7.2.1 Populations

• A species can be defined as a group of similar organisms that can reproduce to give fertile offspring
• Organisms of the same species have the same number of chromosomes in their cells
  • Humans have 46 chromosomes

• The reason that two organisms from a different species cannot produce fertile offspring is due to the fact that different species have a different diploid number of chromosomes in their cells
  • For example, a horse has 64 chromosomes in its cells while a donkey has 62. When the haploid gametes from a horse (32) and a donkey (31) combine, the resulting zygote has 63 chromosomes
  • Cells that have an odd number of chromosomes are not viable. The chromosomes can not form homologous pairs during meiosis to produce gametes

• Members of a species do not live alone
  • Instead, they live in populations

• Species can exist as one or more populations, for example:
  • The American black bear (Ursus americanus) is one species but has multiple populations in America and Canada
  • The Javan rhinoceros (Rhinoceros sondaicus) has only one population on the island of Java in Indonesia

Defining a species

• The system used by biologists to organise living organisms into categories is based on dividing organisms into species
• There are several factors that need to be taken into consideration when defining a species or determining whether two organisms belong to the same species
  • Similarities/differences in observable features (morphology)
  • Similarities/differences in DNA
  • Similarities/differences in RNA
  • Similarities/differences in proteins
  • The ability to interbreed and produce fertile offspring

• A population can be defined as a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed and produce fertile offspring
  • For example, the Bornean orangutan (Pongo pygmaeus) and Müller's gibbon (Hylobates muelleri) live in the same area at the same time but are two species with two different populations
  • They cannot be part of the same population as they are different species (they cannot interbreed and produce fertile offspring)

• Some organisms have very small populations, whereas others have very large populations, for example:
  • The Amur leopard (Panthera pardus orientalis), now found only in the Amur River basin of eastern Russia (having already gone extinct from China and the Korean Peninsula) has a remaining population of around 60 individuals
  • Although humans (Homo sapiens) used to live in separate populations, we are now widely considered as one global population of around 7,800,000,000 individuals