S5 Biology: Life, Earth History, Evolution, Classification, and Populations

Comprehensive vocabulary flashcards covering the S5 Biology B-test study notes, including definitions of life, prebiotic chemistry, geologic eons, mass extinctions, peppered moth microevolution, taxonomy and cladistics, and population ecology models.

Short definition of life

A cellularly organised system that uses metabolism to maintain internal balance, grows and reproduces, and is both a product of and subject to evolution.

Classical definition of life

A definition focusing on living things as complex, self-replicating systems that are able to grow, develop, and respond to stimuli.

Cellular definition of life

A definition stating that living things are composed of one or more cells, which represent the basic fundamental units of life.

Evolutionary definition of life

A definition focusing on living systems as being capable of variation, heredity, evolution, and adaptation over generations.

CHNOPS

The six key chemical elements required for all known life: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur.

Polar solvent

A liquid such as water whose molecules have slightly charged poles (oxygen is slightly negative, hydrogen is slightly positive), allowing it to dissolve ions and molecules so they can move and react.

Phototrophy

The process of obtaining energy from light to fuel cellular and metabolic processes (e.g., photosynthesis).

Chemotrophy

The process of obtaining energy from chemical reactions and compounds rather than from sunlight.

Abiogenesis

The natural process by which the first living systems arose from non-living matter through chemical steps on the early Earth.

LUCA (Last Universal Common Ancestor)

The most recent common ancestor from which all cellular life on Earth today is descended, inferred to have lived about $3.5\text{--}4\text{ billion years ago}$.

Chemoautotroph

An organism that synthesizes its own organic compounds using energy derived from chemical reactions (such as using $H_2$) rather than sunlight.

Deep-sea hydrothermal vents

Hot, oxygen-free mineral fissures on the ocean floor ('black smokers') where hydrogen-rich water escapes, providing chemical energy gradients without sunlight and serving as a hypothesized site for the origin of life.

Miller–Urey experiment

A $1953$ experiment simulating early Earth conditions that showed organic molecules, including amino acids like glycine and alanine, can form from simple inorganic gases and energy.

Three domains of life

The overarching biological classification scheme proposed in $1977$ by Carl Woese based on ribosomal RNA comparisons: Bacteria, Archaea, and Eukarya.

Bacteria

A domain of prokaryotic single-celled organisms without a membrane-bound nucleus or complex organelles, such as E. coli and cyanobacteria.

Archaea

A domain of prokaryotic single-celled organisms that resemble bacteria in appearance but differ fundamentally in membrane lipids, genes, and metabolism, often living as extremophiles.

Eukarya

The domain comprising all organisms whose cells possess a membrane-bound nucleus and internal organelles, including protists, fungi, plants, and animals.

Hadean eon

The earliest geologic eon ($4.6\text{--}4.0\text{ bya}$), marked by the formation of Earth, heavy meteorite bombardment, volcanism, early liquid water, and no accepted evidence of life.

Archean eon

The geologic eon ($4.0\text{--}2.5\text{ bya}$) during which the first unicellular prokaryotic life appeared and cyanobacteria began oxygenic photosynthesis.

Proterozoic eon

The longest geologic eon ($2.5\text{--}0.541\text{ bya}$), characterized by the Great Oxidation Event, the first eukaryotes, the first multicellular life, and 'Snowball Earth'.

Phanerozoic eon

The current geologic eon ($0.541\text{ bya to today}$), meaning 'visible life', marked by the Cambrian explosion, colonization of land, rich fossils, and three eras: Paleozoic, Mesozoic, and Cenozoic.

Stromatolites

Layered structures formed in shallow water by microbial communities of photosynthetic cyanobacteria that trap and cement sediment grains.

Great Oxidation Event (GOE)

The accumulation of biologically produced oxygen in Earth's oceans and atmosphere approximately $2.4\text{ billion years ago}$, which poisoned many obligate anaerobes and enabled aerobic respiration.

Precambrian

An informal collective name for the first three eons (Hadean, Archean, and Proterozoic), spanning about $88\%$ of Earth's history prior to the Cambrian period.

Cambrian explosion

A geologically rapid diversification event approximately $541\text{ million years ago}$ lasting about $13\text{--}25\text{ million years}$, during which almost all major animal body plans appeared in the fossil record.

Mass extinction

A period in which a substantial proportion of global species (often more than $75\%$) disappears worldwide in a geologically short time.

Background extinction

The continuous, normal, slow loss of individual species that happens steadily throughout geologic time.

The Big Five

The five major mass extinction events of the Phanerozoic eon: End-Ordovician ($444\text{ mya}$), Late Devonian ($372\text{ mya}$), End-Permian ($252\text{ mya}$), End-Triassic ($201\text{ mya}$), and End-Cretaceous ($66\text{ mya}$).

