AQA A Level Biology复习笔记7.2.1 Populations

Species Exist in 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


Exam Tip

Defining a species can be notoriously difficult for biologists. For example, there are two types of hawthorn tree that have several physical differences. However, when these plants are planted together they are able to produce fertile offspring. New evidence can throw a spanner in the works, it can cause scientists to reconsider the relationship between certain organisms or to change the binomial name of an organism!

The Definition of a Population

  • 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