Classification, selection and evolution

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Classification, selection and evolution

Question: Shade in all those processes that reshuffle alleles in the population.

Question: Mutation cause new alleles which are usually recessive. Why does it take time for the phenotype that they express to build up in a population?

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Mutation in DNA can occur as a result of the environment. Alternatively, it can occur as a result of DNA replication. A new base change can arise in the DNA resulting in gene mutation. This is probably how the sickle cell anaemia allele probably arose

The diagram shows that mutations in somatic cells as well as usually having no effect on a multicellular organism cannot be passed onto offspring. A malfunctioning cell in a tissue is only one of thousands of similar cells, and it is unlikely that this cell would cause any problems. Most mutated cells are recognised as foreign by the body’s immune systems and are destroyed. Occasionally the mutation may affect the regulation of cell division. If a cell with such a mutation escapes the attack of the immune system, it can produce a lump of cells called a tumour.

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Tumours often cause little harm, but sometimes they can be malignant and cause metastases (secondary cancer).

Question: Explain how cancer cells spread to form secondary cancers

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Overproduction

All organisms have the reproductive potential to increase their population. If all the rabbits produced in a litter were to go on and produce litters themselves (several each year), the following graph would be produced:

This will occur if population numbers remain unchecked (1859 in Australia, 12 pairs of rabbits from Britain were released and this happened). However, as a population of rabbits increases, usually environmental factors come into play that keeps their

numbers down.

Question: These factors could be biotic i.e., caused by living organisms or abiotic i.e., caused by non-living components of the environment. Give five examples of

anything that would normally be limiting factors on rabbit population growth.

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Question: Highlight all biotic factors green. Annotate examples of abiotic factors.

Question: Which environmental factors will exert less of an effect when the population numbers of rabbit decrease?

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Question: Which environmental factors are independent (not influenced by) of the rabbit population?

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Lemming populations are famous for their large increases and decreases (4-year cycle). In some years, populations become so large that lemmings may emigrate ‘en mass’ from overcrowded areas. The reason for the oscillating population size is not known for certain, although it has been suggested that food supply or food quality may be the main cause. As the population rises, food supplies run out, so the

population size ‘crashes. Once the population size has decreased, food supplies begin to recover, and the population size rises again.

Natural selection

Under extreme environmental pressure some rabbits will be more suited or adapted to survive than others. Due to variation (genetic and that brought about by the

environment) some rabbits will acquire features which will give them an advantage in the ‘struggle for existence’.

One feature that may vary is coat colour in rabbits. Let B = allele for normal agouti (brown) colour and let b = recessive white coat colour.

Question: Why should there be more brown rabbits than white? Why should brown rabbits be at an advantage to white rabbits? Under what conditions might the white coat be an advantage?

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Predation by foxes is an example of natural selection i.e., nature selected only those with the best adaptations (fittest) to survive. Selection pressures increase the chances of some alleles being passed on to the next generation and decrease the chances of others. In this case the alleles for agouti coat have a selective advantage over the alleles for white.

Question: The types of gene in a population are referred to as the gene pool. Which gene will be selected for and against (and may ultimately be lost from the population)?

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Question: Skomer is a small island off the coast of Wales. Rabbits have been living on the island for many years. There are no predators on the island.

(a) Rabbits on Skomer are not all agouti. There are quite large rabbits of different colours such as black and white. Suggest why this is so.

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(b) What do you think might be important selection pressures acting on rabbits on Skomer?

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Evolution

Usually, natural selection keeps things the way they are. This is the stabilising selection.

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Agouti rabbits are best adapted to survive predation, so the agouti allele remains the most common coat colour allele in rabbit populations. Unless something changes, then natural selection will ensure that this continues to be the case.

However, if a new environmental factor or a new allele appears, then allele

frequencies may also change. This is called directional selection i.e nature selects in direct favour towards a new allele frequency away from the existing one.

Question: Imagine that the climate changed so that it became a snowy environment (new environmental factor), would directional selection environmental pressure be exerted on the rabbit population?

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Question: One the allele frequency in the gene pool became largely for the white fur allele. Now explain how stabilising selection would help to maintain that frequency.

