What is fitness in biology

Relative fitness is quantified as the average number of surviving progeny of a particular genotype compared with average number of surviving progeny of competing genotypes after a single generation, i. Relative fitness can therefore take any nonnegative value, including 0. While researchers can usually measure absolute fitness, relative fitness is more difficult.

It is often difficult to determine how many individuals of a genotype there were immediately after reproduction. The two concepts are related, and both of them are equivalent when they are divided by the mean fitness, which is weighted by genotype frequencies. This leads to the well known Fisher's fundamental theorem of natural selection.

Fisher's theorem states that: "The rate of increase in the mean fitness of any organism at any time ascribable to natural selection acting through changes in gene frequencies is exactly equal to its genic variance in fitness at that time". This may be somewhat dubious because selection takes place on the individual level, ruling the enrichment of genes Mayr In addition, according to Maynard Smith, a population may reach a state of selective equilibrium, in which case the increase of mean fitness is equal to zero, but not necessarily the variance in fitness.

Because fitness is a coefficient, and a variable may be multiplied by it several times, biologists may work with "log fitness" particularly so before the advent of computers. By taking the logarithm of fitness each term may be added rather than multiplied.

A fitness landscape , first conceptualized by Sewall Wright , is a way of visualising fitness in terms of a three-dimensional surface on which peaks correspond to local fitness maxima; it is often said that natural selection always progresses uphill but can only do so locally. This can result in suboptimal local maxima becoming stable, because natural selection cannot return to the less-fit "valleys" of the landscape on the way to reach higher peaks.

The related concept of genetic load measures the overall fitness of a population of individuals of many genotypes whose fitnesses vary, relative to a hypothetical population in which the most fit genotype has become fixed. If the first human infant with a gene for levitation were struck by lightning in its pram, this would not prove the new genotype to have low fitness, but only that the particular child was unlucky.

Railton , ], and some hold that they do so via a detour into long run relative frequencies. But few are comfortable with such arguments and adopt them only because, at the level of the quantum mechanical probabilistic propensities are indispensable and irreducible [cf.

Lewis, ]. Proponents of probabilistic propensities in biology may envision two possibilities here. No one doubts the potential biological significance of quantum percolation.

It may well be an important source of mutation [cf. Stamos for a discussion]. But the claim that it has a significant role in most fitness differences is not supported by any independent evidence [cf. Millstein and Glymour for a discussion]. The claim that there are brute probabilistic propensities at the level of organismal fitness differences is only slightly more tenable. No one has adduced any evidence that, for instance, the probabilistic generalizations about the behaviour of animals that ethology and behavioural biology provide, are irreducibly probabilistic, instead of simply expressions of the current state of our knowledge and ignorance of the causes and conditions of the behaviour in question.

Rosenberg and Kaplan [] advance an alternative account of the nature and source of objective chances in Darwinian processes, arguing that they are not quantum mechanical in origin but are identical to the objective chances that operate in the processes described by the second law of thermodynamics.

Of course this suggestion trades one problem for another, since the source of thermodynamic chances is a vexed question in the philosophy of physics. There is however a much more serious issue facing the propensity definition of fitness: it has long been held to be difficult to pin down the specific mathematical expression of the probabilistic propensity that is supposed to constitute fitness altogether.

The difficulty reflects features of natural selection that we must accommodate. And if this is so, the probabilistic propensity definition will not defuse the threat of triviality facing the theory of natural selection. We review here the research program of delivering a propensity definition of fitness that has sought to deal with these difficulties.

Take a simple example from Brandon It is also the case that in some biologically actual circumstances—for example, in circumstances in which mean fitnesses are low, increased variance is sometimes selected for Ekbohm, Fagerstrom, and Agren One simple way to do so is to add a ceteris paribus clause to the definition.

But the question must then be raised of how many different exceptions to the original definiens need to be accommodated. Thus Brandon writes , 20 :. But how many such factors are there, and when do they play a non-zero role in fitness? The answer is that the number of such factors is probably indefinitely large. The reason for this is given by the facts about natural selection as Darwin and his successors uncovered them.

It is the set of operational measurements of the property of comparative fitness. A promising candidate is due to Pence and Ramsey A brief account is given here. This equation is also subject to some further conditions Pence and Ramsey argue are biologically reasonable, for example that selection is density dependent, that there is an extinction threshold of some population size below which extinction obtains, and most important that the population dynamics are non-chaotic.

