Evidence that natural selection alters beak size in Geospiza fortis. (a) In dry years, when only large, tough seeds are available, the mean beak size increases. In wet years, when many small seeds are available, smaller beaks become more common. (b) Beak depth is inherited from parents to offspring.
Suppose a bird with a large bill mates with a bird with a small bill. Would the bills of the pair's offspring tend to be larger or smaller than the bills of offspring from a pair of birds with medium-sized bills? Answer: The figure demonstrates that the beak depth of offspring can be predicted by the average beak depth of the parent's bills. Thus, one would expect the offspring to have the same beak depth if their parents' mean beak depth is the same. This is only correct if males and females do not differ in beak depth. In species for which the sexes differ (such as height in humans), then one would need to know both the depth and the sex of the parents and the calculation would be more complicated.
Selection against melanism. The circles indicate the frequency of melanic Biston betularia moths at Caldy Common in England, sampled continuously from 1959 to 1995. Diamonds indicate frequencies of melanic B. betularia in Michigan from 1959 to 1962 and from 1994 to 1995.
What can you conclude from the fact that the frequency of melanic moths decreased to the same degree in the two locations? Answer: Such a parallel trend would suggest that similar processes are operating in both localities. Thus, one would conduct a study to identify similarities. In this case, both areas have experienced coincident reductions in air pollution, which most likely is the cause of the parallel evolutionary trends.
Artificial selection in the laboratory. In this experiment, one population of Drosophila was selected for low numbers of bristles and the other for high numbers. Note that not only did the means of the populations change greatly in 35 generations, but also all individuals in both experimental populations lie outside the range of the initial population.
What would happen if, within a population, both small and large individuals were allowed to breed, but middle-sized ones were not? Answer: Assuming that small and large individuals would breed with each other, then middle-sized offspring would still be born (the result of matings between small and large flies). Nonetheless, there would also be many small and large individuals (the result of small x small and large x large matings). Thus, the frequency distribution of body sizes would be much broader than the distributions in the figures.
Evolutionary change in body size of horses. Lines indicate evolutionary relationships and reveal that although most change involved increases in size, some decreases also occurred.
Why might the evolutionary line leading to Nannippus have experienced an evolutionary decrease in body size? Answer: This evolutionary decrease could occur for many reasons. For example, maybe Nannippus adapted to forested habitats and thus selection favored smaller size, as it had in the ancestral horses, before horses moved into open, grassland habitats. Another possibility is that there were many species of horses present at that time, and different sized horses ate different types of food. By evolving small size, Nannippus may have been able to eat a type of food not eaten by the others.
1). Which of the following best describes the correlation between beak size and the amount of rain that fell on Daphne Major?
a). Birds with small beaks are favored in dry years.
b). All birds are favored equally in wet years.
c). Birds with large beaks are favored during wet years.
d). Birds with large beaks are favored during dry years.
2). In peppered moths, the black coloration is selected when soot covers tree bark; this is a phenomena called
a). artificial selection.
b). convergent evolution.
c). industrial melanism.
d). none of these.
3). Evolutionary change through artificial selection has been demonstrated in all but which of the following?
a). Galápagos finches
d). dog breeding
4). Darwin's examinations of fossils relied on ____________ dating to determine the evolution of species.
d). radioactive isotope
5). The missing links between whales and their hoofed ancestors include
d). all of these.
6). Evolution has occurred in the horse as seen by
a). a reduction in body size.
b). an increase in complexity of ridges on teeth.
c). an increase in the number of toes.
d). all of these.
7). Over time, the same bones in different vertebrates were put to different uses. This falls under the category of
a). missing links.
b). vestigial structures.
c). analogous structures.
d). homologous structures.
8). After examining the evidence related to the evolution of hemoglobin, you might conclude that
a). bird hemoglobin evolved prior to lamprey hemoglobin.
b). frogs are more closely related to lampreys than to birds.
c). evolutionary changes occur at the molecular level.
d). only DNA can be examined for establishing evolutionary differences.
9). An example of convergent evolution is
a). Australian marsupials and placental mammals.
b). the flippers in fish, penguins, and dolphins.
c). the wings in birds, bats, and insects.
d). all of these.
10). The shape of the beaks of Darwin's finches, industrial melanism, and the changes in horse teeth are all examples of
a). artificial selection.
b). natural selection.
c). convergent evolution.
d). homologous structures.
Test Your Visual Understanding
1). The graph illustrates how a radioactive isotope decays over time. Some isotopes decay more quickly than others do (they have shorter half-lives), but all isotope decay follows this same scale-that is, half of the isotope atoms decay with each half-life. For each of the isotopes in the following list, calculate how long it will take each parent sample to decay to 12.5% of the original amount. Also, graph three of the isotopes, plotting the proportion of parent isotope remaining to the number of half-lives, and compare these three graphs with the figure. Are they similar, or are they different?
a). beryllium-11 13.81 seconds
b). oxygen-15 2 minutes
c). sodium-24 15 hours
d). phosphorus-32 14.3 days
e). carbon-14 5,730 years
f). plutonium-239 24,110 years
Answer: It takes 3 half-lives of any isotope to reduce the original amount of parent isotope down to 12.5%, therefore the time it takes for each isotope is their half-life times 3:
a). Beryllium-11 would take 41.43 seconds to reach 12.5%
b). Oxygen-15 would take 6 minutes to reach 12.5%
c). Sodium-24 would take 45 hours to reach 12.5%
d). Phosphorus-32 would take 42.9 days to reach 12.5%
e). Carbon-14 would take 17,190 years to reach 12.5%
f). Plutonium-239 would take 72,330 years to reach 12.5%
All three graphs, regardless of the isotope's rate of decay, would look the same as the graph pictured.
Apply Your Knowledge
1). In a laboratory experiment, researchers selected for an increase and a decrease in protein content of corn seeds. The initial population contained an average of 9.5% protein by weight. As with the artificial selection experiments described in this chapter, corn seeds with the top 20% protein content were crossed and corn seeds with the lowest 20% protein contents were crossed. After 50 generations, the high-protein offspring averaged 19.2% protein, and the low-protein offspring averaged 5.4% protein.
a). What percentage of change was recorded for the high-protein and low-protein populations?
b). Which trait, the high- or the low-protein level, was modified more because of selection? Can you explain why one trait was modified more?
1a). The high-protein population increased by
(19.2 - 9.5) / 9.5 = 1.02 or a 102% increase in protein content
The low-protein population decreased by
(9.5 - 5.4) / 9.5 = 0.43 or a 43% decrease in protein content
1b).The high-protein population was modified more by selection. One reason why this may have occurred is that the plant requires protein to grow and there may be a lower limit of protein content, under which growth ceases. Corn seed with less than this lower limit (maybe around 5.4%) may not grow and reproduce and so there would be no corn seeds with less than 5.4% protein.
2). Why is it incorrect to think of evolution as progressive (i.e., proceeding from lowest or simplest to highest or most complex)?
Answer: Although many evolutionary trends noted thus far have been examples of increasing complexity this is not to say that evolution is progressive, pushing in a single direction. When we examine horse evolution, there is a general trend to larger, more complex animals but throughout the fossil record, there are also examples where the animals became smaller, less complex. The evolution of the hemoglobin molecule doesn't show directionality in its evolution; rather just a modification of what is there (i.e., the amino acid sequence). Another example discussed in this chapter is the evolution of the vertebrate eye. Creatures that may be considered "lower" on the evolutionary tree such as mollusks actually have eyes that are more optimally designed for sensing light. Evolution through natural selection occurs more by the appearance of workable solutions rather than the appearance of optimal designs.