Natural Selection
Resumen
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, how a spray of insecticide can make a pest population harder to kill within a single summer, and how two halves of one species, kept apart by a mountain range or a stretch of sea, can drift so far apart that they can no longer breed together at all.
Objetivos
- 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.
- New species arise as a result of: isolation: two populations of a species become separated, eg geographically; genetic variation: each population has a wide range of alleles that control their characteristics; natural selection: in each population, the alleles that control the characteristics which help the organism to survive are selected; speciation: the populations become so different that successful interbreeding leading to fertile offspring is no longer possible.
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Nota de la lección
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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 alleles are produced by random mutation D. Populations become separate species when they are isolated 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 alleles arising by random mutation C. From the environment altering the DNA of adult body cells D. From organisms choosing which characteristics they need Answer: B
- Two isolated populations of one species are considered to have become separate species when they can no longer: A. Live in the same habitat B. Look similar to one another C. Interbreed to produce fertile offspring D. Compete for the same food 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
- Which sequence gives the correct order of the stages by which a new species arises? A. Speciation, isolation, natural selection, genetic variation B. Isolation, genetic variation, natural selection, speciation C. Genetic variation, speciation, isolation, natural selection D. Natural selection, speciation, genetic variation, isolation Answer: B
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