Pence and Ramsey show how their definition avoids all of the problems discussed above, as well as some other counterexamples to previous proposals, owing to its incorporating and combining all possible daughter populations over indefinitely many reproductive generations. They show how standard operation fitness measures biologists employ can be derived from the definition, including the simple geometric and arithmetic means, along with their higher moments variance, skew, etc.

Suppose that the Pence and Ramsey proposal adequately deals with the counter-example problems that confronted earlier probabilistic propensity definitions of fitness. The question of whether this definition is adequate for trait fitness as well as for individual fitness and fitness differences remains.

To begin with there is matter of whether the definition merely provides units in which to measure fitness the way that plainly operational definitions do, by contrast with definitions of dispositions in terms of their occurrent bases. Moreover, some have argued that the propensity definition is at most only a component of the meaning of trait fitness. As noted above, some philosophers e. From this conclusion it seems an easy matter to infer that as it functions in the theory of natural selection, trait fitness may not be causal force at all.

The Pence-Ramsey proposal holds out the prospect of dealing with a serious difficulty that long daunted the probabilistic propensity definition of fitness. For the counterexamples it deals with made impossible any attempt to turn a generic probabilistic schema for fitness into a complete general definition that is both applicable and adequate to the task of vindicating the truth of the principle of natural selection PNS :. These problems had suggested to some philosophers that we need to rethink the cognitive status of the theory of natural selection altogether.

Brandon, for example, has argued that the theory of natural selection should not be viewed as a body of general laws, but as the prescription for a research program. As such its central claims need not meet standards of testability, and fitness need not be defined in terms that assure the nontriviality, testability, and direct explanatory power of the theory of natural selection.

In that sense, Brandon makes common cause with the later Popper. Popper originally argued the claim that the theory of natural selection was unfalsifiable pseudoscience. Later he came to hold that theory of natural selection was a scientifically respectable but nevertheless untestable organizing principle for biological science Brandon , — On this view, in each particular selective scenario a different specification of the schematic propensity definition of fitness figures in the antecedent of a different and highly restricted principle of natural selection that is applicable only in that scenario.

Properly restricted to the right function and the right set of statistical features of its reproductive rate for a given environment, this version will be a highly specific claim about natural selection for the given population in the given environment.

The notion that there is a very large family of principles of natural selection, each with a restricted range of application, will be attractive to those biologists uncomfortable with a single principle or law of natural selection, and to those philosophers of science who treat the theory of natural selection as a class of models.

See Lloyd , Beatty , and Thompson for exposition of this position. For every such definition instantiated by a population, there is the synthetic truth that the members of the population, or all of its subpopulations, instantiate the definition, and the further broader truth that many populations of otherwise diverse species instantiate a small sub-set of the models, and the yet broader truth that all populations instantiate one of this set of really quite similar models.

Presumably, all these statements of varying degrees of generality need explanation. But, one will want to ask, does this set of generalizations with similar antecedents and identical consequents have something in common, which in turn explains and unifies them, or are they the fundamental principles of the theory of natural selection? The question is obviously rhetorical. A white rabbit in a snow covered environment has camouflage, which protects it from its predators.

The same is true with the black moth living in a in a soot-covered industrial area. The same is true for a bird that can crack nuts in an area where nuts are the main source of food. A female cheetah in Africa has four litters of cubs over her lifetime. Her first litter has six cubs that grow to adulthood and is fathered by the most spotted male in the area. Her second litter has four cubs that grow to adulthood and is fathered by the fastest male in the area.

Her third litter has two cubs that survive to adulthood and is fathered by the strongest male in the area. Her fourth litter has five cubs that survive to adulthood and is fathered by the smartest male in the area.

Which male cheetah has the most biological fitness? The term biological fitness refers to reproductive success and is different than physical fitness. Since the most spotted male fathered the most cubs that survived to adulthood to reproduce themselves, he would be considered the most biologically fit. It is also important to note the inclusion of the "survived to adulthood" aspect since reproductive success is dependent on an organism's offspring being able to reproduce and contribute to the gene pool as well.

For example, if the most spotted male had fathered a litter that initially had nine cubs, but only one of them survived to adulthood to have cubs of its own, he would no longer be considered the most biologically fit. If you've found an issue with this question, please let us know. With the help of the community we can continue to improve our educational resources. If Varsity Tutors takes action in response to an Infringement Notice, it will make a good faith attempt to contact the party that made such content available by means of the most recent email address, if any, provided by such party to Varsity Tutors.

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