Natural selectionOxford AQA IGCSE Biology: Flashcards

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What is the main scientific theory that explains how organisms have evolved?

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What is the main scientific theory that explains how organisms have evolved?: The theory of evolution by natural selection.
What did Lamarck's theory propose?: That changes occurring in an organism during its own lifetime (acquired characteristics) could be inherited by its offspring.
Is Lamarck's theory of inheritance of acquired characteristics correct?: No — we now know that, in the vast majority of cases, this type of inheritance cannot occur.
What is the source of the variation that natural selection acts on?: Differences in individuals' genes within a species.
According to natural selection, which individuals are more likely to survive and reproduce?: Individuals with characteristics most suited to their environment.
What happens to genes that help an individual survive and reproduce?: They are passed on to the next generation, so the advantageous characteristics become more common in the population over time.
Over what kind of time scale does evolution by natural selection usually occur?: A very long time scale, typically thousands to millions of years, over many generations.
What is the first stage in the formation of a new species?: Isolation — two populations of a species become separated, for example geographically, so they can no longer interbreed.
Why might natural selection favour different alleles in two separated populations?: Because the two populations live in different environments or conditions, so different characteristics (and the alleles behind them) help each population survive best in its own environment.
What is speciation?: The point at which two populations have become so different that they can no longer successfully interbreed to produce fertile offspring — they are now separate species.
What test is used to decide whether two populations have become separate species?: Whether they can produce fertile offspring together — if not, they are considered separate species.
List the four stages leading to the formation of a new species.: Isolation of two populations. 2) Continued genetic variation within each population. 3) Natural selection favouring different alleles in each population. 4) Speciation, where interbreeding no longer produces fertile offspring.