Biology: Evolution: Evidence and Mechanisms Worksheet

Question 1

A fossil layer containing trilobites is found below a fossil layer containing early fish. Explain what this order suggests about the relative ages of these organisms and how the fossil record supports evolution.

Accepted Answer

The trilobites are likely older than the early fish because deeper rock layers are usually older if the layers have not been disturbed. The fossil record supports evolution by showing changes in organisms over time and the appearance and disappearance of species in a sequence.

Question 2

The forelimbs of humans, whales, cats, and bats contain the same basic bone pattern, even though they have different functions. Identify this type of evidence and explain what it suggests.

Accepted Answer

This is evidence from homologous structures. Similar bone patterns suggest that these organisms inherited the basic limb structure from a common ancestor, even though the limbs evolved for different functions.

Question 3

Dolphins and sharks both have streamlined bodies and fins, but dolphins are mammals and sharks are fish. Explain why this is an example of convergent evolution rather than close common ancestry.

Accepted Answer

This is convergent evolution because dolphins and sharks evolved similar body shapes independently due to similar aquatic environments and selection pressures. Their similarity does not mean they are closely related.

Question 4

A population of beetles includes green and brown individuals. Birds can see green beetles more easily on tree bark, so they eat more green beetles. Over several generations, brown beetles become more common. Identify the mechanism of evolution and explain the change.

Accepted Answer

The mechanism is natural selection. Brown beetles had a survival advantage because they were harder for birds to see, so they were more likely to survive and reproduce. Over generations, the allele or alleles for brown color became more common.

Question 5

Explain why individual organisms do not evolve, but populations can evolve.

Accepted Answer

Individual organisms may survive, reproduce, or die, but their inherited traits usually do not change during their lifetime. Populations evolve when the frequencies of alleles or traits change across generations.

Question 6

A DNA comparison shows that Species A and Species B have 98 percent of their DNA sequences in common, while Species A and Species C have 82 percent in common. Based on this evidence, which species is more closely related to Species A, and why?

Accepted Answer

Species B is more closely related to Species A because it shares a higher percentage of DNA sequences with Species A. Greater molecular similarity usually indicates a more recent common ancestor.

Question 7

A small group of birds is blown by a storm from a mainland population to an island. The island population starts with only a few alleles from the original population. Name this form of genetic drift and explain its effect.

Accepted Answer

This is the founder effect. Because the island population began with only a few individuals, its allele frequencies may be very different from the mainland population, and genetic variation may be reduced.

Question 8

A disease kills most members of a rabbit population at random. The surviving rabbits reproduce, but the new population has less genetic variation than before. Identify the mechanism and explain why genetic variation decreased.

Accepted Answer

The mechanism is the bottleneck effect, a type of genetic drift. Genetic variation decreased because only a small, random subset of the original population survived and passed on their alleles.

Question 9

In a population, the frequency of allele A is 0.7 and the frequency of allele a is 0.3. Assuming Hardy-Weinberg equilibrium, calculate the expected frequency of heterozygous Aa individuals.

Accepted Answer

The expected heterozygous frequency is 2pq. Using p = 0.7 and q = 0.3, 2pq = 2(0.7)(0.3) = 0.42, so 42 percent of the population is expected to be heterozygous.

Question 10

List two conditions required for Hardy-Weinberg equilibrium and explain why a population that violates these conditions may evolve.

Accepted Answer

Two required conditions are no natural selection and no migration, although other conditions include no mutation, random mating, and a very large population. If these conditions are violated, allele frequencies can change over time, which means the population is evolving.

Question 11

A mutation changes a gene in a way that gives some bacteria resistance to an antibiotic. When the antibiotic is used, resistant bacteria survive and become common. Explain the roles of mutation and natural selection in this process.

Accepted Answer

Mutation created the new resistant variation in some bacteria. Natural selection increased the frequency of that variation because resistant bacteria survived antibiotic treatment and reproduced more successfully than nonresistant bacteria.

Question 12

A phylogenetic tree shows that lizards and snakes share a more recent common ancestor with each other than either shares with turtles. Explain how the tree supports this conclusion.

Accepted Answer

The tree supports this conclusion if the branches for lizards and snakes meet at a node that is closer to the present than the node connecting them with turtles. A more recent node represents a more recent common ancestor.

Question 13

Two populations of the same salamander species are separated by a mountain range. After many generations, they can no longer interbreed successfully. Describe the type of speciation and the role of isolation.

Accepted Answer

This is allopatric speciation. The mountain range created geographic isolation, which prevented gene flow between the populations. Over time, mutations, natural selection, and genetic drift caused enough differences that the populations became separate species.

Question 14

Peacocks with larger, brighter tails attract more mates, but the tails can make it harder to escape predators. Identify the evolutionary mechanism and explain the tradeoff.

Accepted Answer

The mechanism is sexual selection. Large, bright tails may increase reproductive success by attracting mates, but they can also reduce survival by making the animal more visible or less agile around predators.

Question 15

A cave fish population has reduced eyes and very pale body color compared with closely related surface fish. Give one evolutionary explanation for these traits and include a mechanism that could cause them to become common.

Accepted Answer

In dark caves, eyes and pigmentation may provide little advantage, so mutations reducing these traits may not be strongly selected against. Genetic drift could make these traits common in a small cave population, or natural selection could favor energy savings if reduced eyes are beneficial.

Biology: Evolution: Evidence and Mechanisms

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