Biology Chapters 20, 22, 23, and 26

In a population of red or white flowers in Hardy- Weinberg equilibrium, the frequency of red flowers is 91%. What is the frequency of the red allele?

70%

If you could travel back to early Earth and return with a primordial pool sample, what types of molecules would you look for to unpack the origins of early life? Construct an argument for the origins of life based on your predicted molecules.

Amino acids, including glycine and alanine, nitrogenous bases like adenine that are found in DNA and RNA, lipids, and simple carbon compounds like CH20 might be found. If the early life was based on RNA, then ribonucelotides rather than deoxyribonucleotides would be found. Early RNA sequences would have had both enzymatic and information functions. Once the machinery was in place for RNA to encode proteins, amino acid sequences would become less random and gain functions in catalyzing metabolism as well as structural roles and regulating nucleic acid replication, transcription, and translation. Lipids would form early cell membranes and metabolic pathways would continue to evolve within the confines of the cell membrane. Nucleic acid replication and cell division would evolve early also.

What experiments could you design to test other examples of natural selection, such as the evolution of pesticide resistance or background color matching?

Background color matching- camouflage. Males want to be inconspicuous to predators but attractive to potential mates. To test the effects of predation on background color matching in a species of butterfly, one might raise captive populations of butterflies with a normal variation in coloration. After a few generations, add natural predators to half of the enclosures. After several generations, one would expect the butterflies in the predatory environment to have a high degree of background color matching in order to avoid predation, while the non predatory environment would have promoted brightly colored individuals where color would correlate with mating success.

Why might a species be most phenotypically similar to a species that is not its closest evolutionary relative?

Because of convergent evolution; two distantly related species subjected to the same environmental pressures may be more phenotypically similar than two species with different environmental pressures but a more recent common ancestor. Other reasons for the possible dissimilarity between closely related species include oscillating selection and rapid adaptive radiations in which species rapidly adapt to a new available niche.

Identify which of the following statements is false and correct the statement

Brown algae gained chloroplasts by engulfing green algae (endosymbiosis) Corrected statement: Brown algae gained chloroplasts by engulfing red algae (endosymbiosis).

Which of the following events occurred first in eukaryotic evolution?

Compartmentalization and formation of the nucleus.

If you know the genotype frequencies in a population, how can you determine whether the population is in Hardy- Weinberg equilibrium?

Determine the actual allele frequency, which can be calculated based on the genotype frequencies. Use the equation p^2+2pq+q^2=1 to determine the genotype frequencies. If the actual and the expected genotype are the same, you can say the population is in the Hardy- Weinberg equilibrium.

How does directional selection differ from frequency dependent selection?

Directional selection occurs when one phenotype has an adaptive advantage over other phenotypes in a populations, regardless of its relative frequency within the population. Frequency- dependent selection, on the other hand, results when either a common (positive frequency- dependent selection) or rare (negative- frequency dependent selection) has a selective advantage simply by virtue of its commonality or rarity. If a mutation introduces a novel allele into a population, directional selection may result in evolution because the allele is advantageous, not because its rare.

You have discovered a fossil that may be an early bacterial cell. What evidence would be sufficient to convince you that this is indeed an early bacterial cell?

Electron microscopy revealing organic walls and vesicles would provide good evidence, combined with spectroscopic analysis indicating the presence of complex carbon molecules.

Could evolutionary change be punctuated in time (that is, rapid and episodic), but not linked to speciation?

Evolutionary change can be punctuated, but in times of changing environmental pressures we would expect adaptation to occur. The adaptations, however, do not necessarily have to lead to the splitting of a species- instead one species could simply adapt in accordance with the environmental changes to which it is subjected. This would be an example of nonbranching, as opposed to branching, evolution; but again, whether the end- result organism is a different species from its ancestral organism that preceded the punctuated event is subject to interpretation.

How might the initial degree of reproductive isolation affect the probability that reinforcement will occur when two populations come into sympatry?

In order for reinforcement to occur and complete the process of speciation, two populations must have some reproductive barriers in place prior to sympatric. In the absence of this initial reproductive isolation we would expect rapid exchange of genes and this homogenization resulting from gene flow. On the other hand, if two populations are already somewhat reproductively isolated (due to hybrid infertility or a prezygotic barrier such as behavioral isolation), then we would expect natural selection to continue to improving the fitness of the nonhybrid offspring, eventually resulting in speciation.

Why is cladistics more successful at inferring phylogenetic relationships in some cases than in others?

