TOPIC 5: ECOLOGY AND EVOLUTION

5.1 COMMUNITIES AND ECOSYSTEMS

5.1.1 Define species, habitat, population, community, ecosystem and ecology.

Species: a group of organisms that can interbreed and produce a fertile offspring.

Habitat: the environment in which species normally live or the location of a living organisms.

Population: a group of organisms of the same species who live in the same area at the same time.

Community: a group of populations living and interacting with each other in an area.

Ecosystem: a community and its abiotic environment.

Ecology: the study of relationships between living organisms and between organisms and their environment.

5.1.2 Distinguish between autotroph and heterotroph.

Autotroph: an organism that synthesizes its organic molecules from simple inorganic substances. Producers are referred to as authotrops because they make their own food from inorganic sources of carbon. In case of photosynthetic organisms, the carbon source is CO2.

Heterotroph: an organism that obtains organic molecules from other organisms. Consumers are known as heterotrophs. They are organisms that obtain their carbon from organic sources. Heterotrophs occupy higher trophic levels. Animals that ingest prey, and fungi that absorb externally digested organic matter, are two examples of heterotrophs.

4.1.2 Explain how the biosphere consists of interdependent and interrelated ecosystems.

In an ecosystem, organisms feed off of each other. This relation or interaction of organisms can be in the form of a food chain or a food web. The food chain is a linear and simple feeding relation, where one organism has one type of food and is eaten by one type of organism. However, a food web is a more complex and it includes more variety of organisms, each of which can feed on a variety of other organisms and is fed upon by a variety of organisms. These are not the only interactions that compose the biosphere, however. A remarkable diversity of animal interactions, as well as the work of plants, bacteria, fungus, and protists combine to influence the biosphere. Also, organic cycles such as the water cycle, the recycling of the respiratory products of animals (carbon dioxide) in photosynthesis, and the transpiratory return of water to the atmosphere in plants all play major roles as well.

5.1.3 Distinguish between consumers, detritivores and saprotrophs.

Consumer: an organism that ingests other organic matter that is living or recently killed.

Detritivore: an organism that ingests non-living organic matter. - eatworms - feed on decomposed dead organic matter.

Saprotroph: an organism that lives on or in nonliving organic matter, secreting digestive enzymes into it and absorbing the products of digestion.

5.1.4 Describe what is meant by a food chain, giving three examples, each with at least three linkages (four organisms).

Only real examples should be used from natural ecosystems. A→ B indicates that A is being “eaten” by B (that is, the arrow indicates the direction of energy flow). Each food chain should include a producer and consumers, but not decomposers. Named organisms at either species or genus level should be used. Common species names can be used instead of binomial names. General names such as “tree” or “fish” should not be used.

Food chain is a sequence along which food is transferred. Each organism in the sequence feeds on the organism before it. Arrows point toward the consumer, indicating the direction of nutrient and energy flow.

PRODUCER -> PRIMARY CONSUMERS -> SECONDARY CONSUMER -> TETRIARY CONSUMER ->QUARTERNARY CONSUMER

Water lilies -> Moth larvae -> Roach (fish) -> Moorhen

Planktonic algae -> Trumphet snails -> Moorhen

Filamentous algae -> Halipid beetles -> Great diving beetle -> Roach (fish) -> Moorhen

Mosquito larva -> beetle -> mouse -> snake Plankton -> krill -> mullet -> shark Earwig -> lizard

-> shrew -> owl -> Clams -> starfish -> sea otters -> orcas

5. Describe what is meant by a food web.

It is a representation of the complexity of feeding relationships, showing interacting food chains. A food web is more complex than a food chain and it includes a larger variety of organisms. Each of which feed on a variety of other organisms and they are in turn fed on by more organisms. Therefore, if one species becomes extinct the ecosystem will still be able to exist. A drawing will be inserted at a later date of a food web.

5.1.6 Define trophic level

It is a position of organism in the food chain. It is determined by the number of energy-transfer steps prior to that level. The terms “producer”, “primary consumer” and “secondary consumer” are making reference to trophic level. The trophic level that supports all of the others is the producers. Usually these consist of photosynthetic organisms such as terrestrial green plants and phytoplankton.

