The history of life on Earth seems to show a clear trend; for example, it seems intuitive that there is a trend towards increasing complexity in living organisms. More recently evolved organisms, such as

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, appear to be much more complex than organisms, such as

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

, which have existed for a much longer period of time. However, there are

theoretical A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...

and

empirical Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...

problems with this claim. From a theoretical perspective, it appears that there is no reason to expect evolution to result in any largest-scale trends, although small-scale trends, limited in time and space, are expected (

Gould Gould may refer to: People * Gould (name), a surname Places United States * Gould, Arkansas, a city * Gould, Colorado, an unincorporated community * Gould, Ohio, an unincorporated community * Gould, Oklahoma, a town * Gould, West Virginia, a ...

, 1997). From an empirical perspective, it is difficult to measure complexity and, when it has been measured, the evidence does not support a largest-scale trend (McShea, 1996).

History

Many of the founding figures of evolution supported the idea of ''Evolutionary progress'' which has fallen from favour, but the work of

Francisco J. Ayala Francisco José Ayala Pereda (born March 12, 1934) is a Spanish-American evolutionary biologist, philosopher, and former Catholic priest who was a longtime faculty member at the University of California, Irvine and University of California, Dav ...

and

Michael Ruse Michael Ruse (born 21 June 1940) is a British-born Canadian philosopher of science who specializes in the philosophy of biology and works on the relationship between science and religion, the creation–evolution controversy, and the demarcatio ...

suggests is still influential.

Hypothetical largest-scale trends

McShea (1998) discusses eight features of organisms that might indicate largest-scale trends in evolution:

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

, energy intensiveness, evolutionary versatility, developmental depth, structural depth, adaptedness, size,

complexity Complexity characterises the behaviour of a system or model whose components interaction, interact in multiple ways and follow local rules, leading to nonlinearity, randomness, collective dynamics, hierarchy, and emergence. The term is generall ...

. He calls these "live hypotheses", meaning that trends in these features are currently being considered by evolutionary biologists. McShea observes that the most popular hypothesis, among scientists, is that there is a largest-scale trend towards increasing complexity. Evolutionary theorists agree that there are local trends in evolution, such as increasing

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

size in

hominid The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); ''Gorilla'' (the east ...

s, but these directional changes do not persist indefinitely, and trends in opposite directions also occur (

, 1997). Evolution causes organisms to adapt to their local environment; when the environment changes, the direction of the trend may change. The question of whether there is evolutionary progress is better formulated as the question of whether there are any largest-scale trends in evolution (McShea, 1998). That is, is there a consistent directional change throughout the history of life on Earth?

Theoretical perspective

Organisms adapt to their local environment. As long as the local environment is stable, we can expect to observe small-scale trends, as organisms become increasingly adapted to the local environment.

(1997) argues that there are no global (largest-scale) trends in evolution, because traits that are advantageous for some local environment are detrimental for some other local environment. Although it is difficult to measure complexity, it seems uncontroversial that mammals are more complex than bacteria. Gould (1997) agrees, but claims that this apparent largest-scale trend is a statistical artifact. Bacteria represent a minimum level of complexity for life on Earth today. Gould (1997) argues that there is no

selective pressure Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...

for higher levels of complexity, but there is selective pressure against complexity below the level of bacteria. This minimum required level of complexity, combined with random

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

, implies that the average level of complexity of life must increase over time. Gould (1997) uses the analogy of a

random walk In mathematics, a random walk is a random process that describes a path that consists of a succession of random steps on some mathematical space. An elementary example of a random walk is the random walk on the integer number line \mathbb Z ...

that begins near a wall. Although the walk is random, the walker cannot pass through the wall, so we should expect the walker to move increasingly further from the wall as time passes. This does not imply that the walker is driven away from the wall. The wall is analogous to the complexity level of bacteria. We should expect evolution to wander increasingly further from this level of complexity, but it does not imply that evolution is driven towards increasing complexity. In response to Gould's (1997) critique, Turney (2000) presents a computational model in which there is a largest-scale trend towards increasing evolutionary versatility. This trend requires continual change. Although this model shows that largest-scale trends are compatible with evolutionary theory, the model has not yet been empirically confirmed.

Empirical perspective

Evolutionary theory might not predict largest-scale trends, but there may be such trends nonetheless. McShea (1996) looks at the empirical evidence for a trend towards increasing complexity in Metazoan fossils. He concludes that the evidence is not decisive and further investigation is required.

References

*Darwin, C. (1859)
''On the Origin of Species By Means of Natural Selection, or, the Preservation of Favoured Races in the Struggle for Life''
*Gould S.J. (1997). ''Full House: The Spread of Excellence from Plato to Darwin''. New York: Harmony. * (on the applicability of this notion to the study of

social evolution {{unreferenced, date=February 2015 ''Social Evolution'' is the title of an essay by Benjamin Kidd, which became available as a book published by Macmillan and co London in 1894. In it, Kidd discusses the basis for society as an evolving phenomenon ...

). *McShea, D.W. (1996). Metazoan complexity and evolution: Is there a trend? ''Evolution'', 50, 477-492. *McShea, D.W. (1998). Possible largest-scale trends in organismal evolution: Eight "Live Hypotheses". ''Annual Review of Ecology and Systematics'', 29, 293-318. *Markov Alexander V., and Andrey V. Korotayev (2007
"Phanerozoic marine biodiversity follows a hyperbolic trend"
''

Palaeoworld ''Palaeoworld'' is a peer-reviewed academic journal with a focus on palaeontology and stratigraphy research in and around China. It was founded in 1991 by the Nanjing Institute of Geology and Palaeontology at the Chinese Academy of Sciences ( ...

'' 16(4): pp. 311-318. *Turney, P.D. (2000), A simple model of unbounded evolutionary versatility as a largest-scale trend in organismal evolution. ''Artificial Life'', 6, 109-128. {{doi-inline, 10.1162/106454600568357, DOI 10.1162/106454600568357,
OAI arXiv.org:cs/0212021

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