Biodiversity and Evolution
FoundationLife on Earth is wonderfully diverse: modern classification follows phylogenetic relationships into six kingdoms and three domains, names species by binomial nomenclature and identifies them with dichotomous keys; a species is a population that interbreeds to give fertile offspring. Heritable variation, accumulated by natural selection and combined with isolation, forms new species — with fossils, homologous structures and DNA sequences as the evidence.
- Modern classification follows phylogeny into six kingdoms — true bacteria, archaebacteria, protists, fungi, plants, animals — and three domains: Bacteria, Archaea and Eukarya.
- Ranks narrow from kingdom to species; binomial nomenclature names each species genus plus species in Latin, capital genus, small species word, italics throughout.
- A dichotomous key offers a pair of exclusive features at each step until the species is reached; a species is a population that interbreeds to give fertile offspring.
文體小知識：科學知識說明文——介紹生物分類的六界三域系統、雙名法與二叉式檢索表，再以自然選擇解釋物種形成與進化，並列出進化的三類證據。
§Text Passage
1More than two million species share the Earth, from bacteria to blue whales — biodiversity. To make sense of them, scientists build classification systems. Modern classification follows phylogenetic relationships, grouping species by evolutionary kinship; as new evidence emerges the system is revised, showing how scientific knowledge keeps updating.
2The six-kingdom system proposed by Woese divides life into the true bacteria, archaebacteria, protists, fungi, plants and animals; at a higher level, life splits into three domains — Bacteria, Archaea and Eukarya. Below each kingdom come phylum, class, order, family, genus and species, levels descending while shared features increase.
3Binomial nomenclature gives every species one universal scientific name of two Latin words, printed in italics with a capital initial for the genus — humans are Homo sapiens. To identify organisms we use dichotomous keys: each step offers a pair of mutually exclusive features, the user picks the statement matching the specimen and follows the trail to the species name. A species is a naturally interbreeding population whose offspring are fertile.
4Species are not fixed. Populations always carry heritable variation; when a population is split by a geographical barrier, each part accumulates different variants until the two can no longer interbreed — a new species has formed, and this is evolution. The core mechanism is natural selection: variation exists, limited resources force a struggle for survival, better-adapted individuals breed more, and favourable traits build up over generations. Darwin's Galapagos finches, with beak shapes tracking their foods, are the classic case, and antibiotic resistance in bacteria shows evolution happening now.
5The evidence comes from several directions: the fossil record shows the changing sequence of life, but fossils form rarely and the record is incomplete, so other evidence is needed; comparative anatomy finds homologous structures — the same bone pattern in a human arm, a whale's flipper and a bat's wing — pointing to common ancestors; and at the molecular level, the closer the kinship, the more similar the DNA and protein sequences. Together these lines of evidence make evolution the unifying framework of biology.
1Summary 重點摘要
Life on Earth is wonderfully diverse: modern classification follows phylogenetic relationships into six kingdoms and three domains, names species by binomial nomenclature and identifies them with dichotomous keys; a species is a population that interbreeds to give fertile offspring. Heritable variation, accumulated by natural selection and combined with isolation, forms new species — with fossils, homologous structures and DNA sequences as the evidence.
2Vocabulary cards Vocabulary
The richness of species and their genetic differences on Earth, the basis of stable ecosystems.
The science of sorting organisms into hierarchical groups by their similarities and phylogenetic relationships.
Naming each species with two Latin words, genus plus species, printed in italics, as in Homo sapiens.
An identification tool offering a pair of mutually exclusive features at each step, leading step by step to a species name.
A naturally interbreeding population producing fertile offspring — the basic unit of classification.
The process by which populations split into new species through heritable variation and isolation.
The evolutionary mechanism by which better-adapted individuals leave more offspring and favourable traits build up over generations.
The differences among individuals of a species; heritable variation is the precondition of natural selection.
Organs in different organisms sharing the same origin and basic structure but different functions — evidence of common ancestry.
Ancient remains and traces preserved in rock strata, showing the changing sequence of life through time.
3Step by step Steps
- Step 1: Why classify, and phylogenyWith over two million species, biology needs a system. Classification sorts organisms into levels by similarity and difference, easing identification, naming and communication. Modern classification goes beyond looks to phylogenetic relationships — reflecting evolutionary kinship, with closer relatives placed closer together. For example, a whale may look fishlike, but bones, breathing and DNA place it with the mammals, not the fish; classifications revised under new evidence show science updating itself.
- Step 2: Six kingdoms and three domainsThe six-kingdom system proposed by Woese sorts life into true bacteria, archaebacteria, protists, fungi, plants and animals; at a higher level these merge into three domains — Bacteria, Archaea and Eukarya. Each kingdom has its signature: the two bacterial kingdoms are prokaryotes, protists are mostly single-celled eukaryotes, fungi feed by absorbing through hyphae, plants make their own food, animals ingest theirs. For example, bread mould belongs to the fungi, not the plants: it has no chlorophyll, spreads spores and absorbs ready-made food.
