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Summary A Level Biology
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- AQA
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Genetic diversity and adaptations:Genetic diversity is the total number of different alleles in the population. A population is
defined as a group of individuals of the same species that live in the same place and are
able to breed with one another. A species consists of more than one population. The greater
the number of alleles in a population the greater the genetic diversity, and therefore the
greater the chance that a population would survive a change in their habitat.
The niche of a species is its role within the environment. Species which share the same
niche compete with each other. The idea that better adapted species survive is the basis of
natural selection.
Natural selection is the process in which fitter individuals who are better adapted to the
environment survive and pass on the advantageous genes to future generations. Evolution is
the process by which the frequency of alleles in a gene pool changes over time as a result of
natural selection.
Evolution via natural selection:
• There’s a variety of different phenotypes within a population.
• An environmental change occurs and as a result of that the selection pressure changes.
• Some individuals possess advantageous alleles which give them a selective advantage
and allow them to survive and reproduce.
•The advantageous alleles are passed on to their offspring.
• Over time, the frequency of alleles in a population changes and this leads to evolution
Types of Selection:
Selection is the process by which individuals that are better adapted to their environment are
more likely to survive and breed. This means that they can pass on their advantageous
alleles. Every living organism is subject to selection determined by the conditions which they
are living in.
There are two key types of selection, these are:
1. Directional Selection - directional selection occurs when the environmental conditions
change and the phenotypes best suited to the new conditions are more likely to survive. As a
result these individuals will breed and produce offspring. Over time the mean of the
, population will move in the direction of these individuals. An example of this is bacteria being
resistant to antibiotics. A single bacteria will have had a mutation that meant it was not killed
by penicillin as it could produce the enzyme penicillinase. As a result it was able to grow and
populate, and the frequency of the allele that enabled penicillinase production increased in
the population. Therefore the population moved to have greater penicillin resistance.
2. Stabilising Selection - In stabilising selection the phenotypes with successful
characteristics are preserved and those of greater diversity are reduced. This selection
doesn’t occur due to changes in the environment. If the environment stays the same then the
individuals closest to the mean are favoured because they have the alleles that have given
them the survival advantage. The furthest from the mean are selected against. An example
is new born babies’ weights. Those that have a birth weight of around 3kg are more likely to
survive than those at the extremes.
The niche of a species is its role within the environment. Species which share the same
niche compete with each other and a better adapted species survive. The idea that better
adapted species survive is the basis of natural selection.
Organisms are adapted to their environment in various ways:
• Anatomical adaptations are physical adaptations, either external or internal e.g. length of
the Loop of Henlé - in desert mammals this is very long to produce concentrated urine and
minimise water loss.
• Behavioural adaptations are changes in behaviour which improve the organism’s chance
of survival e.g. mating calls.
• Physiological adaptations are processes inside an organism’s body that increase its
chance of survival e.g. regulation of blood flow through the skin.
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Species and taxonomy:
Species are capable of breeding to produce living offspring. We name species using the
binomial naming system. Every species is given a name with two parts, based on Latin and
Greek.
- The generic name is the genus to which the organism belongs. If a species is closely
related it will share the same name
- The specific name is the species to which the organism belongs. When hand writing these
the names must be underlined and the first letter of the generic name must be capitalised,
but not the first letter of the specific name.
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