What conditions are required for natural selection?

Natural selection requires inherited variation within a population, overproduction of offspring (more than the environment can support), a struggle to survive because resources are limited, and differential survival and reproduction in which individuals with favorable inherited traits leave more offspring. Over generations the favorable traits become more common, producing adaptation. The variation originally comes from random mutation; organisms do not choose to adapt.

What is the difference between homologous and analogous structures?

Homologous structures share the same basic underlying structure because of common ancestry, even when they perform different functions, such as the forelimbs of a human, a whale, and a bat. Analogous structures share a function but not a common origin, such as a bird's wing and an insect's wing, which evolved independently. Homologous structures are evidence of common ancestry; analogous structures are not.

How do new species form?

A species is a group that can interbreed to produce fertile offspring. Speciation usually begins with geographic isolation, where a physical barrier splits a population. The isolated groups then face different environments, undergo different mutations, and are shaped by different natural selection, so they diverge genetically over many generations. When they have diverged enough that they can no longer interbreed to produce fertile offspring, they are separate species (reproductive isolation).

How do you read a cladogram?

A cladogram is a branching diagram showing evolutionary relationships, where each branch point represents a common ancestor. Two species that share a more recent (closer) common ancestor are more closely related; the further back you trace to find a shared branch point, the more distantly related they are. A trait placed at a branch point is shared by all the organisms branching off after it. Modern cladograms are built mainly from molecular data (DNA and protein comparisons).

North CarolinaBiology

North Carolina Biology EOC: Evolution and Classification - a complete overview of natural selection, the evidence for evolution, speciation, classification, phylogenetics, and biodiversity

Q: What does the evolution and classification content on the North Carolina Biology EOC cover?

It covers natural selection as the mechanism of evolution (inherited variation, overproduction, the struggle to survive, and differential survival and reproduction, leading to adaptation); the lines of evidence for evolution (fossil record, homologous and vestigial structures, embryology, and molecular evidence); speciation and how reproductive isolation forms new species; classification using the taxonomic hierarchy and binomial nomenclature; phylogenetics, reading cladograms and phylogenetic trees; and biodiversity and the causes of extinction. These align to the LS.Bio.9 and LS.Bio.10 standards in the North Carolina Standard Course of Study.

A deep-dive guide to the Biological Evolution strand on the North Carolina Biology EOC: natural selection and adaptation, the evidence for evolution, speciation, classification and taxonomy, phylogenetics and cladograms, and biodiversity and extinction, with the item types the EOC uses.

Generated by Claude Opus 4.816 min readLS.Bio.9Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What the evolution and classification content demands
Natural selection and adaptation
The evidence for evolution
Speciation
Classification and phylogenetics
Biodiversity and extinction
Check your knowledge

What the evolution and classification content demands

The Biological Evolution strand explains the unity and diversity of life. This guide runs from the mechanism (natural selection), through the evidence for evolution, how new species form (speciation), how we classify organisms, how we diagram their relationships (cladograms), to why biodiversity matters and how species go extinct. The recurring crosscutting concepts are patterns, cause and effect, and stability and change.

This guide ties together the matching topic pages, each with its own practice questions: natural selection and adaptation, the evidence for evolution, speciation and population change, classification and taxonomy, phylogenetics and cladograms, and biodiversity and extinction.

Natural selection and adaptation

Natural selection needs inherited variation, overproduction, a struggle to survive, and differential survival and reproduction: individuals with favorable inherited traits leave more offspring, so those traits become common, producing adaptation. The variation comes from mutation; organisms do not choose to adapt. Antibiotic resistance is the classic example: resistant bacteria survive treatment and reproduce.

The evidence for evolution

Multiple independent lines agree: the fossil record (change over time, transitional forms), homologous structures (same structure, different function, from common ancestry), vestigial structures (reduced remnants), embryology (similar early development), and the strongest, molecular evidence (the more shared DNA and protein, the closer the relationship). Independent lines agreeing make the case powerful.

Speciation

A species can interbreed to produce fertile offspring. Speciation usually begins with geographic isolation; the separated groups diverge through different mutations and selection until they can no longer interbreed (reproductive isolation), forming new species. Environmental change drives ongoing population change, shifting which traits are common.

Classification and phylogenetics

Organisms are classified in a hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species, with three domains (Bacteria, Archaea, Eukarya). Each species has a two-part scientific name (genus capitalized, species lowercase). A cladogram shows relationships: species sharing a more recent branch point are more closely related, and modern trees are built from molecular data.

Biodiversity and extinction

Biodiversity is the variety of species and genes; greater diversity gives resilience to change and benefits humans (food, medicine, ecosystem services). Extinction is the permanent loss of a species; it is natural but greatly accelerated by human activities: habitat destruction, pollution, overharvesting, invasive species, and climate change. Low genetic diversity raises extinction risk.

The evolution and classification content on the North Carolina Biology EOC covers natural selection (inherited variation, overproduction, a struggle to survive, and differential survival and reproduction, producing adaptation, with the variation coming from mutation and antibiotic resistance as the classic example); the lines of evidence for evolution (the fossil record and transitional forms, homologous structures showing common ancestry, vestigial structures, embryology, and the strongest line, molecular DNA and protein similarity, all independent lines agreeing); speciation (a species interbreeds to produce fertile offspring, and geographic isolation followed by divergence leads to reproductive isolation and new species, while environmental change drives population change); classification (the hierarchy of Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species, the three domains Bacteria, Archaea, and Eukarya, and binomial nomenclature with the genus capitalized and species lowercase); phylogenetics (reading cladograms and phylogenetic trees, where a more recent shared branch point means more closely related and traits at branch points are shared by all later branches, built from molecular data); and biodiversity and extinction (biodiversity as the variety of species and genes giving resilience and human benefits, and extinction as the permanent loss of a species, natural but accelerated by habitat destruction, pollution, overharvesting, invasive species, and climate change). The recurring theme is how variation and selection produce the unity and diversity of life.

Check your knowledge

A mix of recall and reasoning questions covering the evolution and classification content. Attempt them under timed conditions, then check against the solutions.

State the four conditions required for natural selection. (4 marks)
Explain why an antibiotic does not "create" resistance in bacteria. (2 marks)
State what homologous structures suggest about the species that have them. (2 marks)
State the usual test for whether two organisms are the same species. (1 mark)
Explain how geographic isolation can lead to a new species. (2 marks)
List the taxonomic hierarchy from broadest to most specific. (2 marks)
In the name Homo sapiens, state which word is the genus. (1 mark)
On a cladogram, how do you tell which two species are most closely related? (2 marks)
Name three human activities that can cause extinction. (3 marks)

Solutions

Q1: Inherited variation; overproduction of offspring; a struggle to survive (limited resources); differential survival and reproduction of the better-suited individuals.
Q2: The resistance already exists by mutation in a few bacteria; the antibiotic selects for it by killing the non-resistant ones, rather than causing it.
Q3: They share the same underlying structure because of a common ancestor, so the species are evolutionarily related.
Q4: They can interbreed to produce fertile offspring.
Q5: A barrier separates a population; the groups face different environments and undergo different mutations and selection, so they diverge until they can no longer interbreed, becoming separate species.
Q6: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
Q7: Homo is the genus (sapiens is the species).
Q8: They share the most recent (closest) common ancestor, the closest branch point on the tree.
Q9: Any three of: habitat destruction, pollution, overhunting or overharvesting, invasive species, and human-caused climate change.

Sources & how we know this

North Carolina Standard Course of Study for Science — North Carolina Department of Public Instruction (2023)
End-of-Course (EOC) program — North Carolina Department of Public Instruction (2024)