Phyletic gradualism
Encyclopedia
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
which theorizes that most speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
is slow, uniform and gradual. When evolution occurs in this mode, it is usually by the steady transformation of a whole species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
into a new one (through a process called anagenesis
Anagenesis
Anagenesis, also known as "phyletic change," is the evolution of species involving an entire population rather than a branching event, as in cladogenesis. When enough mutations have occurred and become stable in a population so that it is significantly differentiated from an ancestral population,...
). In this view there is no clear line of demarcation between an ancestral species and a descendant species, unless splitting
Cladogenesis
Cladogenesis is an evolutionary splitting event in a species in which each branch and its smaller branches forms a "clade", an evolutionary mechanism and a process of adaptive evolution that leads to the development of a greater variety of sister species...
occurs.
History
The word "phyletic" derives from the GreekGreek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
φυλετικός phūletikos, which means relating to evolutionary change in a single line of descent without branching. Phyletic gradualism is in contrast to the theory of punctuated equilibrium
Punctuated equilibrium
Punctuated equilibrium is a theory in evolutionary biology which proposes that most species will exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis...
, which proposes that most evolution is isolated in rare episodes of rapid evolution, which one species splits into two distinct species, followed by a long period of stasis
Stasis
The term stasis may refer to* A state of stability, in which all forces are equal and opposing, therefore they cancel out each other....
or non-change. These models are also in contrast to variable speed evolution, which maintains that different species evolve at different rates, and that there is no reason to stress one rate of change over another.
Zoologist Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
argues that such constant-rate gradualism is not present in the professional literature, thereby the term only serves as a straw-man for punctuated equilibrium advocates. In his book The Blind Watchmaker
The Blind Watchmaker
The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design is a 1986 book by Richard Dawkins in which he presents an explanation of, and argument for, the theory of evolution by means of natural selection. He also presents arguments to refute certain criticisms made on...
, Dawkins argues against the idea that 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...
himself was a constant-rate gradualist, as suggested by Niles Eldredge
Niles Eldredge
Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...
and 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....
. In the first edition of On the Origin of Species, Darwin stated that "Species of different genera and classes have not changed at the same rate, or in the same degree. In the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms... The Silurian Lingula differs but little from the living species of this genus".
Lingula
Lingula (genus)
Lingula is a genus of brachiopods within the class Lingulata. Lingula is known since the Tertiary.-Species:The following species are recognised:*Lingula adamsi Dall, 1873*Lingula anatina Lamarck, 1801*Lingula dregeri Andreae, 1893...
is among the few brachiopods surviving today but also known from fossils over 500 million years old. In the fifth edition of The Origin of Species, Darwin wrote that "the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form."