Seed size in H. exilis

Here is a curiosity I noticed while sorting seeds.

At one of my H. exilis sites (G136) there was a serpentine field beside a raised road. The field had a large population of tiny sunflowers, but there were also a few plants on the gravel embankment beside the road. These plants were much bigger, probably because the soil used in the embankment wasn’t serpentine. I collected seeds from both field and roadside plants.

The roadside seeds are much bigger than the serpentine seeds.

This has several possible explanations:

-Plasticity. The non-serpentine plants are much bigger, being bigger makes their seeds also larger.

-Selection. Bigger seeds are better on the disturbed habitat of the gravel embankment.

-Introgression. Gene flow from H. annuus could be coming in and only persisting on non-serpentine areas, bringing larger seed alleles.