Evolution & Speciation
Evolution Evolution is the change in living things over long periods of time. It may occur through the processes of natural selection and genetic drift.
Natural Selection Natural selection explains why organisms are adapted to their environment. This theory describes a natural process that can produce new adaptations and new species.
Natural Selection 4 conditions: Reproduction Variation in traits between individuals Heredity some of this variation must be heritable (genetic) Individuals differ in fitness (the number of offspring they produce) some individuals are more likely to reproduce than others
Natural Selection If these 4 conditions are met, then natural selection will result. Let’s look at an example…
Natural Selection A cryptic moth: These moths reproduce There is variation in their wing color pattern This color pattern variation is heritable (offspring resemble their parents) http://www.bio.umass.edu/biology/kunkel/catocala.html
Natural Selection 4. Because they use their color pattern as camouflage from bird predators, if individuals do not match their background they will get eaten only moths that look cryptic will survive and reproduce http://www.bio.umass.edu/biology/kunkel/catocala.html
Natural Selection Other examples: Pesticide resistant insects: - only those that can tolerate the pesticide will survive and reproduce - if this pesticide tolerance is heritable, then their offspring will also be tolerant to the pesticide e.g. DDT used to be a very effective pesticide and initially wiped out ~90% of mosquitoes in India. After about 10years, mosquitoes evolved resistance to DDT and now only ~20% of mosquitoes are wiped out by DDT. pesticide Surviving insects Pesticide resistant insects
Natural Selection Other examples: Antibiotic resistant bacteria: - only those that can tolerate the antibiotic will survive and reproduce - if this antibiotic tolerance is heritable, then their offspring will also be tolerant to the antibiotic - have become increasingly prevalent as the use of antibiotics has become widespread e.g. About 70% of bacteria that cause infections in hospitals are resistant to at least one of the drugs most commonly used to treat infections. Add antibiotic Surviving bacteria Antibiotic resistant strains
Natural Selection Types of selection: Directional selection Favors one extreme and shifts the population mean Stabilizing selection Favors the mean and decreases variation around the mean Disruptive selection Favors the extremes and selects against the mean
Responses to Selection # of individuals Coloration light dark Directional selection # of individuals Coloration light dark
Responses to Selection # of individuals Directional selection Stabilizing selection # of individuals Coloration light dark
Responses to Selection # of individuals Directional selection Stabilizing selection Disruptive selection # of individuals Coloration light dark
Genetic Drift Genetic drift is the fluctuation of genotypes in the population due to chance. In small populations, genetic drift can result in non-adaptive evolution.
Genetic Drift Genotype A Genotype B Population 2 6:1 Population 4 1:6 Population 1 1:1 For the population as a whole, the frequency of the 2 genotypes is 1:1 Population 3 1:1
Genetic Drift Genotype A Genotype B Population 2 6:1 Population 4 1:6 Population 1 1:1 For the population as a whole, the frequency of the 2 genotypes is 6:1 the frequency of genotype A has increased due to chance Population 3 1:1
Genetic Drift Genotype A Genotype B Population 2 6:1 Population 4 1:6 Population 1 1:1 For the population as a whole, the frequency of the 2 genotypes is 1:1 the frequency of the 2 genotypes has remained the same due to chance Population 3 1:1
Genetic Drift Endangered species are often reduced to dangerously small numbers and restricted to small, isolated habitats. In small populations, genetic drift erodes genetic variation which limits a species ability to adapt to environmental change (e.g. disease, global warming, etc) and increases its probability of going extinct due to chance events.
Speciation What is a species? A group of interbreeding individuals of common ancestry that are reproductively isolated from other groups
Speciation How might speciation (formation of new species) occur? Allopatric speciation A barrier divides a continuous population into 2 parts Parapatric speciation A hybrid zone divides a continuous population into 2 connected parts Sympatric speciation A population splits in 2 without any geographic separation
Speciation Allopatric speciation Ancestral population A physical barrier divides the ancestral population. Over time, each isolated population diverges due to natural selection or drift Allopatric speciation
Speciation Parapatric Allopatric speciation speciation Ancestral population A physical barrier divides the ancestral population. Over time, each isolated population diverges due to natural selection or drift There is some kind of habitat gradient that results in locally adapted individuals. hybrid zone Parapatric speciation Allopatric speciation
Speciation Parapatric speciation Sympatric speciation Allopatric Ancestral population The extremes in the population are selected for and there is assortative mating. A physical barrier divides the ancestral population. Over time, each isolated population diverges due to natural selection or drift There is some kind of habitat gradient that results in locally adapted individuals. hybrid zone Parapatric speciation Sympatric speciation Allopatric speciation
Allopatric Speciation Blue-headed wrasse Isthmus of Panama Cortez rainbow wrasse http://cas.bellarmine.edu/tietjen/images/speciation.htm
Parapatric Speciation http://cas.bellarmine.edu/tietjen/images/speciation.htm
Sympatric Speciation Is thought to have occurred in Cichlids in a lake in Central America (also in Africa). http://www.cichlidnewsmagazine.com/issues/2001apr/nicaragua.html
The diversity of the animals we see today is the result of processes such as natural selection and genetic drift that lead to speciation over time. Hawaiian honey-creepers