Evolution has a long tradition. Accordingly, it is relevant to start with an introduction to some major ideas in the field. Initially Spencer used the concept of evolution for physical 'transformation'. Darwin added the idea of the evolution of species, by means of natural selection. Later the Modern Synthesis added insights in genetics, and gene-based calculus. Still later new discoveries such as epigenetics, and horizontal gene transfer urged for an Extended Synthesis. Meanwhile, consensus about a definition of evolution sensu lato, that is, not just for biology, is lacking. Yet, an understanding of what we actually mean with 'evolution' is a necessary philosophical basis for telling whether or not such different things as viruses, computer viruses, species, genetic algorithms and ideas ('memes') actually can 'evolve'. On this page we offer a selection of major ideas about evolution. The next page explores the possibility to define evolution in a general way.


For more references and information about this topic:

A rethink of evolution, and the construction of a family of ‘patterns of evolution’ inside and outside biology
Gerard A.J.M. Jagers op Akkerhuis, Hendrik Pieter Spijkerboer, Hans-Peter Koelewijn Academia.edu 2014

Erasmus Darwin

The grandfather of Charles Darwin. He already used the idea of evolution in his poems (1803 "The temple of nature" (posthume)).

Herbert Spencerexternal image 220px-Spencer_Herbert_Age_38.jpg

Spencer used evolution in a very broad sense, defining it as a general process of 'transformation' towards ordered states. Accordingly, it was possible to regard the formation of the universe as an evolutionary process, which naturally included the emergence of life and humanity. After having read Darwin's "Origin" (1859) Spencer coined the phrase "survival of the fittest" in his book Principles of Biology (1898, Vol. 1, 530-531) stating that: "This survival of the fittest which I have here sought to express in mechanical terms, is that which Mr. Darwin has called 'natural selection, or the preservation of favoured races in the struggle for life.' In a later work (Progress: Its Law and Cause', Westminster Review (1857), 67, 446-7.) Spencer described evolution in the following way: "Now, we propose in the first place to show, that this law of organic progress is the law of all progress. Whether it be in the development of the Earth, in the development of Life upon its surface, in the development of Society, of Government, of Manufactures, of Commerce, of Language, Literature, Science, Art, this same evolution of the simple into the complex, through a process of continuous differentiation, holds throughout. From the earliest traceable cosmical changes down to the latest results of civilization, we shall find that the transformation of the homogeneous into the heterogeneous is that in which progress essentially consists." A consequence of using 'continuous differentiation' and 'transformation' as a basis for evolution, is that evolution thus defined includes the crystallisation of snowflakes and the cluttering of galactic dust clouds to stars and planets.

Malthus

Malthus indicated that any population which grows exponentially will eventually suffer from overpopulation. Accordingly, many of the offspring that will be produced must die, or the number of offspring per parent must decrease as a function of population density. According to this viewpoint, the individuals of a succesfull species will eventually become their own worst compatitors.

Darwin and Wallace

Darwin_and_Wallace.jpg
Charles Darwin and Alfred Russel Wallace
Due to the ideas of Darwin and Wallace (1858), the thinking about evolution becamefocused on organisms and their multiplication. Their approach included certain mechanisms that could explain the origin of species, including: 1. reproduction, 2. offspring with variable inherited properties, 3. competition, and 4. less reproduction by less adapted individuals. Differences in reprodutive capacity were analysed by Darwin in terms of "selection", about which Darwin remarked: "This preservation of favourable variations and the rejection of injurious variations, I call natural selection” (Darwin 1859, pp 80-81). Because it compares favourable and injurious variations the concept of selection can be viewed as a difference measure, or as a pattern, but not as a process. This set of ideas offered a new understanding of evolution. At the same time it had a narrowing effect: evolution became stronlgy associated with organisms.

