Change Over Time:
Natural Selection & Variation
1. What is meant by variation when looking at members of a population?
Variations are differences between organisms. In this context, it is phenotypic
variation that is of interest in a population. Variations exist in visible features
such as external colouring or internal functioning (e.g. the ability to produce a
particular enzyme).
2. List the major types of variation that exist between organisms.
Variations can be classified into four major groups:
- Structural variations are easy to observe. Differences in cats’ tail shapes and
sizes are due to structural differences. There are huge differences in feather
patterns in pigeons. In humans differences in ear lobes or facial features are
structural variations.
- Physiological variations are differences in how an individual’s body functions.
Some individuals cope better with heat stress or have greater stamina. The
ability to taste particular chemicals or see certain colours can show variation. In
humans some individuals are red-green colour blind.
- Biochemical variations. The ability to produce pigments of a particular colour
leads to a different coat colour in animals. Consider the range of hair colours in
cats and dogs. These differences are due to biochemical variations. Some people
lack the enzymes necessary to break down the sugars in dairy products. Differing
abilities to produce enzymes are biochemical variations.
- Behavioural variation in pets is easily observed. Some dog breeds respond
well to training whilst for others training is a waste of time. In the human
population many of us are right-handed but many in the population are lefthanded.
3. What are the two major causes of variation?
Environmental influences and genetic makeup are the two major causes of
variation. Genetic influences exist because there is a range of alleles for most
genes. The alleles an individual inherits gives it a certain genetic potential. For
example humans who lack the alleles to produce normal skin pigments will not
have normal skin colour. Environmental conditions can affect genetic
inheritance. Particular plants may have the genetic potential to grow tall and
produce large quantities of fruit. Good growth, however, will not occur without
sufficient water and fertiliser. Differences in genes are the basis of variation but
their expression is altered by changing internal and external environmental
influences.
4. Distinguish between genotypic and phenotypic variation.
Genotypic variation is variation in the types of alleles inherited. Individuals who
have inherited different alleles are said to have different genotypes. Phenotypic
variation is variation in the traits or characteristics displayed by an individual.
As stated earlier, phenotypic differences are often visible to an observer. Internal
functioning and biochemistry, however, are also aspects of phenotype.
5. Do two individuals with the same genotype necessarily have the same
phenotype? Give an example to support your answer.
Individuals with the same genotype do not necessarily have the same phenotype
as environment can affect the expression of genes inherited. For example in
humans the tendency to be obese is autosomal dominant. Two individuals may
have the alleles for obesity. One individual through exercise and sensible eating
patterns may overcome their genetic disposition.
6. Do two individuals with the same phenotype necessarily have the same
genotype? Give an example to support your answer.
Individuals with the same phenotype do not always have the same genotype.
Individuals who show a dominant characteristic can be heterozygous or
homozygous for the alleles responsible. In pea plants there is a gene that
controls plant height. Tall is dominant compared to short. Tall plants can be
heterozygous or homozygous for the allele for tallness.
7. Genetic variation can be monogenic or polygenic. The resulting phenotypic
variation can be discontinuous or continuous. Explain these terms using
examples.
Characteristics determined by one gene are monogenic. For example the control
of ABO blood types in humans is under the control of one gene. The gene has
three alleles. This allows six different genotypes and four different phenotypes.
Because the phenotypes that result fall into discrete groups, the variation is
described as being discontinuous.
Expression of most characteristics is determined by a number of interacting
genes, each with several alleles. These characteristics are polygenic and show
continuous variation. Height in humans is a polygenic trait. Variation in height is
continuous. There are individuals of all heights and no discrete groups.
8. List the various sources and processes that provide genetic variation in a
population. Start with variation that arises during gamete production.
Sources of variation:
- Gene mutation during gamete production (introduces new alleles).
- Chromosomal mutations (changes in chromosome number or size).
- Crossing over during Prophase I of meiosis (shuffle the alleles within a
homologous pair of chromosomes).
- Random assortment of chromosomes during meiosis (one member of each
homologous pair goes to opposite poles during Meiosis I).
- Mate selection (different mates would provide different alleles).
