Largest-scale trends in evolution
Encyclopedia
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 recent organisms, such as mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, appear to be much more complex than older organisms, such as bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. However, there are theoretical
Theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...

 and empirical
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....

 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
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

, 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).

Hypothetical largest-scale trends

McShea (1998) discusses eight features of organisms that might indicate largest-scale trends in evolution:
  • entropy
    Entropy
    Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...

  • energy intensiveness
  • evolutionary versatility
  • developmental depth
  • structural depth
  • adaptedness
  • size
  • complexity
    Complexity
    In general usage, complexity tends to be used to characterize something with many parts in intricate arrangement. The study of these complex linkages is the main goal of complex systems theory. In science there are at this time a number of approaches to characterizing complexity, many of which are...



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.

History of the idea of evolutionary progress

Prominent historical figures who have championed some form of evolutionary progress include Alfred Russel Wallace
Alfred Russel Wallace
Alfred Russel Wallace, OM, FRS was a British naturalist, explorer, geographer, anthropologist and biologist...

, Herbert Spencer
Herbert Spencer
Herbert Spencer was an English philosopher, biologist, sociologist, and prominent classical liberal political theorist of the Victorian era....

, Pierre Teilhard de Chardin, and Henri Bergson
Henri Bergson
Henri-Louis Bergson was a major French philosopher, influential especially in the first half of the 20th century. Bergson convinced many thinkers that immediate experience and intuition are more significant than rationalism and science for understanding reality.He was awarded the 1927 Nobel Prize...

. Even Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

 seems to have believed in some form of progress (Darwin, 1859):


[Chapter 10] The inhabitants of each successive period in the world's history have beaten their predecessors in the race for life, and are, insofar, higher in the scale of nature; and this may account for that vague yet ill-defined sentiment, felt by many palaeontologists, that organisation on the whole has progressed.



[Chapter 14] As all the living forms of life are the lineal descendants of those which lived long before the Silurian epoch, we may feel certain that the ordinary succession by generation has never once been broken, and that no cataclysm has desolated the whole world. Hence we may look with some confidence to a secure future of equally inappreciable length. And as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection.


Ruse
Michael Ruse
Michael Ruse is a philosopher of biology at Florida State University, and is well known for his work on the creationism/evolution controversy and the demarcation problem in science...

 (1997) presents a detailed and carefully researched survey of the idea of progress in evolutionary biology. He argues that belief in evolutionary progress is still prevalent among evolutionary biologists today, although it is often denied or veiled. Ruse (1997) writes, "A major conclusion of this study is that some of the most significant of today's evolutionists are progressionists, and that because of this we find (absolute) progressionism alive and well in their work." He claims that progressionism has harmed the status of evolutionary biology as a mature, professional science.

In examining the issue of evolutionary progress, the first step is to define progress. Ayala
Francisco J. Ayala
Francisco José Ayala Pereda is a Spanish-American biologist and philosopher at the University of California, Irvine. He is a former Dominican priest, ordained in 1960, but left the priesthood that same year. After graduating from the University of Salamanca, he moved to the US in 1961 to study for...

 (1988) defines progress as "systematic change in a feature belonging to all the members of a sequence in such a way that posterior members of the sequence exhibit an improvement of that feature." He argues that there are two elements in this definition, directional change and improvement according to some standard. Whether a directional change constitutes an improvement is not a scientific question; therefore Ayala suggests that science should focus on the question of whether there is directional change, without regard to whether the change is "improvement". This may be compared to Gould's suggestion of "replacing the idea of progress with an operational notion of directionality".

Dawkins, on the other hand, proposes that Darwinian evolution is fundamentally progressive if progress is simply defined as "an increase, not in complexity, intelligence or some other anthropocentric value, but in the accumulating number of features contributing towards whatever adaptation the lineage in question exemplifies."

Evolutionary theorists agree that there are local trends in evolution, such as increasing brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 size in hominids, but these directional changes do not persist indefinitely, and trends in opposite directions also occur (Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

, 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
Largest-scale trends in evolution
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 recent organisms, such as mammals, appear to be much more complex than older organisms, such as bacteria. However, there are...

 (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. Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

 (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
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...

 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
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

, implies that the average level of complexity of life must increase over time. Gould (1997) uses the analogy of a random walk
Random walk
A random walk, sometimes denoted RW, is a mathematical formalisation of a trajectory that consists of taking successive random steps. For example, the path traced by a molecule as it travels in a liquid or a gas, the search path of a foraging animal, the price of a fluctuating stock and the...

 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.

See also

  • Orthogenesis
    Orthogenesis
    Orthogenesis, orthogenetic evolution, progressive evolution or autogenesis, is the hypothesis that life has an innate tendency to evolve in a unilinear fashion due to some internal or external "driving force". The hypothesis is based on essentialism and cosmic teleology and proposes an intrinsic...

  • Full House: The Spread of Excellence From Plato to Darwin
    Full House: The Spread of Excellence From Plato to Darwin
    Full House: The Spread of Excellence from Plato to Darwin is a book by evolutionary biologist Stephen Jay Gould, published in 1996. It was released in the UK as Life's Grandeur, with the same subtitle....

  • Evolutionary biology
  • Michael Ruse
    Michael Ruse
    Michael Ruse is a philosopher of biology at Florida State University, and is well known for his work on the creationism/evolution controversy and the demarcation problem in science...

  • Stephen Jay Gould
    Stephen Jay Gould
    Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

  • Francisco J. Ayala
    Francisco J. Ayala
    Francisco José Ayala Pereda is a Spanish-American biologist and philosopher at the University of California, Irvine. He is a former Dominican priest, ordained in 1960, but left the priesthood that same year. After graduating from the University of Salamanca, he moved to the US in 1961 to study for...


External links

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