Classification & Evolution (OCR A Level Biology): Flashcards

The genus (first word) and the species (second word).

So the Latin name automatically tells you the last two taxonomic ranks the organism belongs to.

All eukaryotes, including the full taxonomic hierarchy of kingdom, phylum, class, order, family, genus and species.

Eukarya contains all eukaryotes, whereas Bacteria and Archaea are both prokaryotic.

Eukarya is made up of eukaryotes (cells with a nucleus), whereas Bacteria and Archaea are prokaryotes (no nucleus).

True

False — Bacteria and Archaea are both prokaryotic; only Eukarya is eukaryotic.

A group of organisms that are able to reproduce to produce fertile offspring.

The two-part scientific name given to an individual species, consisting of the organism's genus and species name in modern Latin.

• The genus name\n- The species name

It allows species to be universally identified.\n\nThe binomial for a species is the same across the entire globe, avoiding confusion caused by differing names.

• Common names often differ between countries\n- They do not always translate directly between different languages\n\nThis can cause confusion about which group of organisms is being discussed.

The Swedish scientist Carl Linnaeus, in the 18th Century.

• Always italicised in writing (or underlined if it is not possible to italicise, e.g. when handwritten)\n- The genus name has a capital letter\n- The species name does not have a capital letter

• Homo = genus name (capitalised)\n- sapiens = species name (lower case)

The genus name is abbreviated to its first letter (capitalised) followed by a full stop.\n\nFor example, Saccharomyces cerevisiae becomes S. cerevisiae.

They belong to the same genus (Saccharomyces) but are different species (cerevisiae and paradoxus).

The binomial naming convention was developed by the Swedish scientist Linnaeus in the 18th Century.

In a binomial name, the genus name is written with a capital letter while the species name is written in lower case.

In modern Latin.

False — a binomial is written in modern Latin.

True

The practice of biological classification, which involves placing organisms into a series of categories or taxa.

The categories into which organisms are placed within the hierarchical classification system (singular: taxon).

The domain.

• Bacteria (prokaryotes)

• Archaea (prokaryotes)

• Eukarya (eukaryotes)

Prokaryotic cells: lack a nucleus.

Eukaryotic cells: have compartmentalised structures, with at least their genetic material segregated from the rest of the cell in a nucleus.

Advances in molecular biology, biochemistry and cell structure studies showed that prokaryotes are not one uniform group.

Based on molecular analyses (particularly of RNA genes, rRNA, protein synthesis, cell membranes and flagella), prokaryotes were found to divide into two separate domains: Bacteria and Archaea.

• Have prokaryotic cells with no nucleus

• Sometimes called extremophile prokaryotes, as they were first discovered in extreme environments

• Membranes contain unique lipids

• Cell walls contain no peptidoglycan

• Ribosomal structure and DNA transcription are more similar to eukaryotes than to bacteria

• Example: Halobacterium salinarum

• Have prokaryotic cells containing no nucleus

• Cell walls always contain peptidoglycan

• Divide by binary fission

• Example: Staphylococcus pneumoniae, which causes pneumonia

• Have eukaryotic cells with nuclei and membrane-bound organelles

• Vary massively in size, from single-celled organisms to large multicellular organisms

• Divide by mitosis

• Can reproduce sexually or asexually

• Example: Canis lupus (wolves)

• Membrane lipids

• Ribosomal RNA

• Cell wall composition

Archaea: branched hydrocarbon chains bonded to glycerol by ether linkages (these lipids are unique and not found in bacterial or eukaryotic cells).

Bacteria: unbranched hydrocarbon chains bonded to glycerol by ester linkages.

Both Archaea and Bacteria have 70S ribosomes, but in Archaea:

• The base sequences of rRNA are more similar to those of Eukarya than Bacteria

• The smaller ribosomal subunit and the primary structure of ribosome proteins are more similar to Eukarya than Bacteria

Organisms from the Bacteria domain always possess cell walls containing peptidoglycan, whereas the cell walls of Archaea do not.

False — although both are prokaryotic, molecular differences place them in two separate domains.

