types of variation Flashcards
types of variation
The widest type of variation is between members of different species - these differences are known as interspecifc variation. For example, a mouse has four legs, teeth, and fur whereas a bird has two legs, two wings, a beak and feathers.
Every organism in the world is different - even identical twins differ in some ways.
Differences between organisms within a species are called intraspecific variation. For example, people vary in height, build, hair colour, and intelligence.
causes of variation
Two factors cause variation:
• An organism’s genetic material - differences in the genetic material an organism inherits from its parents leads to genetic variation.
• The environment in which the organism lives - this causes environmental variation.
genetic causes of variation
Genetic variation is due to the genes (and alleles) an individual possesses. There are several causes for genetic variation being present within a population:
1)Alleles (variants) - genes have different alleles (alternative forms). With a gene for a particular characteristic, different alleles produce different effects. For example, the gene for human blood groups has three different alleles (A, B and 0).
Depending on the parental combination of these alleles (see point 4) four different blood groups can be produced (A, B, AB and 0). Individuals in a species population may inherit different alleles of a gene.
2)Mutations - changes to the DNA sequence and therefore to genes can lead to changes in the proteins that are coded for. These protein changes can affect physical and metabolic characteristics. If a mutation occurs in somatic (body) cells, just the individual is affected. However, if a mutation occurs in the gametes it may be passed on to the organism’s offspring. Both can result in variation.
3)Meiosis - gametes (sex cells - ovum and sperm) are produced by the process of meiosis in organisms that reproduce sexually. Each gamete receives half the genetic content of a parent cell. Before the nucleus divides and chromatids of a chromosome separate, the genetic material inherited from the two parents is ‘mixed up’ by independent assortment and crossing over. This leads to the gametes of an individual showing variation.
4)Sexual reproduction - the offspring produced from two individuals inherits genes (alleles) from each of the parents. Each individual produced therefore differs from the parents.
5)Chance - many different gametes are produced from the parental genome. During sexual reproduction it is a result of chance as to which two combine (often referred to as random fertilisation).
The individuals produced therefore also differ from their siblings
as each contains a unique combination of genetic material.
Points 3, 4 and 5 are all aspects of sexual reproduction. As a result there is much greater variation in organisms that reproduce sexually than asexually. Asexual reproduction results in the production of clones (individuals that are genetically identical to their parents). Genetic variation can only be increased in these organisms as a result of mutation.
An example of a characteristic that is determined purely by genetic variation is your blood group.
The genes passed onto you from your parents
determine if your blood group will be type A, B,
AB or 0.
environmental causes of variation
All organisms are affected by the environment in which they live, although plants may be affected to a greater degree than animals due to their lack of mobility. For example, two rose bushes are planted in different positions in a garden. The one that has greater access to the sun will generally grow larger than one in a shadier position. As the plant cannot move to gain sunlight, it is more affected by the environment than an animal, which could move to another area to look for food or shelter.
An example of a characteristic that is determined purely by environmental variation is the presence (or absence) of any scars on your body. They will have occurred as a result of an accident or disease and have no genetic origin. Scars cannot be inherited from a parent.
environmental and genetic causes
In most cases variation is caused by a combination of both environmental and genetic factors.
If you have very tall parents, you have most likely inherited the genes to also grow to a tall height. However, if you eat a very poor diet or suffer from disease you may only grow to below average height.
Another example of a characteristic that shows both environmental and genetic causes is your skin colour. This is determined by how much of the pigment, melanin, it contains. The more melanin present in your skin, the darker your skin is. Your skin colour at birth is determined purely by genetics - however, when you expose your skin to sunlight you produce more melanin to protect your skin from harmful UV rays.
This results in your skin turning darker.
As many characteristics are caused by a combination of both genetic and environmental causes, it can be very difficult to investigate and draw conclusions about the causes of a variation in any particular case (however, see the Application on studying variation in identical twins). This is often referred to as the ‘nature versus nurture’ argument. For example, many studies have investigated the primary cause of variation in intelligence - genetics or environment? To date, no definitive conclusion has been reached.
