Chapter 23

What are the three main mechanisms that can change the allele frequencies of a population over time?

(1)Natural selection (2)Genetic drift (chance effects that alter allele frequencies) (3)Gene flow (the transfer of alleles between populations)

What is a population?

A population is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

What types of chromosomal alterations may lead to genetic variation in a population?

Errors in meiosis, slippage during DNA replication, the activities of transposable gene elements, and duplications of small DNA segments

Describe stabilizing selection and give an example.

Stabilizing selection acts against both extreme phenotypes and favors the intermediate phenotypes. Stabilizing selection reduces variation. Human birth weight illustrates stabilizing selection. Babies born below 6.6 lbs and above 8.8 lbs have higher mortality rates and a thus selected against.

Describe the bottleneck effect.

Sudden environmental changes such as fire, flood, volcano, etc. may drastically reduce the size of a population. In this remaining population only a fraction of the alleles of the original population are represented. Those remaining alleles remain through chance alone. The result is far reduced genetic variation.

Describe the founder effect.

The founder effect occurs when a few individuals become isolated from a larger population. This small group may establish a new population whose gene pool differs from the source population because they carry only a small portion of alleles represented in the larger, source population.

List the mechanisms that prevent natural selection from reducing genetic variation at those loci by culling all unfavorable alleles.

-Diploidy -Balancing selection -Heterozygote advantage -Frequency-dependent selection

List the four key points about the effects of genetic drift.

-Genetic drift is significant in small populations -Genetic drift can cause allele frequencies to change at random -Genetic drift can lead to a significant loss of genetic variation within populations -Genetic drift can cause harmful alleles to become fixed

What conditions are required for Hardy-Weinberg equilibrium to exist?

-No mutations -Random Mating -No natural selection -Extremely large population size -No gene flow (no immigration or emigration)

Why can't natural selection fashion perfect organisms?

-Selection can act only on existing variations -Evolution is limited by historical constraints -Adaptations are often compromises -Chance, natural selection, and the environment interact

Describe an example of the founder effect in humans.

15 British colonists established a community on the isolated island of Tristan da Cunha in 1814. The allele for a hereditary form of blindness (retinitis pigmentosa) was over represented in this population as compared to the source population. As a result this allele frequency was much higher in the 240 descendants of this isolated colony.

Suppose a population of organisms with 20000 gene loci is fixed at half of these loci and has two alleles at each of the other loci. How many different types of alleles are found in its entire gene pool? Explain.

30000. Half the loci (10000) are fixed, meaning only one type of allele exists for each locus: 10000 x 1= 10000. There are two types of alleles each for the other loci: 10000 x 2=20000. 10000 + 20000= 30000.

What is a cline?

A cline is a graded change in a character along a geographic axis.

Why is adaptive evolution a dynamic process?

Adaptive evolution is a dynamic process because the environment is constantly changing and so, phenotypes that confer an advantage in an environment today may not be advantageous when the environmental conditions change.

How is the process of evolution revealed by the imperfections of living organisms?

Although natural selection can improve the match between organisms and their environments, the evolutionary process can also lead to imperfections in organisms. A central reason for this is that evolution does not design organisms from scratch to match their environments and ways of life but works instead by a process of descent with modifications. Organisms inherit a basic form from their ancestors, and that form is modified by natural selection over time. As a result, such as a bat has wings that are not perfectly designed, but rather represent modifications of forelimbs that bat ancestors used for walking. Imperfections in organisms result from a variety of other constraints, such as lack of genetic variation for the trait in question, and the fact that adaptations often represent compromises (since organisms must do many different things, and a "perfect" design for one activity might impair the performance of another activity).

What is average heterozygosity and why is it important to evolution?

Average heterozygosity is the average percentage of heterozygous alleles in the genome of a species. It is important to evolution because it is used as a quantitative measure of gene variability in a population. Remember, variation is the stuff upon which natural selection acts to drive evolutionary change.

What is balancing selection and what is its significance with regard to evolution?

Balancing selection occurs when natural selection maintains two or more forms in a population. This maintains genetic variation.

Why would mutations in plants and fungi more often be passed to offspring than in most animals?

Because in plants and fungi many different cell lines may produce gametes whereas, in animals, one specific cell line produce gametes

What is geographic variation?

Differences in the genetic composition of separate populations (of the same species) that accumulate when these separate populations are isolated from each other via some geographic barrier such as, a mountain range. This separating geographic barrier prevents interbreeding between the two populations.

