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Summary Natural selection AS biology
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Theories of Natural selection ways of classifying organsims how meiosis introduces genetic variation
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Natural SelectionA mutation that causes a change will not change what the gene is, but it becomes a different allele.
Phenotypes are the measurable state or characteristic which is expressed by an organism due to an
allele or combination of alleles. Genetic diversity is the total number of different alleles in a
population. It can increase due to mutations or immigration of individuals into the population.
Species richness is the number of different species in an area. Species diversity or the index of
N ( N−1)
diversity takes into account number of organisms and is calculated by d= . N is the total
Σ(n ( n−1 ))
number of organisms of all species and n is number of organisms in each species. 3 is a low index of
diversity and 7 is high. A low index indicates an abiotic factor such as pollution. A high index
indicates a biotic factor such as competition, predator-prey relationships, disease, and parasites.
Crop farming creates a monoculture in the land which reduces species diversity. Pesticides and
herbicides kill other organisms, and animals die to fewer niches. A niche is a unique role filled by an
organism within a habitat.
Directional selection causes a shift in the
mean phenotype of the population. An
example of this is antibiotic resistance,
where a bacterium undergoes a random
mutation which causes it to produce a
protein which breaks down antibiotics. When the population is exposed to antibiotics it is more likely
to survive than the others and the antibiotic-resistant population becomes the most common.
Stabilising selection happens when conditions remain stable, and organisms with extreme features
are more likely to die than those that are already the majority population. An example is the heights
of plants. Too small means they will be outcompeted and too big means they are unstable and there
are not enough resources for them to live.
In disruptive selection an event occurs which causes the mean phenotypes to no longer be
advantageous. Organisms naturally selected are ones other than the mean. Different organisms
have different mutations and find different solutions which leads to divergent evolution of more
than one different new population. An example of this is a population of white rabbits in the snow.
If the snow melts they will no longer be camouflaged to predators. Two different populations of grey
and black rabbits would be camouflaged on rocks and would survive, but the white rabbits would be
killed. Another example is organisms living in rocky areas and their size. They are not round the
mean but some are big as they can hide under the rocks, while some are big and can fight predators.
Those of mean size couldn’t do either and would die.
Classification is the process if arranging organisms into groups of various size based on shared
features. Taxonomy focuses on physical features for ease of identification and naming. A hierarchy
is a system in which members of an organisation or society are ranked acceding to relative status
which contains groups within groups called taxa. The groups on the same level are non-
overlapping. To classify organisms in a hierarchy we use, from big to small, Domain, Kingdom,
Phylum, Class, Order, Family, Genus, and then Species. When naming a species you write the
binomial name: The genus with a capital letter the species name with a lower case letter. It should
be written in italics or underlined and, if there is a sub-species name, it should be added at the end.
Starting from kingdom, a human’s full classification is Animal, Chordata, Mammalia, Primates,
Hominids, Homo, Sapiens. Our binomial name is Homo sapiens. Chordata is the phylum of spinal
cords. Dichotomous keys can be used to identify species. They have a set of decisions where there
are two options about the characteristics and you choose the one that best fits the organisms. It can
sometimes be difficult to accurately make a decision and identify the species.
Phylogeny means arranging species into groups based on evolutionary origins so every group shares
a common ancestor. The biological species concept is the definition of a species as ‘a set of
individuals who can reproduce to produce fertile offspring’. The weaknesses of this are that it
doesn’t account for hybrids or organisms which reproduce asexually. Some chains of species exist
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