3.1 Biodiversity and evolution

Biodiversity has three linked levels: variety of habitats, variety and abundance of species, and heritable variation within species.

Ask what is varying before choosing the level. A landscape comparison is habitat diversity; species counts and evenness are species diversity; differences among populations or individuals are genetic diversity.

Four forest types in one region show habitat diversity; different beetle species show species diversity; drought-tolerant alleles show genetic diversity.

Genetic diversity, because the variation is within the species.

Species richness alone is not the whole of biodiversity; level and abundance matter.

Diversity can increase recovery options: habitats provide refuges, species provide overlapping functions and genes provide tolerant variants.

The benefit is conditional on which functions and variants exist, how severe the disturbance is and whether organisms can reach suitable habitat.

After drought, a population with drought-tolerant variants may recover better than a genetically uniform population.

Show a surviving refuge, substitute function or tolerant variant—not diversity as a label alone.

High diversity is not a guarantee; the relevant level and disturbance must match.

Evolution is inherited change in a population across generations, not an adjustment made by one individual during its lifetime.

Acclimatization can change an individual without changing inherited frequencies. Evolution is detected when descendants inherit a different trait or allele distribution.

A person tans in sunlight without evolving; a population becoming more UV-tolerant over generations may reflect evolution.

A heritable frequency shift between generations, not a short-term physiological response.

Individuals do not evolve to meet a need; populations change through inherited variation and selection.

Natural selection changes trait or allele frequencies when heritable variants differ in survival or reproduction under current conditions.

Selection acts each generation; adaptation is the longer-term result. It has no planned goal and a trait is advantageous only relative to the environment.

If darker moths leave more offspring on soot-darkened trees, their frequency can rise over generations.

Heritable variants must have different reproductive success; mere presence of variation is insufficient.

Selection is not organisms choosing useful traits; the environment filters existing inherited differences.

Heritable variation plus overproduction, competition and differential reproduction can change a population over generations.

Resources limit survival. Individuals with traits better suited to current conditions tend, on average, to leave more offspring, so their alleles become more common.

If drought kills more shallow-rooted plants, deep-rooted heritable variants may contribute more seeds next season.

Selection changes reproductive contribution; population frequency shifts accumulate across generations.

‘Survival of the fittest’ means reproductive success in context, not physical strength alone.

Speciation becomes possible when isolation reduces gene flow and divergence eventually prevents production of fertile offspring.

Geographic, ecological or behavioural separation lets mutation, selection and chance accumulate differences. Under the biological species model, reproductive isolation is the decisive test.

A river separates populations; if they later meet but cannot produce fertile offspring, the sequence supports completed speciation.

No. It reduces gene flow, but reproductive isolation must be demonstrated or inferred carefully.

Divergence and speciation are not identical; ask whether gene flow and fertile reproduction remain.

Richness counts species; evenness asks how evenly individuals are distributed among them.

Species diversity uses both. Equal richness can hide dominance, so inspect abundance distribution before ranking communities.

Site A has 25 individuals of each of four species; Site B has 97 of one and one of each other. A is more even and usually more diverse.

Counts or relative abundance; richness alone cannot reveal dominance.

More species does not automatically mean a more even or functionally diverse community.

Simpson’s reciprocal index D = N(N−1) ÷ Σn(n−1); higher D generally indicates greater richness and/or evenness.

N is total individuals and n is each species count. Compute every n(n−1), sum, then divide; compare samples with similar effort and habitat.

For counts 5 and 5, N=10 and D=90/40=2.25; equal abundances raise evenness.

This reciprocal form increases as dominance falls and diversity rises.

Do not compare indices from unequal sampling effort or silently switch to a different Simpson convention.

Useful biodiversity monitoring combines broad local observations with agreed protocols, validation and feedback to the people who contribute them.

Citizens expand coverage; agencies coordinate standards; Indigenous knowledge holders add place-based history; trained parabiologists connect surveys to communities. Record metadata and identification confidence.

Repeated community bird counts with photo verification reveal a seasonal decline that can trigger habitat protection.

Known location/time, method, identification quality and a decision it can inform.

More observations are not automatically better evidence if methods and identifications are inconsistent.

Mutation can create a new allele; sexual reproduction reshuffles existing alleles into new combinations.

Recombination and fertilization increase variation among offspring without inventing a new DNA variant. Selection then changes which variants contribute to future generations.

A copying error creates allele A2; crossing parents with A1 and B1 produces a new A1B1 combination without a new allele.

Mutation; recombination only rearranges variants already present.

‘More variation’ does not specify whether the change is a new allele or a new combination.