True

• Prokaryota

• Protoctista

• Fungi

• Plantae

• Animalia

• Protoctista

• Fungi

• Plantae

• Animalia

• Cells have membrane-bound nuclei separating genetic material from the cytoplasm

• Compartmentalisation within cells due to the presence of other membrane-bound organelles

• Most are unicellular (some form filaments or colonies)

• Cells have cell walls (not made of cellulose) and cytoplasm

• No nucleus and no mitochondria

• Cells divide by binary fission

• Blue-green bacteria and some bacteria are autotrophic (photosynthetic)

• Many bacteria are heterotrophic, feeding by decomposing living or dead organic materials

• All are eukaryotic

• Encompasses all eukaryotic organisms that do not belong to the other three eukaryotic kingdoms (Fungi, Plantae, Animalia)

• They can exist as single-celled organisms or as a group of similar cells

Protozoa (animal-like):

• Cells similar to animal cells

• No cell wall

Algae (plant-like):

• Cells similar to plant cells

• Have cellulose cell walls and chloroplasts

• Eukaryotic cells

• Possess non-cellulose cell walls, often made of chitin

• Do not have cilia

Long threads that grow from the main body of a fungus and form a network of filaments called the mycelium.

Using spores. Larger fungi possess fruiting bodies that release large numbers of spores, which disperse onto the ground nearby.

• Multicellular eukaryotic organisms

• All cells have cellulose cell walls

• Large, usually permanent vacuoles for structural support

• Can differentiate into specialised cells forming tissues and organs

• Possess chloroplasts for photosynthesis (not all cells have them)

• Are autotrophs

• Multicellular eukaryotic organisms

• Differentiate into many specialised cell types forming tissues and organs

• Have small, temporary vacuoles

• Have no cell walls

• Sometimes have cilia

• Are heterotrophs with a wide range of feeding mechanisms

Plants and some prokaryotes are autotrophs, meaning they can synthesise their own organic compounds from inorganic compounds.

Chitin is the polysaccharide that often makes up the non-cellulose cell walls of fungi.

• Are heterotrophs

• Digest dead or decaying matter extracellularly, or act as parasites on living organisms

False — prokaryotic cells have no nucleus and no mitochondria.

False — fungi are heterotrophs that digest dead or decaying matter or live as parasites.

The evolutionary history of organisms.

On the basis of shared visible features.

Species that share a more recent common ancestor are classified together.

Species with a more distant common ancestor are classified in separate groups.

Grouping by shared visible characteristics can place organisms together that are not close evolutionary relatives.

For example, birds and bats might be grouped together because both have wings, but they do not share a recent common ancestor.

Advances in DNA, RNA and protein sequencing.

Molecular analysis allows scientists to build phylogenetic trees showing the relationships between organisms.

• DNA

• mRNA

• Amino acids (of a protein)

• It must be present in a wide range of organisms

• It must show sufficient variation between species

It is an integral protein to respiration (used in the electron transport chain) and is found in all eukaryotic organisms, so it can be compared across a wide range of species.

The more similar the sequences, the more closely related the species are.

Species with very similar sequences separated into separate species more recently than species with less similar sequences.

They have had more time to accumulate mutations in their DNA, mRNA and amino acid sequences.

• Extract DNA from the nuclei of cells (e.g. from blood, skin or fossils)

• Process and analyse the DNA to obtain its base sequence

• Compare the base sequence to that of other organisms

• The more similar the base sequences, the more closely related the species are

• Differences may lie in introns, which are not translated into the protein

• The genetic code is degenerate, so more than one codon can code for the same amino acid

Looking at multiple proteins or multiple regions of the genome allows a more accurate estimate of evolutionary relatedness.

Molecular analysis allows scientists to build phylogenetic tree diagrams that show the evolutionary relationships between organisms.

On the basis of their phylogeny (evolutionary history).

True

False — they do not share a recent common ancestor, so shared wings are not evidence of close relatedness.

The general theory of evolution refers to the way in which species have changed over time and continue to change.\n\nThe theory of evolution by natural selection refers to the specific process by which evolution occurs.

Charles Darwin and Alfred Russel Wallace.

• All organisms produce more offspring than could ever actually survive\n\n- Populations of organisms fluctuate but not significantly (numbers usually stay fairly constant)\n\n- Individuals of the same species show variation in characteristics (intraspecific variation)\n\n- Offspring inherit characteristics from their parents

Variation in characteristics between individuals of the same species.

