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

Selection

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.

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

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?

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

Examples All derived from a wild mustard through artificial selection

Maintenance of variation

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

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)

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

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)

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