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WHY is life so diverse? And why are living things so exquisitely suited to their environments? To understand these two striking features of the natural world you need look no further than evolution. Darwin鈥檚 beautiful idea explains why there are hundreds of thousands of species of beetles and flowering plants, why birds鈥� feathers are ideal for flight and insulation, and why a desert plant possesses hairy leaves to reduce water loss. The Origin of Species was published in 1859, and time has not eroded Darwin鈥檚 insights.

Yet all scientific theories must incorporate new ideas and findings, and evolution is no exception. In recent years, our understanding of biology has taken huge strides. Advances in genetics, epigenetics and developmental biology challenge us to think anew about the relationship between genes, organisms and the environment, with implications for the origins of diversity and the direction and speed of evolution. In particular, new findings undermine the idea, encapsulated by the 鈥渟elfish gene鈥� metaphor, that genes are in the driving seat. Instead, they suggest that organisms play active, constructive roles in their own development and that of their descendants, so that they impose direction on evolution.

Some biologists are trying to shoehorn the new knowledge into traditional evolutionary thinking. Others, myself included, believe a more radical approach may be required. We don鈥檛 deny the roles of genetic inheritance and natural selection, but think we should look at evolution in a markedly different way. .

Our current framework for thinking about evolution emerged only in the 1940s, with the integration of new knowledge about evolutionary processes and biological inheritance. This so-called modern synthesis is at the heart of how most people understand evolution. According to this view, the evolution of the features of an organism 鈥� collectively known as its phenotype 鈥� comes down to random genetic mutation, genetic inheritance and selection of those gene variants that bestow traits best adapted to the environment.

The modern synthesis has served us well: evolutionary biology is developing and thriving. But discoveries made over the past two decades are starting to reveal cracks in some of its central ideas.

Not by genes alone

Take the notion that heredity happens via genes alone. In a classic 19th-century experiment, German biologist August Weismann cut off the tails of generations of mice, bred from the amputees, and found no reduction in tail length. This led to the view that genetic mutations in the germ line (eggs and sperm) are the only changes passed on to the next generation. But recent experiments suggest a more complex picture.

We now know that things other than genes are transmitted from parents to offspring. These include components of the egg, hormones, symbionts (microorganisms that live inside bodies), epigenetic marks (compounds that bind to DNA and turn genes on and off), antibodies, ecological resources and learned knowledge. At least some of these can lead to stable inheritance of phenotypes. For example, the transmission of epigenetic marks across generations is extremely widespread and, in plants, it can account for differences in fruit size, flowering time and many other traits. Epigenetic changes are often induced by changes in conditions within cells or the external environment, such as temperature, stress or diet, and unlike random mutations are often adaptive. Likewise, many animals inherit knowledge from their parents. Cultural inheritance occurs in hundreds of species, not just humans or vertebrates, but invertebrates such as bees and crickets too, creating similarities between even unrelated individuals.

These and many other findings suggest that the current focus on genetic mutations only captures part of the story of adaptive evolution 鈥� the slowly changing part. The broader view shows there are other ways to generate heritable variety. It also undermines the clean separation of development and heredity that Weismann鈥檚 theory promoted. It is time to let go of the idea that the genes we inherit are a blueprint to build our bodies. Genetic information is only one factor influencing how an individual turns out.

And that鈥檚 not all. We now also know that a given set of genes has the potential to produce a variety of phenotypes, depending on the environment in which the organism develops. This ability, called developmental plasticity, used to be dismissed as 鈥渘oise鈥� or mere 鈥渇ine-tuning鈥�, but recent research suggests it may play a far more active role in the evolutionary process. As well as being able to respond in specific ways to particular conditions, organisms seem to have evolved the ability to respond flexibly to whatever conditions they experience. This adaptability results from a sort of Darwinian evolution occurring within organisms. It鈥檚 as if each organism evolves as it develops, by generating new variation and selecting what works. This allows systems such as the immune system, nervous system and behavioural systems (through learning) to adjust to meet whatever environment the individual faces.

A flexible phenotype allows organisms to survive in the short term, and may then initiate evolutionary episodes 鈥� with genetic change following later. Consistent with this idea, several experiments reveal that organisms exposed to new environments develop characteristics that resemble those of closely related species adapted to these same environments.

For instance, marine sticklebacks reared on diets that are either benthic (bottom-feeding) or limnetic (mid-water) grow to resemble populations adapted to life in the corresponding environment. This suggests that adaptations may commonly arise through immediate responses to the environment, with natural selection favouring such individuals and subsequently cementing the useful features through genetic evolution.

