Table 3 - . Key conceptual issues of early Darwinism and their resolution through the Modern Synthesis.

Difficulties of Early Darwinism	Resolution by the Modern Synthesis
Lack of a hereditary mechanism	Integration of Mendelian genetics showed that discrete genes and alleles could explain inheritance and provide material for natural selection (Fisher, Haldane, Wright).
Difficulty explaining continuous variation	Population genetics demonstrated that Mendelian inheritance, combined with multiple loci, could produce continuous variation required for gradual evolution.
Unclear mechanism for adaptation	Quantitative models and empirical data linked allele frequency changes to natural selection, making adaptation mathematically and empirically tractable.
Limited understanding of speciation	Ernst Mayr emphasized reproductive isolation and the biological species concept, explaining how new species arise through accumulation of genetic differences.
Insufficient integration with paleontology	Simpson and others connected fossil records with population-level processes, showing evolutionary patterns over long timescales.
Focus on animals, neglecting plants	G. Ledyard Stebbins extended evolutionary principles to plant biology, integrating botany into the unified framework.
Lack of predictive and quantitative framework	The Modern Synthesis established a coherent, mathematical, and population-based approach, making evolutionary biology predictive and testable.