Download Variation handout - University of Leicester

#VN-­‐S-­‐3 Variation handout
In natural populations, the organisms that are best suited to their environments are the
ones that are most likely to survive and pass their genes onto the next generation. This is
natural selection; the fittest organisms are selected and live long enough to reproduce. For
one organism to be selected over another, it must vary from the other - if every organism
was the same, they would all have an equal chance of surviving!
Key point
Which fly would be better camouflaged in a dark leafy forest, fly 1 or fly 2?
Which fly would be more likely to pass on its genes?
Fly 1
Fly 2
© The Exploratorium,
www.exploratorium.edu Variation, then, is clearly important for evolution. What is also important however is that this
variation is in fact heritable, i.e. that it is possible to pass the variation on to the next
generation. Genetic variation is capable of being inherited by the next generation,
whereas environmental variation will not be seen in the next generation1.
An example of environmental variation is the variation created by hair being bleached by
the sun. If you have the genes for brown hair, then your hair being bleached by the sun
does not change this; you will still have these genes (even though your hair might be a bit
more blonde). No matter how long you spend in the sun, your children will still inherit your
brown hair genes!
1 Unless there is some epigenetic process in place which allows the organism to pass modifications onto its progeny. The Tauber lab 1 The University of Leicester #VN-­‐S-­‐3 Key point
What is the difference between genetic and environmental variation?
Which of these is heritable?
Heritable characteristics are heritable because of genes. Some characteristics have very
few values because the majority of the phenotype is controlled by very few genes. For
example sex determination in humans is mainly controlled by a single gene (SRY), so the
phenotypes produced are almost invariably male or female. Taking a random sample of
5,000 people would likely show you a distribution like in Fig. 1. Traits like this are referred to
as discrete traits.
Figure 1: A likely distribution of sexes for a random sample of 5,000 people
Traits such as human height on the other hand are continuous; heights range from short to
tall with everything in between. This is because many different genes contribute to height,
each by a small amount. If you measured this group of 5,000 people, the distribution might
look like something in Fig 2. This distribution is much closer to the normal distribution there are many heights around the average height, and less at the extreme ends of the
phenotype (short, tall). Traits such as this are referred to as continuous or quantitative
traits, because each gene adds to the value of the trait quantitatively.
The Tauber lab 2 The University of Leicester #VN-­‐S-­‐3 Key point
Can you think of any other discrete traits in humans?
Which would be more obvious on the phenotype, a mutation in one of the genes
responsible for height, or a mutation in SRY (sex-determining region Y)? (Hint: refer
to the histograms).
The Tauber lab 3 The University of Leicester #VN-­‐S-­‐3 The Tauber lab 4 The University of Leicester