Figure 2. Assortative mating, gene flow and diversifying selection drive clinal phenological responses in oaks.

As bud phenology is correlated to the growth rhythm and to flowering in oaks, it is intensively studied in microevolution. The discovery of very large within and between population genetic variation has raised numerous questions about the mechanisms maintaining its evolutionary potential and its contribution to local adaptation. In addition to common evolutionary drivers as gene flow, and selection, assortative mating has a significant contribution to phenological responses of oak populations. This figure illustrates how gene flow and assortative mating shape clinal genetic variation between populations of higher latitude/elevation (a) and lower latitude/elevation (b).

• a)
  and b) Within natural populations early flushing trees tend to mate with early flushing trees, resulting in positive assortative mating. Trees with similar phenotypes regarding date of bud burst preferentially mate within populations. Positive assortative mating is shown by red arrows between trees sharing similar colours on graph a) (colours indicate here the timing of bud burst). However matings resulting from immigrant pollen flow are likely to result in negative assortative mating especially if source populations are farther away in latitude or elevation. Populations at northern latitudes or higher altitude flush on average later than populations from more southern latitudes or from lower elevations because of temperature differences. Hence successful matings resulting from pollen flow can only associate late flushing trees from the south (or low elevation) to early flushing trees in the north (or at higher elevation) (negative assortative mating shown by blue arrows on b). As a result, assortative mating and gene flow contribute to a directional filtering of late flushing genes to the northern (or higher elevation) populations (Soularue & Kremer, 2012; Soularue & Kremer, 2014).
• c)
  Within population positive assortative mating generates higher within population genetic variance, whereas negative assortative mating and gene flow creates genetic clines along temperature gradients in the landscape, as shown here by the genetic divergence of the time of bud burst along an elevational gradient in sessile oak in the Pyrénées (Firmat et al., 2017)