In some cases wherein characters diverge rapidly relative to the frequency of speciation, it can be difficult to construct a phylogeny using cladistics because the most parsimonious phylogeny may not be the most accurate. In most cases, however, cladistics is a very useful tool for inferring phylogenetic relationships among groups of organisms.

In contrast to the archipelago model, how might an adaptive radiation proceed in a case of sympatric speciation by disruptive selection?

In the archipelago model, adaptive radiation occurs as each individual island population adapts to its different environmental pressures. On the other hand, in sympatric speciation resulting from disruptive selection, the traits selected are not necessarily best suited for novel experiment but are best able to reduce competition with other individuals. It is in the latter scenario wherein adaptive radiation due to a key innovation is most likely to occur.

Under what circumstances might evolutionary processes operate in the same direction, and what would be the outcome?

It is difficult to discern in which direction each process is operating within a population- it is much easier to simply see the final cumulative effects of the various agents of evolutionary change. In some cases, more than one evolutionary process will operate in the same direction, with the resulting population changing or evolving more rapidly than it would under only one evolutionary mechanism. Example: mutation may introduce a beneficial allele into a population; gene flow could then spread the new allele to other populations. Natural selection will favor that allele within each population, resulting in relatively rapid evolutionary adaptation of novel phenotype.

Could HIV have arisen in humans and then have been transmitted to other species?

It is possibly, however, the viral analyses and phylogenetic analyses have provided strong evidence that HIV emergence was the other way around; it began as a simian disease and mutated to a human form, and this has occurred several times.

Analyze ways evolutionary innovations prior to the Cambrian may have contributed to the rapid radiation.

Major pre-Cambrian evolutionary innovations that set the stage for rapid radiation included the evolution of eukaryotic cells with chloroplasts and mitochondria, and meiosis that allowed for sexual reproduction. The new gene combinations created by meiosis provided a set of phenotypes for natural selection to act upon.

Why is genetic variation in a population necessary for evolution to occur?

Natural selection occurs when some individuals are better suited to their environment than others. These individuals live longer and reproduce more, leaving more offspring with the traits that enabled their parents to thrive. In essence, genetic variation within a population provides the raw material on which natural selection can act thereby leading to evolution.

How would genetic variation in a population change if heterozygotes had the lowest fitness?

Natural selection should favor both homozygous forms. This would result in disruptive selection and a bimodal distribution of traits within the population. Over enough time, it could lead to a speciation event

How do each of these processes cause populations to vary from Hardy- Weinberg equilibrium?

Natural selection- most influential in bringing about evolutionary change, it is also the one to produce an adaptive change, that is, change that results in the population being better adapted to its environment. Mutation- the only way in which new alleles can be introduced- it is the ultimate source of variation. Because it is a relatively rare event, mutation by itself is not a strong agent of allele frequency change; however in concert with other mechanisms, especially natural selection, it can drastically change the allele frequencies in a population. Gene flow- introduce new alleles into a population from another population of the same species, thus changing the allele frequency within the recipient and donor populations. Genetic drift- random chance factor of evolution- although the results of genetic drift can be negligible in a large population, small populations can undergo drastic changes in allele frequency due to this agent. Nonrandom mating- results in populations that vary from the Hardy- Weinberg equilibrium by not changing allele frequencies but by changing genotype frequencies. Reduces the proportion of heterozygotes in a population.

Explain the difference between negative frequency- dependent and oscillating selection. Over long periods of time, which is more likely to maintain variation in a population?

Negative frequency- dependent selection occurs when rare alleles have a fitness advantage over common alleles. As result, selection is maintained because once an allele becomes rare, selection operates to increase its frequency. Oscillating selection refers to the situation in which the environment changes, first favoring one allele, then another. Over the long term, negative frequency- dependent selection is more likely t preserve variation, because rare alleles are always favored. By contrast, if the environment does not change (oscillate) for an unusually long period of time, this disadvantaged allele may disappear from the population.

Is one of these factors always the most important in determining reproductive success? Explain.

None of these factors is always the most important in determining reproductive success- instead it is the cumulative effects of all three factors that determine an individual's reproductive success.

Does the possession of the same character state by all members of a clade mean that the ancestor of that clade necessarily possessed that character state?

Not necessarily; it is possible that the character changed since the common ancestor and is present in each group due to convergence. Although the most recent common ancestor possessing the character is the most parsimonious, and thus the most likely, explanation, it is possible, especially for small clades, that similar environmental pressures resulted in the emergence of the same character state repeatedly during the course of the clade's evolution.

