Chapter 1

1. The origin of photosynthesis resulted in a gradual build-up of oxygen in the Earth’s atmosphere. Before the Earth had an oxygen rich atmosphere, UV radiation on the Earth’s surface was so intense that it killed any organisms on land; life could only survive if it was protected from UV radiation by water. But as O₂ accumulated in the atmosphere, O₂ molecules began reacting with one another to form ozone (O₃). A layer of ozone gradually built up in the high atmosphere, and by about 500 million years ago it was sufficient to block enough UV radiation that life could colonize land.

2. Among the list of common characteristics of life on Earth, there are some features that we might expect to be general to any origin of life (such as the need to extract energy from the environment and use it to do work), but other features that we expect to be unique to each origin of life. For example, although another origin of life might use a genetic information system of some kind, there is no reason that we would expect the details of how that system functioned to be the same. Another origin of life might well use something other than DNA, or use different nucleotides to make up DNA. Moreover, even if another origin of life used the same basic building blocks to make DNA, the genetic code that specified which combinations of nucleotides encode which amino acids in proteins would be expected to be different. Furthermore, it is unlikely that the same set of amino acids would be used to construct proteins. It is the commonalities of these details across life on Earth that allow us to conclude that all life on Earth has a single common origin.

3. Fish typically have eyes that are used for sight, and eyes obviously require light to function. In a normal surface population, any mutation that disrupts eye-sight would be strongly selected against, because fish with non-functional eyes would be at a disadvantage compared to fish with functional eyes. Such mutations occur, but they are quickly eliminated from a surface population through natural selection. But in a cave population, there is no selection for eyesight, so such mutations are not selected against, and would accumulate in a cave population. In the absence of selection for eye function, we would expect gradual loss of a complex structure like an eye. In fact, many species of cave organisms do gradually evolve eyelessness over time. But how do they compensate for the loss of sight? Usually, there is strong selection for other sensory systems that do not require light, such as cells that detect small vibrations, or chemical scents. The important point is that in the dark environment, selection conditions change, and some features are expected to be lost, and others gained, because different features are favored under natural selection in the new environment.

4. Biologists use quantitative measures of similarities and differences between specimens to establish the relatedness of different species. The more similar, the more likely they have a recent common ancestor and the more different, the more likely their common ancestor is more remote in evolutionary time. Knowledge about phylogenetic relationships is also obtained from the fossil record and more recently from genomic analysis.

1. A hypothesis is a proposed explanation for an observation or a phenomenon. An experiment is a rigorous test of that proposed explanation.

2. In a controlled experiment, all variables are held constant while one variable is manipulated to determine its effect on the system being studied.

3. When questions involve systems in which variables cannot be controlled, which is the case with many questions in natural systems, a comparative approach can be used to establish correlations between the variable of interest and its possible effects.

4. Since all life is related through evolution, different species use similar molecular, biochemical, cellular, physiological, and even behavioral components, systems, and mechanisms. Therefore what is learned from one species is likely to be applicable to other species.

1. Modern biology is used to improve agricultural species—both plants and animals. An example is the genetic improvements of the plants that produce food grains. Biology is also used to treat diseases in agricultural species (e.g., antibiotics given to food animals) and to yield food plants resistant to herbicides.

2. The use of antibiotics creates a situation of artificial selection analogous to natural selection. Any disease organism that survives the antibiotic because of its genetic makeup will transmit that trait to its offspring. As the antibiotic continues to be used, the disease organisms with the resistance trait will multiply, resulting in decreased efficacy of the drug.

3. The Investigating Life thread in this chapter makes evident that global warming presents an enormous threat to reef-building corals, which support a high level of diversity in the marine ecosystem. Another notable example is the plight of the polar bear, which depends on sea ice to be able hunt its prey—seals. With the decrease in sea ice in the Arctic, the hunting areas and therefore hunting abilities of the polar bear have become more and more limited.

1. 

2. For the cool-pool corals, the probability of H₀ = (0.5)17 = 0.00000762939. Therefore we can safely reject H₀ and conclude that there is indeed a significant effect of heat stress on coral bleaching (at P < 0.00001). In other words, if the null hypothesis were true and there were no real effect of heat stress on coral bleaching, we would expect to see this many chlorophyll ratios below 1.0 fewer than 1 time out of 100,000 trials. For the warm-pool corals, there are seven observed values < 1, and none > 1. In this case, the probability of H₀ = (0.5)7 = 0.0078125. Therefore we can again reject H₀ and conclude that there is a significant effect of heat stress on coral bleaching in warm-pool corals as well (this time at P < 0.01).

3. The results of the randomization trials will differ depending on how well the cards are shuffled and how many replicates are compared, but the probability of finding a difference as great as 0.35 (the observed difference) in truly randomized samples of the two groups is very low (P < 0.001). Therefore we can again reject the null hypothesis and conclude that the effects of heat stress on coral bleaching are indeed higher in the corals from cool pools than in those from warm pools.

4. The different distributions of chlorophyll ratios for the cool- and warm-pool corals indicate that although both populations suffer bleaching as a consequence of heat stress, the populations from the cool pools are more sensitive. This suggests that corals from warmer environments might replace those from cooler environments under long-term conditions of global warming.

1. The results show that a population is more likely to go extinct if a deleterious environmental change is sudden and less likely to go extinct if the change is gradual. The complete loss of a species is more likely to happen if a deleterious change in the environment occurs rapidly and wipes out most of the populations of that species. Even if a few individuals survive, they may not be present in large enough numbers to effectively reproduce and maintain a viable population. However, if the environmental change is gradual, the population has time to allow surviving members to reproduce and enhance the proportion of individuals with the ability to survive, which allows the population to undergo adaptation to the change.

2. A-3

   The investigation shows how populations of living things evolve in response to changing environments. It models the process of natural selection that operates on populations and results in evolution. Environmental change and natural selection have helped shape the characteristics of organisms throughout the history of life on Earth.

3. All living things on Earth arose from a common ancestor and share certain characteristics. For example, all living things contain DNA, which encodes the information that passes from generation to generation. DNA is affected by mutation, which produces variation within a population. All living things are also affected by natural selection, which results in adaptation of populations and species over time. Because all living things possess DNA and are affected by natural selection, the results of the investigation are expected to be broadly applicable to life.

4. This was a controlled study because all of the cells were treated the same way (all variables were controlled) except for one condition that was varied. The varied condition was the amount of rifampicin added to the culture medium. (EXAMPLE) A comparative study could track changes in characteristics of a population of fish (or other species) cut in two by the building of a dam or other human-made construction. In a case like this, the study would have to show that the dam separated the original population into distinct groups that were isolated from each other and exposed to different environmental conditions. The study could then compare the characteristics of the separated groups of fish after a period of time to observe how the groups change in response to their respective environments.

5. Yes. The research organization could use the results of the investigation as an example to show that any population of bacteria likely has some proportion of individuals with genetic mutations that allow a degree of resistance to an antibiotic. Over time, selection for resistance will result in an increasing proportion of individual bacteria in that population that can grow in the presence of the antibiotic. Therefore, an antibiotic effective at killing bacteria does not remain effective over time as the bacterial populations evolve resistance to the antibiotic. This means that new antibiotics need to be developed periodically to take the place of older ones that lose their effectiveness as bacteria develop resistance to them. Using this reasoning, the research organization can show the need for a continuous pipeline of new antibiotics to deal with the problem of bacterial resistance.