Evolution of Populations

Evolution as Genetic Change

16-2

Natural Selection on Single Gene Traits• Natural selection never acts on genes,

always on the whole organism• But, if that organism produce a gene

change that affects survival, natural selection will act differently on that organism because of that gene change.

• Ex: Pg 397-Lizard color• Brown vs. Black• Change the survival, change the gene pool

Distribution of Phenotypes in Polygenic Traits• When many genes

affect one trait, you get many phenotypes

• The graph to the right shows the number of individuals (area under the curve) for the different phenotypes

Natural Selection on Polygenic Traits1) Directional Selection: when individuals

at one end of the curve have higher fitness than individuals in the middle or at the other end-Thus the curve will shift left or right

Ex:Beak Shape

Natural Selection on Polygenic Traits2) Stabilizing Selection:when individuals

near the center of the curve have higher fitness than the others-Thus the curve will get narrow and tall

-Survival favors the average

Ex: Birth weight in humans

Natural Selection on Polygenic Traits3) Disruptive Selection:when individuals

at opposite ends of the curve have higher fitness-Thus causes the curve to dip in the middle

-Average struggle to

survive

Ex: Bird Beaks

Genetic Drift

• In small populations, individuals that carry a particular allele may leave more descendants than others, just by chance

• Over time, this can cause a change in phenotype from the original population

• Ex: Founders Effect- small subgroup of individuals populate an area, thus the descendants have their alleles only

                                                                                                                                      

                                 

Genetic Equilibrium=No Evolution/Change• Hardy-Weinberg Principle: allele

frequencies remain the same if…

1. Random Mating Occurs

2. Large Population

3. No immigration or emigration

4. No mutations

5. No natural selection