3.1 Biodiversity and evolutionIB Environmental Systems and Societies HL: Revision notes

Biodiversity is the total diversity of living systems, at three levels: genetic diversity (variety of alleles), species diversity (number and abundance of species) and habitat or ecosystem diversity (variety of habitats and ecosystems).

The components of diversity contribute to the resilience of ecological systems: if genetic diversity is high, some individuals probably carry alleles that allow survival of a disease or change; if species diversity is high, other species can fill the role of one that is lost.

Example: the cheetah has very low genetic diversity, so populations are vulnerable to disease.

Biodiversity arises from evolutionary processes. Natural selection drives evolutionary change and involves variation, overproduction of offspring, competition for limited resources and differences in adaptation that affect rates of survival and reproduction. Allele frequencies change over generations.

Speciation is the generation of new species through evolution: populations are selected in different directions until they can no longer interbreed.

Species diversity is a product of richness (number of species) and evenness (how equal the numbers of each species are).

Simpson's reciprocal index: D = N(N − 1) / Σn(n − 1), where N is the total number of individuals and n the number of each species. Higher D means higher diversity. It compares ecosystems and monitors change over time.

Worked example: counts 12, 8, 6, 4 give N(N − 1) = 870, Σn(n − 1) = 230, so D = 3.8. Counts 27, 1, 1, 1 give D = 1.2: same richness, but one species dominates.

Management strategies to conserve biodiversity need knowledge of global and regional biodiversity: what is present, where, and how it is changing. Surveys and indices allow priorities to be set and success to be monitored, but knowledge is often incomplete and strategies also depend on resources and people.

(HL) Mutation creates new alleles and sexual reproduction (meiosis, crossing over, independent assortment and random fertilisation) combines alleles into new combinations. Both increase genetic diversity.

(HL) Reproductive isolation stops gene flow between populations. It can be achieved by geographical separation (for example populations on different islands) or, for populations in the same area, by ecological differences (different habitats or food) or behavioural differences (such as different courtship signals). Isolated populations diverge and may become new species.

(HL) Biodiversity is spread unevenly across the planet. Certain areas, often called biodiversity hotspots, contain a particularly large proportion of species, especially rare and endangered ones, so protecting them gives a large conservation gain.

(HL) Human activities have changed the selective forces acting on species, causing evolutionary change. Example: poaching in Gorongosa National Park, Mozambique, favoured tuskless female elephants, whose proportion rose from about one in five to about half among survivors. Other examples include antibiotic resistance in bacteria and pesticide resistance in insects.

(HL) Artificial selection reduces genetic diversity, and so reduces the resilience of species, because only individuals with chosen traits breed.

(HL) Earth history extends over about 4.5 billion years. Processes occurring over such an extended timescale have led to the evolution of life.

(HL) Earth history is divided into geological epochs according to the fossil record.

(HL) Mass extinctions are followed by rapid rates of speciation because the extinctions open up niches. After the extinction about 66 million years ago, mammals diversified rapidly.

(HL) The Anthropocene is a proposed geological epoch characterised by rapid environmental change and species extinction due to human activity. Human impacts are planetary and will be detectable in the geological record (plastics, radioactive isotopes, altered sediments). Its formal status and start date are debated.