What is directional selection and what conditions favor directional selection?

Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotype. Changes in environmental conditions or members of a population immigrating to a new environment favor directional selection.

Contrast discrete and quantitative characters.

Discrete characters are "either/or" traits such as, purple or white flowers in pea plants. Quantitative characters vary along a continuum. Quantitative characters usually result from two or more genes acting on a single phenotypic trait (polygenic inheritance). Examples would be height and skin tone in humans.

What is disruptive selection? Illustrate disruptive selection by explaining the example of disruptive selection in black-bellied seedcracker finches.

Disruptive selection occurs when individuals at both extremes of a phenotypic range are favored over the intermediate phenotypes. Black-bellied seedcracker finches in Cameroon display two different beak sizes to take advantage of soft seeds and hard seeds. The finches with intermediate beak sizes are relatively inefficient at feeding on either the soft or the hard seeds and so are selected against.

Describe how gene duplication played a role in mammal's sense of smell.

Duplication of genes encoding olfactory receptors has occurred many times since our distant ancestors, which possessed only one such gene. This has resulted in humans possessing approximately 1000 such genes and mice possessing about 1300. Interestingly, approximately 60% of those genes have become deactivated in humans and 20% deactivated in mice. This indicates that not only has the sense of smell played a more important role in our ancestors but, it is a greater advantage to mice.

What is gene flow?

Gene flow is the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

What is genetic drift? What are two examples of circumstances that can result in genetic drift having a significant impact on a population (not a moose stepping on three CWCW flowers)?

Genetic drift occurs when chance events cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations. Two examples that may result in genetic drift having a significant impact on populations are the founder effect and the bottleneck effect.

Distinguish genetic drift from gene flow in terms of (a) how they occur and (b) their implications for future genetic variation in a population.

Genetic drift results from chance events that cause allele frequencies to fluctuate at random from generation to generation; within a population, this process tends to decrease genetic variation over time. Gene flow is the exchange of alleles between populations, a process that can introduce new alleles to population and hence may increase its genetic variation (albeit slightly, since rates of gene flow are often low).

What are geographically isolated populations?

Geographically isolated populations are separate populations of the same species which are separated by some geographic feature such as, separate islands, lakes, or a opposite sides of mountain range. Geographically isolated populations exchange genetic information only rarely.

Describe heterozygote advantage? Explain how sickle cell disease illustrates heterozygote advantage.

Heterozygote advantage occurs when heterozygotes have increased fitness over both homozygotes. Individuals whom are carriers for the sickle cell allele (heterozygotes) are spared the worst effects of malaria yet do not have full blown sickle cell disease. This provides an advantage over the individuals that are homozygous dominant at that locus because those individuals suffer most severely from malaria. The heterozygotes also have an advantage over the homozygous recessive individuals whom suffer, and die of, sickle cell disease.

Describe diploidy and its significance with regard to evolution.

In diploid organisms recessive alleles are hidden from the action natural selection because their phenotypes are not expressed. This preserves genetic variation. The phenotypes associated with the recessive alleles may not be an advantage under current environmental conditions but may become advantageous when the environment changes.

What is frequency-dependent selection? Illustrate frequency-dependent selection with the example of the scale eating fish of Lake Tanganyika in Africa.

In frequency-dependent selection the fitness of a phenotype depends on how common it is in the population. The scale eating fish attack their prey from either the left or the right depending on which side of their head their mouth is. The side of head on which their mouth is depends on a simple Mendelian mechanism with the allele for right-sided mouths being dominant. Prey species protect from whichever scale eating fish is most common hence selecting against that particular phenotype. As a result, the frequency of each phenotype oscillates over time. This results in each allele showing a frequency of about 50%.

Describe how the example of the mummichog fish illustrates a cline.

In sampling mummichog fish over their geographic range from Maine to Georgia it has been determined that fish living in the colder waters of Maine have a much greater frequency of the allele that codes for lactate dehydrogenase-B (an enzyme that allows the fish to be more metabolically active at colder temperatures). Mummichog fish that live in latitudes with more moderately cold water (Pennsylvania perhaps) have and intermediate frequency of the Ldh-Bb allele. And fish at the southern end of their range have near 0 frequency of the Ldh-Bb allele.

What is intersexual selection?

Intersexual selection, also known as mate choice, occurs members of one sex (usually females) are selective in choosing a mate. Females (usually) respond to morphological and behavioral cues in determining the most fit male with which to mate. One hypothesis proposes that the cues female look for indicate the best genetic fitness.

