UNIT 2 – BIOLOGICAL WORLD Evolution by natural selection


By studying the birds on the Galápagos Islands, Darwin reasoned that nature had selected the genes for the best beak structure over thousands of years. From this, he developed his theory of evolution by natural selection.


• Evolution is the gradual change in the inherited characteristics (controlled by genes) of a species over time.


• Natural selection describes how the organisms best suited to their environment are most likely to survive and reproduce.


:Fig. 5.6 Darwin published his book in 1859


(Fig. 5.7 A finch eating cactus seeds in the Galápagos Islands


Darwin’s theory made the following observations and conclusions: Observations


1. Species produce large numbers of offspring.


2. There is a limited supply of resources in nature, for example, food and mates.


3.


Individuals of a given species are slightly different from each other, as they have inherited genes from their parents for different traits.


4. The number of individuals in a species stays relatively constant from one generation to the next.


? TEST TUBE 8. What is evolution?


9. Two rabbits born to the same parents are being chased by a fox. One rabbit has strong, well-developed hind-limbs. The other rabbit is weaker due to less access to food. Which rabbit is more likely to escape? Use the term ‘survival of the fittest’ in your answer.


10. Why do you think the number of individuals in a species stays relatively constant from one generation to the next? Use the term ‘competition’ in your answer.


:Fig. 5.8 46 Elements of Science


Conclusions


Individuals compete with each other for limited resources – this struggle leads to the ‘survival of the fittest’.


Individuals with the best-adapted traits (and so genes) survive and are able to pass on these genes to their offspring.


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