Perhaps no idea has proven more controversial within evolutionary biology than the idea that evolution manifests progress. To some biologists evidence of progress in the history of life has seemed undeniable. From utterly simple beginnings have come organisms of astounding complexity and sophistication. But to others the hazy notion of evolutionary progress appears distinctly unscientific inasmuch as it is thought to require subjective value judgements that have no place in a science based squarely on empirical facts. Debates over evolutionary progress can be substantially, albeit not entirely, resolved by examining some of the assumptions underlying such debates and by attending more carefully to the definition of key concepts such as ‘direction’ and ‘improvement’. Doing so suggests that acceptance of a moderate form of evolutionary progress can accommodate evidence from the history of life while satisfying standards of scientific objectivity.

Key Concepts:

• The concept of ‘progress’ has been controversial within evolutionary biology.
  
• Resolving disagreements about evolutionary progress requires careful examination of theoretical, empirical and conceptual issues.
  
• Evolutionary progress may be defined as intergenerational directional change embodying improvement in the properties characterising a population of biological entities.
  
• A process has a direction over a given time interval if the value of one of its properties increases during that time interval.
  
• Directional change, which is an observable fact, must be distinguished from directionality, that is, a tendency to change in a specific direction.
  
• It is important to distinguish uniform, net and apex directional change.
  
• Critics of the idea of evolutionary progress complain that ‘improvement’ is a value‐laden term that cannot be given an objective scientific definition.
  
• Improvement in the evolutionary sense is any directional change that contributes to the solution of a problem facing biological entities in a particular environment.
  
• A number of biologists, beginning with Darwin, have argued that evolution manifests progress in the sense defined here.
  
• If the ability of organisms in even one lineage to cope with some problem impacting their chances for survival and reproduction has improved over some time interval, then evolutionary progress, at least in this limited sense, is a scientific fact.