5.1.7 Deduce the trophic level of organisms in a food chain and a food web.

Students should be able to place an organism at the level of producer, primary consumer, secondary consumer, and so on, as the terms herbivore and carnivore are not always applicable.

5.1.8 Construct a food web containing up to 10 organisms, using appropriate information.

5.1.9 State that light is the initial energy source for almost all communities.

Light is the initial energy source for almost all communities.

5.1.10 Explain the energy flow in a food chain

Energy losses between trophic levels include material not consumed or material not assimilated, and heat loss through cell respiration. Energy losses between trophic levels include material not consumed or material not assimilated and heat loss through cell respiration.

5.1.11 State that energy transformations are never 100% efficient.

When energy transformations take place, including those in living organisms, the process is never 100% efficient, commonly between 10-20%.

5.1.12 Explain reasons for the shape of pyramids of energy.

A pyramid of energy shows the flow of energy from one trophic level to the next in a community. The units of pyramids of energy are, therefore, energy per unit area per unit time, for example, kJ m-2 yr-1.

5.1.13 Explain that energy enters and leaves ecosystems, but nutrients must be recycled.

Energy can enter and leave an ecosystem but nutrients must be recycled. Sun light is the main source of energy on this planet. It is absorbed by photosynthesizing organisms, which convert light to chemical energy. Nutrients must be recycled by obtaining them from other organisms or products of organisms.

5.1.14 State that saprotrophic bacteria and fungi (decomposers) recycle nutrients.

These organisms feed on dead organisms and products of living organisms. They secrete enzymes on these materials that cause decomposition, and then they absorb decomposed and digested foods. Examples include many species of bacteria and fungi. These are essential organisms to an ecosystem, since they cause recycling of materials between biotic and abiotic parts of the ecosystem.

5.2 THE GREEN HOUSE EFFECT

5.2.1 Draw and label a diagram of the carbon cycle to show the processes involved.

The details of the carbon cycle should include the interaction of living organisms and the biosphere through the processes of photosynthesis, cell respiration, fossilization and combustion. Recall of specific quantitative data is not required.

TOK: What difference might it make to scientific work if nature were to be regarded as a machine, for example, as a clockwork mechanism, or as an organism, that is, the Gaia hypothesis? How useful are these metaphors?

5.2.2 Analyse the changes in concentration of atmospheric carbon dioxide using historical records.

Data from the Mauna Loa, Hawaii, or Cape Grim, Tasmania, monitoring stations may be used.

5.2.3 Explain the relationship between rises in concentrations of atmospheric carbon dioxide, methane and oxides of nitrogen and the enhanced greenhouse effect.

Students should be aware that the greenhouse effect is a natural phenomenon. Reference should be made to transmission of incoming shorter-wave radiation and re-radiated longer-wave radiation. Knowledge that other gases, including methane and oxides of nitrogen, are greenhouse gases is expected.

5.2.4 Outline the precautionary principle.

The precautionary principle holds that, if the effects of a human-induced change would be very large, perhaps catastrophic, those responsible for the change must prove that it will not do harm before proceeding. This is the reverse of the normal situation, where those who are concerned about the change would have to prove that it will do harm in order to prevent such changes going ahead.

TOK: Parallels could be drawn here between success in deterring crime by increasing the severity of the punishment or by increasing the chance of detection. If the possible consequences of rapid global warming are devastating enough, preventive measures are justified even if it is far from certain that rapid global warming will result from current human activities.

5.2.5 Evaluate the precautionary principle as a justification for strong action in response to the threats posed by the enhanced greenhouse effect.

Aim 8: Consider whether the economic harm of measures taken now to limit global warming could be balanced against the potentially much greater harm for future generations of taking no action now. There are also ethical questions about whether the health and wealth of future human

generations should be jeopardized, and whether it is right to knowingly damage the habitat of, and possibly drive to extinction, species other than humans. The environmental angle here is that the issue of global warming is, by definition, a genuinely global one in terms of causes, consequences and remedies. Only through international cooperation will a solution be found. There is an inequality between those in the world who are contributingmost to the problem and those who will be most harmed.