Describe intrasexual selection.

Intrasexual selection is selection within the same sex. This occurs when members of the same sex compete among themselves for mates of the opposite sex

What is microevolution?

It is evolution on the small scale. More specifically, it is a change in the allele frequencies within a population over time.

What is sexual dimorphism?

Sexual dimorphism is a difference between the two sexes in secondary sexual characteristics. These distinctions include differences in size, color, ornamentation and behavior.

Describe how mutation rates and reproductive cycle time play a role in the adaptability of organisms.

Mutation rates vary between the major groups of organisms with the rate of mutation generally being higher in plants and animals than it is in prokaryotes. This information alone would lead one to conclude that plants and animals should evolve more rapidly than prokaryotes. However, generation time must also be factored in. Because prokaryotes have vastly faster generation times than do plants and animals they can accumulate genetic variation much more rapidly than plants and animals. This accounts, largely, for prokaryotes ability to adapt very rapidly to changing environmental conditions and explains why prokaryotes have evolved to live in a far greater range of habitats than plants and animals. RNA viruses, such as HIV and influenza have both a rapid mutation (owed to their lack of genome proofreading and repair mechanisms) rate and short generation time. This explains why no single drug would be particularly effective against HIV and why new flu shots must be produced annually.

What processes create the genetic variation upon which evolution depends?

Mutation, gene duplication, and other processes such as sexual reproduction which creates new gene combinations (meiosis and fertilization are included in sexual reproduction)

In what sense is natural selection more predictable than genetic drift?

Natural selection is more predictable in that it alters allele frequencies in a nonrandom way: It tends to increase the frequency of alleles that increase the organism's reproductive success in its environment and decrease the frequency of alleles that decrease the organism's reproductive success. Alleles subject to genetic drift increase or decrease in frequency by chance alone, whether or not they are advantageous.

What is sexual selection?

Sexual selection is a form of selection in which individuals with certain inherited characteristics are more likely to obtain mates.

Which, of the three mechanisms that change allele frequencies, consistently improves the match between organisms and their environments? Why?

Natural selection, because it is not a random process

What is neutral variation?

Neutral variations are differences in DNA sequence that do not confer a selective advantage.

Is all phenotypic variation heritable? Explain. Use the example of the Nemoria arizonaria caterpillar to illustrate.

No, phenotype is often the result of an interaction between genes and the environment. For example, the Nemoria arizonaria caterpillar appears very different depending upon its diet. Those caterpillars fed oak flowers ingest chemicals, which cause them to appear like the flowers while those whom eat oak leaves ingest chemicals that cause them to appear as twigs

Would one expect to observe Hardy-Weinberg equilibrium in naturally occurring populations? Explain.

No, such limitations required for Hardy-Weinberg equilibrium are not observed in nature

Do individual organisms evolve? Explain.

No, the evolutionary impact of natural selection is only apparent in the changes of populations over time.

Describe how gene flow explains the survivability of Parus major on the island of Vlieland.

On the island of Vlieland there exists two populations (central and eastern) of the songbird, Parus major. It has been observed that the survival rates for each population differ substantially with lower survival rates in the central population. However, there is significant gene flow between the two populations. This would lead to the prediction that survival rates should become more equal between the two populations over time. So what accounts for these differential survival rates? The answer lies in the gene flow between the mainland and the populations on the island. The central population experiences more gene flow with the mainland than does the eastern population. The genes coming to the central populations bring in alleles not beneficial to survival on Vlieland hence accounting for the variable survival rates.

How might one determine if a population is evolving?

One would determine what the genetic makeup of a population would be if it were not evolving at a particular locus and then compare it to data from a real population. If there are differences between the two data sets, this suggests the real population may be evolving.

Describe the results of geographic isolation on the house mice of Madeira.

Portuguese settlers introduced house mice of Madeira to this island in the 15th century. Populations of these mice became isolated from one another by a mountain range separating them. Each population developed different patterns of chromosomal fusions as a result. However, these chromosomal fusions have not had an effect on the phenotypes of each mouse population and appear to be the result of genetic drift and not natural selection.

Describe how natural selection and, possibly mutation effected Drosophila populations between 1930 and 1960.

Prior to 1930 the incidence of DDT resistance genes in Drosophila was 0% among all specimens collected. By 1960 the incidence of the allele for DDT resistance in Drosophila rose to 37%. This rise in the allele for DDT resistance resulted from natural selection by the introduced toxin, DDT. This allele was either present in the Drosophila population prior to 1930 (and not detected) or arose through random mutation.

