Download Biology 123 SI- Dr. Raut`s Class Session 21

Biology 123 SI- Dr. Raut’s Class
Session 21- 04/15/2015
1. What are the four main sources of genetic variation?
1. The formation of new alleles. New alleles can arise through mutations in the gene. These
mutations must occur in the gametes. Any mutations in the somatic cells will not be passed
down.
2. Altering gene number or position. This includes the deletion of a chromosomal segment,
disruption or rearrangement of loci. This includes much larger sections of chromosomes (entire
genes) moving around compared to number one which likely only includes one nucleotide.
3. Rapid reproduction. The more often new organisms are produced the more often mutations
will occur. Example: if a mutation occurs in every 100,000 genes per generation. Animals and
plants have longer generations, but prokaryotes and viruses have short generations meaning
mutations can add up quickly.
4. Sexual reproduction. The combination of two different sets of chromosomes from two
completely different parents greatly increases variation, as does the process of meiosis.
2. What are the conditions for Hardy-Weinburg Equilibrium?
1. No mutations are occurring- mutations modify the gene pool.
2. Mating is entirely random-if there is preferential mating then genotype frequencies will be
changed.
3. No natural selection- differences in the success and reproduction of animals with certain
alleles will affect the allele frequencies
4. A very large population size- allelic frequencies are more exaggerated in smaller populations.
5. There is no gene flow- moving alleles in and out of a population can alter allele frequencies
3. What are the two Hardy-Weinburg equations? What do they mean?
Allele frequencies: p + q=1.
Genotype: p2 + 2pq + q2 = 1
p2 and q2 correspond to homozygous genotypes. 2pq corresponds to the heterozygous genotype.
Solving for p will tell you the allele frequency of that particular allele, as will solving for q.
4. You have sampled a population with a percentage of the homozygous recessive genotype (aa)
is 36%. Calculate the following:
A. The frequency of “aa” genotype.
If 36% of the population are homozygous recessive then the frequency of “aa” is .36.
B. The frequency of the “a” allele.
q2=.36
√. 36=.6
C. The frequency of the “A” allele.
p + q=1
p + .6=1
1 - .6 = .4
D. The frequency of the “AA” and “Aa” genotypes.
p2 + 2pq + q2 = 1
.42 + 2(.4)(.6) + .62 = 1
.16 + .48 + .36 = 1
“AA”= .16 or 16% of the population
“Aa”= .48 or 48% of the population
E. The frequency of the two possible phenotypes if A has complete dominance.
You could simply add .48 + .16 to get .64.
5. What is natural selection?
Natural selection is the differential success of members of a species due to their ability to survive
and reproduce in their particular environment.
6. Explain genetic drift.
Genetic drift is the process of allelic frequencies fluctuating unpredictably due to chance.
*Note: In natural selection, there is a reason why some characteristics fare better than others, but
in genetic drift, the fluctuations in allelic frequencies are random.
7. Explain both the bottleneck effect and the founder effect.
In the bottleneck effect, a sudden change in environment causes much of the population to die
off. The population has greatly reduced in size and now has a more limited genetic variation. By
chance, some alleles have survived the disaster in higher numbers than others. Also, genetic drift
will have a much higher effect on the population due to its small size. Even after the population
numbers recovered, genetic variation stays low for quite some time.
In the founder effect, a few individuals will become isolated from the rest of the population.
They create their own small population, which will have less genetic variation than the large
population. Genetic drift will be exaggerated due to the small population size. In humans, this
has often resulted in higher instances in diseases in certain parts of the world.
The two effects are fairly similar. They mainly differ in how the small population was formed.
8. What are the four main points to remember regarding genetic drift?
1. Genetic drift is most significant in small populations.
2. Genetic drift can cause the allele frequencies to change at random.
3. Genetic drift can lead to loss of genetic variation within a population.
4. Genetic drift can cause harmful alleles to become fixed.
9. What is relative fitness?
Relative fitness is the contribution an individual makes to the gene pool of the next generation
relative to the contributions of other individuals. A human who has ten children has made quite a
substantial contribution to the population; however, a cat could possibly have ten kittens in just
one pregnancy. If an organism is able to live for a very long time and have many offspring and
has a high relative fitness, then obviously that organism’s genes are favored by natural selection.
10. What are the three different ways that natural selection can alter the frequency distribution of
heritable traits?
Directional selection occurs when the environment favors members of the species that express
one extreme of the phenotypic range.
Disruptive selection occurs when conditions favor both extremes of phenotypic variation, but not
the middle of the range.
Stabilizing selection occurs when conditions really favor the middle of the phenotypic range and
not the extremes.
https://www.youtube.com/watch?v=vCHdT9MWIaA
11. How does sexual selection work?
Sexual selection is a form of natural selection. It often results in sexual dimorphism, which is a
distinct difference between the sexes. In sexual selection, mate choice is a key factor. Some traits
are more desirable than others and will be passed down more often.
12. True or false: Populations keep neutral variation. Why or why not.
True! Populations never get rid of variation unless it is something that reduces relative fitness!
(even then many recessive disorders are kept in the population) While neutral variations may
serve no purpose now, they may help the species survive if the environment changes.
13. Why is natural selection incapable of creating a perfect organism?
1. Selection can only act on existing variations. Characteristics that are not already present in the
population cannot be selected for.
2. Evolution is limited by historical constraints. Each new organism is decent with modification,
not an entirely new organism from scratch.
3. Adaptations are often compromises. Animals do many different things, and therefore,
adaptations help the animal fit its many roles. This means that instead of adaptations making an
animal amazing at one thing, it often makes the animal decent in all of its roles.
4. Chance, natural selection, and the environment interact. Chance also plays a role in what traits
survive and what traits do not.
14. What does the biological species concept state?
A species is a group of populations whose members have the potential to interbreed in nature and
produce viable, fertile offspring, but do not produce viable, fertile offspring with members of
other such groups.
15. What are the two types of reproductive isolation? What are all of the subtypes of these types?
List examples in your answer.
The two main types of reproductive isolation are prezygotic barriers and postzygotic barriers. In
prezygotic barriers, fertilization never occurs (a zygote is not formed.) In postzygotic barriers,
fertilization does occur (a zygote is formed).
Prezygotic barriers:
Habitat isolation: Two species occupy two completely different habitats, and therefore, they
rarely ever encounter each other, if at all. If they are never at the same place at the same time,
they cannot possibly mate. Ex. The two garden snakes: one lives in water and the other is
terrestrial.
Temporal isolation: Two species mate at different times of the year. Ex. The two skunks; one
mates in the late summer, and the other mates in the late winter.
Behavioral isolation: Two species have different mating rituals. Ex. The blue footed boobies
male raises his feet in a dance for the female.
Mechanical isolation: Morphological differences make mating impossible. Ex. The snails shells
twist in opposite directions.
Gametic isolation: Sperm from one species cannot fertilize the eggs of another. Ex. Typically in
aquatic species, the gametes have specific identifying proteins that make which species the
gamete belongs to.
Postzygotic barriers:
Reduced hybrid viability: The genes of the two different parent species interact in ways that
impair the hybrid’s development or survival in its environment. Ex. Salamanders.
Reduced hybrid fertility: Hybrids may live, but they will be sterile. Ex. Mules
Hybrid breakdown: The first generation will be fine and fertile, but the second generation either
dies or is sterile. Ex. Rice.