiotic selective pressures.

• Adaptations illustrate universal ecological and thermodynamic laws governing all homeothermic organisms.

What are the broader implications of these adaptations?

• Environmental gradients directly shape physiological and morphological traits.

• Cultural and biological adaptation are intertwined responses.

• Variations are ecological, not racial, in origin — a continuum of human responses to natural selection.

What key ecological “laws” apply to humans?

1. Law of the Minimum (Liebig): The most limiting resource determines growth or survival.

2. Law of Tolerance (Shelford): Species distribution is bounded by tolerance limits.

3. Thermodynamic Laws: All biological organization requires continuous energy input; inefficiencies produce heat and entropy.