Where does the genetic variation that natural selection acts on come from?
Explain the sources of genetic variation: mutation, meiosis (crossing over and independent assortment) and random fertilisation
A focused answer to the H2 Biology Inheritance and Evolution outcome on variation. Mutation as the source of new alleles, the role of meiosis (crossing over and independent assortment) and random fertilisation in shuffling alleles, and the contrast with environmental variation.
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What this dot point is asking
SEAB wants you to explain where genetic variation comes from: mutation as the ultimate source of new alleles, and meiosis (crossing over and independent assortment) together with random fertilisation as the processes that shuffle existing alleles into new combinations. You should also distinguish genetic from environmental variation. This sets up natural selection.
The answer
Mutation: the ultimate source
A mutation is a change in the base sequence of DNA. It is the only process that creates genuinely new alleles, so it is the ultimate source of genetic variation. Mutations are random and usually rare, but they supply the raw material that the other processes then shuffle.
Meiosis: crossing over and independent assortment
Meiosis generates variation by recombining existing alleles.
- Crossing over in prophase of meiosis I exchanges segments between homologous chromosomes, producing new combinations of alleles on a chromosome.
- Independent assortment at metaphase I means each homologous pair lines up and separates independently, mixing maternal and paternal chromosomes in many combinations among the gametes.
Together these make the gametes genetically varied and nearly all different.
Random fertilisation
Random fertilisation combines two genetically varied gametes from different individuals. Because any of a huge number of different sperm can fertilise any of many different eggs, the offspring show enormous variation.
Genetic versus environmental variation
Variation can also be environmental (caused by surroundings, such as diet affecting growth). Only genetic variation is heritable and so available to natural selection; environmental variation is not passed on.
Examples in context
Example 1. Why siblings differ. Brothers and sisters share the same parents yet differ genetically because each was formed from a different combination of gametes produced by crossing over, independent assortment and random fertilisation. This everyday observation is the direct result of these three processes.
Example 2. Raw material for selection. Natural selection cannot act without variation. Mutation supplies new alleles, and sexual reproduction shuffles them into new combinations, giving populations the genetic diversity on which selection can operate, linking this dot point to evolution.
Try this
Q1. State which source of variation can create entirely new alleles. [1 mark]
- Cue. Mutation.
Q2. Explain how independent assortment during meiosis contributes to genetic variation. [2 marks]
- Cue. Each homologous pair of chromosomes orients and separates independently of the others, so gametes receive many different combinations of maternal and paternal chromosomes.
Q3. Explain why environmental variation does not contribute to evolution by natural selection. [1 mark]
- Cue. Environmental variation is not coded in the DNA, so it is not heritable and cannot be passed to offspring or selected for.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksExplain how meiosis and random fertilisation together generate genetic variation among the offspring of sexually reproducing organisms.Show worked answer →
The answer should cover crossing over, independent assortment, and random fertilisation.
Meiosis introduces variation in two ways. First, crossing over in prophase of meiosis I exchanges segments between homologous chromosomes, producing new combinations of alleles on a chromosome. Second, independent assortment at metaphase I means each homologous pair lines up and separates independently of the others, so the maternal and paternal chromosomes are mixed in many different combinations among the gametes.
These two processes produce gametes that are genetically varied and almost all different from one another and from the parent cell.
Random fertilisation then combines two such varied gametes from different individuals. Because any one of a huge number of genetically different sperm can fertilise any one of a large number of genetically different eggs, the resulting offspring show enormous genetic variation.
Markers reward crossing over producing new allele combinations, independent assortment mixing whole chromosomes, and random fertilisation combining varied gametes to multiply the variation.
Original3 marksExplain why mutation is described as the ultimate source of genetic variation, whereas meiosis and fertilisation are not.Show worked answer →
The answer should distinguish creating new alleles from rearranging existing ones.
Mutation is a change in the base sequence of DNA, which can create entirely new alleles that did not exist before. It is therefore the ultimate (original) source of genetic variation, the only process that produces genuinely new genetic information.
Meiosis (through crossing over and independent assortment) and random fertilisation produce variation by shuffling and recombining alleles that already exist into new combinations. They generate variety among offspring but do not create any new alleles.
Markers reward mutation creating new alleles, the others only recombining existing alleles, and the conclusion that mutation is the original source while the others increase variety from that source.
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