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Rabbits with white fur will be more likely to reproduce, passing on their alleles for white fur to their offspring. The frequency of the allele for white fur increases, at the expense of the allele for agouti. Over many generations, almost all rabbits will become to have white coats rather than agouti.

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Question: Remember from the Transport module when we considered the relationship between surface area to volume ratio? Now consider that there is a sudden change in the environment, and it becomes much colder. Do you think that there might be a change in the selection pressure in a particular direction i.e.

directional selection? If so, show with a bell curve in green on the second graph how the average body mass may change and give your reasoning below:

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Sometimes environmental pressures stay relatively constant for millions of years and stabilising selection favours certain features in an organism to remain generation after

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generation. The tuatara Sphenodon punctatus, is a lizard like reptile that lives in New Zealand. Fossils of a virtually identical animal have been found in rocks 200 million years old. Natural selection has acted to keep the features of this organism the same over all this time.

Question: What might act as directional selection pressure to the tuatara?

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A new allele

Because they are random events, most mutations that occur produce features that are harmful. That is, they produce organisms that are less well adapted to their

environment than ‘normal’ organisms. Other mutations may be ‘neutral’, conferring neither an advantage nor disadvantage on the organisms within which they occur.

Occasionally mutations may produce useful features.

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Question: Annotate this diagram with explanation of the effects of both directional and stabilising selection.

Natural selection therefore gives some alleles a better chance of survival than others.

Over many generations, populations may gradually change, becoming better adapted to their environments e.g. bacterial antibiotic resistance and the moth Biston betularia.

As bacteria have only a single loop of DNA, they have only one copy of each gene, so the mutant allele will have an immediate effect on the phenotype of any bacterium possessing it. Within 24 hours millions of antibiotic resistance bacteria can be produced because they have a selective advantage over others increasing the frequency of this allele in the gene pool.

Such antibiotic-resistant strains of bacteria are continually emerging. The alleles for resistance appear on plasmids that can be passed from one bacteria to another and then replicate into offspring during binary fission. It is even possible to transfer these alleles from one species to another via plasmids. The more we use antibiotics, the greater the selection pressure we exert on bacteria to evolve resistance to them.

By chance one or more Staphylococcus bacteria can acquire the

penicillinase enzyme

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Question: Suggest how each of the following might decrease the chances of an antibiotic strain of bacteria developing:

(a) limiting the use of antibiotics to cases where there is a real need.

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(b) regularly changing the type of antibiotic which is prescribed for a disease.

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The difference between the black and light speckled forms of the moth is caused by a single gene.

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The normal light, speckled colouring is produced by a recessive allele of this gene ‘c’, while the black colour is produced by a dominant allele, ‘C’.

Question: What are all the possible genotypes and phenotypes for moth colouring in Biston betularia?

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Question: What was the stabilising selection pressure acting on this moth to maintain a higher frequency of the light, speckled colouring pre-1849?

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Question: What was the directional selection pressure post 1849 to lower the mean average number of light, speckled moths and increase the mean average number of dark moths in industrial areas?

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Question: In the 1970s with the imposition of clean air laws, the concentration of atmospheric sulphur dioxide started to decrease dramatically favouring the growth of light grey, green and brown algae. What do you think happened?

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The C allele has probably been present in Biston betularia population for a very long time. Until the nineteenth century it remained exceedingly rare. Mutations of the c allele to C allele may have occurred quite frequently, but moths with this allele would have been more readily eaten by birds keeping the population frequency of this gene at a low level.

Sickle cell anaemia

The possession of two copies of the sickle cell allele puts a person at a great selective disadvantage. People who are homozygous for the sickle cell allele are less likely to survive and reproduce. Until recently, almost everyone with sickle cell anaemia died before reaching reproductive age. Yet the frequency of the sickle cell allele is very high in some parts of the world. (parts of East Africa = 50% of babies born are carriers and 14% are homozygous, suffering from sickle cell anaemia).

Reason: These parts of the world have malaria. This is caused by the protoctist parasite, Plasmodium, which can be introduced into a person’s blood when an infected mosquito bites. The parasites enter the red blood cells and multiply inside them. Malaria is the major source of illness and death in many parts of the world.

Now the Plasmodium parasites reproduce inside red blood cells. The allele for sickle cell anaemia is co-dominant to the allele producing normal β polypeptide in the haemoglobin, so the heterozygous genotype will express for a normal phenotype.

