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Revising variation Bill Indge Revising variation Variation may be caused by genes. Revising variation Variation may be caused by the environment. Revising variation Variation may be caused by a combination of genes and environment. Genes Precursor Environment Carotene in food Pink substance in feathers Revising variation Meiosis Sister chromatids Centromere Revising variation Meiosis Revising variation Meiosis: the first division Revising variation Independent segregation Revising variation Meiosis: the second division Revising variation Crossing over Revising variation Crossing over Members of each homologous pair lay side by side. They may twist round each other and break at some point. Revising variation Crossing over The broken segments may join to the other member of the chromosome pair resulting in genetic recombination. Revising variation Genetic bottlenecks Number of seals Revising variation 150 000 100 000 50 000 0 1800 1850 1900 Year 1950 2000 Revising variation Large population Many different alleles Large population Few different alleles Small population Few alleles Revising variation Revising variation 25 Number of bases where gene varies 0 Jaguar Puma Ocelot Margay Tiger Revising variation Domestication Revising variation Wild ancestor Many different alleles Modern breed Few different alleles Revising variation The founder effect Revising variation 100 Percentage of white flowering plants 0 North Wales Lake District New Zealand Revising variation Selection and snails Cepaea nemoralis Revising variation Selection and snails Brown Pink Not banded Yellow Banded • The snails differ in the colour of their shells and in whether or not their shells are banded. • These shell patterns are controlled by genes. • The differences in shell pattern arose by mutation. Revising variation • The snails differ in the colour of their shells and in whether or not their shells are banded. • These shell patterns are controlled by genes. • The differences in shell pattern arose by mutation. • Song thrushes eat more of the snails with conspicuous shells. Revising variation Beech woodland Revising variation Beech woodland Percentage of yellow shells 10 Percentage of banded shells 38 8 36 0 20 4 18 12 0 Revising variation It’s the same every time. • Describe how the organisms vary. • Explain that these variations are controlled by genes. • Explain the importance of mutation in producing genetic variation. • More organisms with the favourable variation will survive and reproduce, passing on the relevant alleles to the next generation. • This will alter the frequency of alleles in the population.
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