[THVInstitute13] Cadmium at Foundry Cove: An Ecology & Evolution Story
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[THVInstitute13] Cadmium at Foundry Cove: An Ecology & Evolution Story
- 1. Cadmium at Foundry Cove: An Ecology & Evolution Story Cornelia Harris
- 2. Today’s Workshop • Quick overview of the Changing Hudson Project • What do students know about evolution? • Lesson overview • Share ideas, Questions
- 3. Research & Education based on Ecosystem Ecology
- 4. Changing Hudson Project: Ecology Curriculum for the High School Classroom
- 5. Changing Hudson Project • Help students understand how the Hudson River estuary changes over time using local data • Train educators through weekend, evening and summer workshops
- 6. Resources: Visualizations, Readings, Lesson Plans with worksheets & labs
- 7. What do students think about how evolution works? • Work in groups • Sort the answers in order of increasing sophistication • Fill out the first column of the worksheet: what characteristics are you using to sort the students’ answers into different levels? • Write the letter of the student responses in the second column (teacher generated rubric); leave the last column blank for the “answers”
- 8. Change in Population Questions Level 4 for change in a population: NY 12th grader: This occurs because the pesticide wipes out all of the non-resistant insects thus leaving the resistant insects to breed and reproduce and become a dominate trait in the population. INTERVIEWER: Okay. How does an insect become resistant? NY 12th grader: It's born with natural variation in genetic population. If there's a 100 bugs maybe 10 are resistant and then when the whole population is sprayed those 10 are left. So when the population eventually recovers those 10 will be the, will now be the entire 100 or they will be make up a large part of it and if the same pesticide is used over and over again eventually it won't have an effect because the entire population will be resistant to it.
- 9. Change in Population Questions Level 1 for change in a population: CO 7th grader: The increase in resistance is most likely caused by the evolving animals in the environment of a farm. Lately we have talked about the changes animals and/or organisms use to change and adapt to their environment. These insects highly adapt to avoid the chemicals. INTERVIEWER: Okay. How do they evolve? CO 7th grader: They evolve by adapting to their surroundings or doing something to make them – make better use of their surroundings. INTERVIEWER: Okay. And so – do you know anything about mechanism? So how does – how do they adapt? CO 7th grader: Probably just by finding different ways to use their surroundings or finding different ways to do something.
- 10. Change in Population: Insecticide ProportionofAnswers 0% 10% 20% 30% 40% 50% 60% 70% Middle School High School Level 1 Level 2 Level 3 Level 4 n = 187 n = 160 L1-4 = 80% L1-4 = 85%
- 11. What do students know and think about how evolution works? • Understanding adaptation by selection and phenotypic plasticity are both important for predicting effects of disturbances, including climate change and other human impacts, on ecosystems • When asked why a population may have changed over time, students describe an individual gaining or losing traits based on some environmental cue • When given enough scaffolding, even young students can understand that there is standing variation in a population and a trait can increase in a population over time through differential reproductive success • We propose understanding the mechanism of adaptation by selection is not what is limiting students understanding of how individuals and populations change: instead we propose that understanding the nature of phenotypic traits - and thus the existence of standing variation in a population - is their largest hurdle. • Key problems that plague students: they don’t really understand what a population is, and they don’t think about multiple generations
- 12. What makes the Hudson a unique river ecosystem?
- 13. What do you already know about the Foundry Cove story? Cadmium in the Cove
- 14. Scientists collecting samples in Foundry Cove
- 15. FOUNDRY COVE SOUTH COVE NORTH COVE 1.0 km N NEW YORK CITY FOUNDRY COVE 200 km N Marathon Battery Factory made nickel-cadmium batteries in Foundry Cove from 1952-1979.
- 16. Heavy Metal Pollution between 1952 and 1979, over 112,000 pounds of cadmiumwas released into Foundry Cove Foundry Cove became “the most cadmium polluted site in the world” nickel and cobalt were released in much smaller amounts
- 17. How could scientists find out where the cadmium went after it left the Marathon Battery outfall?
- 19. = 50,000+ ppm cadmium in sediments of Foundry Cove ppm = parts per million 1975 Marathon Battery Factory
- 20. 1975 Distribution of cadmium in surface sediments in East Foundry Cove 1983 Distribution of cadmium in surface sediments in East Foundry Cove Marathon Battery
- 21. Map activity
- 22. Cadmium in sediments of Foundry Cove What happened to the cadmium between 1979 and 1983? Where could it have gone?
