A3.1 Diversity of organismsIB Biology SL: Revision notes

Variation is a defining feature of life: no two individuals are identical in all their traits. The patterns of variation are complex and form the basis for naming and classifying organisms.

Linnaeus used the morphological species concept: a species is a group of organisms with shared traits (structure and appearance).

Every species has a two-part Latin name. The first part is the genus, the second distinguishes the species within it (e.g. Homo sapiens). Species in the same genus have similar traits. The genus has an initial capital letter; the species name is all lowercase. Names are printed in italics or underlined when handwritten.

The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring. Challenges: it cannot be tested for fossils or for populations that never meet; some separate species occasionally produce fertile hybrids; and competing species definitions exist (morphological, genetic, ecological).

Speciation is the splitting of one species into two or more. It is usually gradual, so diverging populations may be partly different, and deciding whether they are one species or two can be arbitrary (e.g. ring species such as Ensatina salamanders).

Chromosome number varies between species: humans 46, chimpanzees 48. Diploid cells have an even number, as chromosomes are in homologous pairs.

A karyogram shows the chromosomes of a cell arranged in pairs by length, centromere position and banding pattern; the karyotype is the number and type of chromosomes. Evidence that human chromosome 2 formed by fusion of two ancestral chromosomes (chimpanzee 12 and 13): banding patterns match end to end, telomere sequences occur near its middle, and it has a second, inactive centromere. This is a testable hypothesis, unlike statements about purpose or value.

The genome is all the genetic information of an organism. Members of a species share most of their genome; single-nucleotide polymorphisms (SNPs) give some diversity. Between species, genomes vary in size (total amount of DNA) and in base sequence, and variation between species is much larger than within a species.

Genome size does not track complexity well: the marbled lungfish genome (about 130 000 Mbp) is about 42 times larger than the human genome (3100 Mbp).

Sequencing has become dramatically faster and cheaper: the first human genome took 13 years and about US$2.7 billion; now it takes about a day and under US$1000.

• Current uses: research into evolutionary relationships by comparing genomes.
• Potential future uses: personalised medicine, choosing treatments to match an individual's genome.