- Step 3: Hierarchies and binomial nomenclatureThe taxonomic ranks descend from kingdom through phylum, class, order, family and genus to species; the lower the level, the more features members share. Binomial nomenclature gives each species one scientific name — genus first, species second, in Latin, printed in italics with a capital initial on the genus. For example, humans are Homo sapiens and rice is Oryza sativa; members of one genus share its name, as the leopard (Panthera pardus) and lion (Panthera leo) do, instantly signalling close kinship and avoiding the confusion of local common names.
- Step 4: Using and building dichotomous keysThe dichotomous key is the practical identification tool: each step offers a pair of mutually exclusive features — leaves opposite or alternate, say — and the user picks the statement matching the specimen, follows the number to the next step and arrives at the name. Good keys use stable, easily observed features and avoid changeable ones such as colour. For example, a school-garden key might first ask whether leaves are needlelike or broad: needles route to the conifers, broad leaves lead down the broadleaf branch, and a few steps narrow the field to one species.
- Step 5: Species and speciationThe species is the basic unit of classification, defined as a naturally interbreeding population producing fertile offspring. Speciation needs heritable variation plus isolation: once a population is split by mountains, rivers or islands, each part accumulates different variants, gene flow stops, and eventually the two cannot interbreed even if reunited — a new species has formed. For example, finches on separate Galapagos islands, cut off by straits, evolved different beaks and songs, a live demonstration of isolation driving speciation.
- Step 6: How natural selection worksNatural selection runs in four steps: a population already carries heritable variation; limited resources force a struggle for survival; better-adapted individuals survive and leave more offspring, passing on favourable traits; those traits accumulate over generations, steering the population toward adaptation. Darwin and Wallace proposed this independently. For example, in drought years small soft seeds vanish, deep-beaked finches crack the remaining large hard seeds and breed more, and the next generation's beaks average deeper — the environment acts as a sieve for suitable traits.
- Step 7: The evidence for evolutionEvidence comes from several directions. The fossil record shows life's changing sequence — the horse's ancestors walked on several toes that reduced to one — but fossils form only under rare conditions, so the record is incomplete. Comparative anatomy finds homologous structures: the same bone layout in a human arm, a whale's flipper and a bat's wing, pointing to common ancestry. Molecular evidence shows that the closer the kinship, the more alike the DNA and protein sequences. For example, bacteria developing widespread antibiotic resistance within decades of drug use is observable, ongoing evolution — exactly the selection principle of the previous step.
4Diagrams Diagrams
5Worked example Example
Beak depth varies heritably among individuals of a Galapagos finch species. A severe drought greatly reduces small soft seeds, leaving mainly large hard ones; after the drought, the new generation averages deeper beaks than before. (1) Explain the change step by step using the four points of natural selection. (2) State what this has in common with bacteria developing antibiotic resistance.
- Variation: beak depth already differed among individuals, and the difference is heritable — the precondition for selection to act.
- Struggle for survival: drought made food scarce; shallow-beaked birds could not crack the large hard seeds, while deep-beaked birds fed more easily.
- Survival of the fittest: deep-beaked birds survived and bred more, passing the alleles for deep beaks to their offspring.
- Generational accumulation: over several generations the deep-beak alleles rose in frequency and average beak depth increased; likewise, antibiotics kill sensitive bacteria while resistant mutants survive and multiply — the same selection pressure filtering both populations.
- Answer: The finch population began with heritable beak variation; drought forced a struggle in which deep-beaked birds fed and bred more; favourable traits accumulated and average beak depth rose. Antibiotic resistance follows the same mechanism: antibiotics are the selection pressure, sensitive cells die out, and resistant mutants multiply into the mainstream. Natural selection never makes organisms change themselves on demand — it screens pre-existing random variation, and antibiotic resistance is evolution observed in action.
6Common mistakes Mistakes
7Quick check Quick check
Q1List the seven taxonomic ranks from largest to smallest.
Q2What is Homo in Homo sapiens, and what are the writing rules of binomial nomenclature?
Q3How do you use a dichotomous key to identify a plant?
Q4Before invoking natural selection, which two preconditions must be confirmed?
Q5Why does antibiotic abuse speed the rise of resistant bacteria?
8Recap Recap
- Modern classification follows phylogeny into six kingdoms — true bacteria, archaebacteria, protists, fungi, plants, animals — and three domains: Bacteria, Archaea and Eukarya.
- Ranks narrow from kingdom to species; binomial nomenclature names each species genus plus species in Latin, capital genus, small species word, italics throughout.
- A dichotomous key offers a pair of exclusive features at each step until the species is reached; a species is a population that interbreeds to give fertile offspring.
- Natural selection in four steps: heritable variation, struggle for survival, fitter individuals breeding more, traits accumulating — shown by Galapagos finches and antibiotic resistance alike.
- Evidence converges: the fossil record (with limits), homologous structures (common ancestry) and DNA and protein sequence similarity (molecular level).