- 23. Sampling at the train trestle, Foundry Cove Outgoing Tide Incoming
- 24. What happens to cadmium that gets eaten? Cadmium in sediments of Foundry Cove Major Cadmium Pathways Transportedout by tides Eatenby organisms Buriedby sediments carried in by tides or freshwater runoff Redistributedby water Movements within the cove
- 25. Part B of worksheet
- 26. ELA connections • CCS ELA Reading Standards 6-12: Cite specific textual evidence to support analysis of science and technical texts • CCS ELA Reading Standards 6-12: Determine the central ideas or conclusions of a text; provide an accurate summary of the text distinct from prior knowledge or opinions
- 27. Overview of Unit • Lesson 2: Marathon Battery Factory (dilution activity) • Lesson 3: Biomagnification (mobile or poster) • Lesson 4: Survivors at Foundry Cove (what happened to the worms in the toxic mud?) • Lesson 5: Natural Selection • Lesson 6: Evolution & Pollution
- 28. What’s so bad about a little cadmium? • At very high levels, causes Itai-Itai (or ouch-ouch) disease in humans – pain in bones and joints, bones break easily • Can cause cancer and other health problems • Entered the human food web via contaminated blue crabs in the Hudson River • Liver lesions in muskrats in Foundry Cove Lesson 3
- 29. Looking at a Millionth - Activity 100 ppm 1,000 ppm 10,000 ppm 50,000 ppm How big is 1 ppm? How big is 250,000? Lesson 3
- 30. Survivors at Foundry Cove How did the worms in Foundry Cove survive when their environment became highly contaminated by toxic cadmium? 30 Lesson 4
- 31. Scientific experiments were conducted on Limnodrilushoffmeisteri worms. Sampling for worms in Foundry Cove 31 Lesson 4
- 33. Foundry Cove South Cove Very similar environments Extreme Cd contamination 50,000 ppm Lower level of Cd contamination 19 ppm Mud worm dominant mud-dwellers 33 Lesson 4
- 34. Are L. hoffmeisteriworms usually resistant to cadmium? What kind of experiment could be done to find out? Pause here to discuss your ideas. Then describe and sketch an idea for an experiment to answer the question. 34 Lesson 4
- 35. Are all L. hoffmeisteri worms resistant to cadmium? 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Control Area Foundry Cove PercentSurviving Sediment Source Graph A: Numbers of Limnodrilus hoffmeisteri worms surviving sediment exposure after 28 days Foundry Cove Control Area Worms 35 Lesson 4
- 36. “Adaptation”, Evolutionary adaptation or plasticity? • A squirrel lines its nest with grasses and leaves because it has suddenly become very cold outside – “adaptation” • Twin dogs, separated at birth, grow to different sizes because they have access to different types of nutrition – plasticity • A rabbit grows white fur in the winter – the mechanism is an evo adaptation • A species of mosquito has been going dormant later each fall – evo adaptation • Butterfly species in CA are migrating an average of 24 days earlier when compared to 30 years ago – plasticity
- 37. Resistance to Toxins • Physiological plasticity: – In individual organisms – Depends on environmental conditions – Disappears if returned to a clean environment • Heritable trait: – In populations of organisms – Increased through natural selection – Persists in offspring for at least a few generations 37Lesson 4
- 38. Did Foundry Cove worms pass their cadmium resistance to their offspring? 0 10 20 30 40 50 60 70 80 90 100 O 20 30 45 65 90 PercentageofSurvivors Cadmium level in sediment (mg Cd/g of dry sediment) Graph B: Limnodrilus hoffmeisteri Survivors Foundry Cove worms Foundry Cove offspring South Cove worms 38
- 39. Natural Selection at Foundry Cove: Evolution of Cadmium Resistant Worms Part 1 Of Mice, Bugs, and Bacteria Part 2 Mud worms 39 Source: www.smc.edu Source: www.fcps.edu Lesson 5
- 40. How did Foundry Cove worms become resistant to cadmium? Natural Selection Natural selection of an invisible trait such as cadmium resistance is hard to visualize. So let’s first look at natural selection of a visible trait such as fur color. 40 Source: museum2.utep.edu Lesson 5
- 41. Pocket Mice and Predation http://www.hhmi.org/biointeractive/evolution/pocket_mouse_predation.html 41 Source: www.nps.gov Lesson 5
- 42. Pocket Mice and Evolution http://www.hhmi.org/biointeractive/evolution/pocket_mouse_evolution.html 42 Lesson 5
- 43. Existing Variation Variation within a species e.g. Genetic variation for fur color -multiple alleles for color Present in a population or species before natural selection occurs 43 Lesson 5
- 44. Natural Selection in a Population of Pocket Mice • Existing genetic variationmultiple alleles for fur color • Heritable fur color is passed from parent to offspring • Differential succcess dark fur protects mice better than light fur on lava flow habitat • Time many generations of mice have lived and passed on their genes 44 Lesson 5