Why are only shared derived, instead of all derived, characters useful in cladistics for reconstructing phylogenies?

Only shared derived characters indicate that two or more taxa are descended from an ancestor that was not an ancestor of taxa that do not possess that character state in question. Thus, taxa possessing the state are more closely related to each other than they are to taxa without the trait. As the trait discusses, this assumption is usually correct, but not when rates of homoplasy are high.

The Miller- Urey experiment demonstrated that

Organic molecules could have originated in the early atmosphere.

How can epistasis and pleiotropy constrain the evolutionary response to natural selection?

Pleiotropic effects occur with many genes, a single gene has multiple effects on the phenotype of the individual. Whereas natural selection might favor a particular aspect of the pleiotropic gene, it might select against another aspect of the same gene; thus pleiotropy often limits the degree to which a phenotype can be altered by natural selection. Epistasis occurs when the expression of one gene is controlled or altered by the existence or expression of another gene. Thus the outcome of natural selection will depend not just on the genotype of one gene, but the other genotype as well.

How do polyploidy and disruptive selection differ as ways in which sympatric speciation can occur?

Polyploidy occurs instantaneously; in a single generation, the offspring of two different parental species may be reproductively isolated; however, if it is capable of self- fertilization, then it is, according to the biological species concept, a new species. Disruptive selection, on the other hand, requires, many generations as reproductive barriers between the two populations must evolve and be reinforced before the two would be considered separate species.

How is the evolution of reproductive isolation in populations adapting to different environments different from the process of reinforcement?

Reproductive isolation that occurs due to different environments is a factor of natural selection; the environmental pressure favors individuals best suited for that environment. As isolated populations continue to develop, they accumulate differences due to natural selection that eventually result in two populations so different that they are reproductively isolated. Reinforcement, on the other hand, is a process that specifically relates to reproductive isolation. It occurs when natural selection favors nonhybrids because of hybrid infertility or are simply less fit than their parents. In this way, populations that may have been only partly reproductively isolated become completely reproductively isolated.

Given an opportunity to study the diversity of life in the fossil record, which eon would you choose? Support your choice.

The Phanerozoic eon includes the greatest diversity of life and would be the most informative in terms of fossil record reflecting diverse life- forms. Earlier eons have a more limited fossil record, primarily unicellular organisms.

During which times would you expect that geographic isolation would be particularly important in the evolution of life?

The beginning of the Cenozoic era.

How does the ability to exchange genes explain why sympatric species remain distinct and geographic populations of one species remain connected?

The biological species concept states that different species are capable of mating and producing viable, fertile offspring. If sympatric species are unable to do so, they will remain reproductively isolated and thus distinct species. Along the same lines, gene flow between populations of the same species allow for homogenization of the two populations such that they remain the same species.

Suppose that the relationship between birth weight and infant mortality, instead of being at a minimum at intermediate sizes, changed such that babies born at 5 or 10 pounds had the highest survival, with a valley in between such that 7.5 pound babies had low survival rate. How would you expect the distribution of birth weights to change over time?

The distribution would become bimodal, with two peaks an the mean value shifted to the right.

How does the ecological species concept explain this phenomenon?

The ecological species concept states that sympatric species are adapted to use different parts of the environment, and thus hybrids between them would not be well adapted to either habitat and thus would not survive. Even if they did survive and reproduce, genes from one species that made their way into the other species' gene pool would likely be eliminated by natural selection. The concept would explain the connection of geographic populations of a species. As a result, these populations occupy similar parts of the environment and thus experience similar selective pressures.

Which bits of evidence would convince you that life originated as early as 3.3 BYA

Using transmission and scanning electron microscopy, you find evidence of a carbon- based material in what appears to be a cell wall of a fossil isotopically dated as 3.2 BYO

Compare and contrast the events that triggered the glaciation at the end of the Proterozoic with the glaciation early in the Ordovician period.

Weathering is proposed as a key event associated with both glaciations. Weathering in the late Proterozoic was caused by warm, moist air and by shifts in plate tectonics that increased surface area for weathering. In the Ordovician period, land was being colonized for the first time. Weathering of rocks because of atmospheric changes is not sufficient to explain the CO2 shift leading to glaciation. Rather, early land plants secreted and organic acid that weathered rocks, releasing phosphorus that ran into the oceans. In the oceans, phosphorus triggered algal blooms that pulled down CO2 from the atmosphere, triggering a rapid drop in temperature.

Under the biological species concept, is it possible for a species to be polyphyletic?

