A3.1 Diversity of organisms

• Variation occurs between individuals within every species

• Continuous variation often results from polygenes and environmental effects

• Darwin's observations of variation supported natural selection

• Species can be grouped by shared morphology and structural traits

• Morphological species concept is useful in fieldwork, fossils, and many plants

• Sexual dimorphism and within-species variation can make morphology difficult

• Binomial nomenclature gives each species a universal two-part Latin name

• Genus is capitalized; species is lowercase; both are italicized or underlined

• Species in the same genus share similar traits

• Biological species concept defines species by interbreeding and fertile offspring

• It emphasizes gene flow between members of the same species

• Limits include extinct organisms, asexual species, and cases with gene flow between distinct forms

• Speciation is gradual as isolated populations diverge

• Populations may still be theoretically able to interbreed but cannot meet

• Species boundaries can therefore be partly arbitrary

• Chromosome number is usually constant within a species but varies between species

• Diploid chromosome numbers are even because chromosomes occur in homologous pairs

• Examples include fruit fly 8, rice 24, human 46, chimpanzee 48, and dog 78

• Karyotypes show chromosome number and structure in a cell

• Karyograms arrange chromosomes by length, banding pattern, and centromere position

• Human chromosome 2 supports fusion from two ancestral ape chromosomes

• A genome is all genetic information in an organism

• Members of a species share most of their genome but differ at variants

• SNPs are common single-base differences used as biological markers

• Eukaryote genomes vary in size and base sequence

• Between-species variation is much larger than within-species variation

• Not all DNA codes for proteins

• Genome size is the DNA amount in one chromosome set

• Larger genome size does not necessarily mean greater organism complexity

• Polyploidy can greatly increase genome size, especially in plants

• Whole genome sequencing began at scale with the Human Genome Project

• Current uses include evolutionary relationships, gene discovery, and pathogen research

• Future uses include personalized medicine, diagnosis, disease prevention, and vaccine targets

• Biological species concept does not apply well to asexual organisms

• Bacteria gain diversity by mutation and horizontal gene transfer

• Transformation, plasmids, and recombination blur bacterial species boundaries

• Diploid and haploid chromosome numbers are usually constant within a species

• Meiosis conserves chromosome number across generations

• Different chromosome numbers can prevent fertile hybrids, as in mules

• Dichotomous keys identify organisms through paired yes/no choices

• Construct keys from observable features arranged in a character matrix

• Local plant or animal species can be used to build and test keys

• DNA barcoding identifies species using short sequences from selected genes

• eDNA from habitats can be amplified by PCR and compared with databases

• Barcoding is rapid and useful for damaged, partial, or hard-to-identify specimens