Variation
Biology: AQA A Level, topic 7.3.1
Phenotypic Variation
Visible differences in characteristics between individuals are called phenotypic variations. Phenotypic variation is caused by a combination of genetic variation and the environment.
Genetic variation
- There is huge variation between individuals in the same species.
- E.g. A golden retriever looks completely different from a pug but they are both part of the species Canis familiaris.
- This variation is caused by differences in the genetic code. This is called genetic variation.
- Genetic variation happens when individuals in the same species have different alleles.
- Alleles are different versions of the same gene.
Environmental variation
- Environmental factors can also contribute to phenotypic variation.
- The genotype of an individual interacts with the environment.
- E.g. If an individual has the alleles for being tall but is fed a nutrient-poor diet, they will not grow as tall as an individual with the same alleles and a more nutrient-rich diet.
Causes of Genetic Variation
The amount of genetic variation in a population is determined by a number of factors:
Mutation
- Mutation is the main source of genetic variation.
- Mutations are changes in the base sequence. Changes may cause the structure of proteins encoded by DNA to be altered.
- Some mutations are harmful (e.g. the mutation that causes disease), some are beneficial and some have no effect on survival.
Recombination
- Meiosis is the process that produces haploid gametes from diploid cells.
- During meiosis, the genes on homologous chromosomes are reshuffled through a process called recombination or crossing over.
- Recombination produces new combinations of genes. This increases genetic variation.
Independent segregation
- Meiosis also involves the independent segregation of chromosomes into haploid cells.
- The chromosomes in a pair of homologous chromosomes are randomly separated and this creates genetic variation.
Random fertilisation
- During sexual reproduction, the gametes of two unrelated individuals are fused to produce a diploid individual.
- This process is called random fertilisation.
- Random fertilisation produces genetic variation because pairs of chromosomes from two separate individuals are combined.
Test yourself on Variation
What are the two types of variation that give rise to phenotypic variation?
Which of these is NOT a type of environmental variation?
Which of the following statements about phenotypic variation are true?
Which of the following is NOT a cause of genetic variation?
During what process does independent segregation of chromosomes take place?