- 45. Evolution of an Invisible Trait Now, on to the natural selection of cadmium resistance in Foundry Cove mud worms. 45 Source: www.spiegel.de Lesson 5
- 46. Foundry Cove Worms Natural Selection Process Diagram Key = Non-resistant worm = Resistant worm 46 Selection and Growth Toxic levels of cadmium kill 50% of the population First Generation as Young How many are alive? What % are resistant? Lesson 5
- 47. Foundry Cove Worms Natural Selection Process Diagram 47 Key = Non-resistant worm = Resistant worm Selection and Growth Toxic levels of cadmium kill 50% of the population First Generation as Young First Generation as Adults How many are alive? What % are resistant? How many are alive? What % are resistant? Lesson 5
- 48. Foundry Cove Worms Natural Selection Process Diagram 48 Key = Non-resistant worm = Resistant worm Reproduction Surviving adults reproduce, each having one baby Selection and Growth Toxic levels of cadmium kill 50% of the population First Generation as Young First Generation as Adults 1st Gen. Survivors & their Young How many are alive? What % are resistant? How many are alive? What % are resistant? How many are alive? What % are resistant? Lesson 5
- 49. Foundry Cove Worms Natural Selection Process Diagram Selection and Growth Toxic levels of cadmium kill 50% of population Reproduction Surviving adults reproduce, each having one baby 49 Key = Non-resistant worm = Resistant worm 1st Gen. Survivors & Their Young (Copy) 1st& 2nd Gen. Survivors as Adults Survivors & their Young Reproduction Surviving adults reproduce, each having one baby Selection and Growth Toxic levels of cadmium kill 50% of the population First Generation as Young First Generation as Adults 1st Gen. Survivors & their Young How many are alive? What % are resistant? How many are alive? What % are resistant? How many are alive? What % are resistant? How many are alive? What % are resistant? How many are alive? What % are resistant? Copy totals for 1st gen. survivors & young here: Lesson 5
- 50. Scientists noticed fewer muskrats in Foundry Cove during the most polluted years. 1. Do you think muskrats evolved resistance to cadmium? Why or why not? 2. What else might cause a reduction in the number of muskrats in the cove? 50 Source: www.fcps.org Lesson 5
- 51. Lesson 6: Evolution & Pollution • Tomcod resistance to PCBs in the Hudson River • Students get a variety of articles to choose from – they select one, read it & critique
- 52. 1. Scientists have discovered a strange fish that lives in a soup of some of industry's worst pollutants. 2. Bottom-feeding fish in the Hudson River have developed a gene that renders them immune to the toxic effects of PCBs, researchers say. 3. An unpretentious little fish that has learned how to thrive in one of the most polluted rivers in North America has scientists wondering if foul water is driving its evolution and, ironically, whether genetic changes that seem to be taking place might jeopardize its ability to survive in cleaner water. 4. IT IS not often that biologists have a chance to watch natural selection in action. 5. Some fish in New York’s Hudson River have become resistant to several of the waterway’s more toxic pollutants. 52
- 53. 6th grade 11th-12th grade CCSS.ELA-Literacy.RI.6.1 Cite textual evidence to support analysis of what the text says explicitly as well as inferences drawn from the text. CCSS.ELA-Literacy.RI.11-12.1 Cite strong and thorough textual evidence to support analysis of what the text says explicitly as well as inferences drawn from the text, including determining where the text leaves matters uncertain. CCSS.ELA-Literacy.RI.6.3 Analyze in detail how a key individual, event, or idea is introduced, illustrated, and elaborated in a text (e.g., through examples or anecdotes). CCSS.ELA-Literacy.RI.11-12.3 Analyze a complex set of ideas or sequence of events and explain how specific individuals, ideas, or events interact and develop over the course of the text. CCSS.ELA-Literacy.RI.6.5 Analyze how a particular sentence, paragraph, chapter, or section fits into the overall structure of a text and contributes to the development of the ideas. CCSS.ELA-Literacy.RI.11-12.5Analyze and evaluate the effectiveness of the structure an author uses in his or her exposition or argument, including whether the structure makes points clear, convincing, and engaging. CCSS.ELA-Literacy.RI.6.7 Integrate information presented in different media or formats (e.g., visually, quantitatively) as well as in words to develop a coherent understanding of a topic or issue. CCSS.ELA-Literacy.RI.11-12.7Integrate and evaluate multiple sources of information presented in different media or formats (e.g., visually, quantitatively) as well as in words in order to address a question or solve a problem.