5.2.6 Outline the consequences of a global temperature rise on arctic ecosystems.

Effects include increased rates of decomposition of detritus previously trapped in permafrost,

expansion of the range of habitats available to temperate species, loss of ice habitat, changes in distribution of prey species affecting higher trophic levels, and increased success of pest species, including pathogens.

 

4.5.1 Outline the two local or global examples of human impact causing damage to an ecosystem or the biosphere. One example must be the increased greenhouse effect.

The greenhouse effect is a naturally occuring phenomena in the ecosystem of the planet. It is simply the accumulation of carbon dioxide and other gases such as methane in the atmosphere, which traps heat from the sun's radiation and raises planetary temperatures. Recently, however, increased industry and burning of fossil fuels have caused the release of excessive amounts of carbon dioxide into the atmosphere. The planet is now enveloped by a layer of carbon dioxide far thicker than would be there naturally, which allows the sun radiation to enter our atmosphere, but prevents it from going out. This causes the trapping of heat into our atmosphere, and the consequent gradual increase in temperature around the world, hence global warming. This effect is called the greenhouse effect, since the layer of carbon dioxide around our planet has similar effects to the glass walls of a greenhouse in causing increased temperature inside.

The ozone layer is present at about 20 Km above the surface of the earth. It absorbs ultra violet light that radiates from the sun, thus protecting us from the harmful effects of these radiations. Increased industry in the last 20 years or so, have caused the breaking of ozone molecules into oxygen, resulting in a hole in this protective layer. The chemicals responsible for this effect are a group of chlorofluoro carbons (CFCs) that are used in refrigeration, aerosol cans and other types of industry. These compounds are very light and they escape to the upper layers of the atmosphere, reaching the ozone layer and destroying it. A hole in the ozone layer is most prominent over the Antarctic.

Air Polution: The warmth of the earth can be attributed to radiation from the sun. Some of the radiation is absorbed by the earth. Many gases aid in these absorption such as carbon dioxide, methane, and water vapour. They withhold the radiation and form a blanket warming the earth. The effect is like a greenhouse, hence the name: Greenhouse Effect.

However, there has been in increase in carbon dioxide and methane because of combustion of fossil fuels. This has led to an increased greenhouse effect and thus global warming.

Deforestation: In some areas, rainforests are cut to make way for farmland- slash and burn techniques. This has generally a negative effect because rainforest soil is not generally good for farming and can only be used temporally. Moreover, this is a catalyst for soil erosion. Almost everywhere, forested land is cleared for construction and for resources (wood).

4.5.2 Explain the causes and effects of the two examples in 4.5.1, supported by data.

• The greehouse effect is largely a result of human industry and machinery, including automobiles and other vehicles that emit significant amounts of carbon dioxide from the burning of fossil fuels. Its effects have included an increase in global temperature by several degrees over the past decade, a melting of glacial deposits across the globe, and the recent thinning of Artic and Anartic pack ices; all of the effects reported as the much-publicized global warming. Many scientists predict more drastic changes in temperature and environment in the future if current warming patterns continue. Ozone depletion, as previously mentioned, is due to chemicals called CFCs being released into the atmosphere. CFCs, or chlorofluorocarbons, are a compound of chlorine, fluorine and carbon, as the name would suggest. They are found in refrigerants and a variety of aerosol containers. When these compounds are released into the atmosphere, by the action of spraying a can of hair spray, for example, they react with and break apart double-bonded oxygen molecules (ozone). One molecule of CFC can destroy thousands of ozone molecules; thus their large-scale release into the atmosphere during the 1980's and early 1990's was very damaging. The result was the opening of a large hole in the ozone layer (the atmospheric layer responsible for deflecting UV radiation from the sun harmful to most organisms) which was centered over Anartica. For several years the hole moved throughout the Southern Hemisphere, often exposing countries such as Austrailia to dangerously high amounts of UV radiation. Today the hole still exists, but since the banning of the production or use of CFCs it has shrunk considerably due to the repair of the ozone by natural causes.

