A. Summaries

General Summary:

     Migration of individuals into or out of a population introduces, or extracts, alleles.  This disturbs the Hardy-Weignburg equilibrium theory because the alleles are not constant if they are moving in and out of a population, assuming the population that has migrated to the area mates with the population that was originally living there.  Eventually, this can spread around or reduce the frequencies of different alleles and lead to the evolutionary change in a population.

     Nonrandom mating is when there is a preference for one's mate whether it be for their looks, personality, or whatever.  Either way, this is not a random selection process and this means that not every individual will have an equal opportunity to mate.  If individuals prefer other individuals that are similar to themselves, e.g. tall people like tall people,  then homozygosity of alleles is favored. On the other hand, if individuals prefer individuals that are not like themselves, e.g. tall people like short people, then heterozygosity of alleles is favored.  This selection process can be positive if helpful alleles, like being more fit, are favored.  However, this process may also favor unhelpful alleles, like being unfit, which would be detrimental to a population. Also keep in mind that although this process may eventually lead to the change in allele frequencies in a population, nonrandom mating does not directly affect allele frequencies, it affects genotype frequencies.

More Detailed Summary: (for those who like to read)

     Imagine two different groups of rabbits, one group has blue eyes where as the other one has brown eyes.  These two groups of rabbits could share these alleles in a few ways, but the two ways I will focus on is migration between populations, and nonrandom mating.  

     Say that the blue-eyed rabbits lived on the east side of the river and the brown-eyed rabbits lived on the west side of the river (yes, this is my version of the example given in our textbook with deer).  The blue-eyed rabbits could cross over a bridge leading to the west side where the brown-eyed rabbits are, and vice versa. This migration between the different populations of rabbits can alter the frequencies of the alleles, in this case the allele for eye color, in each population because new, or more, alleles are being introduced to a population, and those same alleles are leaving the other population.  This change of frequency of alleles that occurs when individuals move in and out of a population is known as gene flow.  Gene flow occurs when individuals migrate between populations that have different allele frequencies. This bidirectional (movement of alleles to and from both populations) migration tends to reduce the differences in allele frequencies between populations, usually neighboring populations. Population geneticists actually use this to their advantage.  Since populations that mix more frequently would have more similar allele frequencies than populations that are isolated. Migration also leads to an increase of genetic diversity within populations by introducing new mutations or different alleles.  

     Nonrandom mating is another way that genetic variation is affected.  There are many different forms of nonrandom mating, but basically nonrandom mating is when individuals choose their mates based on genotypes and phenotypes opposed to just "randomly mating". Assortative mating is when individuals with similar phenotypes are more likely to mate.  So, back to the rabbit example, this would be like if blue-eyed rabbits were more likely to mate with other blue-eyed rabbits in a mixed populations of rabbits. Or, another example is say short girls prefer short guys.  If these girls tended to mate with short guys more, then there would eventually be a higher frequency of short-alleles.  If this assortative mating occurs with individuals of similar genotypes, this tends to increase the proportion of homozygous individuals and decrease the amount of heterozygous individuals (the opposite situations can occur too where individuals with dissimilar phenotypes mate).  

     Another form of assortative nonrandom mating is when individuals choose their mates based on their genetic history.  This is known as inbreeding; when individuals that are genetically related mate.  This occurs sometimes with humans, like how people of the Royal family used to only mate with other royal members to keep their blood "pure".  Inbreeding is also more likely to occur in nature when population size is small because there is a greater chance that the other individuals in your population will be related to you than it would be if the population size was larger. Inbreeding itself does not affect allele frequencies (the individuals do not choose the allele, like individuals might choose certain phenotypes), but it could eventually affect the allele frequencies because inbreeding usually favors homozygotes since there is a greater chance that the same genes will be passed down from generation to generation.  So when natural selection takes place, inbreeding would accelerate the prevalence of a certain allele in a population.  This increased rate of a prevalence of a gene can either be good, or bad.  If inbreeding resultes in homozygotes that are less fit or are less adapted to the environment such that there is a decrease of reproductive success in a population, this is known as inbreeding depression. 

B. Vocabulary 

gene flow: Occurs when individuals migrate between different populations and results in changes in the genetic composition of the resulting populations.

nonrandom mating: The phenomenon that individuals choose their mates based on their genotypes or phenotypes.

inbreeding: Mating among genetically related relatives.

inbreeding depression: The phenomenon whereby inbreeding produces homozygotes that are less fit, thereby decreasing the reproductive success of a population.

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