This habit sustained for long, has had the result in all members of its race that the forelegs have grown longer than the hind legs and that its neck has become so stretched, that the giraffe, without standing on its hind legs, lifts its head to a height of six meters. In essence, this says that the necks of Giraffes became long as a result of continually stretching to reach high foliage.
Larmarck was incorrect in the hypothesized mechanism, of course, but his example makes clear that naturalists were thinking about the possibility of evolutionary change in the early 's.
Darwin's Theory. Species populations of interbreeding organisms change over time and space. The representatives of species living today differ from those that lived in the recent past, and populations in different geographic regions today differ slightly in form or behavior.
These differences extend into the fossil record, which provides ample support for this claim. All organisms share common ancestors with other organisms. Over time, populations may divide into different species, which share a common ancestral population. Far enough back in time, any pair of organisms shares a common ancestor. For example, humans shared a common ancestor with chimpanzees about eight million years ago, with whales about 60 million years ago, and with kangaroos over million years ago.
Shared ancestry explains the similarities of organisms that are classified together: their similarities reflect the inheritance of traits from a common ancestor. Since then, biologists and paleontologists have documented a broad spectrum of slow to rapid rates of evolutionary change within lineages.
T he primary mechanism of change over time is natural selection, elaborated below. This mechanism causes changes in the properties traits of organisms within lineages from generation to generation. This process is natural selection. The traits that confer an advantage to those individuals who leave more offspring are called adaptations.
In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources. If one of these requirements does not occur, then the trait does not experience natural selection.
Natural selection operates by comparative advantage, not an absolute standard of design. Natural selection can only work on existing variation within a population. Such variations arise by mutation, a change in some part of the genetic code for a trait. Mutations arise by chance and without foresight for the potential advantage or disadvantage of the mutation.
In other words, variations do not arise because they are needed. Let's look at an example to help make natural selection clear. Industrial melanism is a phenomenon that affected over 70 species of moths in England. It has been best studied in the peppered moth, Biston betularia. Prior to , the typical moth of the species had a light pattern see Figure 2. Dark colored or melanic moths were rare and were therefore collectors' items. Figure 2.
Image of Peppered Moth. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits—traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population.
Through this process of natural selection, favorable traits are transmitted through generations. Natural selection can lead to speciation , where one species gives rise to a new and distinctly different species.
It is one of the processes that drives evolution and helps to explain the diversity of life on Earth. He pointed to the pastime of pigeon breeding, a popular hobby in his day, as an example of artificial selection. By choosing which pigeons mated with others, hobbyists created distinct pigeon breeds, with fancy feathers or acrobatic flight, that were different from wild pigeons.
Darwin and other scientists of his day argued that a process much like artificial selection happened in nature, without any human intervention. He argued that natural selection explained how a wide variety of life forms developed over time from a single common ancestor.
Darwin did not know that genes existed, but he could see that many traits are heritable—passed from parents to offspring. Mutations are changes in the structure of the molecules that make up genes, called DNA. The mutation of genes is an important source of genetic variation within a population.
Mutations can be random for example, when replicating cells make an error while copying DNA , or happen as a result of exposure to something in the environment, like harmful chemicals or radiation. Mutations can be harmful, neutral, or sometimes helpful, resulting in a new, advantageous trait.
When mutations occur in germ cells eggs and sperm , they can be passed on to offspring. If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection.
Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct. Dinosaurs are one example. An invasive species , a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species. Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions.
Neither notion, however, is meant to further delineate the circumstances in which selection occurs, or to narrow the scope of application of evolutionary theory for further discussion of these notions, see entry on units and levels of selection. This is evident, for Hull at least, insofar as genes may be both replicators and interactors The view that evolutionary theory is a theory that applies to active germ-line replicators has come under fire from a multitude of directions.
Genes need not be germ-line to undergo selection, as it is at least arguable that the immune system exhibits selection processes Okasha Copying is beside the point, since only similarity across generations, rather than identity, is necessary for evolutionary change Godfrey-Smith , and entries on units and levels of selection and replication and reproduction. For his part, Hull seems to agree with this last point, as he allows that organisms might well count as replicators, at least in cases in which they reproduce asexually Hull 28— Despite the bevy of attacks on replicator selectionism, replicator selectionists have not, to my knowledge at least, been criticized for being too permissive and allowing that systems that do not evolve count as undergoing selection.
