I don't recall one single mutation that's been shown to be helpful.Talkorigins cites a few, such as
- the ability of a bacterium to digest nylon (Negoro et al. 1994; Thomas n.d.; Thwaites 1985);
- evolution in Klebsiella bacteria of a new metabolic pathway for metabolizing 5-carbon sugars (Hartley 1984);
- evolution of multicellularity in a unicellular green alga (Boraas 1983; Boraas et al. 1998);
Hm. Found enough to satisfy me. The abstract and this slightly detailed discussion:
Chlorella vulgaris is a common unicellular green alga that is used as a "lab rat" in labs throughout the world. We've grown the same strain of it for thousands of generations on agar and in liquid culture without it losing its unicellular morphology. Dozens to hundreds of labs have done this.
Steady-state unicellular C. vulgaris cultures were innoculated with the predator Ochromonas vellesiaca, a phagotrophic flagellate. Within less then 100 generations a multicellular form of the Chlorella became dominanant in the culture. (Boraas 1983b, Boraas et al. 1998). The alga first formed globose clusters of tens to hundreds of cells. After 10-20 generations in the presence of the flagellate, eight-celled colonies predominated. These colonies retained the eight-celled morphology indefinitely in continuous culture and when plated onto agar.
The basis of the change appears to be a change in the cell wall. Cell division in normal Chlorella occurs within the cell wall of the maternal cell. The cell undergoes 1-4 divisions to form 2-16 daughter cell. This is followed by a split in the mother cell wall and dispersal of the neonatal cells. In a culture, empty mother cell walls are interspersed with whole cells at a ratio of about 1:4. Empty mother cell walls are not found in cultures of the multicellular form. The colonies are enclosed in a "membrane" which appears to be modified cell wall material.
As was seen in the bacterial cases, this mutation provided Chlorella with resistance to predation at the cost of growth rate. Neonatal colonies are barely small enough for Ochromonas to engulf. After they have grown slightly they are too big to be eaten. In the presence of the predator, the colonial form of Chlorella displaces the unicellular form and persists. When the predator is not present, the unicellular form displaces the colonial form. This makes sense as the colonial form has less surface area exposed to the environment available for nutrient uptake than the unicellular form has.
It explains the specific mutation pathway, a significant helpful effect of the mutation (too big to eat is helpful, plus multicellular just seems significant, even though like any mutation it's sometimes a disadvantage), and why it's likely a mutation and not part of the regular genetic information (the alga breeds true otherwise). That's a pretty strong case to a layman. If you have a good creation resource check it because I couldn't find "Chlorella" on answersingenesis.org.