EVOLUTION OF POPULATIONS Chapter 17

Journal  Hypothesize: What would happen to the frequency (how common or uncommon) of a helpful mutation within a population?  For example, a bug inherits a mutation that helps it to be camouflaged in its surroundings…would this trait become more or lesson common in the population and why?

Natural Selection  Organisms that are best adapted to their environment are more likely to survive and reproduce  Remember!  Adaptation = helpful mutation

Artificial Selection  Humans select and breed organisms for traits that are desirable.  Examples:  Dogs  Plants

Natural Selection & Phenotype  Review: What is a phenotype again???  An observable, physical characteristic of an organism ie. Hair color, eye color, height etc  Natural selection acts directly on the phenotype of an organism.  An organisms is best fit to survive in its environment based on its physical characteristics  Best fit organisms passes its genes down to more offspring

Gene Pools & Allele Frequency  All of the genes within a population including the different alleles for each gene make up the gene pool.  Scientists study gene pools by examining allele frequency or how common a given alleles is in comparison with the total number of alleles for that gene.

How does this relate to evolution?  Evolution involves a change in the frequency of particular alleles in a population over time.  Evolution works on populations as a whole whereas natural selection works on individuals.

How do allele frequencies change?  For single gene traits - natural selection can causes changes in allele frequency and thus, change phenotype frequencies.

How do allele frequencies change?  For polygenic traits - natural selections affects the relative fitness of phenotypes and therefore creates 3 possible selection patterns:  Directional selection  Stabilizing selection  Disruptive selection

Directional Selection  Occurs when organisms at one end of the curve have a higher fitness than those in the middle or other end  Curve shifts in the direction of the best fit animal

Stabilizing Selection  Individuals near the center of the curve are best fit to survive  Curve narrows and remains in the same place

Disruptive Selection  Least fit individuals are at the center of the curve while best fit individuals are on either end.  Curve begins to separate and could fully separate into two distinct phenotypes.