Biology basics & diversity of life · Topic 4 of 17

Natural Selection, Adaptation & Populations

Tap any line to see why it matters

Biology · evolution & populations

Natural Selection, Adaptation & Populations

Inherited traits that help survival and reproduction spread through a population; populations grow by births and immigration and shrink by deaths and emigration.

01 · SelectionHelpful inherited traits leave more offspring

02 · AdaptationAdaptation = inherited trait that aids survival

03 · PopulationChange = births + in − deaths − out

What it is

Natural selection: helpful inherited traits → more offspring → trait spreads must know

Adaptation = an inherited trait that aids survival and reproduction must know

Variation comes from mutation (new alleles) and sexual reproduction

Evidence: fossils, homologous structures, embryos, biogeography, DNA

Carrying capacity (K) = the most individuals a habitat can sustain

Fertility falls with women's education, later marriage, family planning

Step by step

Needs: variation + inheritance + more offspring than resources support

Know by heart

Population change = (births + immigration) − (deaths + emigration) must know

Worked examples

Snowy forest: white hares escape foxes, breed more → more white hares next time

Don't confuse

Homologous = shared ancestry (arm, flipper); analogous = same job, separate origin

Unlimited resources → J-curve (exponential); limited → S-curve (logistic)

Density-dependent: competition, predators, parasites; independent: weather, disasters

J-curve or S-curve?

Commonly confused

	Exponential growth	Logistic growth
Resources	Unlimited	Limited
Graph shape	J-shaped, keeps getting steeper	S-shaped, levels off
Upper limit	None	Carrying capacity (K)

Misconceptions

Populations evolve over generations — one organism does not

Traits gained during life aren't inherited (Lamarck's idea was rejected)

Pearl

Variation, inheritance and too many offspring for the resources: those three together make natural selection happen.

Sources · OpenStax Concepts of Biology §11.1 Discovering How Populations Change · OpenStax Concepts of Biology §11.2 Mechanisms of Evolution · OpenStax Concepts of Biology §11.3 Evidence of Evolution · OpenStax Concepts of Biology §19.2 Population Growth and Regulation · OpenStax Biology 2e §45.3 Environmental Limits to Population Growth (birth rate minus death rate) · OpenStax Introduction to Sociology 3e §20.1 Demography and Population (immigration, emigration) · Population Education — What factors affect the total fertility rate (TFR)? · OpenStax Biology 2e §18.1 Understanding Evolution · TSTC Library — HESI A2 guide (natural selection and adaptation; population growth, birth and fertility factors) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Practice this topic, then test the whole section.

Start HESI prep free

HESI and HESI A2 are registered trademarks of Elsevier. NurseSavvy is not affiliated with or endorsed by Elsevier.