1
What is biological evolution, and how does natural selection allow populations to adapt to changes? Why is genetic diversity important to this process?
INFOGRAPHICS 11.1 AND 11.2
Evolution is defined as a change in ________ ________in a population over time.
Gene frequencies
A population of butterflies used to have small, medium, and large individuals, but several years ago a non-native bird was introduced to the butterfly’s habitat. It eats butterflies but only those that are medium size. Eventually, medium-sized butterflies became rare. This is an example of:
disruptive selection.
stabilizing selection.
artificial selection.
directional selection.
A
“When exposed to pesticides, some Japanese beetles become more pesticide resistant and harder to kill.” There is something wrong with this statement. Reword it to more accurately represent what happens when pesticide application leads to a pesticide resistant population.
The problem here is with the word “become” — this implies that individuals change their vulnerability (and genetic makeup) to the pesticide upon exposure.
“When exposed to pesticides, most Japanese beetles may die but some individuals survive due to natural resistance. The survivors produce the next generation, a population where many (or most) individuals possess the same trait as their surviving parents — pesticide resistance.”
2
How did coevolution (or lack thereof) make most of Guam’s bird species so vulnerable to the brown tree snake?
INFOGRAPHICS 11.3 AND 11.4
Which of the following is an example of coevolution?
Polar bears and Arctic foxes both are white for camouflage on snow.
Dolphins and whales have flippers that are similar in shape and function to the fins of a fish, allowing them both to swim efficiently.
Moths that are preyed upon by bats can hear the ultrasonic sounds the bats use in hunting.
Humans and chimpanzees share 98% of their DNA.
C
Suppose a native Guam bird species began to show evasive behaviors that allowed it to avoid the brown tree snake. Describe a potential coevolution scenario that might have allowed this adaptation to emerge in the population.
Suppose there are individuals who are naturally more wary of movement or disturbance in the trees. They would be less likely to be eaten by the snake and would survive to reproduce. If this trait was controlled by genetics, the surviving birds would pass the evasive behavior trait onto their offspring. Over time, the population contains more and more birds with evasive behaviors as the snakes eat the ones without this trait (who don’t survive to reproduce) and leave behind birds with the trait (who do survive to reproduce). The snakes could eliminate the birds without the evasive behavior completely, leaving only a population of birds with the evasive behavior trait. Essentially, the snake is the selective pressure that favors evasive behavior.
3
How do random events influence the evolution of a population?
INFOGRAPHIC 11.5
Ten thousand years ago, most members of Species A were killed by a series of volcanic eruptions. However, some members escaped; all modern members of Species A are descended from those 100 individuals. This is an example of:
genetic drift.
artificial selection.
mass extinction.
the bottleneck effect.
D
Why might a population, isolated by the founder effect, be more vulnerable to extinction than the original population from which they came?
The founder effect usually produces isolated populations that contain less genetic diversity than the original population. These less diverse populations are less likely to have individuals present that might possess needed traits in the face of environmental change.
4
How do humans, intentionally or accidentally, affect the evolution of populations, and why do some scientists say that Earth is currently experiencing its “sixth mass extinction”?
INFOGRAPHICS 11.6 AND 11.7
Artificial selection differs from natural selection in that:
it cannot produce large changes over time, only minor ones.
humans are the selective pressure, choosing which traits to favor.
it does not depend on genetic variability in the population.
it causes individuals to change their traits rather than causing a change in the population’s gene frequencies.
B
Why do most scientists think that we are in the midst of a sixth mass extinction?
The background extinction rate is approximately 20%.
Most extinctions are occurring in the taiga and tundra.
No new species are being discovered.
Current extinction rates are greater than the background rate.
D
Why can’t all species adjust to changes in their environment and avoid extinction?
The ability of a population to respond to a selective pressure depends on the genetic diversity present in the population. If a needed trait is not present in the population then the entire population could be lost. Environmental changes that occur too quickly may also eliminate a population because the few individuals who might possess a trait that could be helpful are not sufficient to repopulate the population. This is especially problematic for K-selected species who reproduce more slowly but even r-selected species could be lost if the environmental change is too drastic.
Suppose you wanted to use artificial selection to produce a breed of hairless dogs for people who are allergic to dog hair. How would you go about doing this?
You could start with a short-haired breed. You would need to find individual dogs with sparse coats and you would breed these males and females. If this trait is genetic and passed on to offspring, at least some of the offspring should also have sparse coats. You would then choose only the dogs from those litters with the sparsest coats to breed for the next generation. Over the generations, whenever pups were born with even sparser coats (genetic diversity in the puppy’s generation), you would selectively breed only those males and females. Eventually, the hairless trait might emerge in your lineage and you could then breed only the hairless individuals to produce offspring who were also hairless.
5
What are some common misconceptions about evolution?
TABLE 11.1
Which of the following is true of evolutionary processes?
They are goal driven.
New genes arise in response to environmental change.
Evolution acts on existing genetic variation.
Evolution proceeds from simple to complex.
C
Why do we not expect evolution by natural selection to produce perfect adaptations?
Natural selection operates by modifying existing traits to produce new functions or structures and may be constrained by its “starting material”. In addition, some traits are compromises between conflicting needs (such as the flipper of a seal — not ideal for swimming or walking, but suitable for each).
Why is it inappropriate to reject evolutionary explanations for the diversity of life because evolution is “just a theory?
To say something is “just a theory” represents a misunderstanding of the concept of a scientific theory. Rather than seeing it as a well substantiated explanation supported by multiple lines of evidence, this statement is treating it more like a guess, or at best, an untested hypothesis that can be rejected for lack of evidence.
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