Natural Selection
Overzicht
Look closely at any wild population and you will find that no two members of it are quite the same. Some run a little faster. Some are a shade darker. Some survive a cold night that kills their neighbours. Those small differences look trivial until you realise that the environment is quietly sorting through them, generation after generation, keeping some and discarding others. Given enough time, that sorting rebuilds a species.
This lesson takes you through the mechanism behind that rebuilding. You will see why an individual organism never evolves but a population does, why a blacksmith's strong arms are not inherited by his children while a beetle's colour is, why the differences have to be there before the environment can do any choosing, and how a spray of insecticide can make a pest population harder to kill within a single summer even though the same mechanism took millions of years to lengthen a giraffe's neck.
Doelstellingen
- Theories of how organisms have evolved include: the theory of evolution by natural selection; other theories, including that of Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. We now know that in the vast majority of cases this type of inheritance cannot occur.
- Evolution occurs via natural selection.; Individual organisms within a particular species may show a wide range of variation because of differences in their genes.; Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.; The genes that have enabled these individuals to survive are then passed on to the next generation. Students should develop an understanding of the time scales involved in evolution. 40
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- Which statement best describes Lamarck's theory of evolution? A. Characteristics an organism develops during its lifetime are passed to its offspring B. Individuals best suited to the environment survive to breed C. New forms of a gene are produced by mutation D. Populations change because more offspring are produced than can survive Answer: A
- In natural selection, where does the variation between individuals originally come from? A. From individuals practising a useful skill B. From differences in genes, with new forms arising by mutation C. From the environment altering the genes of adult body cells D. From organisms choosing which characteristics they need Answer: B
- Which of these differences between two plants of the same species could be acted on by natural selection? A. One is taller because it was given more fertiliser B. One is paler because it grew in deep shade C. One flowers earlier because of the genes it carries D. One has fewer leaves because an animal ate them Answer: C
- A field of insect pests is sprayed with an insecticide. A few insects survive and breed. What is the best explanation? A. The insecticide caused those insects to mutate B. Those insects learned to avoid the spray C. Those insects already carried an allele giving resistance D. The insects that died had used up their resistance Answer: C
- Two species meet the same new selection pressure. Species X produces a new generation every two weeks and species Y every ten years. Which statement is correct? A. Species X will change more quickly, because selection acts once per generation B. Species Y will change more quickly, because it lives longer C. Both will change at the same rate, because the pressure is the same D. Neither will change, because natural selection needs millions of years Answer: A
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