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Unit 4- Evolution. 8.1 Natural Selection. Overview of section. Types of selection Directional Stabilizing Disruptive Sexual Natural Evolutionary change without selection Genetic drift Genetic bottlenecks Founder effect. Types of selection: Directional.
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Unit 4- Evolution 8.1 Natural Selection
Overview of section • Types of selection • Directional • Stabilizing • Disruptive • Sexual • Natural • Evolutionary change without selection • Genetic drift • Genetic bottlenecks • Founder effect
Types of selection: Directional • Selection that favours increase or decrease away from average phenotype http://avonapbio.pbworks.com/w/page/9429288/Ch-23
Types of selection: Stabilizing • Selection when average phenotype is selected for. http://kenpitts.net/bio/evolution/selections/stabilize.html
Types of selection: Disruptive • Selection that favours extreme individuals over those with average phenotype. http://www.sparknotes.com/biology/evolution/naturalselection/section1.html
Types of selection: sexual • Selection that favours individuals with traits that increase their mating success.
Sexual selection and evolution video http://www.youtube.com/watch?v=gKybAp--n7M
Natural Selection in Action • Tibetan people in the Himalayas • Study on mice: example of genetic component related to behaviour http://fitgevitylifestyle.com/?paged=4
Evolutionary change without selection: Genetic Drift • Change in genetic population due to chance. • Has greater impact in smaller populations http://smabiology.blogspot.com/2009_02_01_archive.html
Evolutionary change without selection: Genetic bottlenecks • Genetic drift that results when there is a dramatic reduction in population.
Genetic bottleneck (cont’d) Example of Bottleneck Effect: -By the end of the 19th century, hunting reduced the population of northern elephant seals to as few as 20 individuals.
Genetic Drift (cont’d) -As a result of this population bottleneck, the genetic variation of the population was greatly reduced. -The population of northern elephant seals has since rebounded due to environmental protection, but it still has low genetic variation, leaving it susceptible to disease, etc.
Evolutionary change without selection: Founder effect Founder Effect: Genetic drift that results when a small number of individuals separate from their original population and start a new population. -new population may have: -reduced genetic variation from the original population -a non-random sample of the genes in the original population
The Founder effect Island The Mainland
The Founder effect Population Island The Mainland
The Founder effect A few individuals colonise a new isolated area The Mainland
There may be a higher frequency of one allele in the founder population just by chance This allele needn’t have been very common in the original population
The island population grows Island
…after a few generations The green allele may be lost completely if individuals fail to leave offspring carrying it Island
…after a few generations The green allele may be lost completely if individuals fail to leave offspring carrying it Island
…after a few generations Mutations may occur creating new alleles Island
…after a few generations The new allele becomes more common Island
The 2 populations now look very different! Island The Mainland
Founder effect (cont’d) • Polydactyly (extra fingers or sometimes toes) is commonly found in the Amish population in Eastern Pennsylvania due to the Founder Effect and a closed population.
The Fugates • Small founding population • Mountain communities • 2 of the founders were carriers of a recessive allele • Blue skin!
Skittles bottleneck game Different coloured skittles represent different alleles. • Count the number of individuals in the population in your bottle, calculate the frequency of each colour allele (%). • Mix your sample of alleles in the bottle. • Let 5 alleles through the bottleneck after a disaster causing a crash in population numbers. • Record the colours & numbers of these 5 alleles. • When each individual in this generation dies it can leave a maximum of 3 offspring (or 3 skittles of the same colour) unless you do not have enough of this particular colour, in which case the individual has failed to reproduce. Record the new allele frequencies. • Remove the original population from the bottle and put in the survivors. • Repeat the process of letting 5 individuals through a bottleneck, recording the allele frequencies, letting them reproduce if possible and recording the allele frequencies again. Repeat instructions 3-7 for 10 disasters
Similarities and differences between the founder effect & bottlenecking