Difficulty: Beginner | Prerequisites: Scientific Method and Biological Organisation (BIO 101 Section 1, Part 1)
This part of Section 1 answers three questions that run through the entire course: What makes something alive? How do we sort living things into groups? And what ties all of life together? The answer to the third question is evolution, which the course treats as biology's single most important unifying idea. You will need the Linnaean classification hierarchy and the characteristics of life for nearly every later topic, and the distinction between natural selection and adaptation shows up repeatedly in genetics, ecology, and speciation chapters. If you are joining late, read the Scientific Method notes first.
Living things share six core characteristics (order, regulation, growth and development, energy use, responsiveness, reproduction). Life is classified using a hierarchical Linnaean system running from Domain down to Species, with three Domains and five traditional Kingdoms. Evolution, driven by natural selection and adaptation, is the unifying theme that explains both life's diversity and its underlying unity.
Evolution
Genetic (heritable) changes in a population or species over generations.
In simple terms, evolution is the way populations change over time as traits that help organisms survive and reproduce become more common.
Natural selection
Differential, or unequal, success in reproduction. Individuals with traits better suited to their environment tend to reproduce more.
Think of it as "survival of the fittest" in the reproductive sense: the organisms that leave more offspring pass on more of their genes. Compare with artificial selection, where humans choose which organisms breed.
Adaptation
The principle that species share a common ancestor and arise through descent with modification.
In simple terms, the traits you see in a species today were shaped over time by changes inherited from ancestral populations.
Macroevolution
Large-scale evolutionary changes accumulated from small changes over long periods of time.
Think of it as the big picture: the emergence of entirely new species or major groups.
Microevolution
Small changes in the relative frequencies of traits (such as colour) within a population.
Think of it as evolution you could observe within a few generations, like a shift in the proportion of dark versus light moths.
Linnaean classification
A hierarchical system for organising life, developed by Carl Linnaeus. Organisms are identified by a two-part scientific name (genus + species).
Domain
The broadest level of classification. The three domains are Bacteria, Archaea, and Eukarya.
Kingdom
The level below Domain. The five traditional kingdoms are Monera (bacteria), Protista (algae and protozoans), Fungi (moulds, yeast, mushrooms), Plantae (plants), and Animalia (animals).
Biological species definition
A population or group of populations whose members have the potential to interbreed and produce fertile offspring.
In simple terms, if two organisms can mate and their young can also reproduce, they belong to the same species.
Evolutionary species concept
A species is a cluster of organisms that share a genealogy, or lineage of descent.
Characteristics of life
The six properties shared by all living things: order, regulation, growth and development, energy utilisation, responsiveness, and reproduction.
Heredity
The passing of traits from parents to offspring through genetic information.
Environment
The external conditions and surroundings that influence an organism's features and behaviour.
All living things share six traits. If something lacks any of these, it is generally not considered alive.
Order – living things are highly organised at every level
Regulation – organisms maintain internal stability (homeostasis)
Growth and development – organisms increase in size and complexity following genetic instructions
Energy utilisation – all life requires energy to carry out its processes
Responsiveness – organisms detect and respond to stimuli in their environment
Reproduction – organisms produce new individuals
Evolution explains the unity of life: all organisms are connected through common ancestry.
It also explains diversity: populations diverge over time as they adapt to different environments.
Features of living organisms are determined by a combination of environment and heredity.
Born 1809.
In 1831, at age 22, began a round-the-world voyage aboard the H.M.S. Beagle as a naturalist.
Published On the Origin of Species in 1859.
Proposed a mechanism for how evolution works.
The Theory of Evolution has two core parts:
Natural selection – differential reproductive success. Organisms better suited to their environment tend to leave more offspring.
Adaptation – species share common ancestors and arise through descent with modification.
There is variation within a population.
Much of that variation is heritable.
Organisms overproduce offspring.
Natural resources are limited, creating selection pressures.
There is a struggle to survive.
Better-adapted traits lead to preferential reproduction.
Small changes accumulate over geologic time.
Macroevolution – large changes accumulated over long periods (e.g. the evolution of mammals from reptilian ancestors).
Microevolution – small shifts in trait frequencies within a population over shorter timescales (e.g. changes in fur colour in a mouse population).
Evolution replaced earlier ideas like spontaneous generation (the belief that life arises from non-living matter without descent). This contrast highlights why testable, evidence-based thinking matters.