• Fossil evidence\n\n- Molecular evidence

Preserved remains of organisms, or other features left by organisms, such as footprints, burrows and faeces.

Fossils and their rocks can be dated, allowing organisms to be placed into a sequence from oldest to youngest.\n\nThis shows how organisms have changed through evolutionary time.

Fossils show similarities between extinct species (how related they are), and between now-extinct ancestral species and present-day species.\n\nThis provides evidence for the gradual change from simple life forms (e.g. Archaea and Bacteria) to complex eukaryotic life forms.

DNA from the nucleus can be sequenced to provide evidence of evolutionary relationships between species, and to show how the genetic code of species has changed as they have evolved.

The more similar the nucleotide sequence, the more closely related the species are.\n\nTwo groups with very similar DNA separated into distinct species more recently than two groups with less similarity in their DNA.

DNA sequence analysis and comparison can be used to create phylogenetic trees that show the evolutionary relationships between species.

Molecular evidence has shown that characteristics are passed on to offspring in genes.

In 1858.

True

False — the more similar the sequences, the more closely related the species are.

The differences that exist between living organisms.

• Between different species or within a single species\n\n- Continuous or discontinuous\n\n- Caused by genetic and/or environmental factors

Variation that exists between individuals of different species.

Variation that exists between individuals of the same species.

Variation within a species allows natural selection to occur.

When species have such similar phenotypes that they are very difficult to distinguish, genetic variation must be used to classify them.

Variation showing differences that fall into discrete, distinguishable categories with no intermediates (e.g. ABO blood groups).

Variation showing a range of values that can fall anywhere between two extremes (e.g. body mass and height).

• Discontinuous variation: a bar chart with bars that are clearly distinct from each other\n\n- Continuous variation: a histogram with bars that touch, often showing a bell-shaped curve

It is caused solely by genetic factors, so phenotype = genotype; the environment has no direct effect.

It is caused by an interaction between genetics and the environment, so phenotype = genotype + environment.

When a large number of genes have a combined effect on the phenotype, they are known as polygenes.

Only alterations to the genetic component of gametes can be inherited; changes caused by the environment alone do not affect the DNA passed to offspring.

• Scarring on the body following an accident\n\n- Weight gain from eating too much and an inactive lifestyle\n\n- Speaking a certain language or accent from being raised in a particular country\n\n- Genetically identical plant clones growing to different heights in different conditions

True

False — continuous variation is shown on a histogram with touching bars; a bar chart is used for discontinuous variation.

False — interspecific variation is between different species; variation within the same species is intraspecific.

The value usually meant when the term "average" is used in biology.

Mean = sum of all measurements ÷ number of measurements

It is sometimes represented by the symbol X̄.

A measure of the spread of data around the mean value.

An unusually high or low value in a data set.

Outliers can make the mean too high or too low to reflect the true patterns in the data.

The mean tells you the average value, but the standard deviation tells you how spread out the data are around that mean.

Together they give a fuller picture of the data than the mean alone.

One or two outliers can make the mean too high or too low, so that it no longer reflects the real patterns in the data.

It is very useful for comparing the consistency between different data sets, because it shows how much the values vary around the mean.

The mean must be calculated before working out the standard deviation.

• Calculate the mean of the data

• Find the difference between each value and the mean

• Square each difference

• Total (sum) the squared differences

• Divide the total by (n − 1), where n is the number of values

• Take the square root of this result to give the standard deviation

You divide by (n − 1), where n is the number of measurements in the data set.

Take the square root of that value.

The result is the standard deviation.

Mean = sum of measurements ÷ number of measurements

Mean = 885 ÷ 15 = 59 mm

A table helps you keep track of all the calculations (the differences from the mean and the squared differences), so you are less likely to make an error.

True

False — outliers can make the mean too high or too low to reflect the true patterns in the data.

A statistical test used to compare the means of two sets of data and determine whether they are significantly different.

• The data must follow a rough normal distribution

• The data must be continuous

• The standard deviations should be approximately equal

The standard deviation (s) of each data set.

A statement of what we would expect if there is no significant difference between two means, i.e. that any differences seen are due to chance.

In the exam the formula for the t-test will be provided, but the formula for calculating the number of degrees of freedom is not provided and must be learnt.

v = (n₁ − 1) + (n₂ − 1)

where n₁ and n₂ are the number of observations in each sample.

