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Patterns of Natural Selection and Speciation Natural Selection Genes provide the source of variation. The environment selects for the best adapted phenotype. An allele is only common where it will provide an advantage. (Natural Selection) Mutations 1. can be neutral, harmful or beneficial 2. Harmful mutations result in dysfunctional proteins, they occur frequently but they are selected against and remain rare. 3. Beneficial mutations allow the cell to produce a new or improved protein and gives the individual a selective advantage. They are rare, but are selected for and become more common over time. Example: Sickle Cell Anemia Serious blood disorder due to single base pair mutation (point mutation) leading to a change in one amino acid that makes up the hemoglobin protein. Results in RBCs being sickle shaped, cannot hold oxygen well Suffer from fatigue, malaise, jaundice, other minor problems Sickle shaped RBCs are more prone to clogging blood vessels which can be fatal. The heterozygous advantage Heterozygous individuals are only mildly affected by the disorder since it is codominant, both normal and sickle RBCs are made. Benefit: more resistant to malaria, since the malaria causing protist cannot infect sickle RBCs and there are not enough normal RBCs Question: Is being heterozygous for sickle cell anemia an advantage or disadvantage?..... Answer: it depends on where you live! Advantage or Disadvantage? Disadvantage in regions where malaria is uncommon (ie. North America) Advantage where malaria is common: Sub-Saharan Africa heterozygous individuals are strongly favoured, heterozygote advantage; they are more likely to survive than either homozygous group. Blue = malaria Red = sickle cell anemia Purple = overlap Question: Would you expect the sickle cell anemia allele to be found more frequently in Sub-Saharan Africa or in North America? Why? Types of Natural Selection Directional selection – The environment favours individuals with an extreme variation of a trait. Occurs when organism moves to a new environment ex. gill nets and salmon fishing in 50s and 60s Stabilizing selection The extremes of a population are selected against and the average is favored. Once well adapted to environment, selection pressures tend to prevent them from changing ex. baby weights (3kg) Disruptive selection The environment selects for extremes and against the average. Example: Peppered moths in London – those living in rural areas were almost all light in colour, while moths in industrialized areas were all dark in colour – no medium coloured moths Peppered moth simulation http://www.techapps.net/interactives/pepperMoths.swf Sexual selection Favours the trait that influences mating success Usually based on female choice and/or male vs. male competition Male competition Male competes against other males for territory, or access to females Anything that gives him an advantage makes him more likely to pass on his genes Sexual Selection Female selection (or male selection) Leads to sexual dimorphism (physical differences between males and females) Male must prove he is genetically good enough Plumage, gifts, nesting site or mating rituals Reproductive isolation and Speciation Speciation: The evolutionary formation of new species. Species: members of groups or populations that interbreed or have the ability to interbreed with each other under natural conditions. reproductively isolated from other groups evolve independently Modes of Speciation Reproductive isolation may lead to speciation. The gene pool is isolated, any mutation and selection that occurs is no longer shared; any significant evolutionary changes that occur in either population (new or old) will result in the formation of separate species. Allopatric speciation Evolution of populations into separate species as a result of geographic isolation. Ex. water, canyon, mountain range, human construction (dams, highways, canals) Allopatric speciation Sympatric speciation Evolution of populations within the same geographic area into separate species, by exploiting a new niche. (ex. Flies that feed on hawthorns vs apples)
