Evolution and hatchery populations

[Laxaria]

For a quiz I got this question:

Question 3: In populations of coho salmon, males that mature in two years tend to produce male offspring that mature in two years. Males that mature in three years tend to have offspring that mature in three years. In hatcheries in the Pacific Northwest, hatchery managers collect adult salmon on their breeding grounds, mate them, rear the offspring in culture for a year, and then release the juveniles to the ocean. When they return as adults again, they repeat the cycle. For many years, hatchery managers only mated the three year old fish.

1a: Will the hatchery populations evolve if managers only mate three year old fish? Please explain your answer fully, based on your understanding of Darwin's postulates.

1b: What will be the eventual phenotype(s) of the hatchery population?

How I understood it was that hatchery managers were taking only 3 year old fish and mating them. Therefore, in a hatchery population, there can only be 3 year old fish, and thus evolution cannot occur. Subsequently, the eventual phenotype in a hatchery population will tend to three.

However, when I got my quiz back, my professor marked it wrong. The answer key specifically stated that evolution had occurred in the hatchery population because of a heritable trait and variable reproductive success, but it did not bother breaking down justifying how this is true. Interestingly, she also claimed that the eventual phenotypes will trend to three years I tried talking to her, but I did not get a convincing answer as to why evolution occurred here.

So, I want to pose this question. Am I missing something here?

[Darby]

I feel like I have a pretty good grasp of evolutionary principles, and I have no idea what the questioner thinks they are dealing with here. How do they differentiate the groups in a wild-caught population? Is it collection times, size (quicker maturation doesn't necessarily mean smaller breeders), or some other attribute? There's so much here that's unclear.

If it's collection times, they aren't breeding 3-year-old fish, but fish that are there on a 3-year cycle. Assuming that ONLY hatchery fish are involved (no idea why that would be true, but whatever), the breeding cycle is just a statistical tendency, so the collected stock are not necessarily all 3-year breeders, some could be the outliers in the 2-year stock whose offspring would not always be present. (and every 6 years - every other sampling - the collected population should be a mixture)

Anyway, we're looking at an artificial selection process for a single trait, protected for a year but with a wild release, the the trend toward a "typical" 3-year phenotype makes some sense.

I think that maybe you're assuming that a behaviorally uniform population (3-year breeders) is genetically uniform, but there will be some "abnormal" 2-year breeders in there to start, but of THEIR offspring, tending toward 2-year, more won't be picked up later than the 3-year-leanings' offspring. That is the evolution (a term not usually applied to such an an unnatural circumstance) of the group toward a more uniform type.

[Laxaria]

I got back the answer key and this is what I got from the key:

Yes, they will evolve. Age at maturity is a heritable trait (for example, two year old males 
produce two year old offspring). The hatchery managers impose differential reproductive 
success on individuals within the population; three year old fish will have higher numbers of 
offspring.

I would be seeing her personally to figure out how this question works out, but I definitely would like a better understanding of what is at play before I do see her. I definitely do not see what is happening and I really have no clue either.

Thanks for trying though! I do agree that there are too many variables and thus too many assumptions to make.

[AstraSequi]

The hatchery managers impose differential reproductive success on individuals within the population; three year old fish will have higher numbers of offspring.

This sounds reasonable - the males that mature in three years will leave more offspring behind, and thus the population will shift to have a greater proportion of fish that mature in three years. If this is done effectively for enough generations, eventually all the fish in the population (if there is no significant selective advantage for the two-year phenotype) will mature in three years.

I definitely think the question could have been presented better though. For example, it assumes that the fish the hatchery managers collect breed more than the ones that they don't collect. It also assumes sufficient gene flow (many three-year fish mating with two-year fish), but the managers are working against this.

[Laxaria]

the males that mature in three years will leave more offspring behind

That's the bit which confuses me. I'm guessing that only males which mature in 3 years are mated, and males which mature in 2 years are not. Subsequently, the population of males that mature in 3 years increase because it is selected for by the hatchery managers. Is this reasoning sound?

[AstraSequi]

Sounds good, yeah.