3.1 Biodiversity and Evolution

Species diversity in a community is a product of two separate variables. Richness is the number of species present. Evenness is how similar the population sizes of each species are to one another. Two communities can have identical richness and still feel very differently diverse, because of evenness.

Richness and evenness combine into a single number, Simpson's reciprocal index, covered as its own application-of-skills item in the Skills tab.

Effective management strategies to conserve biodiversity depend on knowledge of global and regional biodiversity, and that knowledge has to come from somewhere. Government-funded agencies and voluntary organizations run formal biodiversity surveys. Citizen science lets members of the public contribute observations at a scale professional researchers alone could never cover. Training local and indigenous people, including as parabiologists, builds monitoring capacity embedded in the region itself rather than dependent on outside researchers, and draws on ecological knowledge that outside surveys often miss entirely.

Simpson's reciprocal index gives a single quantitative measure of species diversity, combining richness and evenness, so that different ecosystems can be compared, or a single ecosystem can be monitored for change over time. If provided with data, calculate:

N is the total number of individuals of all species; n is the number of individuals of a single species, and Σn(n−1) means: work out n(n−1) for every species, then add those values together. A higher D value means greater diversity (more richness, more evenness), with 1 being the lowest value the index can produce, a community made up of a single species.

Try it: adjust the four species counts below and watch N, Σn(n−1) and D recalculate live to see how richness and evenness together drive the index.

Worked example, by hand: 4 species with counts 4, 3, 2, 1 (N = 10). Σn(n−1) = (4×3) + (3×2) + (2×1) + (1×0) = 12 + 6 + 2 + 0 = 20. N(N−1) = 10×9 = 90. D = 90 ÷ 20 = 4.5.

Before you crunch numbers, being able to judge which sample is likely more diverse just from its description is a testable skill. Three more scenarios to practice on.

Genetic diversity, the raw material natural selection acts on, increases through two distinct processes. Mutation generates entirely new variants of genes, changes to the DNA sequence itself. Sexual reproduction does not create new gene variants, but it generates new combinations of the variants that already exist, by mixing genetic material from two parents.

Diagram: Jonsta247, via Wikimedia Commons (CC BY-SA 4.0).

Diagram: via Wikimedia Commons (CC BY-SA license).

Reproductive isolation, the barrier that turns a diverging population into a truly separate species, can be achieved in two different ways: geographical separation, or, for populations that still share the same area, ecological or behavioral differences.

Ordinary evolutionary change within a single species, such as giraffes evolving longer necks over generations, is not speciation on its own: a new species only forms once a population splits and reproductive isolation evolves between the two branches.

Practice: classify each new scenario by which kind of reproductive isolation it illustrates.

This also explains why isolated islands so often show high rates of endemism. Once a founding population is cut off from the mainland, gene flow with mainland relatives stops entirely, and the island's often-empty ecological niches leave room for the isolated population to diversify in ways it never would have on a crowded mainland. Madagascar, next, is the clearest possible illustration.

Biodiversity is spread unevenly across the planet. Certain areas contain a particularly large proportion of the world's species, especially species that are rare and endemic, and many of these hotspots sit in tropical biomes.

Map: NoniMF / Coral Triangle Initiative, via Wikimedia Commons (CC BY-SA 4.0).

Madagascar shows how geographic isolation, the same mechanism behind allopatric speciation, produces extreme endemism on land. The Coral Triangle shows the same unevenness applies at sea, driven instead by an exceptionally stable, species-rich marine environment at the convergence of two ocean basins.

Human activities have altered the selective forces acting on species within ecosystems, producing measurable evolutionary change. The clearest documented case is African elephants in Gorongosa National Park, Mozambique.

Artificial selection is the deliberate act of choosing individual plants or animals for breeding, unlike natural selection, which is not deliberate. Because breeders repeatedly select for the same narrow set of traits, artificial selection reduces genetic diversity, and with it, the resilience of the selected population. The Cavendish banana, now the dominant banana variety in global trade, is propagated by cloning rather than seed, so essentially every plant is genetically identical. That uniformity is why Fusarium wilt (Tropical Race 4), a soil fungus, has been able to spread through Cavendish plantations worldwide with no resistant genetic variation to fall back on, the same vulnerability that wiped out the previous dominant variety, the Gros Michel, in the mid-20th century.

Practice: classify each scenario as natural or artificial selection. Remember: the question is whether breeding was a deliberate human choice, not just whether humans were involved.

Earth history extends over roughly 4.5 billion years, and processes acting over that extended timescale have driven the evolution of life. The fossil record is the primary evidence for reconstructing that history: preserved remains show which organisms existed when, and how life has changed over geological time.

Earth history is divided into geological epochs according to the fossil record, at increasingly fine resolution: eons, which are subdivided into eras, then periods, then epochs. The boundary between one epoch and the next is marked by a significant shift in the fossil record, typically reflecting environmental change severe enough to cause extinctions, followed by the evolution of new species.

Five mass extinctions have occurred in Earth's past, each followed by unusually rapid rates of speciation, since the sudden loss of species reopens ecological niches for survivors to diversify into.

Figure: OpenStax, in Wilson & Primack, Conservation Biology in Sub-Saharan Africa (2019), via Wikimedia Commons (CC BY 4.0).

Practice: match each cause below to the mass extinction it triggered.

A sixth mass extinction is underway now, driven by human activity rather than tectonics, volcanism or an impact. It is this contrast, five natural events against one ongoing anthropogenic one, that sets up the next question: does human impact deserve its own named chapter in the geological timescale?

The Anthropocene is a proposed geological epoch characterized by rapid environmental change and species extinction due to human activity, positioned as a successor to the current epoch, the Holocene. There is real scientific debate over whether, and when, it begins. Proposed starting markers ("golden spikes") have included the 1610 dip in atmospheric carbon dioxide linked to the population collapse of the Americas after European arrival, and a mid-20th-century marker tied to the post-1945 explosion in industrial activity, nuclear testing, and synthetic materials, often called the Great Acceleration.

Independent of the formal vote, human impacts are having a planetary effect that is detectable in the geological record, the core evidence base the Anthropocene argument rests on. Sort each observation below into the category of evidence it represents.