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Variation
Differences amongst organisms
Learning objectives To define continuous
variation To define discontinuous
variation To understand which two
factors contribute to variation.
To understand that continuous exposure to selection pressures causes permanent adaptations.
Why do people vary so much? Every male has the potential
to make 8 million different combinations of chromosome in their sperm
The same is true of women and eggs.
So altogether a couple can have a maximum of 64 million genetically different children!
No wonder there are so many variations in humans
Sometimes genetic mutations cause variation when egg or sperm are formed.
There is variation within species With some species like
humans it easy to see, with others. E.g. panda’s its not so obvious.
The variations are caused by the individuals genetic make up (allele pairings)
These influence everything, from the enzymes it makes to the colour of its fur/hair etc,
Failure to make a particular enzyme often causes a disease or dietary disorders in a species.
There is variation between species Lots of variation between
organisms is what makes species different species!
They end up isolated genetically, reproductively and/or geographically making them so different that they are classified as a different species.
Geographical isolation is sometimes referred to as Allopatric
Differences between species can be less than you think: there is only 4% difference between humans and the chimpanzee
Giraffe and Okapi (sympatric)
Continuous variation Is variation in a species in
which there are extreme measurable examples (e.g. very short/very tall and lots of intermediate values in between
It usually forms a bell shaped curve (called a normal distribution) especially if a large sample is taken.
Continuous variation is usually a polygenic phenomenon (i.e lots of genes control it)
The environment often influences it too.
Discontinuous variation These are very distinct
differences which cannot be measured on a scale e.g. eye colour, blood group, attached or unattached earlobes etc.
There are no intermediate values
They are usually coded by one gene pair (alleles) only.
Environmetal influences These combined with your genes can affect
continuous variation Examples are: Diet Exercise Diseases you catch Alcohol consumption Smoking and drug abuse Education
Environmental Influences are not passed on. If you break an arm and your arm ends up
deformed. If end up massively obese because
you regularly eat McDonalds If you get rickets and have bow legs If you dye your hair These characteristics will not be passed on to your
children because they are not genetic, they are caused by your interactions with the environment.
Adaptations to the environment The features which an organism has must be well
suited to its environment. If not it risks extinction Adaptations fit into 4 main categories:
Structural
Physiological
Biochemical
Behavioural
ExamplesAdaptation Rhizobium Venus
flytrapFennec fox
Structural Nitrogenase (fixes nitrogen)
Adapted leaves form traps
Large ears
Behavioural Moves through soil and into roots
Insects trigger leaves to trap it
nocturnal
Physiological Exchanges amino acids for energy molecules with host plant
Secretes enzymes onto dead insects
Ears have large surface area to lose heat
Examining Structural Variation
Look at the three skulls What structural
variations do you notice?
How do you think these adaptations help the hominid survive better in its environment?
Questions
Define the term variation (give one example in humans) 2 marks
Explain the difference between variation between species and variation within species 2 marks
What shape is a normal distribution typically found in continuous variation? 1 mark
How is continuous variation different to discontinuous variation 2 mark
Give one example of an adaptation in a species and how it fits the organism to its environment 2 marks