(Remember: this formula is not given in the exam.)

The greater the t value calculated (for any degree of freedom), the lower the probability that chance caused the difference between the two sample means.

• The difference between the two means is statistically significant

• There is a less than 5% probability that the difference is due to chance

• The null hypothesis is rejected

• There is no significant difference between the two means

• The probability that the difference is due to chance is higher than 5%

• The null hypothesis is accepted

A probability level of 0.05 (the 95% confidence level), i.e. a 5% cut-off for the probability that a difference is due to chance.

You must state that the difference between the data sets is significant (or not) — not simply that "the data is significant".

If there is a statistically significant difference between the means, the observation is not down to chance and the null hypothesis can be rejected.

True

False — the null hypothesis is rejected, because the difference between the means is statistically significant.

The t-test determines whether the means of two data sets are significantly different from one another.

The statement that there is no significant difference between the two sets of data (e.g. no significant difference between the ear lengths of populations A and B); any difference observed is due to chance.

Sum all the values in the data set and divide by the sample size.\n\nPopulation A: x̄₁ = 885 ÷ 15 = 59 mm\n\nPopulation B: x̄₂ = 870 ÷ 15 = 58 mm

Using the formula:\n\n\n\n- Find the difference between each value and the mean, (x − x̄)\n\n- Square each difference and sum them to get ∑(x − x̄)²\n\n- Divide by (n − 1), then take the square root

Population A: ∑(x − x̄)² = 36, so s₁ = √(36 ÷ 14) = √2.57 = 1.60\n\nPopulation B: ∑(x − x̄)² = 22, so s₂ = √(22 ÷ 14) = √1.57 = 1.25

v = (n₁ − 1) + (n₂ − 1)\n\nIn the worked example: v = 14 + 14 = 28

A value calculated from the sample sizes, using v = (n₁ − 1) + (n₂ − 1), which determines the row used when reading the critical values table.

A probability level of 0.05 (5%) is used. The calculated t value is compared with the critical value at p = 0.05.

The t value (1.90) is less than the critical value (2.05).\n\nThis means there is no significant difference between the means of the two data sets.

• A reference to the t value, the critical value, the degrees of freedom and the probability level\n\n- A statement of whether or not there is a significant difference between the means of the two data sets\n\n- Whether the null hypothesis is accepted or rejected

The t value of 1.90 is less than the critical value of 2.05 at 28 degrees of freedom and a probability level of 0.05.\n\nTherefore there is no significant difference between the data sets, and we accept the null hypothesis. Any difference between the mean ear lengths of populations A and B is due to chance.

If the calculated t value is less than the critical value, the null hypothesis is accepted.

Degrees of freedom is calculated using v = (n₁ − 1) + (n₂ − 1) and is used to read the critical values table.

We accept a less than 5% probability that any difference between the two data sets is due to chance.

True

False — the null hypothesis states there is no significant difference between the two data sets.

A statistical test that determines whether there is a correlation between two variables when the data do not show a normal distribution.

When investigating a possible correlation between two variables whose data are not normally distributed.

• Plot a scatter graph and identify possible linear correlation\n\n- State a null hypothesis\n\n- Use the equation to calculate the Spearman's rank correlation coefficient, rs\n\n- Compare the calculated value with the critical value from a table at the 0.05 probability level

• rs = Spearman's rank correlation coefficient\n\n- D = difference in rank\n\n- n = number of samples

That there is no correlation between the two variables being investigated.

The smallest data value is given rank 1.

Compare the calculated rs with the critical value for the number of samples (n) at the 0.05 probability level.\n\nIf the calculated value is greater than the critical value, the null hypothesis can be rejected, meaning there is a correlation between the two variables.

The 0.05 probability level.

If the calculated value of rs is greater than the critical value, the null hypothesis can be rejected.

Spearman's rank correlation is used when data are not normally distributed.

Because correlation does not always mean causation - a correlation between two variables does not prove that one causes the other.

No - the formula is provided in the exam. You must still be able to carry out the calculation and know when the test is appropriate to use.

False - it is used when data are not normally distributed.

True

A characteristic that aids an organism's survival in its environment.

• Anatomical\n- Physiological\n- Behavioural

A physical feature of an organism that aids its survival.\n\nE.g. the white fur of a polar bear provides camouflage in the snow, so it is less likely to be detected by prey.

