Darwin, Natural Selection,
and the Definition of Life
Evolution as explained by Darwin is the modern equivalent of the Bible for those of us who wonder about how things got to be the way they are. His theory of natural selection explained so much about the life that abounds on our planet, and consequently proved useful in so many ways, that its essence soon became accepted as common sense, at least in the more scientifically oriented elements of global society.
But even Darwin didn’t explain everything, because he couldn’t. He could not explain how characteristics were inherited because the world did not yet have genetics. Nor did he have cosmology, or population genetics, or other related sciences that have proliferated since his time. What he did give us was a path that led us through the jungle of our ignorance to a place on the other side, as opposed to the paths others were proposing, paths that just left us milling around inside. What we need now is more paths like the one Darwin gave us that help us get closer to those mysteries on the other side that Darwin exposed for us.
Darwin identified natural selection as an evolutionary process that explains how living things came to be on our planet, without divine intervention, through descent with modifications. That is, living things reproduce themselves, producing copies that are not exactly identical to themselves. The copies that are best adapted to the environment are the ones most likely to survive and reproduce. Over time, this can produce significant changes in the form and the behavior of the life form. The process looks purposeful, but in reality it is no more guided by some overarching intelligence than the law of gravity.
Natural selection is a complicated theory, especially when we examine the copying mechanisms, and the ways the environment and the life form can change each other. But its essence is simple enough so that the concept of life itself can usefully be defined in its terms. That which is subject to natural selection we can consider alive, while everything else is not.1
Natural selection is an evolutionary, anti-entropic process in that it concentrates energy and produces complexity out of diversity, but it is not the only such process. Human intelligence has produced a new environment within which a different anti-entropic process exists, which is known as cultural evolution. We’ll get to that later.
Natural Selection at the Cellular Level
Single-celled organisms generally reproduce when individual cells reach a certain size and split, each half replicating the parent. If the environment is favorable and each half has good prospects for growing up and splitting, then pretty soon the environment is full of them. Once in a while, minor changes occur within individual units during the splitting process, which can be called copying errors. Other changes, even less frequent, may be caused by external forces like radiation, which can be called mutations. Most of these changes have an adverse effect on that cell’s odds of surviving. Those changes may survive for a few generations, if the environment is favorable enough to tolerate their departure from the norm, but the organisms carrying them (or rather, their descendants) eventually get weeded out of the population to the extent they are competing for resources with other, better adapted cells.
Rarely, some changes will occur that favor the organism’s survival. In simpler times, survival depended on the principle of “eat or be eaten”, so any modification of some component of the organism’s structure that enhanced its capacity to sniff out the neighborhood and distinguish between food and foe would likely survive over many generations, and might even spread through the entire population. Such adaptively advantageous changes are the catalysts that make evolution through natural selection possible. They may be so rare that it is hard to conceive of their happening at all. But even if a change only happens to one individual in a million in its particular neighborhood, consider the possibilities if there are a million neighborhoods in the same pool, continuing for a million generations. The odds are good that eventually enough of the lucky mutants will survive to allow natural selection to cut in.
As a general rule, evolution through selection speeds up when the environment changes in ways that put unfamiliar stresses on the population. The new environment may favor copying errors or mutations that are just emerging, or it may make lucky winners out of a minority that already had changes that either had been mildly advantageous or that were latent.
The causes of change in the environment can come on slowly, like an ice age, or abruptly, like a volcanic eruption. The change itself can be generated by life within the ecosystem, if for example a new predator or a new food source appears on the scene. Or the change can be self-generated, if for example a species is so successful it outgrows its home turf and the pressure of its own population forces some individuals out into foreign turf. That can introduce new variables into the process if the new environment differs from the old one, or if the emigrants are not entirely representative of the home population.
If there is no outlet for surplus population, and the environment remains stable, an optimum population will eventually be reached, establishing a durable equilibrium.
Very rarely, something happens that changes the basic parameters of the evolutionary process. Sexual reproduction was such a change.2 It altered the basis for the copying function that enabled the basic cycle of birth and death punctuated by reproduction that is the essence of life as we know it. It did this by transforming reproduction from a relatively straightforward matter of copying to something more like a crap shoot. In the latter, you start with two identical objects, but when you roll them you don’t know whether you’ll end up with snake eyes or box cars or some number in between. In sexual reproduction each parent’s characteristics get mixed together, and one never knows in advance how the offspring will turn out, except that whatever can be identified as a genetically inherited feature will be attributable to one or the other.
When you roll a pair of dice the number of possible combinations is limited. When living organisms mate the range of possible outcomes is several orders of magnitude larger, because the complexities of the DNA code in each individual vastly exceed the simple six possibilities engraved on a single die.
Sexual reproduction was a vast improvement over reproduction by fission from an evolutionary point of view because it greatly increased the possibilities for useful variations to emerge through natural selection. It provided a quantum jump in the odds that any given life form would respond adaptively to changes in the environment. It is certainly one of the most important innovations in the long history of the evolution of life on earth.3
Once sexual reproduction had taken hold in our planet, natural selection came into its own as the primary engine of evolution. It ran amok, producing the huge archipelago of life forms that exist today.
 Cf. The Origins of Life by John Maynard Smith and Eors Szathmary, 1999. My definition of life as that which is produced by natural selection is a simplified version of theirs.
 For simplicity’s sake I have omitted several major breakthroughs involved in the evolution of sexual reproduction. These included “…the development of chromosomes from independent replicators, the emergence of eukaryotic cells from prokaryotic cells, the evolution of multicellular organisms from unicellular organisms, and the development of eusocial colonies.” (Turchin et al. p. 4, Cliodynamics (2012) V.3, Issue 2.)
 The Selfish Gene, by Richard Dawkins, opens with an admirably detailed but lucid explanation of how life evolved during this phase.