Yes. An example, assume two populations of a species become geographically isolated from one another in similar environments, and each population diverges and speciation occurs, with one group retaining its ancestral traits and the other deriving new traits. The ancestral group in each population may be part of the same biological species but would be considered polyphyletic because to include their common ancestor would also necessitate including the other, more- derived species (which may have diverged enough to be reproductively isolated).

What would you conclude if you found a population not in Hardy- Weinberg equilibrium? What would be your next step?

You would conclude that one or more of the five evolutionary agent were acting to cause the lack of equilibrium. The next step would be to design studies to test hypotheses about which assumption is not being met.

Many factors can limit the ability of natural selection to cause evolutionary change, including

a conflict between reproduction and survival as seen in Trinidadian guppies. Lack of genetic variation. Pleiotropy.

Assortative mating

affects genotype frequencies expected under Hardy- Weinberg equilibrium.

A paraphyletic group includes

an ancestor and some of its descendants.

In order to determine polarity for different states of a character

an outgroup must be identified.

Leopard frogs from different geographic populations of the Rana pipens complex

are different species shown to have pre- and postzygotic isolating mechanisms.

The forelimb of a bird and the forelimb of a rhinoceros

are homologous and symplesiomorphic

The chloroplasts of brown algae

are surrounded by two membranes.

Character displacement

arises through competition and natural selection, favoring divergence in resource use.

Natural selection can lead to speciation

because the evolutionary changes that two populations acquire while adapting to different habitats may have the effect of making them reproductively isolated.

Prezygotic isolation

becomes stronger as a result of reinforcement.

Genetic drift and natural selection can both lead to rapid rates of evolution however,

both processes of evolution can be slowed by gene flow.

Hybridization between incompletely isolated populations

can serve as a mechanism for preserving gene flow between populations, thus preventing speciation.

Isotopic decay of C14

can support the claim that carbon was fixed by organisms as early as 3.8 BYA.

Overall similarity of phenotypes may not always reflect evolutionary relationships

due to convergent evolution. because variation in rates of evolutionary change of different kinds of characters. due to homoplasy.

Natural selection can

enhance the probability of speciation and reproductive isolation, and act against hybrid survival and reproduction.

Parsimony suggests that parental care in birds, crocodiles, and some dinosaurs

evolved once in an ancestor common to all three groups.

Founder effects and bottlenecks are

forms of genetic drift

If reinforcement is weak and hybrids are not completely infertile,

genetic divergence between populations many be overcome by gene flow.

The principle of parisomy

helps evolutionary biologists distinguish among competing phylogenetic hypotheses.

Sieve tubes and sieve elements are

homoplastic because their common ancestor was singled celled.

Prezygotic isolating mechanisms include all of the following except

hybrid sterility

Stabilizing selection differs from directional selection because

in the former, phenotypic variation is reduced but the average phenotype stays the same, whereas in the latter both the variation and the mean phenotype change.

Allopatric speciation

involves geographic isolation of some kind

Adaptive radiation

is the evolution of closely related species adapted to use different parts of the environment.

Although we do not know how early life arose, the following likely happened:

lipids organized to form cell membranes

Problems with the biological species concept include the fact that

many species reproduce asexually.

A global glaciation would be unlikely to occur if

millions of acres of forest were cleared.

Speciation by allopolyploidy

occurs after hybridization between two species.

A taxonomic group that contains species that have similar phenotypes due to convergent evolution is

polyphyletic

Relative fitness

refers to the survival rate of one phenotype compared to that of another.

Reproductive isolation is

required by the biological species concept.

Cladistics

required distinguishing similarity due to inheritance from a common ancestor from other reasons for similarity.

In a paraphyletic group

some species are more closely related to species outside the group than they are to some species within the group.

Species recognized by the phylogenetic species concept

sometimes also would be recognized as species by the biological species concept.

When the environment changes from year to year and different phenotypes have different fitness in different environments

the effect of natural selection may oscillate from year to year, favoring alternative phenotypes in different years.

Rapid rates of character change relative to the rate of speciation pose a problem for cladistics because

the frequency with which distatnly related species evolve the same derived character state may be high. Evolutionary reversals may occur frequently. Homoplasy will be common.

Gradualism and punctuated equilibrium are

two ends of the continuum of the rate of evolutionary change over time.

For natural selection to result in evolutionary change

variation must exist in a population, reproductive success of different phenotypes must differ, and variation must be inherited from one generation to the next.

Plate tectonics can contribute to

volcanoes and earthquakes. Formation of supercontinents. Increased weathering and C02 sequestration.