## 1. Definitions of Evolution



The definition of evolution given at the outset of this entry is very
general; there are more specific ones in the literature, some of which
do not fit this general characterization. Here is a sampling.



Although the work of Charles Darwin (see the entry on
 Darwinism) is usually the starting 
point for contemporary understandings
of evolution, interestingly, he does not use the term in the first
edition of *On the Origin of Species*, referring instead to
"descent with modification". In the early-mid 20th
century, the "modern synthesis" gave birth to
 population genetics,
 which provided a mathematization of Darwinian evolutionary theory in
light of Mendelian genetics (see also the entry on
 ecological genetics).
 This yielded a prevalent--probably the most
prevalent--understanding of evolution as "any change in the
frequency of alleles within a population from one generation to the
next". Note, however, that this definition refers to evolution
only in a microevolutionary context and thus doesn't reference
the emergence of new species (and their new characteristics), although
it is intended to underlie those macroevolutionary changes (see the
entry on 
 philosophy of macroevolution). 



In a popular textbook, Douglas Futuyma gives a more expansive
definition: 



> 
> 
> [biological evolution] is change in the properties of groups of
> organisms over the course of generations...it embraces everything
> from slight changes in the proportions of different forms of a gene
> within a population to the alterations that led from the earliest
> organism to dinosaurs, bees, oaks, and humans. (2005: 2) 
> 
> 
> 



Note also that Futuyma's definition, unlike the population
genetics' definition, does not limit itself to changes in
*alleles;* John Endler's definition is similar in this
respect: 



> 
> 
> Evolution may be defined as any net directional change or any
> cumulative change in the characteristics of organisms or populations
> over many generations--in other words, descent with
> modification... It explicitly includes the origin as well as the
> spread of alleles, variants, trait values, or character states.
> (Endler 1986: 5) 
> 
> 
> 



Yet even this definition is not expansive enough; molecular evolution
focuses on the molecular changes within macromolecules such as DNA and
RNA. 



In a very different vein, Leigh van Valen characterized evolution as
"the control of development by ecology" (1973, 488); this
anticipates those who emphasize the importance of development in
evolution, including proponents of "evo-devo" (see
 the entry on evolution and development).
 Today, some have called for an "extended evolutionary
synthesis" in light of developmental biology and other recent
findings in evolutionary biology.



Although this entry focuses on biological evolution, philosophers and
biologists have also sought to extend evolutionary ideas to the
cultural realm. Figuring out how and whether to extend the definition
of evolution to this realm is part of the study of
 cultural evolution.
 



In spite of this diversity of definitions, there has been very little
philosophical analysis of the term "evolution" itself.
This dearth forms a stark contrast to the voluminous literature in the
philosophy of evolution; indeed, for a long time the
 philosophy of biology
 was focused almost entirely on evolution. Thankfully, that is no
longer the case, with philosophers turning their attentions to issues
in genetics, 
 molecular biology,
 cell biology, 
 ecology, 
 developmental biology,
 microbiology, and
more. It may be, as Theodosius Dobzhansky famously said, that
"Nothing in biology makes sense except in the light of
evolution" (1973: 125), but much of biology is not evolutionary
biology. Still, though, philosophy of evolution remains a growing and
vibrant area within the philosophy of biology.


## 2. Modes of Evolution



It is essential to understand that biologists recognize many ways that
evolution can occur, evolution by
 natural selection
 being just one of them, although it is often held to be the most
prevalent one. Evolution can also occur through genetic drift,
mutation, or migration. It can also occur through sexual selection,
which some consider to be a form of natural selection and others
consider to be distinct from natural selection (the latter having been
Darwin's 1859, 1874 view). Evolutionary theory, then, can be
taken to be the study (including, but not limited to, mathematical
models) of these and other modes of evolution.



To see why it makes sense to think of multiple modes of evolution,
consider again one of the definitions of evolution presented above,
where evolution is understood as "any change in the frequency of
alleles within a population from one generation to the next".
With natural selection, the frequency of alleles that confer greater
fitness would tend to increase over those which confer lesser fitness.
Sexual selection would be the same, but with fitness understood
strictly in terms of mating ability. With genetic drift, a form of
evolution that involves chance (see the entry on
 genetic drift
 for explanation), there could be an increase in the frequency
of alleles that confer greater fitness, an increase in the frequency
of alleles that confer lesser fitness, or an increase in the frequency
of alleles whose manifestation (if any) was neutral. If organisms
migrate from one population to another, it is likely that there will
be a change in the frequency of alleles in both populations. And if
there is a mutation from one allele to another, then the frequency of
alleles in the population will likewise change, albeit by a small
amount. Distinguishing these different modes of evolution allows
biologists to track the various factors that are relevant to
evolutionary changes in a population.



The careful reader may have noted that the previous paragraph invoked
probabilistic language: what *tends* to happen, what
*could* happen, what is *likely* to happen. Indeed,
mathematical evolutionary models today (see the entry on
 population genetics)
 are typically statistical models. This fact about evolutionary
models has given rise to a debate in the philosophy of evolution over
whether natural selection and genetic drift should be be understood as
causes of evolution, as most biologists conceive them, or as mere
statistical summaries of lower-level causes: births, deaths, etc. (The
 natural selection
 and
 genetic drift
 entries give more information about this debate). It is for this
reason that this entry uses the more neutral phrase "modes of
evolution" so as not to beg any questions under dispute between
the causalist and the statisticalist.