Speciation evidence is strongest when a barrier or ecological choice reduces gene flow and divergence follows.

Geographic separation can split populations; the separation of bonobos and common chimpanzees is a geographical example. Ecological or behavioural choices can reduce mating even in one region, as host choice does in apple-maggot flies. Islands add founder effects, low immigration and distinct niches.

Apple-maggot flies mating on different host plants show reduced gene flow, but that is an early-stage case, not automatically complete speciation.

Persistent reproductive isolation and inability to produce fertile offspring, not separation alone.

An island endemic is evidence of isolation and divergence, not proof that every island lineage is a separate species.

A hotspot matters when many species—especially endemic and threatened ones—are concentrated where habitat loss is high.

Use species richness, endemism and threat evidence together. Tropical forests often score highly, but high biodiversity alone does not prove a formal hotspot designation.

A region with 500 endemic plants and rapid habitat loss may be a stronger priority than a species-rich region with intact habitat.

Threat or habitat-loss criteria; richness alone is insufficient.

‘Tropical’ is not a formal hotspot label by itself; check the stated criteria and data.

Human actions can change selection pressures, shifting the frequency of inherited traits without creating those traits.

Show the chain: activity changes mortality or reproduction; existing heritable variants contribute unequally; offspring frequencies change. The result depends on the pressure and starting variation.

Poaching in Gorongosa removed more tusked elephants before reproduction, so tuskless females became more common.

No. It filtered existing variation by changing survival and reproduction.

Natural selection is not intentional breeding; human pressure can select without anyone choosing a trait.

Artificial selection deliberately increases chosen traits, but a narrow breeding pool can reduce resilience to future stress.

Repeatedly selecting similar parents may raise yield while losing alleles. Genetically uniform crops or livestock can share susceptibility to one pathogen, pest or climate shock.

A high-yield crop line performs well until a new fungus infects every genetically similar plant.

Maintain diverse varieties and breeding populations while selecting for useful traits.

Artificial selection is not automatically bad; evaluate yield gains, genetic breadth and risk together.

Earth history extends over about 4.5 billion years. Dated fossil layers let us order when forms appear, change and disappear, providing evidence for the evolution of life across geological time.

Compare older and younger strata, but account for preservation and sampling bias. A well-dated appearance or transition is stronger evidence than an absence from the record.

If a shell form occurs in younger layers but not older ones, it may have appeared later—unless older rocks were not preserved or sampled.

Fossilization and sampling are incomplete, so absence may reflect the record rather than true absence.

The fossil record is evidence, not a complete movie of every generation.

Eons contain eras, eras contain periods, and periods contain epochs; boundaries mark major geological or biological change.

Use the hierarchy to locate scale, then inspect fossils and rocks for extinction, appearance or environmental disruption. It organizes evidence rather than replacing it.

A layer showing abrupt loss of many fossil groups may mark a boundary associated with major environmental change.

A dated, correlatable rock change plus a coherent fossil turnover, not memorized names alone.

The units are nested scales, not interchangeable labels or equal-length intervals.

A mass extinction removes many species and roles rapidly; survivors may diversify into newly vacant niches.

Physical drivers can interact—tectonics, eruptions, climate, sea level or impact. Afterward, reduced competition and divergent selection can support adaptive radiation. The current crisis is dominated by human pressures.

If a lineage survives while major competitors disappear, descendants may split into forms using different resources.

Extinction first removes functions and stability; diversification takes many generations and is uncertain.

Vacant niches do not guarantee recovery; suitable conditions, variation and time are required.

The Anthropocene is a proposed, debated epoch; candidate start markers must be sharply dated, globally correlatable and preserved.

Candidates emphasize different human signals: a 1610 CO₂ dip, ~1950 fly ash and a 1964 carbon-14 peak. Compare what each records and how globally it can be identified.

A 1964 radionuclide peak is sharply dated, while a regional land-use signal may be harder to correlate globally.

A clear signal in many locations with a defensible geological archive and causal human link.

Do not present one candidate as settled fact; the epoch and start date remain debated.

Human activity supports an Anthropocene argument when it leaves a widespread signal that enters and persists in geological archives.

Check source, transport, geographic spread and preservation: plastics and synthetic chemicals, radionuclides, transported remains, altered sediment and novel materials can each leave different traces.

A globally dispersed radionuclide layer with a known nuclear-testing source is stronger evidence than one local landfill fragment.

Agreement reduces the chance that one local or short-lived record is being overinterpreted.

A durable object alone does not define an epoch; scale, correlation and stratigraphic preservation matter.