But germ-line replicators may exert a causal influence on their probability of being copied without spreading in a natural population as a result, as in some cases of frequency-dependent selection of systems already at equilibrium. In cases of frequency-dependent selection, variant genes cause their own reproduction, but the extent of influence on reproduction is a function of their frequency.
Suppose each type spreads when it is rarer. Because causing replication may not lead to differential replication in these and other cases, replicator selectionists do not effectively take evolution to be necessary for selection while Lewontin and those who follow his basic approach typically do do so.
One natural way to arbitrate the issue of whether systems that undergo selection must evolve is to attend to the point of statements of principles of natural selection, or statements of the requirements for selection.
Many theorists take it that the point of these principles is to set out the scope of a theory in the special sciences that deals with selection and evolution, evolutionary theory. Lewontin claims that the theory of evolution by natural selection rests on his three principles Equally, Godfrey-Smith claims that statements of conditions for evolution by natural selection exhibit the coherence of evolutionary theory and capture some of its core principles For these writers, the or at least a point of the principles seems to be to capture the domain of application of the theory we have inherited from Darwin.
Darwin would have been surprised to hear that his theory of natural selection was circumscribed so as to apply only to evolving populations. He himself constructed an explanation of a persistent polymorphism, heterostyly, using his own theory. Plants exhibiting heterostyly develop two, or sometimes three, different forms of flower whose reproductive organisms vary in a number of ways, principally length. Some plants exhibit different forms of flower on the same plant, while some are dimorphic and trimorphic, with only one sort of flower per plant.
Darwin interpreted the flower variations as conducive to intercrossing, which he thought was beneficial, at least for many organisms. Populations should not evolve directionally such that a single form of flower spreads throughout the population; instead, multiple variants should be retained, a polymorphism.
Darwin thinks it clear that heterostyly is an adaptation:. The benefit which heterostyled dimorphic plants derive from the existence of the two forms is sufficiently obvious […. Darwin 30; thanks to Jim Lennox for this reference. Even though the population is not evolving, but instead remaining the same over time, it exhibits an adaptation that consists in this persistent lack of change, an adaptation that Darwin thought explicable using his theory.
These sorts of behaviors result from specific assignments of values for theoretical parameters in many of the very same models that are used to explain simple directional selection where a single variant spreads throughout a population, as in the wolf case discussed in the introduction. The point is that systems seemingly governed by evolutionary theory exhibit a variety of different sorts of dynamics, and this variety includes both different sorts of evolution, including at least cyclical and directional, as well as a lack of evolution at all, as in cases of stabilizing selection.
Consider in particular how the difference between stabilizing and directional selection in the simplest deterministic models of diploid evolution lies in the value of a single parameter in the genotypic selection model, heterozygote fitness:. If we hold evolution as a condition for selection, we will issue the curious ruling that a system governed by the first sort of model falls within the scope of evolutionary theory while a system governed by the second sort of model only does so up until it reaches a stable intermediate state but then no longer.
Moreover, populations exhibiting stable polymorphisms resulting from heterozygote superiority, or overdominance, are just one case among many different sorts of systems that equally exhibit stable polymorphisms. The above models are deterministic, while the dynamics of natural systems are to some extent random. A system governed by both the deterministic equations and the binomial sampling equation is said to undergo drift; all natural systems do so. For more on drift, effective population size, and randomness in evolutionary theory, see entry on genetic drift.
A system exhibiting heterozygote superiority whose dynamics are a function of the binomial sampling equation will not simply rest at its stable intermediate frequency but will hover around it, in some generations evolving toward it, more rarely evolving away, and in some generations exhibiting no evolution at all.
Which of these cases are cases in which the system undergoes natural selection in the capacious sense? That is, which cases are cases in which the system falls within the purview of evolutionary theory? A natural answer is all of them.
To answer in this way, however, we must not make evolution necessary for natural selection. This last pattern of argument can be extended. Indeed, given that every natural system undergoing selection also undergoes drift, evolutionary theory is arguably applicable also to systems that undergo drift even in the absence of selection in the focused sense.
Is the point at which the values equalize so momentous that it marks the point at which systems governed by the equations cease to fall within the purview of one theory and instead fall within the purview of another?
If Brandon is right, then conditions for the application of evolutionary theory must not even include conditions for selection in the focused sense, much less conditions for evolutionary change. The point of stating conditions for evolution by natural selection need not be to state the conditions of deployment of a particular theory in the special sciences.