A biological process within an organism that aids its survival.\n\nE.g. mosquitoes produce chemicals that stop the host's blood from clotting when they bite, allowing them to feed more easily.

The way an organism behaves in order to aid its survival.\n\nE.g. reptiles bask in the sun to absorb heat.

An anatomical adaptation, because it is a physical feature of the organism.\n\nIt provides camouflage in the snow, so the polar bear is less likely to be detected by prey.

A behavioural adaptation, because it relates to the way the organism behaves.

Mosquitoes producing chemicals that stop the host's blood clotting is an example of a physiological adaptation.

The process by which unrelated organisms from different taxonomic groups develop similar adaptations, despite not sharing a recent common ancestor.

Because of convergent evolution.\n\nThe shared adaptations arise by natural selection when unrelated species face the same selection pressures in similar environments.

• Two species live in different parts of the world with similar environments\n\n- The species experience the same selection pressures\n\n- The same characteristics are advantageous in both environments, so individuals with these characteristics are more likely to survive and reproduce\n\n- Over time, the advantageous characteristics become widespread in both populations

Convergent evolution occurs when unrelated species facing the same selection pressures develop similar adaptations.

False — convergent evolution occurs in unrelated species that do not share a recent common ancestor.

True

The differences in DNA base sequences between individual organisms within a species population.

The ability of an organism to survive and pass on its alleles to its offspring.

An environmental factor that affects an organism's chance of survival, increasing the chance of individuals with a more advantageous phenotype surviving and reproducing over others.

There needs to be some level of genetic diversity within a population for natural selection to occur.

Differences in the alleles possessed by individuals result in differences in phenotypes, some of which may be advantageous.

Differences in the alleles possessed by individuals within a population result in differences in phenotypes.

These phenotypes may be advantageous, disadvantageous or neutral compared with others.

They possess adaptations that make them better suited to their environment, increasing their chance of surviving and passing on their alleles.

• A population with a large gene pool (high genetic diversity) has a strong ability to adapt to change

• A population with a small gene pool (very low genetic diversity) is much less able to adapt to environmental changes and can become vulnerable to extinction

Low genetic variation means the species is less likely to be able to respond (survive) in the event of any environmental changes, making it more vulnerable to extinction.

When there are no environmental factors or population checks acting on the population — for example, plentiful resources, no disease, no competition and no natural predators.

If all the offspring of every individual survived to adulthood and reproduced, the population would grow exponentially.

• Random mutation produces new alleles of a gene

• Under certain environmental conditions, a new allele may benefit its possessor, leading to an increased chance of survival and reproductive success

• The advantageous allele is passed on to the next generation

• Over several generations, the new allele increases in frequency in the population

Random mutation can produce new alleles of a gene, some of which may benefit their possessor.

• Variation in fur colour exists: brown fur (dominant allele) and white fur (recessive allele)

• Predators such as foxes act as a selection pressure

• White rabbits camouflage less well, so are more likely to be seen and eaten; brown rabbits have a selection advantage

• Brown rabbits are more likely to survive to reproductive age and pass on their alleles

• Over many generations, the frequency of the brown fur allele increases and that of the white fur allele decreases

A new species may evolve, as the isolated population is unable to interbreed with the original population.

True

False — a small gene pool means low genetic diversity, so the population is less able to adapt and is more vulnerable to extinction.

A chemical substance that inhibits or kills bacterial cells with little or no harm to human tissue, used to aid the body's immune system in fighting a bacterial infection.

Any environmental factor that influences the survival of a particular species and so drives natural selection in that species (e.g. an antibiotic or an insecticide).

• There is variation within the bacterial population caused by random mutations

• A chance mutation makes some bacteria resistant to the antibiotic

• When the antibiotic is applied, the non-resistant bacteria die but the resistant bacteria survive

• The survivors reproduce with less competition, passing on the resistance genes at a much greater frequency

• Over generations the whole population becomes antibiotic-resistant — evolution by natural selection

A mutation can change an existing gene so it gives rise to a nucleotide sequence that codes for a slightly different protein that is not affected by the antibiotic being used.

They have acquired genes that code for the enzyme β-lactamase (also known as penicillinase), which breaks down penicillin.

