In some plant groups, actually, seed dispersal may be secondary in facilitating migration to the other major mode of gene flow, which is pollen. Pollen is by far more buoyant and dispersible than seed, with an ability to travel thousands of miles in a few days. So if a long-distance pollen grain encounters a compatible micropyle or stigma, it has a chance to effectively transport an entire genome copy tens or hundreds of miles and inject this genetic material into a distant gene pool in a single generation.
Seed is far less likely to make this long a journey, and if it does happen to make a long-distance leap, a seed has to germinate, grow and compete without any local genetic complement to facilitate its success. A pollen grain, on the other hand, implants a single copy of its home genome into an ovule containing a copy of a genome adapted to conditions in the distant locality. The resultant F1, if successful enough to reach maturity, can then enter elements of its paternal copy into the distant gene pool where selection can act on them. If the distant locality happens to have become more like the pollen grain’s source locality, as would occasionally be the case with major climate change, large regions of the pollen grain’s genome may be favored, even to the point of reconstituting most of it in a subsequent diploid individual. So in species with broad ranges or genera with intercompatible species, the deck is strongly stacked in favor of pollen as the primary mode of long-distance gene flow. The longer the distance, the more likely pollen is to facilitate gene (and species!!) migration.
The advantages of pollen as a dispersal agent, combined with the necessity of gene flow and migration, are likely the reason that genera or sub-genera of most temperate and boreal woody plants, as well as many herbs, contain species that are fully intercompatible. The migratory function of this inter-compatibility is supported in molecular and morphological data from our three species of alder here in the Willamette Valley.
PCA analysis of nuclear data from 96 individual alders, including all three species and putative hybrids, reveals multiple likely recent hybrids and suggests substantial introgression linking all three species. All of this is relevant to the genetic make-up and diversity within alders, as well as potentially critical to the migration and adaptation of alders to changing climate. Similar evidence of the occurrence and functions of hybridization and introgression can be found in spruce (https://link.springer.com/article/10.1007/s11295-014-0817-y), oak (https://nph.onlinelibrary.wiley.com/doi/abs/10.1046/j.1469-8137.2003.00944.x), poplar (https://onlinelibrary.wiley.com/doi/abs/10.1111/mec.14820) and other wind-pollinated tree genera. This would appear to be the rule, not the exception, in anemophilous trees, which by the way make up the vast majority of boreal and temperate forests.
So we need to pay attention to this. We can’t achieve climate-adapted forests just by moving seeds around. The chances that seed from distant locations is adapted to conditions here and now, including climate, photo-period, soils and everything else, is essentially nil. Actual “climate-adapted” trees (and other plants) that are going to do well here have to also tune into every other element of the local habitat and this requires selection acting on diversity over at least a few generations. Seed movement might be a tool we can use to inject more diversity and start the process, but we would actually approach our goal more quickly by moving not seed, but rather single genome copies – pollen – to new locations, where selection can more quickly begin to screen out advantageous elements. This saves a whole generation, which in trees especially may be a lot of time.
Migration is not just about seed movement, facilitated by humans or otherwise. It’s about gene flow and selection. And pollen is a more effective agent of gene flow.