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Classification is the act of arranging organisms into groups based on their similarities and differences.
Makes it easier for scientists to identify and study them.


Taxonomic hierarchy

- similar organisms are first sorted into one of three large groups called domains. E.g. plants, animals and fungi are in the Eukarya domain.

- similar organisms are then sorted into slightly smaller groups called kingdoms. E.g. all animals are in the animal kingdom.

- similar organisms from that kingdom are sorted into a phylum.

- similar organisms from each phylum are grouped into a class...

- as you move down, there are fewer organisms in each group.

- hierarchy ends with species.


Dumb King Philip Came Over From Great Spain (Taxonomic hierarchy)



5 kingdoms of the 5 kingdom classification system

Prokaryote (bacteria)

Protoctista (algae, protozoa)

Fungi (mushrooms, yeast)





- phylogeny is the study of the evolutionary history of groups of organisms. Tells us who’s related to who and how closely.

- all organisms evolved from shared common ancestors.

- branch points on a phylogenic tree show common ancestors from which a different group diverged.

- closely related species diverged away from each other most recently.


Evidence showing how similar organisms are

Molecular evidence- similarities in proteins and DNA. More closely related organisms will have more similar molecules.

Embryological evidence- similarities in the early stages of an organisms development.

Anatomical evidence- similarities in the structure and function of different body parts.

Behavioural evidence- similarities in behaviour and social organisation of organisms.


Molecular evidence led to the proposal of the 3 domain system

- three domain system was proposed due to new evidence, mainly molecular.

- e.g. prokaryotae were reclassified into two domains because new evidence showed large differences between archaea and bacteria. Evidence included:

molecular evidence- enzyme RNA polymerase is different in bacteria and archaea.
Cell membrane evidence- bonds of the lipids in the cell membranes of bacteria and archaea are different.

- Most scientists now agree that archaea and bacteria evolved separately.



Variation is the differences that exist between individuals.


Intraspecific and interspecific variation

Intraspecific variation is variation within a species.

Interspecific variation is the variation between different species.


Continuous and discontinuous variation

Continuous variation is when individuals in a population vary within a range, with no distinct categories. E.g. height, mass of leaves, volume of milk yield.

Discontinuous variation is when there are two or more distinct categories. Each individual falls into only one of these, with no intermediates. For example: blood group, plant colour, antibiotic resistance.


Genetic factors causing variation

- different species have different genes.

- individuals of the same species have the same genes, but different versions called alleles.

- genes and alleles an organism has make up its genotype.

- differences in genotype result in variation in phenotype.

- you inherit genes from your parents. Means variation caused by genetic factors is inherited.


Environmental factors causing variation

- variation can be caused by differences in the environment. E.g. climate, food, lifestyle.

- characteristics controlled by environmental factors can change over an organisms life.

- examples of variation caused only by environmental factors include accents and whether people have pierced ears.


Genetic and environmental factors causing variation

Genetic factors determine the characteristics an organism’s born with, but environmental factors can influence how some characteristics develop.
E.g. height is determined by genes, but diet/nutritional availability affect how tall an organism actually grows.
Flagellum- genes determine if a microorganism can grow s flagellum, but some will only grow them in certain environments.


Can use mean to look for variation between samples

- to investigate variation, you normally take samples of a population.
- man is an average of the values collected in a sample. Can be used to tell if there’s variation between samples.
- most samples will include values either side of the mean, so you end up with a bell shaped graph. This is called a normal distribution.


Standard deviation tells you about variation within a sample

- standard deviation tells you how much the values in a single sample vary. Measure of the spread of values about the mean.

- large standard deviation means the values in the sample vary a lot. Small standard deviation tells you most of the sample is around the mean value, so varies little.



- being adapted to an environment means an organism has features that increases its chances of survival and reproduction, and also the chances of its offspring reproducing successfully.

- these features are adaptations and can be behavioural, physiological or anatomical.

- adaptations develop because of evolution by natural selection.

- in each generation, best adapted individuals are more likely to survive and reproduce, passing on their adaptions to their offspring.


Behavioural adaptations

Behavioural adaptations are ways an organism acts that increase its chance of survival. For example:

- possums sometimes play dead if they’re being threatened by a predator, thus increasing their chances of survival.
- scorpions dance before mating, masking sure they attract a mate of the same species, increasing likelihood of successful mating.


