Continuing Evolution
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AP Biology › Continuing Evolution
The fossil record provides strong evidence for evolution. Which aspect of the fossil record best supports the concept that evolution is a continuous, ongoing process rather than an event that occurred only in the distant past?
The presence of transitional fossils that show intermediate anatomical features between ancestral and more recently evolved groups of organisms.
The existence of fossils of extinct organisms, which proves that life forms have changed over Earth's history.
The consistent ordering of fossils in rock strata, with simpler organisms in older layers and more complex ones in younger layers.
The discovery of living fossils, organisms that have remained morphologically unchanged for millions of years.
Explanation
Transitional fossils, such as Archaeopteryx (linking reptiles and birds) or Tiktaalik (linking fish and amphibians), demonstrate the incremental changes that occur over time as one lineage evolves into another. This provides a snapshot of evolution in progress, supporting its continuous nature. Choice A shows that change occurs but not necessarily that it's continuous. Choice B demonstrates evolutionary stasis in some lineages, not ongoing change. Choice C shows a general pattern of change over time but doesn't illustrate the continuous, step-by-step process as well as transitional forms do.
Which of the following best explains, from an evolutionary perspective, why the second antibiotic treatment was ineffective?
The initial antibiotic treatment acted as a selective pressure, allowing a few naturally resistant bacteria to survive and reproduce, leading to a population that is now largely resistant.
The antibiotic caused specific mutations to arise in the bacterial DNA that conferred resistance, allowing the population to adapt directly to the presence of the drug.
The patient's immune system was weakened by the initial infection and antibiotic use, preventing it from effectively combating the bacterial population during the second treatment.
The bacteria that survived the first treatment developed an immunity to the antibiotic and passed this acquired trait on to their offspring, making the new population immune.
Explanation
This is a classic example of natural selection. The antibiotic kills susceptible bacteria, but individuals with pre-existing resistance survive and reproduce. This differential survival and reproduction lead to an increase in the frequency of the resistance allele in the population over time. Choice B describes an incorrect, Lamarckian view of acquired characteristics. Choice C offers a physiological explanation for the patient's condition but not an evolutionary explanation for the bacteria's resistance. Choice D is incorrect because mutations are random events, not directed responses to environmental pressures like antibiotics.
The divergence of house sparrow populations in North America demonstrates that evolution is an ongoing process by showing...
how different environmental conditions can lead to different selective pressures and result in adaptive divergence among populations.
that all introduced species undergo rapid and identical changes regardless of their new environment.
that individual sparrows can change their body size in response to temperature and pass that change to offspring.
that the founder effect is the only mechanism driving change in introduced populations.
Explanation
This is an example of local adaptation. The different climates in North America impose different selective pressures. In the north, larger body size is advantageous for heat conservation, while in the south, it is not. This has led to the measurable divergence of populations since their introduction, providing a clear example of ongoing adaptation. Choice A is incorrect, as the changes are different depending on the environment. Choice C is incorrect; while the founder effect was part of the initial introduction, natural selection is driving the subsequent divergence. Choice D is an incorrect, Lamarckian explanation.
A scientist claims that the current increase in atmospheric carbon dioxide is acting as a selective pressure on plant populations worldwide. Which of the following observations would provide the strongest evidence for continuing evolution in response to this pressure?
The total global biomass of plants has increased over the last 50 years due to the fertilizing effect of increased CO2.
A survey of historical herbarium specimens shows a decrease in the density of stomata on the leaves of a particular plant species over the last century.
An individual plant grown in a high-CO2 chamber shows a higher rate of photosynthesis than when it was grown in normal air.
Many plant species are shifting their geographic ranges to higher latitudes or altitudes as global temperatures rise.
Explanation
A change in the heritable traits of a population over time is evidence of evolution. A decrease in stomatal density over many generations, as documented by historical specimens, would be a morphological adaptation to high CO2 levels (as plants need fewer 'mouths' to get the same amount of CO2) and would indicate a genetic shift in the population. Choice A describes phenotypic plasticity (an individual's response), not evolution. Choice C describes an ecological response (range shift), not necessarily genetic adaptation within a population. Choice D is a large-scale ecological observation that doesn't directly show heritable change within a species.
