• 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

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