Although there is widespread agreement that there are multiple modes
of evolution, much contemporary work in biology and philosophy of
biology has been focused on natural selection. Whether this focus is a
good thing or not is in part what the debate over
 adaptationism
 is about. That is, do we have reason to think that natural selection
is the most prevalent or most important mode of evolution? Should
scientific methodologies be geared toward testing natural selection
hypotheses or toward a variety of possible evolutionary modes? The
focus on natural selection has also led to a large literature on the
concept of
 fitness,
 given that population genetics' definitions and other
definitions of natural selection typically invoke fitness; a natural
selection explanation of why *X* was more successful than
*Y* might invoke *X*'s higher fitness. What fitness
means, what entities it applies to (genes, organisms, groups,
individuals, types), what sort of probabilities it invokes, if any,
and how it should be calculated, are all under philosophical dispute.
There is also a large literature on conceptually and empirically
distinguishing natural selection from genetic drift. Migration,
mutation (as a mode of evolution), and sexual selection have received
less attention from philosophers of biology.


## 3. Other Topics in the Philosophy of Evolution



Some of the work in the philosophy of evolution deals with
controversial issues. There is, of course, the debate over
 creationism.
 The vast majority of philosophers agree that creationism has *significantly* less evidence in its favor as compared to the abundant evidence in favor of evolution. They also agree that
creationism ought not to be taught in a public school science classroom,
but they sometimes disagree over the reasons why. For example, is
it because it fails some criteria of science? If so, which criteria?
Or is it because of the lack of evidence? Or is it because of its
religious basis? Debates over
 sociobiology
 and
 evolutionary psychology--areas
 that seek to explain human behavior and psychology as evolved
characteristics--have likewise stirred up controversy over their
scientific status. Proponents have also been accused of employing an
excessive and uncritical adaptationism and resting on sexist or other
problematic biases (on the latter, see the entry on
 feminist philosophy of biology).



Another nexus of topics in the philosophy of evolution involves
 heredity and heritability.
 Although it was not explicitly emphasized in the definitions of
evolution given above, evolution is usually taken to be about
*heritable* changes over time, i.e., characteristics that are
able to be passed from one generation to the next. But there has been
some discussion over which entities can properly be said to be
heritable. Genes are uncontroversial, but are seen as too limited by
some, who would consider phenomena such as learning and cultural
transmission, epigenetic inheritance, and ecological inheritance to be
heritable as well. The term "heritability" can likewise
cause confusion, as it is a technical term within evolutionary theory,
and understanding the term and its implications is not trivial.
Classically, heredity has been thought of in terms of the
 genotype/phenotype distinction,
 with genotypes being seen as heritable and phenotypes being seen as
not heritable. But to accept that distinction seems to accept a
distinction between
 innate and acquired characteristics,
 and that distinction has been challenged, or at least shown to be
more complicated than it would seem at first glance. Heredity also
raises questions about
 biological information--do
 genotypes pass along *information*, and if so, in what
sense?



Relatedly, for heredity to be a part of evolution, there must be
 replication
 of entities, or at least reproduction (with the former being a
special case of the latter that involves copying). In order to allow
for a more general theory of evolution, many authors will speak of
replicators (or reproducers) and vehicles (or interactors) rather than
the more limited and specific terms "genes" and
"organisms". With these terms in hand, one can more easily
begin to discuss (as many have) questions over
 units and levels of selection:
 does selection occur at the level of the gene, the organism, the
group, the species, or all of the above? These units of selection
(replicators/reproducers or vehicles/interactors) are often taken to
be biological individuals (see the entry on
 the biological notion of individual)
 as a necessary condition for being units of selection at all. 



Interestingly, another major area in which biological individuality
has played a large role is in debates over the nature of species.
 That is, many philosophers of biology maintain that species are
properly construed as individuals. Species, often referred to as
"units of evolution"--groups of organisms that evolve
in a unified way--are nonetheless rarely seen as units of
*selection*. In Elisabeth Lloyd's terminology (see the
entry on
 units and levels of selection),
 this is presumably because species are rarely seen as
replicators/reproducers or vehicles/interactors but are commonly seen
as beneficiaries of evolution by natural selection. In addition to
sorting out whether species are individuals and what sort of units of
evolution (if any) they might be, there are many-decades' worth
of papers trying to characterize the species concept, whether in terms
of interbreeding, phylogeny, morphology, ecology, or some other set of
characteristics. Here, as in many other areas of the philosophy of
biology, there have also been arguments for a pluralistic
approach.



Yet another area of discussion is
 evolutionary game theory--an
 application of the mathematical theory of games to biological and
other evolutionary contexts. It has provided a source of putative
explanations for human and other behaviors; evolutionary psychology,
mentioned above, is one area that frequently makes use of a game
theory approach. Among the more challenging behaviors that
evolutionary game theory has sought to explain is
 altruism.
 With altruism, we again encounter questions about the level at which
selection is operating (organisms or groups) because of questions
about which entities selection is benefiting or harming.