Notas del editor
- The Hudson River is just beyond the right edge of the photo. A small bit of Foundry Cove is visible at the bottom right. South Cove is visible in the distance.
- In the 1970’s, scientists discovered that Foundry Cove, near Cold Spring, NY was contaminated by extremely high levels of very toxic metals: cadmium, nickel, and chromium. The Marathon Battery Factory was discharging its industrial wastes into the cove and had been doing so for over two decades. Of the three metals, cadmium is the most toxic of the three and was released in the greatest quantity, and scientific research has focused on it.
- All three of these metals are toxic to humans and other animals.
- Scientists collected sediment samples from Foundry Cove and near the Constitution Marsh Audubon Sanctuary, just two miles to the south. Ask: Why would scientists collect samples from both Foundry Cove and Constitution Marsh? What might they be trying to learn?Over several decades, scientists collected data on cadmium levels in sediments,organisms, and water.
- The outfall is where metal-polluted wastewater was dumped into Foundry Cove. Point out the railroad trestle , which is the only place that Hudson River water moves in and out on the tides. Prior to the construction of the railroad, the whole cove was open and water flowed freely between the cove and the river. Orient students to other parts of the map: water in Foundry Cove and in the Hudson, marshy area (dark), land, iso-line (dotted lines), and sample sites (black dots). The iso-lines mark boundaries between different levels of contamination, similar to lines marking elevations on a topographic map. For example, the space between 1000 and 3000 show the area of the cove where sediment levels of cadmium are between 1000 ppm and 3000 ppm. Volunteers can come to the screen, point to a sample site, and estimate the ppm for that location, based on information in the map. Have students explain how they arrived at their estimation.Ask students: where is the cadmium level highest? Lowest? Why do you think that might be? (Hint: tides) Over time, sediments in the cove slowly migrate from the back of the cove to its mouth at the train trestle, pulled by the tides.
- Ask students what differences they notice in the two maps. Point out approximately where the marsh would be in the 1983 map. (If you have a SmartBoard or similar technology, you or a student can sketch in approximately where the marshline would be in the second map. Ask: how do you think this might have happened? What do you think caused the changes in contamination levels? Where might the cadmium have gone? The Marathon Battery Factory ceased operations in 1979. Cadmium is an element that is denser than water. That means it doesn’t break down and it tends to sink. No clean up activities were conducted between 1975 and 1983. Natural factors were responsible for changes in cadmium levels. The two biggest factors are tidal movement and sediment deposition. Sediment is delivered by the tides and by Foundry Cove Creek and other sources of freshwater runoff and snowmelt.
- As you saw in the two maps we just looked at, there is less cadmium in Foundry Cove. Ask: Where do you think the cadmium went?Possible answers: carried out by tides, covered by sediments, carried to different parts of the cove by water movements, eaten by animals
- Scientists collected water from both the Hudson River and Foundry Cove and analyzed suspended particles to learn about how cadmium moved into and out of the cove. Cadmium was found in very low levels in the water. Most of it remained in the sediments. Overall, there was a small net flow of cadmium out of the cove, meaning that more cadmium left the cove on outgoing tides than was carried back into the cove on incoming tides.
- Toxin pathways need to be studied in order to understand risks to environmental health and human health. Environmental field research informs ecologists, toxicologists, and resource managers about where, when, and how toxins move from place to place. Once organisms become contaminated, the toxin can accumulate in living tissues and move up the food chain. Biomagnification of toxins results in higher levels in those animals at the top of the food chain. A famous case is that of DDT, when high levels in bald eagles, pelicans, osprey, and other top predators nearly caused their extinction.