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. It is important to note that natural selection acts upon the phenotype directly and indirectly the genotype that controls that phenotype.

Suppose two large plant populations exchange pollen and seeds. In one population, individuals of genotype AA are most common (9000 AA, 900 Aa, 100 aa), while the opposite is true in the other population (100 AA, 900 Aa, 9000 aa). If neither allele has a selective advantage, what will happen over time to the allele and genotype frequencies of these populations?

Selection is not important at this locus; furthermore, the populations are not small, and hence the effects of genetic drift should not be pronounced. Gene flow is occurring via the movement of pollen and seeds. Thus, allele and genotype frequencies in these populations should become more similar over time, with regards to this locus, as a result of gene flow.

What is the gene pool of a population?

The gene pool of a population is all the copies of every type of allele at every locus in all members of the population.

Describe how the change in gene frequency for DDT resistance in Drosophila populations illustrates adaptive evolution.

The introduction of DDT to the environment was a major selective force placed upon populations of insects, including Drosophila. In order for populations of Drosophila to survive they needed to evolve adaptations to this new challenge. The change (increase) in the allele frequency for DDT resistance makes Drosophila a better match to its environment by increasing its fitness in said environment.

What mechanisms provide genetic variation in the process of sex and what is there result?

The three mechanisms that provide genetic variation through sex are crossing over, independent assortment of chromosomes, and fertilization. These processes rearrange existing alleles into fresh combinations in each generation. This provides the genetic variation that makes evolution possible.

Consider a population in which heterozygotes at a certain locus have an extreme phenotype that confers a selective advantage. Does such a situation represent directional, disruptive, or stabilizing selection? Explain your answer.

The three modes of natural selection are defined in terms of selective advantage of different phenotypes, not different genotypes. Thus, the type of selection represented by heterozygote advantage depends on the phenotype of the heterozygotes. In this question, because heterozygous individuals have a more extreme phenotype than either homozygote, heterozygote advantage represents directional selection.

A locus that affects susceptibility to a degenerative brain disease has two alleles, A and a. In a population, 16 people have genotype AA, 92 have genotype Aa, and 12 have genotype aa. Is this population evolving? Explain.

There are 120 individuals in the population, so there are 240 alleles. Of these, there are 124 'A' alleles- 32 from the 16 'AA' individuals and 92 from the 92 'Aa' individuals. Thus, the frequency of the 'A' allele is p=124/240=0.52; hence the frequency of the 'a' allele is q=0.48. Based on the Hardy-Weinberg equation, if the population were not involving, the frequency of genotype 'AA' should be p2 =0.52x0.52=0.27; the frequency of genotype 'aa' should be q2=0.48x0.48=0.23. In a population of 120 individuals, these expected genotype frequencies lead us to predict that there would be 32 'AA' individuals (0.27x120), 60 'Aa' individuals (0.5x120), and 28 'aa' individuals (0.23x120). The actual numbers for this population (16 'AA', 92 'Aa', and 12 'aa') deviate from these expectations (fewer homozygotes and more heterozygotes than expected). This indicates that the population is not in Hardy-Weinberg equilibrium and hence may be evolving at this locus.

What is the prerequisite for evolution by natural selection?

Variation of heritable traits

How do scientists determine gene variability?

Through analysis of protein products of genes through gel electrophoresis Additionally, DNA sequences may be compared

Describe how human actions may create severe bottlenecks for other species.

Through human action including destruction of habitat humans have caused many populations to be severely reduced. This causes a bottleneck for the effected populations. One example of such a bottleneck is illustrated in greater prairie chicken populations in Illinois. In the 19th and 20th centuries vast areas of land in Illinois were converted to farmland. This resulted in a population crash (bottleneck) and loss of genetic variation. It was observed that less than 50% of the prairie chicken eggs hatched resulting from harmful alleles being overrepresented in the remaining population and a loss of alleles.

Would individuals who are heterozygous for the sickle cell allele be selected for or against in a region free from malaria? Explain.

Under prolonged low oxygen conditions, some of the red blood cells of a heterozygote may sickle, leading to harmful effects. This does not occur in individuals with two normal hemoglobin alleles, suggesting that there may be selection against heterozygotes in malaria free regions. However, since heterozygotes are healthy under most conditions, selection against them is unlikely to be strong.

What is the relative fitness of a sterile mule? Explain.

Zero, because fitness includes reproductive contribution to the next generation and a mule cannot produce offspring.