I posted this question, “Signal Advance”: Unsure of meaning or contextual use, on Biology SE, as the phrase was used in a biology text that I am reading, Recombinant DNA; Genes and Genomes - A Short Course - Third Edition by James D. Watson, et. al. I came across this paragraph in the discussion about discrete factors of inheritance in Mendelian genetics:
The character shown by a plant depends on the pair of factors it receives from its parents. If both factors it receives are the same, that is, two dominant (yellow seed) or two recessive (green seed), then the plant exhibits those traits - yellow seeds for the former and green seeds for the latter. If one factor is dominant and the other recessive, then the dominant character is expressed; in this case, a plant with a factor for yellow seed and one for green seed will show the dominant trait, yellow seed. But the factor for green seed persists, even if the plant is yellow, and can reappear unchanged in later generations. This was a signal advance. In the 19th century it was believed that the traits of parents were blended in their offspring and, once mixed, the contributing traits could not be recovered. At the time, this was a severe problem for Charles Darwin's theory of evolution through natural selection. His critics pointed to that any favorable variation arising in an individual would have no long-term effect, because it would immediately be lost by blending.
I had never come across the phrase signal advance before and wasn't sure of its meaning based on its context in this paragraph.
The context was difficult to ascertain, as the first part of the paragraph referred to a genetics concept, where signal could be a noun and advance could be a verb, but the second part of the paragraph discussed the hypothesis and how it provided evidence to change the thinking of the day.
However, after reading the question in the comments, I am beginning to think that signal was possibly being used as an adjective and advance was the noun that it modified.
I looked for the definition of signal when used as an adjective, and this definition may be applicable:
unusual; notable; outstanding: a signal exploit.
If that is the case, then the sentence likely means this was notable progress.
I'd appreciate any feedback, especially if someone has come across this usage in scientific writing.