There is also experimental evidence in insects, fish and amphibians that environmentally induced forms can evolve reproductive isolation, meaning that after a while they can no longer interbreed with other members of their species 鈥� a key step towards speciation. So developmental plasticity may play a critical role in both adaptation and speciation.

Features of development also undermine orthodox ideas about what factors influence the direction of evolution. The modern synthesis places natural selection in control, regarding it as the sole explanation for adaptation. Evolutionary biologists have tended to think that evolution is not biased in any particular direction, since genetic mutation is assumed to occur at random. However, this idea is challenged by 鈥渄evelopmental bias鈥� 鈥� the fact that certain characteristics can develop more easily than others. This raises the intriguing possibility that the diversity of life may not only reflect the survival of the fittest but also the arrival of the frequent-est.

Developmental bias could help explain some fascinating quirks of evolution. Consider parallel radiation, in which a species in one location diversifies into several distinct forms and, independently, the same diversification occurs in a different location. A famous example is cichlid fishes living in Lakes Malawi and Tanganyika in Africa. Here many species exhibit striking similarities in body shape with different species from the other lake, despite being more closely related to species from their own lake. These body shapes are adaptive, so natural selection has certainly been at play. But the forms we see are not necessarily the only possible adaptive solutions. This suggests there are features of cichlid development that make certain forms particularly likely to arise. Developmental bias could also help explain why cichlids 鈥� and some other groups of organisms 鈥� are so diverse. It is perhaps because they are particularly good at producing novel variants that can exploit ecological opportunities.

This creative role for development contrasts with its traditional role of imposing constraints on adaptation. Constraints explain the absence of evolution or adaptation, so have been of limited interest. Many evolutionary biologists are now questioning whether this is the best way to think. Perhaps, rather than merely setting limits on what forms are available for selection, developmental bias directs evolution by generating the tramlines along which the engine of selection can proceed.

Not passive observers

There is a further way in which organisms might direct their own evolution. Selection is portrayed as a process in which external agents, such as environmental conditions, sort between alternative variants according to their suitability. This is too passive. Organisms are not merely buffeted around by the forces of nature; through their habitat choices and the way they modify their environment, they play active roles in determining which of their characteristics are useful. So they create some of the conditions of their existence, and this influences their evolution.

By building a nest, which reduces temperature fluctuations, for example, a bird weakens selection on the need for physiological regulation of egg temperature, but creates selection for refinements in nest design. Likewise, selection shapes a mammal that digs a burrow less for ways to counter predation, and more for resistance to fungal diseases. Such niche construction is not random, but systematic and directional. The animal manipulates the environment in a consistent, reliable way, to suit itself. In doing so, it biases the action of natural selection, imposing a direction on its own evolution, in much the same way that an animal breeder selects for particular traits in livestock.

Taken together, these discoveries challenge some of the fundamental assumptions of the modern synthesis (see 鈥�Modern vs postmodern鈥�). This new approach gives organisms a central role in their own evolution, and suggests that novel variation frequently begins not with mutation, but with changes in phenotypes. It indicates that the direction of evolution does not depend on selection alone.

There are two ways to view these new findings: we can try to incorporate them into the old framework, or we can extend the framework. Most evolutionary biologists take the first course, viewing plasticity and niche construction as being under genetic control, and seeing non-genetic inheritance as rare, unstable or functionally equivalent to genes. This view allows genes and selection to retain their explanatory prominence, at the price of downplaying new evidence. The alternative approach is to accept that the modern synthesis struggles to account for the new findings and to propose a broader alternative 鈥� an extended evolutionary synthesis. The two types of explanation can then be compared for their predictive power and ability to explain evidence, as well as their productivity in spawning new research questions and methods.

Evolutionary biologists who embrace this second approach are now acting on it. Earlier this year, an international consortium of 50 biologists and philosophers from eight universities announced a new research programme to investigate the evolutionary consequences of non-genetic inheritance, developmental plasticity and bias, and niche construction (see 鈥�Time for change?鈥�). These are exciting times for evolutionary biology, as the full ramifications of these ideas are explored rigorously for the first time. It remains to be seen whether our efforts will change the orthodox view. What is certain is that over the coming years, these advances will increasingly become the focus for evolutionary biologists.

My own view is that a new conceptualisation of evolution is emerging. The selfish gene has proved to be a powerful and instructive metaphor, but the evidence now suggests it is misleading. Far from being master molecules, genes turn out to be just one of many channels through which cells respond to environmental inputs, and just one of several sources of heredity. Organisms are not the 鈥渢hrowaway survival machines鈥� envisaged by Richard Dawkins and others, but instead often take the lead in their own evolution, dragging genetic change along in their wake. Move over selfish gene, and make way for the orchestrating organism.

This article appeared in print under the headline 鈥淓volution evolves鈥