18.
Variation and selection
Variation can be:
● Difference between species - for constructing keys
● Differences within a species
○ Although we all belong to the same species, there are many differences
between us
There is variation of things that we can see but also in features that we can’t see, such as
the types of haemoglobin that people have. This is phenotypic variation.
There is also the genetic variation, the differences between the genotypes of the individuals
PHENOTYPIC VARIATION GENETIC VARIATION
difference in external and internal Differences in genotypes
appearance
There are two types of phenotypic variations:
● Continuous variation:
Influenced by genes and environment. In a classroom you would
find a range of heights from the shortest to the tallest. If you divide
them into groups you can plot a frequency histogram (mean height
will be in the middle of the range corresponding to the largest group
→ normal distribution.). It is called continuous variation because
there is a continuous range of
heights from shortest to tallest.
Features you can measure
In animals and plants
● Discontinuous variation:
Influenced by genes alone. In this type of variation there is usually a small
number of phenotypes and no intermediates and you plot this variation as a
bar chart. For example, if you collect data in the classroom about attached
and not attached earlobes or about blood groups.
Inheritance versus environment
Two identical twins may be different in many ways because they developed differently and
have been exposed to different environmental influences
18.3 - Mutations
A mutation is a change in a gene or a chromosome that may cause a change in a
phenotypic characteristic, it’s a change in DNA’s base sequence that changes the phenotype
of the thing that that base codes for. This mutation can be harmful, beneficial or have no
effect. Mutation is the way in which new alleles are formed.
CAUSES:
● Damage to DNA.
● Failure in the copying process that occurs before nuclear
division.
● They occur naturally at random, but the rate they occur
is increased by exposure to ionizing radiation and some
chemicals.
18.4 - Adaptive feature
An adaptive feature is an inherited feature that helps an organism to survive and reproduce
and having offspring, which are likely to show the same features, in its environment.
thick white coat in winter for insulation
18.5 - Natural selection
Natural selection is the process that gives organisms that are well adapted to their
environment a greater chance to breed and pass on their alleles to the next generation than
those that are well adapted. There is a struggle for existence, due to competition, predation
and disease.
Types of natural selection:
- Variations are the differences within a population. Some mutations may give an
advantage to the individual that expresses them. Gene mutation, meiosis, mating
between individuals with different genotypes and fertilization give rise to variation
between individuals in every generation.
- Overpopulation of offspring. Many species produce millions of eggs/seeds for
offspring. However, populations remain stable because many of the young offspring
(that can’t protect themselves) die from predators (for plants, seeds are eaten by
birds or other animals), diseases and starvation. Animals that don’t give such a great
number of offspring tend to be mammals or birds that take care of the newborns until
they can defend themselves and look for food.
- The competition to get resources to survive (the ‘struggle for existence’ or ‘for
survival’). This competition is fierce between individuals from the same species.
- Reproduction. Best-adapted individuals are more likely to survive and reproduce.
They also have a greater chance to pass on their alleles to the next generation.
Individuals that are not so well adapted may die before they have a chance to
reproduce or they have few offspring.
18.7 - Selective breeding
Artificial selection has produced new varieties of animals and plants with increased
economic importance.
● Humans choose a feature of an animal or plant to improve ( production of meat, milk;
disease resistance, high yield)
● Animals and plants showing these features are bred to produce the next generation.
● The process of selective breeding continues for many generations
Summary: Artificial selection involves humans finding organisms with desirable features,
crossing them and then selecting the best from the next generation.
