Most biologists take for granted the idea that all life evolved by natural selection over billions of years. They get on with researching and teaching in disciplines that rest squarely on that foundation, secure in the knowledge that natural selection is a fact, in the same way that the Earth orbits the Sun is a fact.
Given that the concepts and realities of Darwinian evolution are still challenged, albeit rarely by biologists, a succinct briefing on why evolution by natural selection is an empirically validated principle is useful for people to have to hand. We offer here 15 examples published by Nature over the past decade or so to illustrate the breadth, depth and power of evolutionary thinking. We are happy to offer this resource freely and encourage its free dissemination.

15 EVOLUTIONARY GEMS
Henry Gee, Rory Howlett and Philip Campbell*

Gems from the fossil record
1 Land-living ancestors of whales
2 From water to land
3 The origin of feathers
4 The evolutionary history of teeth
5 The origin of the vertebrate skeleton
Gems from habitats
6 Natural selection in speciation
7 Natural selection in lizards
8 A case of co- evolution
9 Differential dispersal in wild birds
10 Selective survival in wild guppies
11 Evolutionary history matters
Gems from molecular processes
12 Darwin’s Galapagos finches
13 Microevolution meets macroevolution
14 Toxin resistance in snakes and clams
15 Variation versus stability

*Henry Gee is a senior editor for Nature; Rory Howlett is a consultant editor for Nature; Philip Campbell is editor-in-chief of Nature.

Spontaneous behaviors and evoked responses are traditionally viewed as separate and distinctly different phenomena. This distinction underlies the standard dichotomies between reflex and voluntary behaviors, and between evoked responses and the behavioral states or contexts in which they occur. In the typical tightly controlled laboratory environment, it is convenient to focus on stimulus-evoked behaviors and to treat spontaneous behaviors as simply nuisances to be minimized or averaged out. This focus ignores the critical role of spontaneous activity in creating the behaviors evoked by specific stimuli.
Drs. Wolpaw and Brembs will discuss recent vertebrate and invertebrate research indicating that spontaneous and evoked behaviors are not only inextricably linked but are, in important respects, no longer distinguishable from each other. This erosion of the traditional distinction is driven by the recognition that spontaneous variability is a necessary feature of all adaptive behaviors, and that even the simplest reflexes and stereotypic responses are controlled by sensory feedback. In contrast to the standard stimulus-response concept, the central thesis is that spontaneous behaviors provide a substrate that interacts with the environment so as to produce and maintain adaptive (i.e., appropriate) behaviors.
Dr. Wolpaw will discuss evidence that the quintessential evoked behaviors of vertebrates – reflexes mediated by spinal pathways – are highly plastic and display dependencies on past experience like those of spontaneous behaviors. The stimuli that evoke these reflexes are basically probes that assess ongoing spontaneous activity, and the resultant sensory feedback modifies that activity. Furthermore, the high variability of these reflexes underlies their adaptive capacities. He will also consider how the artificial behaviors created by brain-computer interfaces (BCIs) – behaviors produced by brain signals rather than muscles – arise largely from the interaction of spontaneous brain activity with sensory feedback. By connecting spontaneous activity to external consequences, BCIs create another situation in which spontaneous activity is shaped into adaptive behavior.
Dr. Brembs will discuss studies in several invertebrate species indicating that evoked behaviors are derived traits with reduced features, and have evolved from initially spontaneous behaviors. For example, in the fruit fly Drosophila, mathematical analyses of the temporal structure of spontaneous behavioral variability and genetic dissection of several feedback-based learning systems indicate that behavioral variability is not noise, but rather is essential to the brain’s capacity to use sensory feedback to create and maintain effective forward models for behavioral control. This function is implemented by a hierarchical interaction of learning systems (partially mediated by the mushroom-bodies) that shapes spontaneous actions into habitual responses.
In sum, converging evidence from disparate taxa and seemingly unrelated preparations indicates that spontaneous behaviors and evoked responses are opposite ends of the same spectrum. The data suggest that behavioral flexibility arises from an interaction of spontaneous and evoked behaviors that reflects individual history and genetic endowment and is continuously updated. This interaction maintains a balance between flexible exploration and efficient exploitation and ensures adaptive behavioral choice.

K. Simons's Editorial "The misused impact factor" (10 October, p. 165) reminded me of the Tolstoy story in which his brother tells him not to think about a white bear: When you say, "Don't think about it," it becomes hard to think of anything else. Instead, AAAS and Science should take the lead by defining some declarative "bibliometric postulates" that specify a code of conduct for the use of simplified indices. Postulates might include "numerical factors applied to journals should not be used to evaluate individuals" or "numerical journal factors should never be quoted without disclaimers explaining that they include falsified papers." Only an organization like Science with the backing of the non-commercially beholden AAAS has the profile to defend itself and scientists from enslavement to the impact factor's absurdly one-dimensional parameterization of achievement.

No prestige should be attached to the level of funding that an investigator has managed to secure. The most basic of truths must be emphasized: Money is a means, not an end. We do not do science to get money. We get money to do science. Funding cannot be a measure of productivity, because scientists do not produce research dollars. Research dollars are produced by taxpayers [...]. The amount of money spent by a researcher is not a measure of his productivity, but of his consumption, and might even be counted on the negative side of the ledger when he is evaluated.