However, the fact that sickle cell allele can express itself, somehow affects the ability of the parasite to reproduce inside red blood cells.

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Question: Which genotype would be of selective advantage in two ways to an individual with respect to these alleles in areas where malaria is endemic? Would both these selection pressures exist in areas where malaria was not endemic? Explain appropriately.

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Evidence: Heterozygous people with malaria only have about one third the number of Plasmodium in their blood as normal homozygotes. In one study, a sample of 100 children who died from malaria, all except one were normal homozygotes, although within the population as a whole 20% of people were homozygotes.

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So both H and H alleles remain in the population where malaria is an important environmental factor. In places where malaria was never present, selection against people with the heterozygous genotype has almost completely removed the sickle cell allele from the population.

Question: If drugs were totally effective against malaria in endemic areas, would they constitute stabilising or directional selection pressure?

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Artificial selection

Question: complete using the words that follow:

Sometimes, the most important selection pressures on organisms are those applied by humans. When humans purposefully apply selection pressures to populations, the process is known as _________ selection

Thousands of years ago, people tried to improve their cattle. Desired features included ________ (making the animal easier to control), fast growth rates and high milk yields. These original wild cattle from which individuals were first domesticated are thought to have looked like the modern __________ White breed. Individuals showing one or more of the desired characteristics were chosen for breeding with each other. Some of the _______ conferring the features were passed onto their offspring.

Again the ‘best’ animals from this generation were chosen for breeding. Over many generations, alleles conferring the desired characteristics increased in frequency, while those conferring characteristics not desired by the breeder decreased in

_________, often losing ‘disadvantageous’ alleles totally. In this way many different breeds have been produced, such as Guernsey. _________ cows have much larger udders than the Chillingham White breed and produce large quantities of fat-rich ____.

Possible words: Chillingham, Guernsey, docility, alleles, frequency, milk, artificial

The Darwin-Wallace theory of evolution by natural selection

Charles Darwin and Alfred Russel Wallace 1856 said that nature selects only the best adapted organisms to survive. These then pass on their features onto offspring resulting in gradually changing features with a gradually changing environment.

When features of organisms change in this way, we say that they are evolving.

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Question: Darwin and Wallace collected a lot of evidence before they came to this conclusion. For each of the following observations, explain how it helped them to formulate their theory of natural selection as being the mechanism of evolution.

(1) Organisms produce more offspring than are needed to replace their parents.

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(2) Natural populations tend to remain stable in size over long periods.

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(3) There is variation among the individuals of a given species.

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Nowadays we understand that natural selection is acting on phenotypes (features) and alleles or groups of alleles. Darwin’s ideas were recorded in his book On the Origin of Species. However, he did not explain how new species could be produced, a process called Speciation.

Species and speciation

Can natural selection result in speciation?

A species is a group of organisms, with similar morphological, physiological, biochemical and behavioural features which can interbreed to produce fertile offspring and are reproductively isolated from other species.

In other words, members of a species have the same structural (morphological) and internal working mechanisms (physiological). If they reproduce sexually, they can only do so with members of the same species.

Biologists believe that natural selection is the force that changes allele frequencies by natural selection and in doing so ultimately results in new species evolving. The previous examples (antibiotic resistance in bacteria / wing colour in peppard moths) did not show this. However, it is believed that by a process of gradual change in two groups of the same species, significant differences between them can evolve which results in separate independent speciation.

Question: Donkey sperm can penetrate horse sperm producing offspring called mules.

Can donkeys and horses be considered the same species? Explain fully.

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What happens if it is difficult to show whether or not two individuals can breed and therefore belong to the same species?

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Reasons:

(1) Perhaps the organisms are dead, museum specimens or fossils.

(2) Perhaps the individuals are of the same sex.

(3) Perhaps there is not the time to interbreed them, or they will not breed in captivity.

(4) Perhaps they breed asexually.

(5) Perhaps they are immature and unable to breed.

Morphological (structural) differences and DNA sequencing can be rapidly determined compared to physiological, biochemical and behavioural differences.

