Patterns of Natural Selection and Speciation
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)
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
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 http://youtu.be/4j7GSu99LmY
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)
Microevolution: Changes in Allele Frequencies
Small population size –> Genetic Drift Migration –> Gene Flow Mutation Natural Selection Nonrandom mating
Genetic Drift In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. It happens to ALL populations—there’s no avoiding the vagaries of chance..
The Bottleneck Effect Population bottlenecks occur when a population’s size is reduced for at least one generation. It is a type of genetic drift. Undergoing a bottleneck can reduce a population’s genetic variation by a lot, even if the bottleneck doesn’t last for very many generations.
An example of a bottleneck: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
The Founder Effect A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have: reduced genetic variation from the original population. a non-random sample of the genes in the original population. Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community, which prevents new genetic variation from entering the population.
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)
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
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 http://youtu.be/4j7GSu99LmY
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)
Microevolution: Changes in Allele Frequencies
Small population size –> Genetic Drift Migration –> Gene Flow Mutation Natural Selection Nonrandom mating
Genetic Drift In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. It happens to ALL populations—there’s no avoiding the vagaries of chance..
The Bottleneck Effect Population bottlenecks occur when a population’s size is reduced for at least one generation. It is a type of genetic drift. Undergoing a bottleneck can reduce a population’s genetic variation by a lot, even if the bottleneck doesn’t last for very many generations.
An example of a bottleneck: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
The Founder Effect A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have: reduced genetic variation from the original population. a non-random sample of the genes in the original population. Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community, which prevents new genetic variation from entering the population.
