Environmental Selective Pressure

 Investigate changes in a population of organisms due to selection pressures over time, for example: 

  • cane toads in Australia
  • prickly pear distribution in Australia


A baby cane toad

Cane toads in Australia:

The cane toad was introduced into Australia in 1935 by the Bureau of Sugar Experiment stations to help control the grey-black beetle and french beetle which were a pest eating sugar crops. These two invasive species were supposed to be eaten by the cane toads to help control them as an alternative to using toxins. However, the cane toads lacked any natural predators hence they ate everything, grew and reproduced prolifically.

Since there release in Northern Queensland they have been moving at approximately 60 km a year, expanding there distributional spread in Australia. Australia does not have any predators for the cane toad since it secretes a toxin which is able to kill most animals. In Mainland America, where cane toads are native to, predators of the cane told evolved a resistance to the toxin hence there numbers were regulated by other predators eating them. Additionally, Cane toads have spread so profusely due to ideal warm and humid climate found in Queensland. These conditions have been beneficial to their increase in distribution and abundance.

The black kite has successfully developed behavioral adaptations against the cane toads, learning to flip them on their back and eat them out from their belly’s to avoid the toxin they secrete from their backs. Whilst some snake species have developed structural adaption of smaller jaws so they can not eat the large ones which contain a large amount of toxins. Hence causing the evolutionary changes in native Australian predators.

Prickly Pear

Prickly pear in Australia:

Prickly pear first appeared in Australia throughout the 19th century, originally as agricultural fences. However, they lacked natural predators and the conditions were ideal for them to rapidly reproduce hence they populated rapidly rendering plots of lands unsuitable for agriculture. The Cactoblastis cactorum Moth was introduced from South America since its larvae feed on the prickly pear hence allowing it to be controlled.

Prickly pear is a dense growing plant which smothers out other native plants whilst providing a safe harbour for pests such as fox’s. Their fruit has been known to be eaten by fruit fly’s which are a natural pest to a lot of agricultural crops and some native plants hence negatively effecting native plants.


Question 1:

Contrast the use of pest control for the grey-blacked beetle and the prickly pear. Why do foreign predators often need to be introduced? [4 marks]

Question 2:

The following two maps demonstrate the increased distribution of the cane toad in Australia.

Explain the trends and patterns in population dynamics present and predict what you think will happen in future years. [3 marks]

Question 3:

Using the examples of the prickly pear and cane toad in Australia, how does the lack of selective pressures effect the population of an organism over time. [4 marks]

Up Next: The theory of evolution by natural selection