Because bacteria only have one copy of each gene, so there is no second allele to mask the mutation — the mutant gene is expressed straight away.

• Bacteria reproduce asexually by binary fission, each daughter cell receiving a copy of the chromosome

• This happens very rapidly (on average every 20 minutes)

• If one bacterium has a resistance gene, all of its descendants inherit it

• This spreads resistance within a single bacterial population

• Plasmids (small rings of DNA) often carry antibiotic-resistance genes

• Plasmids are transferred between bacteria during conjugation, when a thin tube forms between two bacteria to exchange DNA

• This can pass resistance to other bacteria, even of a different species

• It spreads resistance within or between bacterial populations, producing multi-resistant 'superbugs' such as MRSA

The exchange of DNA between two bacteria during horizontal transmission occurs during a process called conjugation.

Horizontal transmission can spread antibiotic resistance within a population, between populations of the same species, or occasionally between different species.

Through the overuse of antibiotics when not necessary and their incorrect use, for example:

• Treating non-serious infections

• Routine treatment of animals in agriculture

• Failure to finish the prescribed course of antibiotics

• Only prescribe antibiotics when absolutely necessary

• Ensure patients complete their full course of antibiotics

• Rotate which antibiotics are used so one type is not used continuously

• Hold back some antibiotics as a 'last resort'

• Invest more in research into new antibiotics

A chemical that kills pests of any kind, including insect pests, pathogenic organisms or weeds. Types include insecticides, herbicides, fungicides, molluscicides and rodenticides.

Insecticides sprayed on crops act as a selective agent. Any insect with a chance mutation making it resistant to the insecticide will survive and reproduce, passing on the resistance gene. Over generations the resistant allele becomes more common in the population — natural selection.

False — vertical transmission only spreads resistance within a single bacterial population; horizontal transmission can spread it between species.

True

A strain of bacteria that has become resistant to several different antibiotics (multiple resistance), making the infections it causes very difficult to treat.

Methicillin-resistant Staphylococcus aureus — a strain of Staphylococcus aureus that has developed resistance to methicillin and other antibiotics (e.g. penicillin). It is the most common example of a superbug.

When bacteria carry plasmids containing resistance genes for several different antibiotics, so they are resistant to more than one antibiotic at the same time.

The frequent overuse of antibiotics exerts a selective pressure on bacteria.

This favours the survival and reproduction of resistant individuals, driving the evolution of antibiotic resistance.

• Overuse of antibiotics, including antibiotics being prescribed when they are not necessary

• Large-scale use of antibiotics in farming to prevent disease in livestock kept in close quarters, even when the animals are not sick

The resistance can be passed on to a pathogenic species by horizontal transmission (transfer of resistance genes, e.g. on plasmids, between bacteria).

So that all the bacteria are killed and none are left alive to mutate into resistant strains.

Antibiotics have no effect on viruses, so prescribing them provides no benefit.

It also gives bacteria an unnecessary opportunity to be exposed to antibiotics and develop resistance.

• Avoiding the overuse of antibiotics and only prescribing them when absolutely necessary

• Testing the bacteria first to prescribe the correct antibiotic

• Using narrow-spectrum antibiotics that are highly specific to the infection rather than wide-spectrum ones

• Changing the type of antibiotic prescribed for a given infection so the same one is not always used, reducing the chance of a resistant strain developing

• Not using antibiotics for non-serious infections the immune system will clear, or for viral infections

• Ensuring good hygiene practices, such as handwashing and the use of hand sanitisers

• Isolating infected patients to prevent spread, especially on surgical wards where MRSA can infect surgical wounds

Reducing the use of wide-spectrum antibiotics and instead using narrow-spectrum antibiotics, which are highly specific to the infection, helps limit the development of resistance.

It threatens the security of future food supplies, as resistant pests can damage crops.

This is especially serious for human populations that already face food shortages.

• Using insecticides sparingly or on rotation

• Using a combination of pesticides to delay the emergence and spread of resistance

• Using biological control (introducing a natural parasite or predator of the pest)

• Using crops that have been selectively bred or genetically modified to be pest-resistant

By using antibiotics frequently, humans exert a selective pressure on bacteria, which supports the evolution of antibiotic resistance.

False — antibiotics have no effect on viruses, so using them only gives bacteria an unnecessary chance to develop resistance.

True