4.5.3 Discuss measures which could be taken to contain or reduce the impact of the two examples, with reference to the functioning of the ecosystem.

The best method currently agreed upon to resolve the greenhouse effect issue is a twofold proposal. The first involves attempts to reduce the production of greenhouse gases by international treaties on the amount of gases emitted, such as the Kyoto Treaty, the use of alternative fuel and energy sources that emit little or no greenhouse gases, and improved filtering for industrial and automotive gases already produced. The second involves allowing the environment to stabilize this problem itself. This includes checking the destruction of forests and other photosynthetic environs and organisms, as these naturally regulate the amount of carbon dioxide in the atmosphere.

In reference to the Greenhouse Effect: Cleaner fuels that emit less carbon dioxide and other gases, especially in motor vehicles, could ease pressure on the environment, allowing natural effects to work the problem out. Photosynthesis should be encouraged to reduce carbon dioxide from the air, and also emissions from the burning of fossil fuels should be reduced.

In reference to deforestation: People having fewer children (2 at most) would stop the increase in the human population. If the population were to remain the same, new housing construction would be unnecessary. Also, re-planting deforested areas and giving them time to grow would serve to rectify the problem to an extent.

5.3 POPULATIONS

5.3.1 Outline how population size is affected by natality, immigration, mortality and emigration.

Population size can be affected by natality (birth) because as birth rate increases, the population increases. The increase in a population is exponential, as the population increases so does the birth rate. Immigration is the arrival to the population from another area. This adds to the numbers in the total population. Mortality is death, and the mortality rate, like the birth rate, increases as the population increases. This, along with emigration (migration of population to another area) can help to stabilize population growth. In order for a population to be stable in size, Natality + immigration = mortality + emigration.

• If (natality + immigration) > (mortality + emigration) then a population is increasing. These factors determine whether a population is increasing or decreasing.

5.3.2 Draw and label a graph showing a sigmoid (S-shaped) population growth curve.

5.3.3 Explain the reasons for the exponential growth phase, the plateau phase and the transitional phase between these two phases.

• The exponential growth phase exists because that is when the population has already begun to grow, but not a lot yet, and it rises quickly because there are no limiting factors yet and the resources are in unlimited amounts. The plateau phase begins when the organism hits it's carrying compacity, which is the maximum number of organisms in a population that can be supported by the environment at a certain time, in a certain ecosystem. The transitional phase in between these two phases occurs because this is when the limiting factors in the environment start to limit the increase, slowing the population increase.

• During the Exponential phase the population increases exponentially because the natality rate is higher than the mortality rate. The resources needed by the population such as food and space are abundant, and diseases and predators are rare.

• During the Transitional Phase, the birth rate begins to decrease. Natality is still larger than mortality, but the difference between them is slowly decreasing.

• During the Plateau phase, available resources become so low that no further reproduction can take place. Mortality starts to become larger than natality. A species may have reached its Carrying Capacity.

5.3.4 List three factors that set limits to population increase.

Three factors that set limits to population increase are the availability of nutrients, the number of predators, and the accumulation of waste materials.

• Available resources.

• Disease

• Space available

• Predators

1. Define carrying capacity

Carrying capacity is the number of organisms in a population that can be supported by the environment at a certain time, in a certain ecosystem. Carrying Capacity: The maximum number of organisms of a species, or the maximum population size which an environment is able to support.

5.4 EVOLUTION

5.4.1 Define evolution.

Evolution is the cumulative change in the heritable characteristics of a population. If we accept not only that species can evolve, but also that new species arise by evolution from preexisting ones, then the whole of life can be seen as unified by its common origins.

Variation within our species is the result of different selection pressures operating in different parts of the world, yet this variation is not so vast to justify a construct such as race having a biological or scientific basis.

5.4.2 Outline the evidence for evolution provided by the fossil record, selective breeding of domesticated animals and homologous structures.

5.4.3 State that populations tend to produce more offspring than the environment can support.

This increases the chance of survival of the population as a whole--a single death is less disastrous in a population of 1,000 than it is in a population of 10.

