Natural Selection and Specation Key words Genetic drift

Natural Selection and Specation

Key words • Genetic drift • Bottleneck effect • Founder effect • Gene pool • Gene flow on • Descent with modificati

The Peppered Moth – example of evolution

Sources of Genetic Variation • The 2 main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction

Mutations • Any change in a sequence of DNA • Mutations can occur because of – Mistakes in DNA replication – Radiation or chemicals in the environment • Some mutations dont affect the phenotype but some do

Variation in Populations • Variation via different phenotypes naturally exist within a population • Some genotypes may offer a selective advantage, while others a disadvantage • Variations within a population can be small, but are the basis of evolution

Gene Pools • The sum of alleles within a population is referred to as the gene pool • Biologically, a gene pool is a group of individuals of the same species that live in the same geographical area and interbreed to produce fertile offspring • Range of gene pool restricted by alleles available – not all organisms have the genes to produce tails, wings etc • Some genes do not contribute to variation – they are fixed within the population • Only 15 -20% of genes are used in evolutionary change

Allele Frequencies • Genetic polymorphs – different phenotypes that exist due to different alleles • Polymorph frequency affected by: – – – – Mutations in alleles Immigration Emigration Reproduction rate Genetic drift The bottleneck effect The founder effect

Migration and Gene Flow • Populations in a genetic sense are defined by reproductive and genetic isolation • Although some populations exist in isolation, migration usually takes place into and out of a population • If interbreeding occurs, gene flow takes place

Migration and Gene Flow • Indigenous Australians have some alleles for blood group gene that exist in frequencies different from other populations • Do not possess allele IB therefore cannot be B or AB blood groups • Isolation for over 50, 000 years means limited gene flow • Increased genetic flow has lead this to change

Genetic Drift • Natural Selection is not the only source of evolutionary change • In small populations, an allele can become more or less common by chance

Genetic Drift • These individuals may carry alleles in different relative frequencies than did the larger population from which they came • If so, the population that they found will be genetically different from the parent population • This cause is not natural selection, but chance

Bottleneck Effect • Catastrophic events can severely reduce the size of a gene pool • Surviving population may persist, however this means that genes are lost and surviving population not representative of founding gene pool

Founder effect • A situation in which allele frequency changes as a result of the migration of a small subgroup of a population • Results in less genetic diversity in new population

Questions • In groups, or with a partner, discuss questions 1 – 8 to test your knowledge of what we have learned so far. Use the terminology in this section to articulate your answers. • Discussion helps retention of new concepts, so get involved!!