How does the distribution of lactase persistence support the idea that evolution is a continuous process that responds to environmental pressures?
It demonstrates that all human populations are slowly and uniformly evolving toward the ability to digest lactose.
It suggests that the ability to digest lactose is a temporary adaptation that will disappear if populations stop consuming dairy products.
It indicates that a specific cultural practice—dairy farming—created a strong selective pressure that favored a particular genetic trait in certain populations.
It shows that the trait appeared randomly and its prevalence is unrelated to diet or geography, indicating continuous random change.
Explanation
This example shows that evolution is ongoing and can be driven by cultural changes. In environments where dairy farming was practiced, individuals with the mutation for lactase persistence had a significant nutritional advantage, leading to increased survival and reproduction. This selective pressure caused the allele frequency for the trait to increase, but only in those specific populations. Choice A is incorrect because the evolution is not uniform. Choice B is incorrect because the distribution is strongly correlated with dairy farming. Choice D is speculative and does not explain the current evidence.
In a city park, a population of pigeons includes two alleles at a gene affecting beak depth: D (deeper) and d (shallower). After a shift in available food toward larger, harder seeds, researchers sample allele frequencies each breeding season for 5 seasons. Allele D increases from 0.33 to 0.58. Which observation best demonstrates continuing evolution in the pigeon population?
The average seed size in the park increased when a new tree species was planted.
Some pigeons switched from seeds to insects during summer when insects were abundant.
Pigeons spent more time foraging near benches where visitors dropped food scraps.
Individual pigeons developed stronger jaw muscles after eating harder seeds for several weeks.
Allele D increased from 0.33 to 0.58 across five breeding seasons in the park population.
Explanation
Continuing evolution refers to genetic shifts in populations adapting to new conditions, such as these pigeons facing harder seeds. The correct answer, choice B, demonstrates this as allele D for deeper beaks rose from 0.33 to 0.58 over five breeding seasons, implying better food handling led to higher survival and reproduction. This population trend reflects natural selection favoring deeper beaks in the changed food environment. Consistent sampling across seasons highlights the heritable nature of the change. Choice A tempts with muscle development from eating harder seeds, a misconception of acquired traits or plasticity rather than genetic evolution. A transferable approach is to examine allele frequency data over generations to confirm evolution, avoiding confusion with non-heritable changes.
A population of rabbits includes two alleles at a coat-color locus: W (white) and B (brown). In 2017, snow cover duration decreased due to warmer winters. Researchers sampled the same population each spring: 2016 $f(B)=0.12$, 2018 $f(B)=0.20$, 2020 $f(B)=0.29$, 2022 $f(B)=0.33$. Predators more easily detect white coats on snow-free ground. Which observation best demonstrates continuing evolution in this rabbit population?
Some rabbits change hiding behavior by using shrubs more on snow-free days.
Juvenile rabbits grow more slowly when winter temperatures are above average.
The frequency of allele B increases in the population over successive years.
More predators are observed hunting in open areas during low-snow winters.
Individual rabbits shed fur earlier in warm years than in cold years.
Explanation
This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the B allele increasing from 0.12 in 2016 to 0.20 in 2018, 0.29 in 2020, and 0.33 in 2022, indicating a population trend toward brown coats as snow cover decreases. This change reflects natural selection favoring brown rabbits that are less detectable by predators on snow-free ground, enhancing survival and reproduction. Spring sampling of the same population tracks the genetic response to warmer winters. A tempting distractor is choice A, which describes individual rabbits shedding fur earlier in warm years, but this is wrong because it represents seasonal phenotypic plasticity, not genetic evolution across generations. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.
A population of field mice includes two hemoglobin alleles, H1 and H2. In 2000, $f(H2)=0.10$ in a lowland population. A drought from 2001–2004 reduced available water, and the same population was sampled each year: 2002 $f(H2)=0.16$, 2004 $f(H2)=0.23$, 2006 $f(H2)=0.24$. H2 is associated with improved oxygen delivery during dehydration stress. Which observation best demonstrates evolution occurring in this mouse population?
