Changes in assumptions:
| modern synthesis | extended evolutionary synthesis |
|---|---|
| the pre-eminance of natural selection | reciprocal causation (organisms shape, and shaped by, selective and developmental environments) |
| genetic inheritance | inclusive inheritance |
| random genetic variation | non-random phenotypic variation |
| gradualism | variable rates of change |
| gene-centred perspective | organism-centred perspective |
| micro-evolution | macro-evolution |
For classical MS, the major directing or creative influence in evolution is natural selection, which alone explains why the properties of organisms match the properties of their environments (adaptation). Genes constitute the only general inheritance system. Acquired characters are not inherited. There is no relationship between the direction in which mutations occur - and hence the supply of phenotypic variants - and the direction that would lead to enhanced fitness. Evolution via mutations of large effects is unlikely because such mutations have diruptive pleiotropic effects. Phenotypic transitions typically occur through multiple small steps, leading to gradual evolutionary change. Evolution requires, and is often defined as, change in gene frequencies. Populations evolve through changes in gene frequencies brought about through natural selection, drift, mutation and gene flow. Macro-evolutionary patterns are explained by micro-evolutionary processes of selection, drift, mutation and gene flow.
For ESS, developmental processes, operating through developmental bias and niche construction, share with natural selection some responsibility for the direction and rate of evolution and contribute to organism-environment complementarity. Inheritance extends beyond genes to encompass (transgenerational) epigenetic inheritance, physiological inheritance, ecological inheritance, social (hehavioural) transmission and cultural inheritance. Acquired characters can play evolutionary roles by biasing phenotypic variants subject to selection, modifying environments and contributing to heritability. Developmental bias, resulting from non-random mutation or phenotypic accommodation, means that some phenotypic variants are more likely than others. Developmental systems facilitate well-integrated, functional phenotypic responses to mutation or environmental induction. Variants of large effect are possible, allowing for rapid evolutionary change. Saltation can occur either through mutations in major regulatory control genes expressed in tissue-, module- or compartment-specific manners, or when developmental processes respond to environmental challenges with change in coordinated suites of traits, or through nonlinear threshold effects. Developmental systems can facilitate adaptive varaiation and modify selective environments. Evolution redefined as a transgenerational change in the distribution of heritable traits of a population. There is a broadened notion of evolutionary process and inheritance. Additional evolutionary processes, including developmental bias and ecological inheritance, help explain macro-evolutionary patterns and contribute to evolvability.