Results

We simulated the success of establishment of the pathogen population during colonization of a new host and, when pertinent, the evolution of its compound phenotype under the new selective pressure. Parsing these components allowed a better understanding of the individual and combined influence of distinct rates of novelty emergence (μ ), propagule size (N0 ), and reproductive rate (b )_ on the success of establishment of a new association and the behavior of the fitness and size of the population of pathogen following colonization of a new host resource. All analyzed parameters were evaluated under variable distance of the propagule compound phenotype from the host (d0 ) and revealed their influence on the probability of establishment of the pathogen population in the new host (Fig. 2). For a single propagule (N0 =1), the increase of d0gradually reduces the probability of establishment (Fig. 2a and b) – which was an expected result since the survival probability decays following this distance (black curves in Figs. 2 represent Eq. 1 for the propagule, di,n =d0 ). For the pathogen population to colonize the new host, it needs to survive successive selection events - therefore the probability of establishment is lower than the survival probability recovered for a single colonizing individual to persist until the first reproduction (the black curve in Figure 1).
Greater reproduction rates (b ) favor the pathogen establishment (Fig. 2a). As b increases, the establishment success approaches the probability of one individual surviving the selective forces of the new host species (in Fig. 2a; compare non-black probability curves approaching the black curve as b increases). For high brates (e.g. b = 7.5), the probability of establishment of the pathogen population will be the same as that expected for a single individual surviving until the first reproductive event of the simulation - and the probability of survival will depend only on the effect of d0 .
Only high novelty rate values (10-2 and 10-1) had a measurable effect on the populational probability of establishment - all other variations of novelty rate had practically the same low effect on the probability (Fig 2b). For novelty rates between 0.0 and 10-3, the probability of success practically did not differ, reaching 0 for d0 \(\approx\)1 (propagule compound phenotype app. one standard deviation distant from the optimum imposed). The effect of the increasing novelty rate between these values is more evident on the population growth; the population reaches the carrying capacity about twice faster whenμ =10-4 than when μ =0 (Fig 3). Less than 10% of establishment success was detected for non-synergic simulations when d0 = 2, despite the novelty rate (Fig. 2b).
Simulations have shown that a small increase in the propagule size (from 1 to 10) greatly expanded the diversity of compound phenotypes which resulted in a probability of success greater than 90% for pathogens with a d0 <0.9 (Fig. 2c). For larger propagule sizes, this success extends up tod0 \(\approx\)1.2. The propagule-size effect was significant for the survival rate of the compound phenotypes that do not meet the optimum imposed by the host, maximizing the survival rate of the neighboring phenotypes as the propagule size increases. This high probability effect quickly diminishes, depicting a cliff-like pattern for phenotypes survival probabilities higher thand0 \(\approx\)1.2, independent of the propagule size.
Finally, the simultaneous maximization of all parameters (B=7.5;μ =0.1; N0 =200) resulted in a synergetic effect on the probability of success of colonization (Fig. 2). Under this scenario, even host lineages representing distant resources (resources that are less compatible with the pathogen requirements/capacity) have a high probability of colonization, far exceeding the probability observed for the populational parameters of the pathogens tested independently (Fig. 2).
As expected, based in every simulated scenario with a non-null μ , the emergence of phenotypic novelties in the generations following colonization allowed the compound phenotypes to evolve towards and stabilize around the optimal fitness value imposed by the host (Fig. 3). The greater the novelty rate (μ ), the faster the evolution towards the optimum, also increasing the diversity of compound phenotypes (Fig. 3, μ =10-2). During simulations, population size rapidly reaches the established carrying capacity. Even though higher values of μ favors population growth, the carrying capacity is achieved much earlier than phenotype stabilization for all scenarios (Fig. 3). Surprisingly, even in the absence of novelties (Fig. 3, μ=0 ) many simulated pathogen populations persisted and achieved the carrying capacity in the newly colonized host species.
Varying rates of the emergence of evolutionary novelties revealed also an unexpected outcome on the qualitative profile of the populations, following colonization. High rates of emergence resulted in the retention of compound phenotypes (variants) present in the initial and previous populations during populational growth, with correspondingly larger load (something analogous to the concept of genetic load; Wallace 1970) (Fig. 4a). Lower rates of novelty emergence resulted in populations that depict smaller phenotypic variability, with greater loss of pre-existing phenotypes (Fig. 4b). By maximizing every other parameter, the expansion of phenotypes is even larger, indicating that the increased maintenance of ancestral phenotypes is also influenced by other populational parameters besides μ .