Godfrey-Smith mentions that the principles may be important to discussions of extensions of evolutionary principles to new domains. Statements of the conditions for evolution by natural selection might have value for other reasons.
But evolutionary theory is, despite the name, at least arguably a theory that is applicable to more systems than just those that evolve, as the replicator selectionists would have it. One of the two chief uses of the notion of natural selection is as an interpretation of one or another quantity in formal models of evolutionary processes; this is the focused sense distinguished above.
Two different quantities are called selection in different formal models widely discussed by philosophers. This is standard textbook usage Rice ; Hedrick The recursive structure of these models is important.
They can be used to infer how a system will behave into the future though of course only if causes of the variables in the system do not change their values in dynamically-relevant ways that are not explicitly modeled in the recursive equations. Writers working with type recursion models have developed explicit interpretations of their theoretical terms, including the fitness variables quantifying selection.
So, for instance, Beatty and Millstein defend the view that the fitness coefficients representing selection in type recursions should be understood as modeling a discriminate sampling process, while drift, controlled by effective population size, should be understood as indiscriminate sampling Beatty ; Millstein Philosophers have also contended that particular terms in models of systems featuring the formation of groups or collectives should be understood as quantifying the influence of selection at different levels.
Kerr and Godfrey-Smith discuss one such system of recursions; Jantzen defends an alternative parameterization of group selection as part of different system of equations. See also Krupp for causal-graphical conceptualization of the notion of group selection. For much more on multi-level selection, see entry on units and levels of selection. The other formal model of particular interest to philosophers is the Price Equation. The Price Equation represents the extent of evolution in a system with respect to a given trait across a single generation using statistical functions:.
In the Price Equation, selection is associated with the first right-hand side quantity, while the second represents transmission bias.
Identities among algebraic functions of statistical functions make possible the mathematical manipulation of the Price Equation such that it may feature a variety of different quantities. As with type recursions, quantities in various transformations of the Price Equation are equated with selection at different levels for different systems; Okasha, following Price, treats the covariance of the fitness of collectives with the phenotype of collectives as collective-level selection, while the average of the within-collective covariances between particle character and particular fitness is identified with particle-level selection.
The Price Equation can equally be manipulated to yield distinct notions of inheritance; Bourrat distinguishes temporal, persistence, and generational heritabilities and argues for the temporal notion as appropriate for the purposes of stating conditions for evolution by natural selection Bourrat The distinction between type recursions and the Price Equation is important, because selection is interpreted differently in each.
The two formalisms will issue in different verdicts about whether, and the extent to which, focused selection operates within a single system. To see this, consider how type recursions are structured such that inferences about dynamics over multiple generations may be made by means of them. If fitness coefficients in these models quantify selection, and these take fixed values as they do in the genotypic selection model considered above and a great many others , then the extent of selection will remain the same over the time period governed by the model: the fitness variables remain at fixed values so selection remains an unchanging influence.
Consider, for instance, the extent to which the population evolves, according to the genotypic selection model above, when the following values are plugged into the model:. If we understand selection as quantified by the fitness coefficients in this sort of set-up, then the whole time, selection operates in a constant fashion, since the fitness coefficients remain fixed.
In particular, the operation of selection is the same when the system is evolving toward its stable equilibrium as when it remains at that stable equilibrium.
By contrast, the covariance term in Price Equation model of the system will diminish in value until it reaches zero as the system evolves to its equilibrium state. When selection is identified with the covariance between type and reproduction, the frequency of the different types matters to the extent of selection.
When selection is identified with fitness variables in type recursions, the frequency of different types has no influence on the extent of selection in the system. Thus, the different interpretations of selection that correspond to different quantities in different formal models are actually incompatible. We should expect, then, at least one of these interpretations of selection to fail, since focused selection cannot be two different things at once, at least if what counts as natural selection is non-arbitrary.
One way to reconcile these competing interpretations of selection is to make first right-hand side term in the Price Equation quantify the extent of the influence of selection in a system. If we assume that focused selection accounts for whatever covariance exists between parental offspring number and phenotype, then we may treat the first right-hand side term of the Price Equation as a measure of the extent of the influence of focused selection, at least at a given type frequency see Okasha This approach puts the logical house in order, allowing for a univocal concept of selection, but it does so at the expense of other commitments.
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