1. Selection + Maintenance of variation
Section 27.3 Cont’d – Nov 24, 2014

2. Selection

3. Selection
Both natural selection and artificial selection are mechanisms of change in the gene pool of a population
Natural selection – those individuals who are best suited to their environment survive and reproduce
Artificial selection – humans ensure that individuals with the more desirable traits are allows to reproduce.

4. Artificial selection
Artificial selection is a form of non-random mating
One of the causes of change to a gene pool
For centuries, breeders – select plants or animals that best represent the properties they wish for in future generations
Examples?
More productive milk cows
Earlier ripening fruits
Greater grain yields
Faster racehorses
Charles Darwin was able to use artificial selection as a model for change in the natural world

5. Examples What causes a squash, a squash?
Acorn, butternut, hubbard, pumpkin, spaghetti
All members of same species, Curcurbita maxima
How do you think humans were able to produce such a variety of squash?

6. Examples Brassica oleracea
Broccoli, broccoflower, cauliflower, cabbage, brussel sprouts, kale, kohlrabi, collard greens, Chinese broccoli, broccoli, rapini

7. Examples
All derived from a wild mustard through artificial selection

8. Maintenance of variation

9. Genotypic variation
“A population nearly always shows some genotypic variation”
Beneficial for adaptation to new conditions and/or a changing environment
What if they CANNOT adapt?
Limited variation with change conditions – may become extinct

10. Genotypic variation
Ways genetic variation is maintained, despite selection working to reduce it:
Mutation generates new alleles
Recombination and independent assortment shuffle alleles during gamete formation
Fertilization creates new combinations of alleles in a gene pool
Gene flow may occur
Natural selection favors certain phenotypes, but other types may still remain in reduced frequency (polymorphism even results from disruptive selection)

11. Diploidy and heterozygote
Only expressed alleles are subject to natural selection
In diploid organisms, recessive alleles can be protected from being eliminated in heterozygotes
Sickle-cell disease is an example of balanced polymorphism

12. example Sickle-cell disease is an example of balanced polymorphism
Heterozygotes can have an advantage over both homozygous conditions (normal or sickle cell disease) due to its protection against malaria
Recessive allele has a high frequency in areas where malaria is prevalent
Malaria parasite unable to live in red blood cells of heterozygote (sickle shape = red blood cells lose potassium and parasite dies)

13. Complete “Check Your Progress”
pg. 558 #1-2
Handout
“Where Did All the Four-leaf Clovers Go?”
Quiz Review