Notes 

Population Genetics Chapter 24

Population Genetics-The study of genes and genotypes in a population.

Population-A group of individuals of the same species that occupy the same environment.

Gene pool = all the alleles for every gene in a given population.

Polymorphism = the phenomenon that a trait displays variation within a population.

Polymorphic gene=  a gene that commonly exists as two or more alleles in a population.

Monomorphic gene= a single allele in a population. When 99% or more of the alleles of a given gene are identical in a population.

Single-nucleotide polymorphism= SNP = smallest type of genetic change that can occur within a given gene, and also the most common.

Personalized medicine = a medical practice in which information about a patients genotype is used to tailor their medical care.

Allele frequency=   Number of copies of a specific allele in a population

                               Total number of all alleles for that gene in a population

Genotype frequency=   Number of individuals with a particular genotype  in a population

                                     Total number of all individuals in a population

See pages 492-493

*Allele and genotype frequencies always sum to less than or equal to one.

*Monomorphic genes have allele frequencies closer to one, but polymorphic genes have alleles closer to zero.

Hardy-Weinberg equation= relates allele and genotype frequencies

                             P2 + 2pq +q2= 1

                             P2= the genotype frequency of CRCR homozygotes

                             2pq= genotype frequency of CRCW heterozygotes

                             q2= genotype frequency of CWCW homozygotes

Hardy-Weinberg equation is based on the following assumptions:

A)No new mutations

B)No natural selection occurs; that is, no survival or reproductive advantage exists for any of the genotypes.

C)The population is so large that allele frequencies do not change due to random chance.

D)No migration occurs between different populations.

E)Random mating occurs-Members of the population mate with each other without regard to their phenotypes and genotypes.

*In reality, no population will satisfy these criteria completely.

Disequilibrium-Indicates that evolutionary mechanisms are affecting populations-evolution is occurring.

*These genes are not in Hardy-Weinberg equilibrium.

Microevolution= describe changes in a populations gene pool. See table 24.1 p. 494

24.2 Natural Selection

Natural selection= the process on which beneficial traits that are heritable become more common in successive generations.

*In 1926 Russian geneticist Sergei Chetverikov was the first to suggest that random mutations are the raw material for evolution.

Adaptations= changes in populations of living organisms that promote their survival and reproduction in a particular environment.

Reproductive success= likelihood of an individual contributing fertile offspring to the next generation.

*Generally based on two factors:

1.Characteristics make organisms better adapted to their environment and more likely to survive.

2.Traits that increase the ability to find a mate produce viable gamates and offspring.

Fitness= the relative likelihood that a genotype will contribute to the gene pool of the next generation as compared with the other genotypes.(Fertility Rate)

*p.495 List principles of natural selection 1-4.*

Mean fitness of the population= average reproductive success of members of a population.

*Over time the population should become more adapted to the native environment and more likely to reproduce.

How would global weather patterns change this?

Directional selection= a pattern of natural selection that favors individuals at one extreme of a phenotypic distribution.

*Figure 24.3 on page 497.

*What may cause directional selection:

1.A new allele from a mutation may result in higher fitness.

2.Prolonged change in environmental conditions. Ex. Peppered moth.

Stabilizing selection= the survival of individuals with intermediate phenotypes and selects against those with extreme phenotypes

*Clutch size(# of eggs laid) in birds-Studied by David Lack in 1947.

Diversifying selection= disruptive selection= the survival of two or more different genotypes that produce different phenotypes.

*Occurs in areas with heterogenous environments.  Ex. Bentgrass in South Wales-Book p.497

Balancing selection= a type of natural selection that maintains genetic diversity in a population.

Balanced polymorphism= the phenomenon in which two or more alleles are kept in balance and maintained in a population over the course of many generations.

*Sickle cell disease. Figure 24.6 p.498

 Heterozygote advantage= when balancing selection can favor the heterozygote over either corresponding homozygote.

Negative frequency-dependent selection= a pattern of natural selection in which the fitness of a genotype decreases when its frequency becomes higher; the result is a balanced polymorphism.

*Usually found among animals that are prey.

*Larger lizards compete to get more food, but larger lizards are more likely eaten.

24.3 Sexual Selection

Sexual selection= a type of natural selection that is directed at certain traits of sexually reproducing species that make it more likely for individuals to find or choose a mate and/or engage in successful mating.

Secondary sex characteristics-Evolution of traits which increase the probability of mating.(Usually Males.)

Sexual dimorphisms-A process that results in differences in the appearances of the two sexes of the same species.

Intrasexual selection= members of one sex directly compete with each other for the opportunity to mate with individuals of the opposite sex.

*The champion male mates.

*Traits that may result are big horns on male sheep, larger antlers on male moose, and enlarged claws of the fiddler crab.

Also, talk about apes and gorillas.

Intersexual selection= a type of sexual selection that often results in showy characteristics in males.

*Females choose mate based on some characteristic.  (Colorful feathers of the male.)

Elk exhibit both Intrasexual and intersexual slection.

*Males become aggressive, bugle, and spar with other males.

*Females usually choose the strongest male.

Cryptic female choice-The female genital tract selects sperm that is genetically different.

*This increases genetic diversity and decreases the chance of inbreeding and have more genetic diseases.

**Common among ducks and reptiles that females mate with multiple males.

***Many plants prevent pollen from the same plant to fertilize the flower.(Must cross pollinate.)

Female guppies prefer brightly colored males, but brightly colored fish are seen and eaten.

*Few predators result in brightly colored males.

*Many predators result in drab males.

24.4 Genetic drift.

Genetic drift= random genetic drift= changes in allele frequencies due to random chance.(Results when allele frequency hits 0% or 100 %)

*Developed by Sewall Wright in the 1930’s

*Genetic drift has a greater impact in small populations.

Random chance=Also know as random sampling error.

Bottleneck effect= allele frequencies of the resulting population change due to genetic drift.

*This may result from earthquakes, floods, fires, drought and human destruction of habitat.

Founder effect= when a small group of individuals separates from a larger population and establishes a colony in a new location.

Galopogoes Finches are examples. (p.474)

Amish in Lancaster County, Pennsylvania.(p.503)

Samoan Island People.

Neutral variation= does not affect reproductive success.

Neutral theory of evolution= most genetic variation is due to the accumulation of neutral mutations that have attained high frequencies in a population via genetic drift.

*Concept developed by Motoo Kimura in 1968.

Non-Darwinian evolution=(Survival of the luckiest)- states that both genotypes are equal in fitness; however, such new mutations can spread throughout a population due to genetic drift.

24.5 Migration and Nonrandom Mating

Gene flow= the movement of alleles into or out of a population.

Nonrandom mating= the phenomenon that individuals choose their mates based on their genotypes or phenotypes.

Inbreeding= the mating of two genetically related individuals.

Inbreeding depression= produced homozygotes that are less fit, thereby decreasing the reproductive success of the population.