Ch. 15 Notes

“Changing Allele Frequencies”

• Changing allele frequencies permit both LONG AND SHORT-TERM CHANGES to occur in a species/population.

• Gene frequencies are affected by both NATURAL and UNNATURAL events.• Natural events such as severe storms, EARTHQUAKES and

DISEASES can shrink the gene pool.• Unnatural events (caused by MAN) can also have

immediate and lasting affects on the available genes in a population. (wars, DEFORESTATION, nuclear testing, OVER-FISHING)

Conditions That Change Allele Frequencies

•Nonrandom mating•Migration•Genetic drift•Mutation•Selection

Nonrandom mating• Nonrandom (SELECTIVE) Mating

• Nonrandom mating indicated individuals of one genotype reproduce more often with each other.

• May be due to• Ethnic or religious preferences• Isolated communities• Cultures in which consanguinity (marriages between

relatives – cousins) is more prominent

• EX: The males of some bird species must make ELABORATE NESTS to lure in potential female partners.

• Sexual selection selects for traits that improve mating success.

• Humans choose their mates based on physical appearance, ethnic background, intelligence and shared interests.

• Some traits ARE RANDOMLY MIXED- MOST BLOOD TYPES are not affected by preferential mating. (Mr. Smith didn't meet Mrs. Smith by asking if they had compatible blood types.) These randomly mixed traits are often considered UNIMPORTANT or are NOT OBSERVED at all.

Examples

• Arnold• Genghis Khan• Albanism in Hopi Indians• Tay Sachs among Ashkenazim Jews• Endogamy or marriage within the

community

Migration• Individuals migrate and move genes from one area to

another.• Genetic effects of migration are reflected in current

populations.• Changes in allele frequency can be mapped across

geographic or linguistic regions • Allele frequency differences between current

populations may be correlated to historical events or cultural differences. • Native American alleles started to become MIXED with

European and African alleles after the arrival of Columbus in 1492.

• Migration may also reduce the VARIETY or frequency of alleles in the "HOME" population.

• Gene flow moves alleles from one population to another.

• A CLINE is a change in allele frequency from one population to a neighboring one. There are linguistic & geographic clues to the gene frequencies.

• Examples: • Along the Nile : genetic variations due

to language & cultural barriers in ancient neighboring kingdoms.

• A mountain range may create a cline.

Genetic Drift

• The "change in allele frequency that occurs when a SMALL GROUP separates from the LARGER is termed genetic drift."

• Genetic Drift occurs when a population becomes SUFFICIENTLY SMALL TO CHANGE THE NORMAL allele frequencies. The change in population can be the result of behavior (the AMISH) or natural disasters (the LEMURS in the movie "Dinosaurs").

• Genetic drift changes allele frequencies due to chance alone.

FOUNDER EFFECT• The founder effect is the result of small

groups leaving a population and creating new populations (if those poor lemurs in "Dinosaurs" would have started a new population, it would have suffered from the founder effect).

• They maintain their original small gene pool.

• Increases incidence of otherwise rare traits.

Founder Effect

• Examples:• French Canadians lack many breast

cancer genes • DUNKERS = physical features (attached

earlobes)• Amish & Mennonites (many recessive

disorders)

POPULATION BOTTLENECKS

• Extreme cases of founder effect are called POPULATION BOTTLENECKS.

• Large population size is drastically reduced in size.

• Rebounds in population size occur with descendants of a limited number of survivors.

• After a population bottleneck occurs there are a very LIMITED NUMBER OF SOME ALLELES and a high frequency of others to pass to the next generation. This can have severe a long-lasting effects. • EX: CHEETAHS (can graft skin from one to another

without rejection), & EUROPEAN JEWS (Tay Sachs)

Mutation• Allele frequencies change in response to

mutation.• Can introduce new alleles.• Can convert one allele to another• Mutation has a minor impact unless

couple with another effects such as small population size

• Mutations produce the genetic variation needed for evolution.

Natural Selection

• Is the differential survival and reproduction of individuals with a particular phenotype.

• Natural selection may result in • An increase (positive selection) or • decrease (negative selection) in the

frequency of an allele.

• Natural selection selects for traits advantageous for survival.

• In nature, populations evolve.

– expected in all populations most of the time

– respond to changing environments

SPECIATIONSPECIATIONKEY CONCEPTNew species can arise when populations are isolated.