Even this is sometimes not clear cut. Ideally, to show that natural selection leads to differences in speciation, two populations of the same species need to be

geographically separated and then subject to different environmental conditions for years. For example, the Hawaiian and Galapagos islands are famous for their spectacular array of species of all kinds of animals and plants found nowhere else in the world.

Allopatric speciation

Question: Geographic isolation requires a barrier of some kind between two populations of the same species. Give three examples.

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Geographic separation can be caused by:

(a) Large stretches of water: A species of bird blown hundreds of miles off course from mainland America to the Hawaiian Islands would be subject to very different selection pressures.

(b) Dense forest: If large tracks of forest are cut down for agricultural land, populations of a species could become geographically isolated in ‘islands remaining forest’.

(c) Mountain ranges: over millions of years these can separate two populations of the same species.

Speciation which happens like this, when two species are separated from each other geographically is called allopatric speciation.

Complete the text with the words that follow:

Rationale: Directional selection pressure in two __________ locations may be different. Whole collections of ______ may be lost from one population and yet retained in the other. Similarly, the frequency of new alleles in one population may increase relative to the other due to ________ selection pressure in that environment.

Wide scale changes in ________, biochemistry and physiology can occur in the two populations. This may result in ______ from one population being unable to

________ eggs from the other population.

Possible words: fertilise, morphology, alleles, directional, geographic

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Sympatric speciation

New species can arise without the original populations being separated by a geographic barrier. This is known as sympatric speciation.

Question: What problems might triploid or tetraploid organism have during meiosis?

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Question: What would be produced at fertilisation if (a) two diploid gametes fuse (b) a diploid and a haploid gamete fuses?

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Question: Explain why diploid multicellular organisms can exist and produce offspring along with the tetraploid organisms that might evolve from them. Why must they be of a different species?

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Thus, by sympatric speciation producing polyploidy in offspring, a new species can evolve in just one generation.

The kind of polyploidy described here contained four sets of chromosomes all from the same species. It is said to be autopolyploid (‘auto’ means self). Polyploids can also be formed containing two sets of chromosomes from one species and two sets from another closely related species. These are called allopolyploids (‘allo’ means

‘other’ or ‘different’). Look carefully at the diagrams on page 18.

Question: Can teraploid multicellular organisms which are allopolyploids and therefore able to produce diploid gametes without any problems, able to breed with

‘normal’ gametes from their original diploid parents i.e haploid? Explain as fully as you can.

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One well documented instance of speciation through allopolyploidy is the cord grass Spartina anglica. This is a vigorous grass that grows in salt marshes.

Question: Complete using words that follow and then annotate the diagram on page 18 using Spartina anglica as an example.

Before 1830, the species of Spartina that grew in _____ marshes was Spartina martima. Then in 1829, a different species called Spartina alterniflora was imported from ________. Spartina martima and _______ ________ hybridised, producing a new species called Spartina townsendii. This is a ______ plant, with one set of chromosomes from Spartina maritima and one set from Spartina alterniflora. It is _______, because the two sets of chromosomes from its parents cannot pair up, so it cannot undergo _______ properly. Nor can it interbreed with either of its two parents, which is what makes it a different species.

Although it is sterile, it has been able to spread rapidly, reproducing ________ by producing long underground stems called rhizomes, from which new plants can grow.

Later, probably about 1892, faulty cell division in Spartina townsendii somehow produced cells with double the number of _________. A ________ plant was

produced, probably from the fusion of two diploid gametes from Spartina townsendii.

So this tetraploid plant has two sets of chromosomes that originally came from _______ _______, and two sets from Spartina alterniflora. It is an _________.

These chromosomes can pair up with each other, two and two during _______, so this tetraploid is fertile. It has been named Spartina anglica. It is now more vigorous than any of the other three species, and has spread so widely, that is has practically

replaced them in England.

Possible words: salt, sterile, America, Spartina alterniflora, meiosis, diploid, asexually, chromosomes, Spartina maritima, tetraploid, allotetraploid.

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Classification

Taxonomy is the study of the classification of organisms. We put organisms into groups which have similar features that result from common ancestral origins.

Similar features in different organisms that were derived from a common ancestor are called homologous features. An example is the limb bones of vertebrates.

Different species which share many homologous features show that they relatively more recently evolved from a common ancestor than if they had fewer features in common. We therefore put these species together into a group called a genus. Thus, we put horses, zebras and donkeys together in the genus Equus. They probably evolved from the same species a long time ago.

