A3.1 Diversity of organisms

Variation occurs among individuals in every species. Darwin recognized that, when some inherited variants affect survival or reproduction, their frequencies can change across generations by natural selection.

Appearance varies within species. Sexual dimorphism can make males and females look unlike, while unrelated species can converge on similar forms. A boundary should therefore use several traits and, where possible, independent evidence.

A scientific species name has two parts: the genus followed by the specific epithet. In Panthera leo, Panthera names the genus and leo distinguishes the species within that genus.

• Capitalize the genus; write the specific epithet in lowercase.
• Italicize both words when typed; underline each word separately when handwritten.
• After the first full use, the genus may be abbreviated: P. leo.

The shared genus indicates a group of similar, closely related species. The binomial prevents ambiguity between local common names, but the name records a classification—it is not itself evidence that the species boundary is correct.

Under the biological species concept, members of one species can interbreed and produce viable, fertile offspring. Successful reproduction lets alleles move through a shared gene pool.

This concept cannot be tested directly for fossils, and it does not fit organisms that reproduce only asexually. Distinct forms may also retain limited gene flow, so interbreeding evidence must be interpreted with ecology and evolutionary history.

In a typical diploid cell, chromosomes occur as homologous pairs—one member of each pair inherited from each parent—so the diploid number is usually even and normally constant within a species.

Chromosome number varies widely between species but does not rank organism size or complexity. Closely related species can differ by a chromosome fusion or split while retaining many homologous genes.

A karyotype is the chromosome number and structural features of a cell. A karyogram is an arranged image in which homologous chromosomes are paired using length, banding pattern and centromere position.

A genome is all genetic information in an organism: nuclear DNA plus mitochondrial DNA and, in plants, chloroplast DNA. Messenger RNA and transfer RNA are products of genes, not additional parts of the genome.

Most SNPs are in non-coding DNA and have no known phenotypic effect. Some lie in genes or regulatory regions and can affect function, drug response or disease risk; association alone does not prove causation.

Eukaryote genomes vary in both sequence and total DNA. Sequence variation between species is much larger than variation within one species, while repeated and other non-coding DNA can contribute greatly to genome size.

Whole-genome duplication produces polyploid organisms with several chromosome sets. This can greatly enlarge plant genomes without making the organism proportionally more complex, so a genome-size comparison is not a complexity ranking.

Whole-genome sequencing determines the order of bases across an organism’s complete DNA. The Human Genome Project established large-scale reference sequencing and accelerated bioinformatics for storing, comparing and interpreting sequence data.

Current and developing uses include:

• comparing evolutionary relationships
• locating genes and variants associated with disease
• identifying and tracking pathogens
• finding possible diagnostic, treatment or vaccine targets
• guiding prevention or treatment using an individual’s genetic profile

A sequence match or statistical association generates a hypothesis. Clinical effect, causation and safety require independent evidence; sequencing alone does not diagnose every disease or guarantee that a proposed target will work.

Bacteria reproduce asexually, so ‘interbreed and produce fertile offspring’ cannot define their species. Mutation creates new variants within lineages, while horizontal gene transfer moves DNA between cells and sometimes across established taxonomic boundaries.

Because ancestry is partly tree-like and partly network-like, no mating test encloses a single bacterial gene pool. Bacterial groups are therefore distinguished using combinations of genome sequence, evolutionary history, physiology and ecology.

Meiosis separates homologous pairs to produce haploid gametes; fertilization combines two haploid sets and restores the characteristic diploid number. This cycle prevents chromosome number doubling in every generation.

Each row asks about one observable contrast, and every answer either gives an identification or points to exactly one next couplet. The key identifies only organisms represented when it was designed; it does not prove that the resulting names are evolutionarily closest.

A key can fail when the specimen is outside its scope, damaged or at a different life stage; when a trait varies with sex or environment; or when terminology and hidden physiological features cannot be judged reliably in the field.

A detection shows that matching DNA was present in the sample. It does not by itself prove population size, current survival or ecological importance; missing reference sequences, degradation and contamination can also change the result.