Lesson 1 of 5 / Variation, selection and species
Supply the variation selection needs
Which processes make new alleles and which reshuffle existing ones?
I can explain why heritable variation from mutation, meiosis and sexual reproduction is needed for natural selection.
I can explain the importance of variation from mutation, meiosis and sexual reproduction in selection.Syllabus
Syllabus 8876, Core 4(a). Explain the importance of variation from mutation, meiosis and sexual reproduction in selection.
About 5 min
Make a guess. You are not marked.
Which process creates a brand-new allele: crossing over or mutation?
Show the answer
Mutation.
Mutation changes the DNA sequence, creating a new allele. Crossing over reshuffles alleles that already exist.
The key idea
Mutation can create new alleles. Meiosis and fertilisation create new combinations. Heritable differences allow unequal reproductive success to change later generations.
Explore the idea
Create an allele or change a combination?
A DNA change creates a new sequence variant. Its effect is not directed by what the organism needs.
Gamete examples assume independently segregating loci. They illustrate sources of variation, not detailed meiotic stages.
Explanation
Individuals in a population differ. Some differences are caused by alleles, some by environment, and many by both. Natural selection can produce an evolutionary response when differences affecting survival or reproduction have a heritable basis.
Mutation changes DNA and is the ultimate source of new alleles. A mutation may be beneficial, harmful or have little effect in a particular environment. It does not arise because the organism knows which change would be useful.
During meiosis, crossing over and the independent segregation of homologous chromosomes produce different allele combinations in gametes. Random fertilisation brings together different gametes. These processes usually rearrange existing alleles rather than inventing a new sequence at each gene.
If all individuals were genetically identical for a relevant characteristic, selection could not favour one inherited variant over another for that characteristic. An acquired difference, such as a leaf being damaged, does not by itself supply a heritable allele for future generations.
Step by step
- 1
Create an allele
A DNA mutation can introduce a new variant.
- 2
Recombine alleles
Meiosis and fertilisation create different combinations among offspring.
- 3
Connect to selection
Inherited differences can affect how many descendants individuals leave.
Worked example
A new allele or a new combination?
Parents with existing alleles A, a, B and b produce an offspring with a combination absent from either parental genotype. Must a mutation have happened?
One way to explain it
No. Meiosis and fertilisation can combine existing alleles in a new way. Evidence of a genuinely new DNA sequence would be needed to infer a new allele arising by mutation.
Why this answer works
- Distinguish an allele sequence from a combination of alleles.
- New offspring variation does not require a new mutation at every gene.
Watch out for this
A student says: "Crossing over normally creates a useful new allele whenever the environment changes." What is wrong with this?
Show the answer
Crossing over reshuffles DNA between homologous chromosomes. Mutation supplies new sequence variants; neither process is directed towards an organism's needs.
Check your understanding
Which difference most directly supplies heritable variation for selection?
Show the answer
Individuals carry different alleles that affect survival and can pass to offspring.
This connects variation to both reproductive success and inheritance.
Quick recall
Which processes make new alleles and which reshuffle existing ones?