End-Permian extinction ('Great Dying')

The most catastrophic mass extinction in Earth's history ($252\text{ mya}$), triggered by massive Siberian Traps volcanism, resulting in the loss of about $96\%$ of marine species.

Chicxulub crater

A buried impact crater roughly $180\text{ km}$ across located on Mexico's Yucatán peninsula, caused by an asteroid collision $66\text{ million years ago}$ that triggered the End-Cretaceous extinction.

Cenote

A natural sinkhole formed by the collapse of limestone uncovering groundwater; an arc of cenotes on the Yucatán peninsula traces the fractured rim of the buried Chicxulub crater.

Disaster taxa

Small, widespread, hardy generalist species (such as Lystrosaurus after the End-Permian) that briefly become extremely common immediately following a mass extinction.

Adaptive radiation

The rapid evolutionary diversification of surviving lineages into many distinct species that fill empty ecological niches following an extinction event.

Industrial melanism

The evolutionary phenomenon in which dark (melanic) forms of a species become increasingly common in industrial areas polluted with soot.

Biston betularia

The scientific name of the peppered moth, a textbook microevolutionary example of natural selection driven by changes in environmental camouflage.

Typica

The speckled, light-coloured morph of the peppered moth (Biston betularia) that is camouflaged on clean, lichen-covered tree bark.

Carbonaria

The melanic, dark-coloured morph of the peppered moth (Biston betularia) that is camouflaged on soot-blackened tree bark.

Selection pressure

An environmental factor or agent (such as predatory birds eating visible moths) that alters the survival and reproductive success of individuals based on their traits.

Microevolution

Directly observable, generation-to-generation changes in the inherited trait or allele frequencies within a biological population over time.

Homologous structures

Shared anatomical structures derived from a common ancestor that have been modified for different functions across divergent species (e.g., human arm, bat wing, whale flipper).

Binomial nomenclature

The formal system created by Carl Linnaeus for naming species with two Latin parts: a capitalized genus followed by a lowercase species epithet, written in italics (e.g., Homo sapiens).

Taxonomic ranks

The nested hierarchy of biological classification levels: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Cladogram

A branching diagram that illustrates evolutionary relationships among organisms grouped by shared derived characteristics inherited from common ancestors.

Node (cladogram)

A branching point on a cladogram representing the most recent common ancestor of all lineages that branch off above it.

Clade

A monophyletic group consisting of a single common ancestor and all of its evolutionary descendants.

Outgroup

A reference organism or lineage on a cladogram that branches off first and possesses none of the derived traits shared by the other groups.

Derived trait

A newly evolved physical or genetic feature marked on a cladogram that is shared by all descendant lineages situated above it.

Population

All individuals of one species living within the same geographical area at the same time.

Exponential growth (J-curve)

Unrestricted population growth under unlimited resources where the population multiplies by a constant factor every generation ($N = N_0 \times R^t$).

Carrying capacity ($K$)

The maximum population size of a species that a particular environment can sustainably support over an extended duration.

Logistic growth (S-curve)

Population growth characterized by an initial lag phase, rapid exponential acceleration, and a plateau where growth slows and stabilizes around carrying capacity ($K$).

Limiting factor

Any essential environmental resource or condition in shortest supply (such as food, water, space, or predators) that restricts the further growth of a population.

Two-fold cost of sex

The reproductive efficiency penalty of sexual reproduction where half the population consists of males that do not directly bear offspring, allowing equivalent asexual populations to grow twice as fast per generation.

Intraspecific competition

Competition for shared, limited environmental resources occurring among individuals of the same species.

Interspecific competition

Competition for shared, limited environmental resources occurring between individuals belonging to different species.

Ecological niche

The complete role and lifestyle of a species within its environment, including what it eats, where and when it feeds, required conditions, and its interspecies interactions.

Competitive exclusion principle (Gause's principle)

The ecological rule stating that two distinct species competing for identical resources cannot occupy exactly the same niche indefinitely without one outcompeting and eliminating the other.

Malthus's principle of population

The thesis published in $1798$ by Thomas Robert Malthus stating that unchecked human population expands in a geometrical ratio while food subsistence increases only in an arithmetical ratio.

Positive checks (Malthusian)

Factors that raise the death rate and reduce population back to resource limits, such as famine, disease, and war.

Preventive checks (Malthusian)

Voluntary decisions and socioeconomic behaviours that reduce the birth rate, such as delaying marriage and having fewer children.

Struggle for existence

The competition among organisms for finite resources resulting from the production of more offspring than can survive, leading to differential survival and reproduction.

Demographic transition

The historical drop in fertility rates observed as societies experience falling child mortality, increased wealth, and improved education for women.