DP IB Biology: SL

Revision Notes

Syllabus Edition

First teaching 2014

Last exams 2024

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4.1.1 Species & Population

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Species

  • A species can be defined as:

A group of organisms that can interbreed to produce fertile offspring

  • The ability to breed and produce fertile offspring is a useful method of distinguishing species for organisms that reproduce sexually but can be difficult to apply in some situations
    • Organisms that reproduce by asexual reproduction, such as bacteria, cannot be classified using this method
    • On rare occasions, animals of different species breed together and produce fertile offspring, such as the so-called 'wholphin'; the fertile offspring from a cross between a melon-headed whale and a common bottlenose dolphin
      • According to the species rule above the wholphin would be a new species, but while scientists do believe that hybridisation can lead to new species it needs to be a frequent event for this to occur, and wholphins are rare
      • Note that the melon-headed whale is actually a species of dolphin, so the name 'wholphin' is a bit inaccurate!
  • The imperfect nature of this method of classifying species means that other characteristics are often used at the same time
    • Organisms of the same species share similar morphology
    • DNA sequences can be compared, with a certain level of similarity indicating that organisms are the same species

Populations

  • A population can be defined as:

A group of organisms of the same species living in an area at one time

  • A population can be isolated from other populations of the same species due to living in a different area
  • This isolation means that members of the separate populations cannot breed together and gene exchange or gene flow cannot take place between them
  • As long as these isolated populations could, in theory, interbreed to produce fertile offspring, they are the same species
  • If the environmental conditions affecting each population are different, then natural selection could act differently on each population and eventually lead to speciation
    • Genetic drift can also lead to speciation
  • Once speciation has taken place, the two species can no longer produce fertile offspring; they are reproductively isolated

Allopatric speciation in trees (1), downloadable AS & A Level Biology revision notesAllopatric speciation in trees (2), downloadable AS & A Level Biology revision notesAllopatric speciation in trees (3), downloadable AS & A Level Biology revision notes

Reproductive isolation of two populations of trees can occur when the populations are separated for a long period of time

Exam Tip

Make sure that you can state the definition for a species - organisms belong to the same species if they can interbreed to produce fertile offspring

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Naomi H

Author: Naomi H

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.