Specific adaptation to strong competitors can offset the negative effects of population size reductions

Abstract

Competition plays a crucial role in determining adaptation of species, yet we know little as to how adaptation is affected by the strength of competition. On the one hand, strong competition typically results in population size reductions, which can hamper adaptation owing to a shortage of beneficial mutations; on the other hand, specificity of adaptation to competitors may offset the negative evolutionary consequences of such population size effects. Here, we investigate how competition strength affects population fitness in the bacterium Pseudomonas fluorescens Our results demonstrate that strong competition constrains adaptation of focal populations, which can be partially explained by population size reductions. However, fitness assays also reveal specific adaptation of focal populations to particular competitors varying in competitive ability. Additionally, this specific adaptation can offset the negative effects of competitor-mediated population size reductions under strong competition. Our study, therefore, highlights the importance of opposing effects of strong competition on species adaptation, which may lead to different outcomes of colonization under intense and relaxed competitive environments in the context of population dispersal.

Keywords: Pseudomonas fluorescens; adaptation; competition strength; population size reduction.

Figure 1.

A graphic illustration of the design of selection experiment, where focal populations evolved for eight transfers with no (N), inferior (I), equivalent (E) or superior (S) competitors. Each treatment consisted of 12 replicate microcosms. (Online version in colour.)

Figure 2.

Mean bacterial cell densities ± s.e.m., where focal populations evolved with no competitors (N: blue squares), inferior competitors (I: green circles), equivalent competitors (E: orange triangles), or superior competitors (S: red inverted triangles). Different letters denote significant between-treatment differences in mean focal population densities over time (Tukey multiple comparison, p_adj < 0.05). (Online version in colour.)

Figure 3.

The relative fitness of focal populations when competing with the following reference populations: (a) inferior competitors, (b) equivalent competitors, and (c) superior competitors. The x-axis titles indicate selection regimes (i.e. focal populations evolving with no (N), inferior (I), equivalent (E), and superior (S) competitors). Note the difference in y-axis value ranges. Within each panel, different letters denote significant differences in the relative fitness of focal populations between treatments (Tukey multiple comparison, p_adj < 0.05).

Figure 4.

The relationship between the extent of adaptation (the relative fitness averaged across the three reference populations) and population size (averaged over time) of focal populations. The symbols represent different treatments, where focal populations evolved with no competitors (N: blue squares), with inferior competitors (I: green circles), with equivalent competitors (E: orange triangles), or with superior competitors (S: red inverted triangles). The regression line is based on data pooled across the four treatments. (Online version in colour.)

Figure 5.

The relationship between the extent of adaptation (the relative fitness measured by the inferior competitors (a), equivalent competitors (b), and superior competitors (c) as reference populations) and population size (averaged over time) of focal populations. The symbols represent different treatments, where focal populations evolved with no competitors (N: blue squares), with inferior competitors (I: green circles), with equivalent competitors (E: orange triangles), or with superior competitors (S: red inverted triangles). Significant relationships are depicted by regression lines. Note the difference in y-axis value ranges. (Online version in colour.)