Physiological adaptations

Physiological adaptations are processes inside an organisms body that increase its chance of survival. For example:

- brown bears hibernate, lowering their rate of metabolism over winter, thus conserving energy, so they don’t need to look for food for months, increasing chances of survival.
- some bacteria produce antibiotics, killing other species of bacteria in the area, reducing competition.


Anatomical adaptations

Anatomical adaptations are structural features of an organisms body that increases its chance of survival. For example:

- otters have a streamlined shape, making it easier to glide through water, making it easier to catch prey.
- whales have a thick layer of blubber, keeping them warm in the cold sea.


Different taxonomic groups may have similar features

- organisms from different taxonomic groups may have similar features, despite not being closely related.

- usually because the organisms have evolved in similar environments to fill similar ecological niches.

- e.g. marsupial and placental moles.


Marsupial mammals

- have a short gestation period (pregnancy)

- don’t develop a full placenta

- born early in their development and climb into their mothers pouch. Here they become attached to a teat and receive milk while they continue to develop.


Placental mammals

- have a longer gestation period (pregnancy)

- develop a placenta during pregnancy, allowing for the exchange of nutrients and waste produces between the foetus and mother.

- born more fully developed.


Marsupial and placental moles

- marsupial and placental moles aren’t closely related. They evolved independently on different continents.

- they do share some anatomical features though (look alike), because they both evolved to live in similar environments:

- both types of moles live in tunnels in the ground and burrow to reach their food. Adaptations to this lifestyle include:

- smalle/nonexistent eyes
- no external ears, so streamlined head for burrowing
- scooped-shaped, powerful front paws, good for digging.
- claws specialised for digging.
- tube shaped body and cone shaped head, making it easier to push through soil.


Darwin’s 4 key observations

- organisms produce more offspring than survive.
- there’s variation in characteristics or members of the same species.
- some of these characteristics can be passed from one generation to the next.
- individuals best adapted to their environment are more likely to survive.


Darwin’s theory of evolution by natural selection

- individuals within a population show variation in their phenotypes.

- selection pressures (predation, disease, competition...) create a struggle for survival.

- individuals with better adaptations are more likely to survive and have reproductive success, passing on their advantageous genes.

- over time, proportion of population possessing the advantageous adaptions increases.

- over generations, this leads to evolution, as favourable adaptations become more common in the population.


Fossil record evidence to support evolution

- fossils are the remains of organisms preserved in rocks.
- by arranging fossils in chronological order, gradual changes in organisms can be observed that provide evidence of evolution.


DNA evidence to support evolution

- theory of evolution suggests all organisms evolved from shared common ancestors.
- closely related species diverged more recently.
- evolution is caused by gradual changes in the base sequence of an organisms’ DNA.
- organisms that diverged away from each other more recently, should have more similar DNA, as less time has passed for changes in the DNA sequence to occur.


Molecular evidence to support evolution

- in addition to DNA, similarity in other molecules provide evidence.
- scientists compre the sequence of amino acids in proteins, and compare antibodies.
- organisms that diverged away from each other more recently have more similar molecules, as less time has passed for changes in proteins and other molecules to occur.


Evolution of pesticide resistance can be explained by natural selection

- there’s variation in a population of insects. Genetic mutations create alleles that make some insects naturally resistant to a pesticide.
- if the insect population is only exposed to that pesticide, individuals with resistance will survive and reproduce.
- alleles which cause the pesticide resistance will be passed on to the next generation, so the population will evolve and more individuals will carry the allele than in the previous generation.


Evolution of pest resistance has implications for humans

Crop infestations with pesticide-resistant insects are header to control. Some insects are resistant to lists of different pesticides, so hard for farmers to kill specific pests.

If disease-carrying insects become pesticide-resistant, the spread of disease could increase.

A population of insects could evolve resistance to all pesticides in use. Takes time and money to develop new pesticides.


Evolution or drug resistance has implications for humans

- scientists have observed evolution of antibiotic resistance in many species of bacteria.

- other pathogens have evolved resistance to specific drugs too.

- infections caused by drug-resistant microorganisms are harder to treat, especially if the microorganism is resistant to lots of different drugs. Can take doctors a while to figure out which drug will get rid of the infection.

- could come a point where a pathogen becomes resistant to all the drugs we currently use against it and it’s difficult/costly to develop new drugs.