The Extended Evolutionary Synthesis

Image credits – clockwise from top-left: ^{1 2 3 4 5 6}

Extending the Synthesis

The Extended Evolutionary Synthesis is a development of evolutionary biology in which new knowledge and thinking are changing the view of how organisms adapt and evolve. Integration of genetics with the Darwin Wallace theory of natural selection in the Modern Evolutionary Synthesis (MES) provided a very successful explanation of the origins of life that remains core to evolutionary science today. But in the 1970s the beguiling sense that the MES approached a complete description of evolution’s mechanism began to be disturbed by new thinking and research. For example, in two important publications Francois Jacob and Steven Jay Gould, from different perspectives, both challenged assumptions about the source of creativity in biological adaptation. These arguments opened new directions in research on how evolution takes place, not least with regard to the role of embryological development in evolutionary change. The photographs above illustrate an interesting example of this.

The oddness of centipede legs

The photographs above illustrate an interesting example of the science that the Extended Evolutionary Synthesis involves. Centipedes are a large animal group with thousands of different species. They have segmented bodies with each segment bearing a pair of legs. The numbers of legs different centipede species have varies a lot, ranging from 15 to well over a hundred pairs. But, despite that wide range, there is a rule about centipede leg numbers: the number of body segments, and hence leg-pairs, is always odd. It is hard to imagine a competitive advantage coming from having an odd, rather than even, number of leg-pairs. That presents a problem for the idea that natural selection is the source of evolutionary direction: if there is no advantage how can there have been selection for the odd leg-pair rule? Instead, this centipede characteristic may be an example of bias in embryological development. Bias may arise from restraints on the ways in which embryological development of centipedes can proceed, leading to restrictions in the forms of their morphology that are possible.

This implies that evolutionary direction can be influenced by developmental factors as well as natural selection. By development we mean the growth and differentiation of multicellular organisms from fertilized eggs. Developmental biology is the study of this; it includes embryology but isn’t limited to it, covering wider processes as well. Since it is about how organisms are formed, development is one of the most important, most fundamental components of biology. Despite this, developmental processes were not incorporated into the MES model, and were at best peripheral in much of the evolutionary biological research that stemmed from it. Evolutionary developmental biology (the term is often shortened to “evo devo”) is a modern aspect of biological research that concerns how the processes, the mechanisms of biological development evolved, together with how they themselves influence evolution. It is a major addition to the MES and how it now continues to be extended.

In the examples that follow we describe some of the profound and in some cases amazing biology the Extended Evolutionary Synthesis has revealed, particularly with regard to how environments directly influence adaptation and evolution. We start with an example from distant time.

Scroll to bottom of page to comment

Image credits

1. Wellima; https://uk.inaturalist.org/observations/146465407 [↩]
2. Trey Dornn; https://uk.inaturalist.org/photos/275975502 [↩]
3. Marie Lou Legrand; https://uk.inaturalist.org/observations/63625848 [↩]
4. wake_nc; https://uk.inaturalist.org/photos/264976426 [↩]
5. Merav Vonshak; https://uk.inaturalist.org/observations/79739487 [↩]
6. Jane Orgee; https://uk.inaturalist.org/observations/150014400 [↩]