Introduction to Biology — Ecology and Evolutionary Theory

Foundations of Ecology

Ecology:

The study of the interactions between organisms and their environment
Levels of study: organism → population → community → ecosystem → biosphere

Abiotic environmental factors:

Temperature: determines enzyme activity and metabolic rate
Light: drives photosynthesis and circadian rhythms
Precipitation: shapes species distribution (defines biomes)
Soil composition, pH, wind, ocean currents

Biomes:

Tropical rainforest: hot and humid, high biodiversity
Savanna: alternating dry and wet seasons, abundant grazing animals
Temperate deciduous forest: distinct four seasons
Coniferous forest (taiga): high latitude, low species diversity
Tundra: permafrost, polar regions
Desert: less than 250mm of annual rainfall
Coral reef and hydrothermal-vent ecosystems: distinctive marine environments

Ecological niche:

The functional role a species occupies within an ecosystem
Fundamental niche vs. realized niche (narrowed by competition)
Niche overlap → competitive exclusion

Population and Community Ecology

Population:

A group of the same species living in the same place
Population size, density, distribution pattern, and age structure

Population growth models:

Exponential growth (J-shaped): dN/dt = rN r: the intrinsic rate of increase / assumes unlimited resources
Logistic growth (S-shaped): dN/dt = rN(K-N)/K K: carrying capacity growth rate is maximal at N = K/2

Population regulation:

Density-dependent factors: their effect strengthens as density rises competition, predation, disease, parasitism
Density-independent factors: drought, flood, fire

Survivorship curves:

Type I (convex): mortality concentrated in old age (humans, large mammals)
Type II (linear): mortality is roughly even across ages (birds, reptiles)
Type III (concave): mortality concentrated early, survivors live long (fish, oysters)

Community:

The set of all populations in a given location
Species diversity: richness and evenness of species
Dominant species vs. keystone species (few in number but a large influence on the community)

Interspecies interactions:

Competition (−/−): competing for resources
Predation (+/−): predator-prey coevolution
Symbiosis: mutualism (+/+), commensalism (+/0), parasitism (+/−)

Succession:

Primary succession: begins on bare ground (a volcanic island, retreating glacier)
Secondary succession: begins after an existing community is destroyed (recovery after a wildfire)
Climax community: a stable end state that no longer changes

Ecosystem Energy and Matter Cycling

Ecosystem:

A community plus its abiotic environment
Energy: flows in one direction (no recycling)
Matter: cycles between organisms and the abiotic environment

Energy flow:

Solar energy → primary producers (photosynthesis) → primary consumers → secondary consumers
Ecological efficiency (the 10% rule): only about 10% of energy passes to the next trophic level
Gross primary production (GPP) − respiration = net primary production (NPP)
Ecological pyramids: energy, biomass, and number pyramids

Food webs and trophic levels:

Trophic level: level 1: plants (producers) level 2: herbivores (primary consumers) level 3 and above: carnivores (secondary, tertiary consumers)
Apex predator: maintains ecosystem balance

Biogeochemical cycles:

Carbon cycle:

Photosynthesis: absorbs CO2 → synthesizes organic matter
Respiration, combustion, decomposition: release CO2
Fossil-fuel combustion: disrupts the carbon cycle → drives climate change

Nitrogen cycle:

Nitrogen fixation: nitrogen-fixing bacteria (root nodules), lightning → NH3 or NO3-
Nitrification: NH4+ → NO2- → NO3-
Denitrification: NO3- → N2 (denitrifying bacteria)

Phosphorus cycle:

No significant gaseous phase in the atmosphere (cycles through sedimentary rock, soil, water)
Eutrophication: excess P and N inflow → algal blooms → oxygen depletion

Darwin’s Theory of Evolution by Natural Selection

Darwin’s theory of evolution (1859, On the Origin of Species):

Natural selection: traits well-suited to the environment survive and reproduce
Common descent: all living things branch from a common ancestor
Gradual change: accumulates over long time spans

Requirements for natural selection:

Variation: individuals differ in their traits
Heritability: variation is passed to offspring
Selection: variation produces differences in survival and reproduction
Time: change accumulates across generations

Types of natural selection:

Directional selection: favors one extreme trait → shifts the mean
Stabilizing selection: favors the intermediate trait → reduces variation example: human birth weight
Disruptive selection: favors both extremes → increases trait divergence

Sexual selection:

Selecting traits that are advantageous in competition for mates
Female choice: a peacock’s tail, a bird’s song
Male-male competition: a stag’s antlers, an elephant seal’s body size

Adaptation:

A trait that has evolved to fit the environment through natural selection
Morphological, physiological, and behavioral adaptation
Convergent evolution: unrelated lineages evolve similar traits in similar environments example: the streamlined body shape of dolphins vs. fish
Coevolution: two interacting species evolve together

The Modern Synthesis and Taxonomy

The modern synthesis:

Integrates Darwinian evolution + Mendelian genetics + population genetics

Population genetics:

Gene pool: all the alleles within a population
A change in allele frequency = evolution

Hardy-Weinberg equilibrium:

The allele frequencies of an idealized population in which evolution is not occurring
Conditions: random mating · no mutation · no migration · no genetic drift · no natural selection
p² + 2pq + q² = 1 (p + q = 1)
Real populations: deviate from equilibrium → prompting a search for the evolutionary factor at work

Factors driving evolution:

Natural selection: directional change
Genetic drift: random frequency change in a small population bottleneck effect, founder effect
Mutation: supplies new alleles (the ultimate source of evolution)
Gene flow: exchange of genes through migration between populations

Speciation:

Geographic isolation → allopatric speciation
Without isolation → sympatric speciation: polyploidy, habitat differentiation

Taxonomy:

Three domains: Bacteria, Archaea, Eukarya
Kingdom → phylum → class → order → family → genus → species
Cladistics: classifying lineages by shared derived traits
DNA barcoding: identifying species from short DNA sequences

Frequently Asked Questions

Q. Does evolution occur at the level of the individual or the population? A. Evolution occurs at the population level. An individual does not evolve during its lifetime. Natural selection acts on individuals (selecting traits favorable for survival and reproduction), but evolutionary change is a shift in allele frequency within a population across generations. When antibiotic-resistant bacteria emerge, for instance, it is not that individual bacteria changed — bacteria that were already resistant were selected, raising the frequency of the resistance gene in the population. So the phrase “a giraffe stretched its neck to reach food” is a mistaken Lamarckian description of inherited acquired traits; the accurate Darwinian description is “giraffes with longer necks survived and reproduced more through natural selection.”

Q. Hardy-Weinberg equilibrium never actually holds in nature — so why study it? A. Hardy-Weinberg equilibrium matters because it defines the idealized baseline state in which evolution is not occurring. Analyzing the extent and direction of a real population’s deviation from that equilibrium reveals which evolutionary force is at work. If the expected and observed frequencies differ, for example, that suggests natural selection, genetic drift, gene flow, or non-random mating is acting. Medically, it’s used to calculate the frequency of a disease allele in a population — knowing the frequency of cystic fibrosis patients (q²), for instance, lets clinicians calculate the carrier frequency (2pq), which is useful in genetic counseling.