Describe how mutation and genetic recombination increase genetic variation, and the possible effects of mutations (NGSSS SC.912.L.15.15; Reporting Category 2, Classification, Heredity, and Evolution).

What this topic is asking

The NGSSS benchmark SC.912.L.15.15 asks you to describe how mutation and genetic recombination increase genetic variation. For the Florida Biology 1 EOC you need to know what a mutation is, that its effect can be harmful, neutral, or beneficial, and how sexual reproduction (recombination in meiosis plus fertilization) shuffles alleles into new combinations. The central idea is that this variation is the raw material that evolution acts on.

Mutation: the source of new alleles

A mutation in a body (somatic) cell affects only that organism, but a mutation in a gamete (egg or sperm) can be passed to offspring and so can contribute to evolution.

Effects of mutations: harmful, neutral, or beneficial

A common misconception is that all mutations are bad. In fact, a mutation's effect depends on what it does to the resulting protein:

• Harmful. The mutation disrupts a protein's function, possibly causing a genetic disorder or, if it affects cell-cycle genes, cancer.
• Neutral. The mutation has no noticeable effect (for example, it does not change the protein, or changes it in a way that does not matter). Most mutations are neutral.
• Beneficial. The mutation gives the organism an advantage in its environment (for example, antibiotic resistance in bacteria), which natural selection can then favor.

Whether a mutation is helpful or harmful often depends on the environment: a trait that is useless or harmful in one setting can be advantageous in another.

Genetic recombination

Besides mutation, sexual reproduction generates enormous variation by recombining existing alleles into new combinations. There are three contributors, all linked to meiosis and fertilization:

• Crossing over. Homologous chromosomes exchange matching segments during meiosis, making new allele combinations on a chromosome.
• Independent assortment. Chromosome pairs line up randomly during meiosis, so gametes get different mixes of chromosomes.
• Random fertilization. Any sperm can fertilize any egg, so two genetically different gametes combine at random.

This is why sexually reproducing offspring differ from their parents and from each other, while organisms that reproduce asexually (by mitosis or binary fission) produce identical copies with no new variation.

Try this

Q1. State the three possible effects of a mutation. [2]

• Cue. A mutation can be harmful, neutral, or beneficial.

Q2. Explain how genetic recombination increases variation in sexually reproducing organisms. [2]

• Cue. Crossing over and independent assortment in meiosis, plus the random joining of gametes at fertilization, shuffle existing alleles into new combinations.