Life is unified by a hierarchical classification scheme. Groupings run from most inclusive (top) to least inclusive (bottom):
Domain
Kingdom
Phylum / Division
Class
Order
Family
Genus
Species
Organisms are formally identified by their genus and species names (binomial nomenclature).
Bacteria – single-celled prokaryotes
Archaea – single-celled prokaryotes, often found in extreme environments
Eukarya – organisms with membrane-bound nuclei (includes protists, fungi, plants, animals)
Monera – bacteria
Protista – algae and protozoans
Fungi – moulds, yeast, mushrooms
Plantae – plants
Animalia – animals
Two definitions are used:
Biological species definition – a population or group of populations whose members can interbreed and produce fertile offspring.
Evolutionary species concept – a cluster of organisms sharing a genealogy or lineage of descent.
The biological species definition is the one most commonly tested in introductory courses.
Natural selection is the basis for antibiotic resistance in bacteria: when antibiotics kill susceptible bacteria, resistant individuals survive and reproduce, making the population harder to treat. Understanding classification is essential in medicine (identifying pathogens), conservation (cataloguing endangered species), and agriculture (breeding programmes that rely on knowing genetic relationships between crop varieties).
"Evolution means an individual organism changes during its lifetime." Evolution happens to populations over generations, not to individuals. An individual organism does not evolve.
"Natural selection means only the strongest survive." "Fitness" in biology refers to reproductive success, not physical strength. An organism that is well-camouflaged and produces many offspring is "fitter" than a strong one that does not reproduce.
"Humans evolved from monkeys." Humans and modern primates share a common ancestor. That is a different claim.
"A scientific theory can be promoted to a law if enough evidence accumulates." Theories and laws are different types of scientific statements. A theory explains why; a law describes what. One does not become the other.
⚠️ Be able to list all six characteristics of life. Missing even one costs marks.
⚠️ Know the full Linnaean hierarchy from Domain to Species, in order.
⚠️ The biological species definition ("interbreed and produce fertile offspring") is a classic exam question.
⚠️ Be prepared to distinguish natural selection from adaptation, and macroevolution from microevolution.
⚠️ Expect a question asking you to name the three Domains and the five Kingdoms.
⚠️ The seven principles underlying natural selection are frequently tested as a list. Memorise them.
True or False: Evolution occurs in individual organisms. False. Evolution occurs in populations over generations.
Fill in the blank: The two parts of the Theory of Evolution are natural selection and __________. Adaptation.
True or False: A scientific law can explain why a phenomenon occurs. False. A law predicts what will happen but does not explain why.
Fill in the blank: The three Domains of life are Bacteria, Archaea, and __________. Eukarya.
True or False: Under the biological species definition, two organisms belong to the same species if they can interbreed and produce fertile offspring. True.
Q: List the six characteristics of life.
A: Order, regulation, growth and development, energy utilisation, responsiveness, and reproduction.
Q: What is the biological species definition?
A: A population or group of populations whose members have the potential to interbreed and produce fertile offspring.
Q: Explain the difference between macroevolution and microevolution.
A: Macroevolution refers to large-scale changes accumulated over long periods (e.g. emergence of new species). Microevolution refers to small changes in trait frequencies within a population over shorter timescales.
Q: Name the seven principles that underlie natural selection.
A: Variation within a population; heritable traits; overproduction of offspring; selection pressures from limited resources; struggle to survive; preferential reproduction of better-adapted traits; small changes accumulating over geologic time.
Q: What are the three Domains and five Kingdoms of life?
A: Domains: Bacteria, Archaea, Eukarya. Kingdoms: Monera (bacteria), Protista (algae and protozoans), Fungi (moulds, yeast, mushrooms), Plantae (plants), Animalia (animals).
Q: How does natural selection differ from artificial selection?
A: In natural selection, environmental pressures determine which organisms reproduce most successfully. In artificial selection, humans deliberately choose which organisms breed based on desired traits.
Q: Why is evolution considered the unifying theme of biology?
A: Because it explains both the unity of life (all organisms share common ancestry) and the diversity of life (populations diverge as they adapt to different environments over time).
The characteristics of life connect to cell biology (the cell is the unit that carries out all six characteristics) and to ecology (responsiveness and energy utilisation are central to how organisms interact with their environment).
Natural selection and adaptation link directly to genetics chapters, where you will learn how DNA mutations create the variation that selection acts upon.
The Linnaean classification system connects to phylogenetics and cladistics, which you will encounter in later units on biodiversity and evolutionary relationships.
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