Question: Every species is given a two-word Latin name, called a binomial, made up of an organism’s genus, followed by that of its species. Latin is a universally

accepted language which avoids confusion that might arise from localised names in different countries. This system was first devised by Carl Linnaeus (1700s). Give examples below:

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Question: The diagram below shows the different taxonomic groups. Highlight in green all those to which the zebra belong.

Complete the following text using the words that follow:

This classification reflects the __________ history of the zebra and its relationship with other organisms. A common zebra is extremely closely related to all other common zebras, and all common zebras are believed to share a common ancestor.

The common zebra is closely related to all other organisms belonging to the genus ______ – they, too, almost certainly share a common ancestor, but this goes much further back than the ancestor giving rise to the species ______ _______. Going even further back, all of the organisms in the family _______ probably had a common ancestor, and long before that so, perhaps, did all the organisms in the order _________.

The evolutionary history of living organisms is known as _________ and the

biological classification system attempts to classify organisms according to what we think this history may be. It is sometimes difficult to define a species.

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Even fewer _________ similarities exist between groups higher up the classification ________ – the further back in time that groups split apart from one another. Mistakes can be made and have to be continually rectified!

Possible words: phylogeny, hierarchy, homologous, Equus, Equus burchelli, Equidae, Perissodactyla, evolutionary

The five kingdoms

In 1988, Margulis and Schwartz proposed that the living world be divided into five kingdoms. These are Prokaryotae, Protoctista, Fungi, Plantae and Animalia.

Prokaryotes have a single loop of DNA, sometimes referred to as a bacterial

chromosome, containing nucleic acid but no histones, and this is free in the cytoplasm rather than contained within a nucleus. They may be autotrophic or heterotrophic.

Question: The diagram above shows a eukaryotic cell which belongs to the kingdom Plantae.

(a) What structures does it contain that are present in all eukaryotic cells?

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(b) What structures does it contain that are specific to the Plantae?

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In 1988, Margulis and Schwartz proposed that the living world be divided into five kingdoms. These are Prokaryotae, Protoctista, Fungi, Plantae and Animalia.

Protoctists are simple eukaryote organisms.

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Many are unicellular, while others are filamentous (made up of chains of single cells joined end to end), colonial (for example made up of balls of cells) or thalloid (made up of sheets of cells, such as those found in the seaweed fronds). They may be autotrophic or heterotrophic. Most protoctists are aquatic or live in moist conditions such as the soil.

Question: What evidence shows that the Prokaryotae evolved before the Protoctista?

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Fungi are eukaryotic organisms that feed heterotrophically. They have cell walls, but unlike those of plants, their cell walls contain materials such as chitin, never cellulose.

Fungi are saprophytes or parasites. Some, such as yeasts, are unicellular. Others have bodies made of threads called hyphae, forming a mass called a mycelium.

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Question: What are the main differences between protoctista and fungi?

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Plants are multicellular eukaryotic organisms that feed by photosynthesis. They have cells with walls made of cellulose. Some cells contain chloroplasts, and many contain large sap-filled vacuoles. Mosses, liverworts and ferns belong in this kingdom, as well as conifers and flowering plants.

Question: What are the main differences between plants and fungi?

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Animals: are multicellular eukaryotic organisms that feed heterotrophically. Their cells do not have cell walls, and never contain chloroplasts or large sap-filled vacuoles.

Animals include relatively simple organisms such as jelly fish and flatworms as well as more complex ones such as arthropods and mammals.

Question: What are the main differences between plants and animals?

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Although this five-kingdom system is widely used, many biologists state that not all protoctista are related to each other. They were put into this group because they would not fit into any other. Also nowadays we realise that the Prokaryotae consists of two distinct groups of organisms – the true bacteria and the ‘ancient’ bacteria – that are probably as different from one another as plants are from animals. It is likely that one day this kingdom will be divided into two.

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Question: Complete the following table showing the main differences between the five kingdoms.

Feature Prokaryotae Protoctista Fungi Plantae Animalia Type of cells

Presence or absence of cell wall

Cell wall material if present

Unicellular or muticellular

Method of feeding

Example of species or genus in this kingdom