Cambridge International A Level Biology Revision Guide

Chapter 17: Selection and evolution
Figure 17.7 Rabbits living in dense populations are
more likely to get myxomatosis than those in less
crowded conditions.
These environmental factors act to reduce the rate of
growth of the rabbit population. Of all the rabbits born,
many will die from lack of food, or be killed by predators,
or die from myxomatosis. Only a small proportion
of young will grow to adulthood and reproduce, so
population growth slows.
If the pressure of the environmental factors is
sufficiently great, then the population size will decrease.
Only when the numbers of rabbits have fallen considerably
will the numbers be able to grow again. Over a period
of time, the population will oscillate about a mean level.
Figure 17.8 shows this kind of pattern in a lemming
population over 11 years. The oscillations in lemming
populations are particularly marked; in other species, they
are usually less spectacular.
This type of pattern is shown by the populations of
many organisms. The number of young produced is far
greater than the number which will survive to adulthood.
Many young die before reaching reproductive age.
What determines which will be the few rabbits to
survive, and which will die? It may be just luck. However,
some rabbits will be born with a better chance of survival
than others. Variation within a population of rabbits
means that some will have features which give them an
advantage in the ‘struggle for existence’.
One feature that may vary is coat colour. Most rabbits
have alleles which give the normal agouti (brown) colour.
A few, however, may be homozygous for the recessive
allele which gives white coat. Such white rabbits will stand
out distinctly from the others, and are more likely to be
picked out by a predator such as a fox. They are less likely
to survive than agouti rabbits. The chances of a white
rabbit reproducing and passing on its alleles for white coat
to its offspring are very small, so the allele for white coat
will remain very rare in the population. The term ‘fitnessʼ
is often used to refer to the extent to which organisms are
adapted to their environment. Fitness is the capacity of an
organism to survive and transmit its genotype to
its offspring.
403
Population size
3-year
interval
4-year
interval
1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957
Year
Figure 17.8 Lemming populations are famous for their large increases and decreases. In some years, populations
become so large that lemmings may emigrate in one group from overcrowded areas. The reason for the oscillating
population size is not known for certain, although it has been suggested that food supply or food quality may
be the main cause. As the population size rises, food supplies run out, so the population size ‘crashes’. Once the
population size has decreased, food supplies begin to recover, and the population size rises again.