A3.1 Diversity of organisms

Variation means members of a species differ in traits, and it is a defining feature of life because populations are not exact copies.

Differences can arise from mutation, recombination, gene flow and environmental effects. Heritable variation supplies material for evolution; non-heritable differences can still affect an individual without changing the next generation’s gene pool.

Separate sources of variation:

• genetic and potentially heritable
• environmental and usually acquired
• continuous or discontinuous traits
• variation within or between populations

Two genetically similar plants can grow to different heights in different light, while a DNA mutation can create a heritable difference in pigment.

Variation is not automatically an adaptation; it must be heritable and affect reproductive success to drive selection.

The morphological species concept groups organisms as one species when they share a characteristic set of observable, inherited structural traits.

This original Linnaean approach is practical for field observations, preserved specimens and fossils. However, natural variation, life stages and sexual dimorphism can make members of one species look different, while unrelated species can look similar.

Binomial nomenclature gives every species a universal two-part name: the genus begins with a capital letter, the second word is lowercase, and both words are italicized when typed or underlined when handwritten. Species placed in the same genus share similar traits.

In Panthera leo and Panthera tigris, Panthera is the shared genus and the lowercase second words distinguish lion from tiger.

A shared appearance supports a morphological classification but does not by itself demonstrate interbreeding. The biological species concept uses reproductive evidence instead.

The biological species concept defines a species as a group of organisms that can breed with one another and produce fertile offspring.

Successful reproduction allows gene flow within the species, whereas reproductive isolation limits gene flow between species. During speciation, separated populations usually diverge gradually rather than becoming different species in one step.

To apply the concept: identify whether breeding is possible, determine whether offspring are viable and fertile, and distinguish actual reproductive isolation from populations merely being unable to meet. Because divergence is continuous, the point at which two populations receive different species names can be partly arbitrary.

Two geographically isolated populations may never meet in nature but could still produce fertile offspring if brought together. Their separation alone therefore does not prove that they are different biological species.

The concept is difficult to test for fossils, asexual organisms and geographically isolated populations, and competing species definitions may give a different boundary.

Chromosome number is usually constant within a species but varies widely between species. Humans have 46 chromosomes and chimpanzees have 48; diploid chromosome numbers are normally even because chromosomes occur as homologous pairs.

Karyotyping records the number and structural appearance of chromosomes. A karyogram is the ordered image produced by pairing homologous chromosomes and arranging them using length, banding pattern and centromere position.

A karyogram can reveal chromosome-number differences and structural similarities. Evidence for human chromosome 2 fusion includes its correspondence to two ancestral ape chromosomes, internal telomeric sequences at the predicted fusion site and remnants of a second centromere.

When pairing chromosomes in a karyogram, first match banding patterns, then confirm comparable length and centromere position rather than relying on size alone.

The observations support a testable fusion hypothesis; they should not be described as proof based only on humans having two fewer chromosomes than chimpanzees.

A genome is all the genetic information of an organism: protein-coding genes, non-coding DNA and, where present, non-nuclear DNA such as plasmids or organelle genomes.

Members of the same species share most genes and genome organisation but differ in variants. A single-nucleotide polymorphism (SNP) is a one-base difference that can act as a genetic marker. Variation between species is generally much greater than variation within one species, although the two distributions can overlap.

Across eukaryotes, genomes differ in total size, base sequence and chromosome organisation. A larger genome does not automatically mean a more complex organism because much DNA is non-coding and genome size can be affected by repeated sequences or polyploidy. In comparisons, state both the level (within or between species) and the feature being compared (sequence, size or organisation).

Genome size is the total amount of DNA in one haploid chromosome set. It can be compared across taxonomic groups, but a larger genome does not necessarily indicate a more complex organism.

Non-coding repeated DNA and polyploidy can increase genome size without adding a proportional number of functional genes. Database comparisons must therefore use consistent units and distinguish haploid from diploid measurements.

The Human Genome Project helped establish large-scale whole-genome sequencing. As sequencing has become faster and less expensive, current uses include studying evolutionary relationships and identifying genes or variants; a potential expanding use is personalized medicine.

A researcher can compare homologous genome sequences from several species, measure their sequence differences and use the pattern as evidence for closer or more distant evolutionary relationships.

A genome sequence provides data, not a diagnosis or treatment by itself. An interpretation must connect a validated genetic difference to evidence about function, ancestry or health.

The biological species concept is based on sexual reproduction, so it does not work well for asexual organisms or for bacteria that exchange genes horizontally.

Bacteria gain diversity through mutation and horizontal gene transfer, including transformation, plasmid transfer and recombination. Genes can therefore cross lineage boundaries without the organisms breeding as animals or plants do.

In many sexually reproducing species, haploid and diploid chromosome numbers are shared traits because meiosis and fertilization conserve chromosome number across generations. If closely related parents have different chromosome numbers, homologous pairing in a hybrid's meiosis can fail, reducing fertility.

A horse and donkey can produce a mule, but mismatched parental chromosome sets disrupt homologous pairing during meiosis, so the hybrid is usually infertile.

Chromosome number can support a species distinction but is not a universal definition: different species can share a chromosome number, and bacterial boundaries require evidence other than interbreeding.

Choose the identification tool that matches the available evidence.

• Dichotomous key: repeatedly choose between paired statements based on observable traits. Use reliable, relatively immutable features such as structures or reproductive processes; size, colour and behaviour may vary with age or environment. Each choice should lead to one narrower group until the specimen is identified.
• DNA barcoding: amplify and sequence a short standard region of DNA from a specimen, then compare it with a reference database to identify the species.
• Environmental DNA (eDNA): collect DNA traces from water, soil or another habitat, amplify barcode regions by PCR and match sequences to a database; it can detect organisms that are not directly observed.

Keys are useful when visible features are intact and reliable. Barcoding or eDNA is more useful when specimens are damaged, partial, cryptic or absent. Local plant or animal species can be used to build and test a key, but a database match is evidence of sequence similarity, not automatically proof that every individual in the habitat was sampled.