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

Example Change crabs finches moth
Cause of change

• 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.
• White moths were common and black moths were rare.

• During the industrial revolution, as soot and pollution increased, light colored lichens on the bark darkened or died off.
• It was observed that the dark moths increased until they became the 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)
• Selection (on those individuals)

• 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 .

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

• There are two main types of selection:
• 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 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

• 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 resistance
–
Typical of a changing environment

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

– 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!)

• 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 change
– Genetic drift – change due to chance in small populations
• Bottleneck effect
• Founder effect
– Gene flow = immigration and emigration
– Nonrandom mating
– Mutation

• 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.

• 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.

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

– 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 :
– 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 .

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Steps of evolution by natural selection