Evolution EVOLUTION OF POPULATIONS

Three Cases of Variation What specifically causes the change in each of these populations of organisms:What specifically causes the change in each of these populations of organisms: –Samurai crabs –Galapagos finches –Peppered moth

ExampleChange Cause of change crabs finches moth

The Peppered Moth The Peppered Moth from England exists in two forms – black and mottled gray.The Peppered Moth from England exists in two forms – black and mottled gray. Before the industrial revolution (in 1850) most trees were light colored because of a lichen that grew on their bark.Before the industrial revolution (in 1850) most trees were light colored because of a lichen that grew on their bark. White moths were common and black moths were rare.White moths were common and black moths were rare.

The Peppered Moth… During the industrial revolution, as soot and pollution increased, light colored lichens on the bark darkened or died off.During the industrial revolution, as soot and pollution increased, light colored lichens on the bark darkened or died off. It was observed that theIt was observed that the dark moths increased dark moths increased until they becamethe until they becamethe more common form. more common form.

In order for changes such as these to occur in a population, what two things need to be present in the population?

Variation (between individuals)Variation (between individuals) Selection (on those individuals)Selection (on those individuals)

Variation of Traits in a Population Within a population, individuals vary in observable traits.Within a population, individuals vary in observable traits. Often these traits are cover a range of appearances (ex. height) that can be represented by a bell curve.Often these traits are cover a range of appearances (ex. height) that can be represented by a bell curve.

Sources of Variation Variations arise in two main ways:Variations arise in two main ways: –Mutation – the only way to get a new trait –Genetic Recombination Why no two siblings (except twins) are identicalWhy no two siblings (except twins) are identical

Selection There are two main types of selection:There are two main types of selection: Artificial selection is used by humans for breeding useful traits into a population. Artificial selection is used by humans for breeding useful traits into a population. –Example – seedless fruit; turkeys with more white meat Breeds of dogs

Natural Selection Natural selection is the process by which individuals that are better suited to their environment survive and reproduce most successfully. Natural selection is the process by which individuals that are better suited to their environment survive and reproduce most successfully. –Acts only on populations not individuals –Acts only on appearances (phenotype) not genes (directly) Examples: pesticide resistance; antibiotic resistance in bacteria

Natural Selection There are three types of selection on multiple gene traits:There are three types of selection on multiple gene traits: – Directional selection - the range of shifts toward one extreme. examples include the peppered moth; Galapagos finch, antibiotic/pesticide resistanceexamples include the peppered moth; Galapagos finch, antibiotic/pesticide resistance – Typical of a changing environment

Natural Selection – Stabilizing selection - the range of phenotypes narrows; extremes are eliminated. Examples include birth weight in humans, clutch size in birds, hatching weight in birdsExamples include birth weight in humans, clutch size in birds, hatching weight in birds – –Typical in a stable environment

Natural Selection – Disruptive selection - the range of phenotypes “splits” and moves toward extremes; intermediate form is eliminated. examples include shell color in limpets, Galapagos finches with large or small beaks (none with medium size!)examples include shell color in limpets, Galapagos finches with large or small beaks (none with medium size!)

Other Causes of Change Summary : any type of selection will cause population to change over time. Summary : any type of selection will cause population to change over time. In addition to selection, there are other factors that will cause a population to changeIn addition to selection, there are other factors that will cause a population to change –Genetic drift – change due to chance in small populations Bottleneck effectBottleneck effect Founder effectFounder effect –Gene flow = immigration and emigration –Nonrandom mating –Mutation

Bottleneck Effect Disasters such as fires may drastically reduce the size of a population reducing the size of the gene pool.Disasters such as fires may drastically reduce the size of a population reducing the size of the gene pool. –By chance, certain alleles may be more or less prevalent in the survivors. –Some alleles may be eliminated altogether. This decreases genetic variation in a population.This decreases genetic variation in a population.

Founder Effect Founder effect –This occurs when a small subunit of a population begins a fresh population (due to migration, or a large die-off). The “founders” don’t have the same allele frequency as the original population. Founder effect –This occurs when a small subunit of a population begins a fresh population (due to migration, or a large die-off). The “founders” don’t have the same allele frequency as the original population.

Immigration & Emigration Migration - movement of individuals (and their genes) from one population to another = gene flow Migration - movement of individuals (and their genes) from one population to another = gene flow

Nonrandom Mating – Nonrandom mating - any type of mate selection –Example: sexual selection Most species have distinctly male and female phenotypes Differences include fur, plumage, antlers Organisms select mates that provide resources or “good genes” for the chooser It is usually the female that is the “agent of selection” causing males to develop elaborate decorations

Population Stability To review: the conditions that WILL cause change are : To review: the conditions that WILL cause change are : –natural selection –mutation –immigration or emigration –nonrandom mating If none of the previous conditions exist in a large population, the population’s genetic make up should stay the same.If none of the previous conditions exist in a large population, the population’s genetic make up should stay the same.

Steps of evolution by natural selection