A3.1 Diversity of organismsIB Biology HL: Revision notes

Variation is a defining feature of life: no two individuals are identical in all traits. Linnaeus's morphological species concept groups organisms with shared traits. In the binomial system, the first part of the name is the genus (initial capital) and the second the species (lowercase), e.g. Quercus robur. Species in the same genus have similar traits.

The biological species concept: a species is a group of organisms that can interbreed and produce fertile offspring. Challenges include fossils, populations that never meet, fertile hybrids between recognised species, and the existence of competing definitions.

Speciation (one species splitting into two or more) is usually gradual, so deciding whether diverging populations are one species or two can be arbitrary.

Chromosome numbers vary: humans 46, chimpanzees 48; diploid numbers are even. Karyograms arrange chromosomes by length, centromere position and banding pattern. Human chromosome 2 is hypothesised to have formed by fusion of two ancestral chromosomes (chimpanzee 12 and 13): matching bands, internal telomere sequences and a second centromere support this testable hypothesis.

The genome is all of an organism's genetic information. Members of a species share most of it, with SNPs giving diversity; genomes of different species differ in size and base sequence, and genome size does not match complexity. Whole genome sequencing is now fast and cheap: used in research on evolutionary relationships, with personalised medicine a future use.

The biological species concept does not work well where organisms do not breed sexually or where genes pass between species.

• Asexual species (e.g. bdelloid rotifers, many bacteria) never interbreed, so the interbreeding test cannot be applied.
• Bacteria exchange genes by horizontal gene transfer (e.g. plasmids carrying antibiotic resistance), even between different species, so gene pools are not isolated.

Members of a species share the same chromosome number. When closely related species with different chromosome numbers cross, the hybrid's chromosomes cannot all pair in meiosis, so it is unlikely to be fertile. Example: horse (64) × donkey (62) gives a mule (63), which is almost always sterile. Species with the same number (e.g. the two British oaks, 24) are more likely to produce fertile hybrids.

A dichotomous key identifies organisms through a series of couplets, each with two contrasting statements that lead to another couplet or a name. Use clear-cut, consistent features, not variable or seasonal ones; n species need n − 1 couplets. Test the key on new specimens and revise it.

Environmental DNA (eDNA) is DNA shed into water or soil. A short barcode gene region that differs between species is amplified by PCR, sequenced and matched against a reference database, allowing the biodiversity of a habitat to be investigated rapidly without capturing organisms. Limits: incomplete databases, DNA from dead or transported organisms, and no data on abundance.