5.4.4 Explain that the consequence of the potential overproduction of offspring is a struggle for survival.

• Populations of living organisms tend to increase exponentially.

1. More offspring are produced than the environment can support. There is a struggle for important resources such as food and space. Intraspecific competition. Some individuals survive and others die.

2. Characteristics in organisms differ from one another. Some have characteristics which make them better suited to survive in their environment. These are the most likely to survive.

5.4.5 State that the members of a species show variation.

The members of a species show variation.

5.4.6 Explain how sexual reproduction promotes variation in a species.

Variation is essential for natural selection and therefore for evolution. Although mutation is the original source of new genes or alleles, sexual reproduction promotes variation by allowing the formation of new combinations of alleles. Two stages in sexual reproduction promote variation.

1. Meiosis allows a huge variety of genetically different gametes to be produced by each individual

2. Fertilization allows alleles from two different individuals to be brought together in one new individual.

3. Sexually reproduction promote variations because, unlike the cloning that occurs in asexual reproduction ,every offspring in a genetic combination of his of her parents. This allows for infinite possibilites, as one can easily see by looking at the people around them. During meiosis, many different gametes are created because chromosomes are independently assorted during meiosis. Then, during fertilization, one of the many gametes from the mother joins with one of the many gametes from the father, creating a new and unique combination of genes.

5.4.7 Explain how natural selection leads to evolution.

The much better-adapted individuals pass on their characteristics to more offspring than the less well adapted individuals. The results of natural selection therefore accumulate. As one generation follows another, the characteristics of the species gradually change, the species evolve.

5.4.8 Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria.

Other examples could include: the changes in size and shape of the beaks of Galapagos finches;pesticide resistance, industrial melanism or heavymetal tolerance in plants.

4.3.6 Explain how natural selection leads to the increased reproduction of individuals with favorable heritable variations

• Combining the ideas of the struggle to survive, and the great variation in organisms, we can see that a group of different organisms are all fighting to occupy a certain niche (a place in the ecosystem). If organism A is better suited for this environment than organism B, organism A will survive and reproduce more than organism B. It is very important to understand that longer life is not a "goal" of natural selection. An organism that is better suited to an environment will be able to reproduce and pass on thier superior genes. The ability of better sutied organisms to reproduce more than other organisms that are not as suited for thier environment allows for the better suited organisms to produce more organisms with those same genes. These organisms have inherited the superior genes, so the amount of organisms with superior genes has increased.

Discuss the theory that species evolve by natural selection.

There is a struggle for existence in populations. There are a limited amount of resources to suffice a population of organisms. They must face both interspecific and intraspecific competition in order to obtain these resources.

Some organisms have more useful characteristics than others that makes them more adapted to their environment, and give them a better ability at obtaining these limited resources.

These creatures will survive because of this trait, reproduce, and there is high chance it's offspring will also survive as well. Overtime, more members of the population will acquire this trait and thus overtime the whole species may have evolved.

'Explain two examples of evolution in response to environmental change; one must be multiple antibiotic resistance to bacteria.

Before Penicillin was invented, bacteria was the leading cause of death. However, once it began to be used, since it's an antibiotic, some individuals of bacteria may carry the gene Penicillinase, which codes for an enzyme that deactivates Penicillin, making them resistant to an antibiotic such as Penicillin. Thus, when it is indeed used, they will be the only ones left to reproduce and new bacteria will also be resistant to the antibiotic.

The Peppered Moth is another example of evolution in response to environmental change. When Britain began industrialising, soot would come from factories and land on trees. A species of peppered moth with a lighter colour vanished and those with a darker colour flourished because they could hide themselves easily.

4.3.8 Explain two examples of evolution in response to environmental change; one must be multiple antibiotic resistance in bacteria

Example 1: Two varieties of the moth Biston betularia exist in the forms of different body color. One is black, the other is speckled. The black moth is easily seen by predators while the speckled one is camoulflaged. When on a tree covered in lichens, the peppered moth blends in very well. The number of speckled moths was greater than the number of black moths, because the speckled genes made the speckled moths more suitable for thier environment of lichenous trees. Because they were able to camouflage, they could evade predators more than black moths could, which allowed them to reproduce more moths with the genes for speckled color. Then, the trees began to get covered in suit due to the industrialization, and the black moth was able to be more camouflaged than the speckled moths. Because of this more black moths than speckled moths evaded predators, allowing them to produce more black moths. So the population of black moths then increased and the speckled moth population decreased.