Some mice migrate from nearby hills into the lowlands during the drought.
The population’s allele frequency for H2 increases across multiple sampling years.
Young mice weigh less at weaning during the driest years of the study.
Individual mice drink less water during drought years than during wetter years.
Mice increase burrow use during hot days to reduce water loss.
Explanation
This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the H2 allele increasing from 0.10 in 2000 to 0.16 in 2002, 0.23 in 2004, and 0.24 in 2006, indicating a population trend toward better oxygen delivery during drought stress. This shift reflects natural selection favoring mice with the H2 allele that cope better with dehydration, resulting in higher survival and reproductive success. The consistent sampling during and after the drought period highlights how environmental pressure drives genetic change in the population. A tempting distractor is choice A, which describes individual mice drinking less during droughts, but this is wrong because it represents behavioral adaptation or physiological response within a lifetime, not heritable genetic evolution. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.
A population of mosquitoes is exposed to insecticide-treated bed nets starting in 2011. A sodium-channel allele (kdr) confers reduced sensitivity to the insecticide. In 2011, $f(kdr)=0.08$ in the village population; in 2013, $f(kdr)=0.21$; in 2016, $f(kdr)=0.39$; in 2020, $f(kdr)=0.52$. Mosquitoes were sampled from the same set of households each year. Which observation best demonstrates continuing evolution in this mosquito population?
Individual mosquitoes avoid entering houses with bed nets during a single night.
The frequency of the kdr allele increases over years in the village population.
Some adult mosquitoes live longer when provided sugar water in laboratory cages.
More mosquitoes are captured during the rainy season than during the dry season.
Mosquito larvae develop faster in warmer puddles than in cooler puddles.
Explanation
This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the kdr allele increasing from 0.08 in 2011 to 0.21 in 2013, 0.39 in 2016, and 0.52 in 2020, indicating a population trend toward insecticide resistance with bed net use. This change reflects natural selection favoring mosquitoes with reduced sensitivity, allowing better survival and reproduction despite exposure. Consistent sampling from the same households tracks the genetic adaptation over time. A tempting distractor is choice A, which describes individual mosquitoes avoiding houses with nets, but this is wrong because it represents behavioral avoidance or learning, not a heritable shift in allele frequencies. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.
A population of wild sunflowers grows along roadsides where de-icing salt is applied each winter. Salt-tolerance is associated with allele T at a transporter gene. Researchers sampled plants from the same 2 km stretch of road: in 2012, $f(T)=0.14$; in 2015, $f(T)=0.26$; in 2018, $f(T)=0.37$; in 2021, $f(T)=0.40$. Plants with allele T produce more seeds in salty soils than plants without T. Which observation best demonstrates evolution in this sunflower population?
Road salt application varies between winters depending on snowfall amounts.
Some plants grow taller in shaded patches than in sunny patches along the road.
Seedlings grow more slowly when transplanted from the roadside into a greenhouse.
Individual sunflowers wilt less on salty days because their leaves close stomata.
The frequency of allele T increases across years in the roadside population.
Explanation
This question assesses the skill of understanding continuing evolution, which involves ongoing changes in allele frequencies within a population over generations due to selective pressures. The correct answer, choice B, demonstrates evolution because it shows the frequency of the T allele increasing from 0.14 in 2012 to 0.26 in 2015, 0.37 in 2018, and 0.40 in 2021, indicating a population trend toward salt tolerance along salted roadsides. This shift reflects natural selection favoring plants with the T allele that produce more seeds in salty soils, enhancing reproductive success. Sampling from the same road stretch ensures the trend is a genetic response to de-icing salt. A tempting distractor is choice A, which describes individual sunflowers wilting less by closing stomata, but this is wrong because it represents physiological acclimation, not heritable genetic change. To identify evidence of continuing evolution in similar questions, focus on data showing shifts in allele or genotype frequencies over multiple generations rather than short-term individual responses.