Mendel and de Vries

Darwin and contemporaries could not yet explain heredity. Most explanations focused on the idea of 'blending inheriance', in which traits could be blended as different colors of paint. This changed when Gregor Mendel (1865) and de Vries (1889) studied the statistical laws governing the redistribution of the parental properties in the offspring. Their studies suggested that the heridity of properties was 'discontinuous': a specific property was or was not inherited by the offspring. In relation to these results, the existence of physical structures that carryed genetic properties was proposed by Mendel. De Vries named these carryers pangenes, which was shortened to 'genes' by Johannsen.

The Modern Synthesis.

Following the acceptence of genes as the structural basis for inheritance, it took between 1918 and about 1950 for evolutionary theory to incorporate these insights and establish a new, generally accepted theoretical framework, which was called the 'Modern synthesis'. A summary of the modern synthesis was presented by Ernst Mayr (1982, pp. 479–480), indicating key facts and the inferences drawn from them:
  • Every species is fertile enough that if all offspring survived to reproduce the population would grow.
  • Despite periodic fluctuations, populations remain roughly the same size.
  • Resources such as food are limited and are relatively stable over time.
  • A struggle for survival ensues.
  • Individuals in a population vary significantly from one another.
  • Much of this variation is inheritable.
  • Individuals less suited to the environment are less likely to survive and less likely to reproduce; individuals more suited to the environment are more likely to survive and more likely to reproduce and leave their inheritable traits to future generations, which produces the process of natural selection.
  • This slowly effected process results in populations changing to adapt to their environments, and ultimately, these variations accumulate over time to form new species.
One may add to the points two and tree that species are not only limited by food, but also by predation.

The DNA double helix

The structure of the double helix of DNA was discovered by James. D. Watson and Francis Crick (1953). It marked the start of the era of the molecular genetics.

An 'Extended Synthesis'

Extended_synthesis_Pigliucci.jpg
Figure 1: The Extended Synthesis: Pigliucci and Müller 2010
As is explained in 'The Extended Synthesis' (Pigliucci and Müller, 2010), there is a need for a new integrative theory because "Much of the confusion and resistance to new ideas may derive from the fact that evolutionary biologists of course have gradually updated their thinking beyond the Modern Synthesis, without necessarily paying too much attention to the fact that in so doing, they have stepped well outside of its original boundaries." As is indicated by the outer rim of Figure 1, a range of new concepts has been added to evolutionary thinking, the implications of which are explained in the book by Pigliucci and Müller.

Symbiogenesis and endosymbiosis

Endosymbiosis refers to one or more cells that live as obligate symbionts inside another cell. Also a multicellular organism consists of cells that must work together to survive and reproduce. These topics can be considered part of multilevel selection as mentioned in the Extended Synthesis, but this does not automatically include cooperation and facilitation. Important contributions to the emergence of multicellular units in evolution, and corresponding new levels of natural organisation have been made e.g. by Kropotkin (1890-1896, Mutual aid, a factor of evolution), Kozo Polansky (1920, Symbiogenesis), Margulis (1970, Origin of eukaryotic cells) and, while focusing on competition reduction, the so called 'major evolutionary transitions' of Maynard Smith and Sathmary (1995).

Extending the concept of evolution

The scientific understanding of topics that are relevant for biological evolution increases rapidly. In this light, any desire to generalise evolution even further may seem a bit counterintuitive. But do we really understand evolution? For example, the extended synthesis offers many relevant mechanisms, but does it offer a simple definition fitting any and all evolutionary events? Such a generalised definition is not just a luxury, but is a basic requirement for knowing whether a certain process in fact represents evolution, or not. How about viruses, memes (the alledged units of social evolution) and genetic algorithms? If Darwin does not include their evolution, what generalisation does? And some people call the slow changes of a river flood bed evolution, but is it? Clearly, if a simple definition of evolution can be found, it should distinguish evolutionary processes from any and all other processes. The search for a shortest possible core-definition of evolution, and the attempt to link different evolution approaches, has not come to an end, as will be discussed on the next page.