Chapter 20 - Population Genetics

Random and nonrandom occurrences both cause evolution.
- Mutations are a random process, but natural selection, which operates on the phenotypes that arise from those mutations, is not.
- This chapter will go over some of the mechanisms that cause populations to develop, as well as some of the equations that may be used to represent population changes.
- The study of genetic diversity within populations and the factors that might induce changes in allele frequencies within a population is known as population genetics.
- Natural selection, gene flow, and genetic drift are the three fundamental factors that drive changes in allele frequencies in a population.
The transmission of alleles from one population to another is referred to as gene flow.
- Individuals migrating into a population might induce gene flow.
- If these people have different alleles than the rest of the population, the allele frequency in the rest of the population will vary.
- Gene flow in plants can occur by pollen transfer (through wind or animals) into new plant populations.
The random loss of alleles in a population is referred to as genetic drift. Smaller populations are more prone to experience genetic drift.
- Assume that an allele is present in 10% of the population.
- If there are 1,000 people in the population, 100 of them will carry that allele.
- At least one of those 100 people is likely to live and reproduce, passing on that allele to the following generation.
- Consider a population of ten people.
- Only in that small population.
The bottleneck effect is one probable source of genetic drift.
- When the size of a population is drastically decreased for one or more generations, this is referred to as a population bottleneck.
- Natural calamities such as fires, floods, and volcanic eruptions can result in population bottlenecks.
- ^^Population bottlenecks can also be caused by man-made events like as overhunting or fast habitat loss.^^
Because the population size is reduced after these bottlenecks, the surviving population is considerably less likely to carry all of the alleles that the bigger population had before to the bottleneck, and hence has less genetic diversity.
- During the 1800s, the northern elephant seal population underwent a demographic bottleneck.
- Hunting had decreased the population of northern elephant seals to less than 30 individuals by the 1890s.
- The protection of this species in the twentieth century increased their numbers, although they are not as diversified as the southern elephant seals, which were not exposed to heavy hunting in the 1800s.
The founder effect is another source of genetic drift.
- When a few individuals of a larger population establish a new population, this is known as the founder effect.
- These few people of the larger population frequently have less genetic diversity than the larger population, or they may be a nonrandom sample of the larger population.
In the original population, two distinct alleles (shown by black circles and white squares) are present at about equal rates.
- Population A was created by only individuals who possessed the allele indicated by white squares, resulting in less genetic diversity than existed in the initial population.
Similarly, the founding individuals of population C.
- exclusively held the allele represented by black circles, resulting in less genetic diversity in population C than in the original population; this is also a considerably different allele frequency than observed in population A.
- @@The Amish population in Pennsylvania is an example of the founder effect. In the mid-1700s, the first members of this community arrived from Europe.@@
- The approximately 200 founding individuals of this community have a greater frequency of the allele that causes Ellis-Van Creveld syndrome, a rare type of dwarfism.
- As a result, the frequency of this allele in this group is relatively high.
- Some populations have constant allele frequencies and do not evolve.
Populations in a stable environment may not be subjected to the selection pressures that drive evolution.
- Godfrey Hardy and Wilhelm Weinberg, both scientists, created equations that explain these stable populations.
- Five requirements must be satisfied for a population to have stable allele frequencies:

High population size—A large population size minimizes the likelihood of genetic drift. A shift in allele frequencies (induced by genetic drift) is significantly more frequent in small populations.
Random mating—Random mating reduces the likelihood of sexual selection modifying allele frequencies.
No gene flow—In order for allele frequencies to stay constant, no individuals must enter or leave the group. The introduction of additional people, or the removal of members from a population, may alter the allele frequencies.
No selection—In order to maintain allele frequencies steady, all phenotypes in the population must have equal reproductive success. If one trait offers a survival benefit, the alleles associated with that phenotype will grow more common in the population. @@Because there are 80 people in the population and each person contributes two alleles for the trait, there are a total of 80 2 = 160 alleles for the trait in this population.@@

Each homozygous dominant person gives two copies of the dominant allele to the population, resulting in a total of ten and two dominant alleles.
- Each heterozygous person contributes one copy of the dominant allele, resulting in a total of 46 1 = 46 dominant alleles from heterozygotes.
- Individuals that are homozygous recessive do not contribute any dominant alleles to the population.
- As a result, the frequency of the dominant allele is.
%%Each homozygous recessive person adds two copies of the recessive gene to the population, resulting in a total of 24 2 = 48 recessive alleles.%%
- Individuals that are heterozygous add one copy of the recessive gene to the population, for a total of 46 1 = 46 recessive alleles.
- The frequency of the recessive allele is then calculated.
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