IB Environmental systems and societies 2.1A Organisms Notes

Species: The Basic Unit of Biodiversity

1. Ecology is studied at rising levels of organisation: individual, population, community, ecosystem, biosphere.
2. This article covers the individual and the species, the foundation on which every higher level is built.

What a Species Is: The Biological Species Concept

1. Members of a species share a gene pool, so they can exchange genetic material through reproduction.
2. The most common definition is the Biological Species Concept (BSC), which sets three tests.
   1. Interbreeding: members can reproduce with each other in nature.
   2. Fertile offspring: their offspring can also reproduce.
   3. Reproductive isolation: different species cannot naturally interbreed to produce viable, fertile offspring.

Example

• A horse and a donkey produce a mule, which is sterile.
• A lion and a tiger can breed, but their hybrid (a liger) is usually infertile, so they stay separate species.

Limitations of the Biological Species Concept

1. Geographically isolated populations may evolve apart yet still count as one species, so the test is hard to apply.
2. Asexual organisms such as bacteria reproduce without interbreeding, so the BSC cannot be used.
3. Extinct species cannot be tested for reproductive compatibility.
4. Incomplete barriers let some species hybridise under certain conditions, blurring the line.

Binomial Naming: One Species, One Universal Name

1. A universal naming system avoids the confusion of local common names, which vary by language and region.
2. Each species has a two-part Latin name (binomial nomenclature).
   1. Genus name: capitalised, for example Homo.
   2. Species name: lowercase, for example sapiens.
3. Both names are italicised when typed or underlined when handwritten.

Example

• Panthera leo is the lion.
• Panthera tigris is the tiger, sharing the genus Panthera but with its own species name.

Naming Sits Within a Wider Classification

1. The species name is the smallest rung of the Linnaean hierarchy (Domain down to Species).
2. The full classification system and how organisms are grouped is covered in 2.1L (Classification methods).

Identifying Organisms in the Field: Dichotomous Keys

1. A dichotomous key identifies an organism through a series of paired either/or choices.
2. Each choice narrows the possibilities until only one species remains.
3. Keys are quick for known groups but weak for new, microscopic, or look-alike species.

Why This Skill Matters for ESS

1. Accurate identification underpins biodiversity surveys and conservation decisions in fieldwork.
2. Deeper methods such as DNA barcoding and reference collections are covered in 2.1L.