2Natural SelectionGenes 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)
3Mutations can be neutral, harmful or beneficial Harmful mutations result in dysfunctional proteins, they occur frequently but they are selected against and remain rare.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.
4Example: 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 wellSuffer from fatigue, malaise, jaundice, other minor problemsSickle shaped RBCs are more prone to clogging blood vessels which can be fatal.
5The 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 RBCsQuestion: Is being heterozygous for sickle cell anemia an advantage or disadvantage?.....Answer: it depends on where you live!
6Advantage or Disadvantage? Disadvantage in regions where malaria is uncommon (ie. North America)Advantage where malaria is common: Sub-Saharan Africaheterozygous individuals are strongly favoured, heterozygote advantage; they are more likely to survive than either homozygous group.
8Question:Would you expect the sickle cell anemia allele to be found more frequently in Sub-Saharan Africa or in North America?Why?
9Types of Natural Selection Directional selection SBI 4U: Metablic ProcessesTypes of Natural Selection Directional selection– The environment favours individuals with an extreme variation of a trait.Occurs when organism moves to a new environmentex. gill nets and salmon fishing in 50s and 60sGill nets in the 1050s were made of hemp and had larger mesh size (e.g. only catching larger fish) therefore large fish (an extreme variation of a trait) were selected against while average – smaller sized fish were able to swim through and surviveSection 1.3
10Stabilizing 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 changingex. baby weights (3kg)
11SBI 4U: Metablic Processes Disruptive selectionThe 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 mothsSection 1.3
13Sexual selection Favours the trait that influences mating success Usually based on female choice and/or male vs. male competitionMale competitionMale competes against other males for territory, or access to femalesAnything that gives him an advantage makes him more likely to pass on his genes
14SBI 4U: Metablic Processes Sexual SelectionFemale selection (or male selection)Leads to sexual dimorphism (physical differences between males and females)Male must prove he is genetically good enoughPlumage, gifts, nesting site or mating ritualsSelection makes many organisms go to extreme lengths for sex: peacocks (top left) maintain elaborate tails, elephant seals (top right) fight over territories, fruit flies perform dances, and some species deliver persuasive gifts.Section 1.3
15Reproductive 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 groupsevolve independently
16Modes of SpeciationReproductive 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.
17Allopatric speciation Evolution of populations into separate species as a result of geographic isolation. Ex. water, canyon, mountain range, human construction (dams, highways, canals)
19SBI 4U: Metablic Processes Sympatric speciationEvolution of populations within the same geographic area into separate species, by exploiting a new niche. (ex. Flies that feed on hawthorns vs apples)For example, 200 years ago, the ancestors of apple maggot flies laid their eggs only on hawthorns—but today, these flies lay eggs on hawthorns (which are native to America) and domestic apples (which were introduced to America by immigrants and bred). Females generally choose to lay their eggs on the type of fruit they grew up in, and males tend to look for mates on the type of fruit they grew up in. So hawthorn flies generally end up mating with other hawthorn flies and apple flies generally end up mating with other apple flies. This means that gene flow between parts of the population that mate on different types of fruit is reduced. This host shift from hawthorns to apples may be the first step toward sympatric speciation—in fewer than 200 years, some genetic differences between these two groups of flies have evolved.Section 1.3