The Role of Isolation in Forming New Species (DP IB Environmental Systems & Societies (ESS))

The Role of Isolation in Forming New Species

• The theory of evolution states that species do not stay the same, but change over time; this can lead to the process of speciation

  • Speciation can be defined as the development of new species from pre-existing species over time

• In order for speciation to occur two populations of the same species must be genetically isolated from each other in some way

  • When this happens, there can no longer be an exchange of genes between the two populations

  • The exchange of genes is sometimes known as gene flow

• Isolation of populations usually occurs due to geographical isolation, which can be caused by environmental changes forming barriers such as:

  • Mountain formation

  • Changes in rivers

  • Sea level change

  • Climatic change

  • Plate movements

• Populations that become isolated from each other may face different selection pressures in their environment e.g. different predators or different food sources

  • The different environmental conditions for the two populations might mean that different genes are advantageous, so different genes are more likely to be passed on and become more frequent in each population; this is the process of natural selection

  • The gene frequencies in the two populations change over time

• Over time, the two populations may begin to differ physiologically, behaviourally and morphologically to such an extent that they can no longer interbreed to produce fertile offspring; speciation has occurred

Example of Geographic isolation Leading to Speciation

The geographical barrier created by a newly formed mountain range can lead to speciation in trees

• An area contains a large single population of trees

• A new mountain range forms that divides this single population into two separate populations (A and B)

• The geographical barrier prevents the two populations from interbreeding so there is no gene flow between them

• The two populations experience different environments, so different genes become advantageous

  • For example, population B is now partially shaded by the mountains, so the trees in this population need to grow taller and contain more chlorophyll, in order to maximise the amount of sunlight they can absorb for photosynthesis

  • Genes that contribute to greater tree height and increased chlorophyll production become advantageous for population B

• Different genes are therefore more likely to be passed on in each population

• Different genes become more frequent in each population

  • Over time, this causes the tree in population B to become taller and darker (the leaves contain a higher concentration of chlorophyll)

• Over thousands of years the divided populations form two distinct species that can no longer interbreed to produce fertile offspring