Example 2: Resistance to antibiotics in bacteria. If a culture of bacteria is sprayed with antibiotics, most of the bacteria is killed. A small number that naturally have genes resistant to antibiotics, will remain. It is important to note that these bacteria did not "learn" to resist antibiotics. These bacteria has mutated genes that somehow allowed them to resist antibiotics. These bacteria will reproduce and pass on thier resistant genes. Natural selection chose the antibiotic resistant ones, so those are the only ones to exist. This can become a problem when trying to kill a bacterial infection in a patient, because if the bacteria is resistant to the antibiotics given, then they can't be killed. Someone would have to come up with a new antibiotic that it is not resistant to, which can be difficult.

4.3.7 Discuss the theory that species evolve by natural selection

In order to answer this question, the ideas aforementioned should be used.

If more organisms are produced that have "superior" genes, genes that make the organisms more suited for their environment, then they are able to produce more organisms with superior genes. This causes the population become more and more made of these superior organisms. When a population of a species changes as a result of natural selection, the species has evolved.

4.3.3 Explain that the consequence of the potential overproduction of offspring is the struggle for survival.

The world has limited resources. Organisms produce many more offspring than can live off of these limited resources. Therefore, there is a struggle to survive between offspring. This allows for natural selection, because those best suited for their environment survive and pass on their better-suited genes.

5.5 CLASSIFICATION

5.5.1 Outline the binomial system of nomenclature.

TOK: The adoption of a system of binomial nomenclature is largely due to Swedish botanist and physician Carolus Linnaeus (1707-1778). Linnaeus also defined four groups of humans, and the divisions were based on both physical and social traits. By 21st-century standards, his descriptions can be regarded as racist. How does the social context of scientific work affect the methods and findings of research? Is it necessary to consider the social context when evaluating ethical aspects of knowledge claims?

Organisms are given two names in this system (binomial). The first name indicates the genus and the second indicates the species. The genus is written in a capital letter and the species in small letters. Also the two names are usually printed or underlined. Naming organisms in this way facilitates the process of identification and helps in overcoming language barriers between scientists.

Called binomial because two names are used.

1. First name is genus, with first name being a capital.

2. Second name is species, with no capital.

3. Italics are used when the name is printed.

4. The name is underlined if it is handwritten.

5.5.2 List seven levels in the hierarchy of taxa—kingdom, phylum, class, order, family, genus and species—using an example from two different kingdoms for each level.

• Kingdom: Animalia | Plantae

• Phylum: Chordata | Conferophyta

• Class: Mammalia | Pinopsida

• Order: Cetacea | Pinales

• Family: Balaenopteridae | Taxodiaceae

• Genus: Baleaenoptera | Sequoia

• Species: musculus | sempervirens

5.5.3 Distinguish between the following phyla of plants, using simple external recognition features: bryophyta, filicinophyta, coniferophyta and angiospermophyta.

5.5.4 Distinguish between the following phyla of animals, using simple external recognition features: porifera, cnidaria, platyhelminthes, annelida, mollusca and arthropoda.

5.5.5 Apply and design a key for a group of up to eight organisms.

Keys are most commonly used to identify plants, insects, and birds. These are often area specific, for example, the Plants of Northern Europe. Keys are usually constructed in the following ways:

• The key consists of a series of numbered stages

• Each stage consists of a pair of alternative characteristics

• Some alternatives give the nest stage of a key to go to

• Some alternatives give the identification

5.5.6 State that organisms are classified into the kingdoms, Prokaryotae, Protoctista, Fungi, Plantae, and Animilia.

• Prokaryotae- bacteria

• Protoctista- Including unicellar organisms like Amoeba and Algae

• Fungi- Including moulds and yeasts

• Plantae- Including conifer, ferns, mosses and flowering plants

• Animalia- Multicellular and locomotive, including sponges, corals, birds and mammals

Describe the values of classifying organisms.