- Gamete selection (many gametes are produced but few are involved in
fertilisation).
9. Which is the more important source of variation – recombination during
meiosis or mutation? Justify your opinion explaining the significance of each.
Mutation is the more important source of new genetic variation as it introduces
new possibilities as new alleles. Recombination of alleles is important in
shuffling alleles into different combinations. There is no additional variation but
new combination of alleles could lead to a better set of characteristics in
offspring.
10. How are ‘Natural Selection’ and ‘Evolution’ related?
‘Evolution’ occurs as a result of ‘Natural Selection’. Evolution refers to the
changes in species over time. Natural selection is the process that results in the
change.
11. If natural selection is to occur, what must exist in a population?
For natural selection to occur, there must be phenotypic variation (based on
genetic variation) in a population. If all individuals were phenotypically identical,
selection for better-suited phenotypes could not occur.
12. Is selection at the phenotypic or genotypic level? Explain what sorts of
individuals are most likely to survive in a particular environment.
Natural selection occurs at the phenotypic level. The individuals with
phenotypes that suit them best to the environment will tend to be selected. Less
well-suited individuals tend not to survive and reproduce. As phenotype is
largely determined by genotype, alleles that lead to better-suited phenotypes
will become more common. The selection, however, is at the phenotypic level.
13. Does the ‘fitness’ of an individual vary and if so what determines an
individual’s fitness?
The genetic fitness of an individual depends on the environment (selecting
agents) in which it lives. Polar bears are well adapted to life in the Arctic Circle. A
polar bear placed in the Simpson Desert would not survive long!
14. Which individuals in a population are most likely to survive in a particular
environment?
Those with the best set of adaptations for that environment.
15. Which individuals in a population are most likely to reproduce in a particular
environment?
Individuals that are well suited to their environment are more likely to
reproduce. Poorly suited individuals are less likely to survive and reproduce. If
they do reproduce on average they would produce fewer offspring.
16. If an individual is to pass on its alleles (genetic information) to the next
generation, how long must it live?
To pass alleles on to the next generation an individual has to survive long enough
to reproduce.
17. What is meant by the terms gene pool and allele frequency?
The gene pool is literally the pool of genes for a population. A population is a
group of individuals of one species living in one area. Within a population there
is genetic variation. That is, there are alternative forms of genes present – alleles.
Calculation of frequency of particular alleles in the population’s gene pool can
occur. Over a number of generations, alleles that give the individual an
advantage will increase in frequency in the gene pool.
18. In terms of the gene pool and allele frequency, what is the difference between
gene flow and genetic drift? When will the two processes have the greatest effect
in allele frequency?
Gene flow refers to the movement of genes in and out of the gene pool. This
occurs through immigration (movement in) and emigration (movement out).
Alleles can be added or removed from a population’s gene pool. Genetic drift is a
random change that can lead to changes in allele frequency. In a population there
may be a number of phenotypes with equal fitness. Over a number of
generations one phenotype (and the alleles that determine it) could become
more frequent just by chance. Genetic drift is of importance in smaller
populations where random selection could significantly alter an allele’s
frequency.
19. Is there any real difference in the process of ‘Natural Selection’ when
compared to ‘Artificial Selection’?
There is no major difference between ‘Natural’ and ‘Artificial Selection’. They
both produce evolutionary change. The difference is the selecting agent. In
natural selection it is the environment at large. In the case of artificial selection it
could be a dairy farmer keeping certain cows and removing others from their
herd.
20. What is a species?
The term species is a human concept used to classify organisms. Classification is
often subjective, as organisms are hard to pigeonhole. Therefore no definition is
perfect!
A species is a group of organisms that are structurally and biochemically similar.
Members share more characteristics in common than with any other individuals.
Members of a species interbreed to produce vigorous fertile offspring. Members
of a species share the same gene pool or are connected by gene flow to other
populations’ gene pools.
The species definition is a matter of debate and there is no absolute definition.
21. What is the most important test used to distinguish members of a species?
The test used most is to determine whether individual organisms mate to
produce fertile offspring. This test is not always possible!