- Itai-itai illness first recorded in Japan in the 20th century in people who lived and farmed near a cadmium mine in Japan. Itai-itai, cancer, and other serious health problems are rare, but the risk to human health is great enough that sport and commercial fishing for blue crabs and fish in the Hudson was restricted due to contamination with cadmium and other toxins, notably mercury and PCBs.Muskrats were not studied at Foundry Cove, though data on ppm was tested and recorded. Anecdotal observations by scientists: they noticed that muskrats were rarely seen in Foundry Cove when it was highly contaminated, though they were seen at nearby, cleaner sites. Muskrats are more common in the cove today.
- 1 ppm isn’t visible in this graphic – it’s 1/100th of one dot!Each visible dot = 100 ppmEach row in each square has 10 dots, with each representing 100 ppm, so each row represents 1000One square = 1000 x 10 = or 10,00050,000 = five squares of 10,000250,000 ppm = one quarter of the large square. This was the highest ppm cadmium ever recorded in sediments, and was found right at the outfall site in the 1970’s.
- Different species respond to toxins in their environment in different ways, and the worms in Foundry Cove have a special story to tell.
- Even though Foundry Cove was highly contaminated in the 1980s, L. hoffmeisteri was the most common macrobenthic organism in both Foundry and South coves. Scientists were surprised by this discovery and wanted to find out if L. hoffmeisteri are generally so tolerant of cadmium, or if there was something special going on with the population of Foundry Cove worms.Scientists had already learned that some plant species have evolved resistance to metals, but they did not know much about the evolution of metal resistance in animals. At the time (1980s), a genetic basis for resistance in animals had been found only in a population of isopods living in a metal-polluted river in England.
- Samples of gut contents of L. hoffmeisteri had elevated levels of both cadmium and nickel. Scientists concluded that at least some of the cadmium in Foundry Cove was bioavailable and could be absorbed by body tissues.
- The overall density of macrobenthic organisms, including L. hoffmeisteri worms, in Foundry Cove was just about the same as that in South Cove, a very similar environment about a mile to the south. However, the taxonomic richness (diversity) was lower at Foundry Cove. According to the current EPA guidelines, the actionable level of cadmium is 1 ppm. Depending on the location and other factors, the EPA can require cleanup of cadmium over 1 ppm.
- Discuss these questions and then have students complete the first two pages of their worksheets.
- Ask students to explain the results shown here.Sediments and worms were collected from both Foundry Cove and the control area (South Cove). Most worms from both sites survived 28 days in the cleaner sediment from the control site. When kept in cadmium-contaminated sediments from FC, almost all worms from the control area died, while 90% of the worms from FC survived.This is a common type of experimental design, called a ‘reciprocal transplant’ experiment. In this type of experimental design, the researcher attempts to determine if the population of interest maintains a certain trait even when grown under different environmental conditions—in particular, the environment of a separate target population, such as a control population. The control population is also transplanted to the experimental population’s site. This design can help us learn whether the there’s something in particular about the experimental that population determines the expression of the trait or whether ‘site’ determines the trait’s expression.
- PhysiologicalPlasticity (aka phenotypic plasticity): Taking anti-malaria medications provides some protection to individuals, reducing the chance of getting malaria, but the protective effects cease when medication in stopped. The protection from malaria is temporary, not genetic, and cannot be passed to offspring. Heritable trait: People who carry one sickle cell gene are resistant to malaria, though not completely protected from the disease. Compared to non-carriers, they have about 1/10 the risk of dying from malaria. In a group of people living in an area with high rates of malaria it benefits the population as a whole to have people who are resistant to the disease and therefore more likely to reach reproductive age in good health.** Note that for people NOT living in malaria-prone areas, having this sickle cell gene is NOT adaptive. Therefore, be careful using the word ‘adaptation.’ Adaptations describe inherited traits that are performing useful functions. There are lots of instances of inherited traits that are either no longer performing a useful function or perhaps never were.
- Scientists conducted research to find out which type of resistance was allowing the mud worms to survive in FC. Malaria provides an example in people:Plasticity (aka phenotypic plasticity) / acclimation: Modifying one’s behavior in response to the environment is an easy-to-see example of plasticity. For example, travelers often take anti-malaria medications in high-malaria areas. This provides some protection to the individual traveler, reducing his or her chance of getting malaria, but the protective effects cease when medication in stopped. The protection from malaria is temporary, not genetic, and cannot be passed to offspring. Heritable trait: People who carry one sickle cell gene are resistant to malaria, though not completely protected from the disease. Compared to non-carriers, they have about 1/10 the risk of dying from malaria. In a group of people living in an area with high rates of malaria it benefits the population as a whole to have people who are resistant to the disease and therefore more likely to reach reproductive age in good health.** Note that for people NOT living in malaria-prone areas, having this sickle cell gene is NOT adaptive. Therefore, be careful using the word ‘adaptation.’ Adaptations describe inherited traits that are performing useful functions. There are lots of instances of inherited traits that are either no longer performing a useful function or perhaps never were.