• Species Classification:It is easier to find out which species an organism belongs to when you have other organisms to compare it too.

• You can make assumptions about characteristics of a species in general.

• Evolutionary links, you can make assumptions about traits of a common ancestor. You can also predict how they evolved.

PALEONTOLOGY

The formation of fossils.

Fossils are generally of rock that had replaced the preserved organisms ot its traces.

It usually occurs when the organism is covered quickly so it is preserved.

Sediment, forming sedimentary rock, is then laid down.

FORMATION OF FOSSILS

Not all fossils are petrified

Some are preserved by dehydration (mummified), in ice in peat bogs, in tar beds or trapped in amber.

FINDING FOSSILS

• the discovery of fossils is greatly assisted where there has been natural erosion, which exposes the deeper older layers containing the fossils.

• Useful sites include gorges, quarries, caves and desert areas.

• Therefore fossils are only formed under certain conditions and then have to be uncovered

• The chance that a body will be fossilized is rare and the chance that it will be discovered in even rarer

• The fossil record is far from complete

• This may account for the `missing links' and for apparently restricted distribution of many species

• But paleontologists can improve their chances by searching systematically in places where fossils are likely to be found.

FOSSIL DNA

The current limit for fossil DNA appears to be about 100 000 years old.

Oxygen and water damage the molecule with time.

RELATIVE DATING

• sedimentary rock is laid down in layers or strata the deepest usually being the oldest

• this sequence forms the stratigraphy of the rock and together with the fossils and artefacts which are present, give a relative dating.

• However, due t earth movements in the past and to the great time spans and migrations of some organisms, this method is not very accurate.

ABSOLUTE DATING

• accurate dating can be obtained using radiometric dating.

• This uses the phenomenon of radioactive decay of isotopes.

• When sedimentation occurs radioactive isotopes are incorporated

• These decay to form other atoms at a known rate

• This rate Is measure as the half-life of the isotope, defined as the time taken for half the parent atoms to decay to the daughter atoms.

POTASSIUM _ ARGON METHOD

• potassium - 40 (40-k) decays to form Argon-40 (40-Ar), which is trapped in the rocks.

• The amount of argon is measured, so that this is known as an accumulation method

• The half-life of 40-k is 1.3X106 years, so it is useful for dating very old rocks (as old as the Earth) the minimum age being 100 000 years.

• The limitation is the degree of precision of the measuring devices.

• volcanic rock is particularly useful for this technique

• when it melts the rock releases any 40-Ar it has in it, setting the clock to zero

• then when the molten rock crystalises it becomes impermeable which traps 40-Ar gas so it cannot escape

• with time the 40-Ar builds up and the 40-k diminishes

• volcanic rock, however, does not contain fossils

• so when fossisls are dated using this method their association with the lava flow or ash fall needs to be established.

CARBON - 14 METHOD

• carbon-14 ( 14-C) decays to form nitrogen-14

• carbon -14 is formed in the upper atmosphere by the action of cosmic rays on Nitrogen-14

• 14-c is oxidized to 14-CO, that gets taken up by plants, in photosynthesis. The 14-c becomes incorporated in living tissue and travels up the food chain like other isotopes of carbon (e.g. 12-c)

• Whilst an organism is living it incorporates a known amount of carbon-14

• At death, no more is taken in, and so the amount declines as the 14-c, decays back to 14-n

• DEATH STARTS THE CLOCK

CARBON 14 METHOD

The ration of 14-c to 12-c is measured. 12-c is a stable isotope which does not decay. So as time goes by the ratio of 14-c/12-c gets smaller

The half-life of 14-c is 5730 years, so it is used to date bery recent remains, the maximum age being 50 000 years (there is not much 14.c left after 9 half-lifes)

The amount of 14-c in the atmosphere varies with the amount of bombardment of the atmosphere by cosmic rays. Therefore, correction factors are used which have been calculated using other methods (e.g. dendrochronology - tree ring dating)

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