- The Foundry Cove worms and the South Cove worms were field collected. The Foundry Cove offspring are second generation offspring of field collected parents. This graph shows the percentage of worms of each of the three types that survived 28-day exposure to sediments with the indicated levels of cadmium.In sediments with 0 mg/g cadmium, survival rates for all types are similar. At 20 mg Cd/g, survival of FC worms and their offspring is similar, while just a few SC worms survive. At 30 mg Cd/g, survival of FC offspring drops more than that of FC worms, but survival is still much higher than SC worms. At 45 mg Cd/g and above, no SC worms survive the 28 days, while even at 90 mg Cd/g, a small percentage of the FC worms and the FC offspring still survive.This experiment showed that cadmium resistance in the population of worms in FC was indeed inherited, since the second generation offspring also display the trait. Though the resistance was less pronounced in the offspring than in the adults, it is clearly inherited. Additional lab experiments showed that in about 4 generations, a population of non-resistant L. hoffmeisteri evolved resistance.
- Before learning about the evolution of cadmium-resistance, students will look at three other examples of evolution by natural selection.
- Invisible traits are such things as resistance to cadmium (in mud worms) or chicken pox (in humans). Other examples also include what people sometimes refer to as “bad genes” --- genes that make one more susceptible to developing things like heart disease, diabetes, or breast cancer.
- Two short videos provide a visual model for a natural selection event that occurred in a pocket mouse population in the desert region of the southwestern U.S.
- This video provides a simple representation of the natural selection story in one pocket mouse population. The volcanic eruption referred to in the video happened about 1000 years ago.
- Students often think that the traits an organism “needs” to survive are caused by the environmental change. While the concept of existing variation is fairly easy to understand here because students are familiar with the idea of color variation (multiple alleles) from their own lives, this understanding often gets lost in unfamiliar cases. Ask students for other examples of existing variation. Ask for examples in plants (e.g. roses), food (e.g. apples), and invisible traits (smell of roses, taste of apples, susceptibility to heart disease in humans).
- These four conditions are needed for selection (both natural and artificial) selection to occur: **genetic variation, **the trait must be heritable**differential reproduction &/or survival due to presence of the trait (selective pressure)**time (multiple generations). Students often think that natural selection occurs in individual organisms rather than in a population or species. Natural selection occurs over many generations, not within an individual.
- The next four slides can be used to introduce the Natural Selection Process Diagram in the student handout. Depending on your students, you might use the first two slides to show students how the diagram works and then let them finish alone on their own, or you may want to have the class follow along and complete their diagram as you view the four slides.
- This shows young Foundry Cove worms before the first introduction of cadmium into their environment. Cadmium-resistant worms make up 5% in this diagram.The students should be sure to answer the questions below the worms before/after each event. These are also on their worksheet.20 alive; 5% resistant
- Many of the non-resistant adults die without reproducing. In this model, the cadmium kills more than half of the non-resistant worms.10 alive10% resistant
- Now the population size increases to twenty, but the proportion stays the same. Two out of 20 worms (10%) of the second generation are resistant.
- The second row starts with the surviving adults and their young (from the end of the first row). Once again, cadmium kills more than half of the non-resistant worms. Of the ten surviving adults, two (20%) are resistant. The population size increases to twenty, but the proportion stays the same, so 4 out of 20 worms (20%) of the second generation are resistant.Again, exposure to cadmium kills many worms before they can mature and reproduce. The percentage of cadmium-resistant worms in the third generation has increased to 20%.
- Many muskrats from the area had liver lesions and high levels of cadmium in their kidneys, and there is no evidence that the population was resistant.Cadmium may have killed muskrats. It’s also possible that another cause unrelated to cadmium, such as disease, may have reduced their numbers.Evolution is not a solution to toxins in our environment. Resistance to contaminants has been found in very few organisms, while a whole range of detrimental effects of many toxic pollutants are well documented in many species and in many environments.