Video Lecture

Adaptation and Fitness

Adaptations, by definition, help organisms survive in their environment and their ecological niches. Adaptations may be anatomical, physiological, or behavioral. Anatomical adaptations concern the physical characteristics of an organism. Physiological adaptations allow the animal, for example, to produce toxins or venom. Behavior adaptations comprise inherited behavior (instincts and fixed action patterns), the tendency to display a particular behavior, and the ability to learn.

There are two ways to understand the term adaptation. One is phenotypic adaptation, where some organisms are better at dealing with the environment: they adapt better and fit better. The second meaning of adaptation concerns genetics. The fittest pass their fittest genes to their progeny, and over time, we get a fitter (better adapted) population (that is if the environment remains constant). 

Adaptations are the product of a (natural) selection of continuous, small, random changes in traits. Natural selection favors the variants best suited for their environment and produces a genetic change in the gene pool of a population. In this sense, we can say that the population adapts genetically to its circumstances, irrespective of the de facto adaptation happening to the single individual.

No single definition of adaptation or adaptability exists. When an organism is said to be “adapted to particular habitats,” it can live and reproduce in those circumstances. A structural or functional trait, or more generally, a feature of an organism’s developmental pattern, is an adaptive trait if it increases the likelihood of the organism surviving and reproducing (Dobzhansky, 1956). Adaptability refers to the ability of an organism or population to remain or become physiologically or genetically adapted to a specific range of conditions. Adaptedness is the state of being adapted. Adaptation refers to the process of being adapted (Dobzhansky, 1968). An individual is adapted for survival at its peak, often before and throughout the reproductive age, declining afterward. Ling-lived species typically show longer reproductive cycles.

The following definitions, put forth by evolutionary biologist Theodosius Dobzhansky, are now generally agreed upon: 

1. Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats (Dobzhansky, 1968).

2. Adaptedness is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats (Dobzhansky, 1970).

3. “An adaptive trait is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.” (Dobzhansky, 1968).

Flexibility, Acclimation, and Learning

Flexibility, acclimation, and learning are all life-long changes that are not inherited; however, as we will see, learning demonstrates specific adaptive characteristics.

Flexibility depends on how well an organism can survive in various environments. Phenotypic plasticity, an organism’s capacity to adapt its phenotype to changes in its environment or to relocate to a different environment, is the source of flexibility (Price et al., 2003). Flexibility varies from individual to individual and is inherited. A highly specialized animal or plant requires a particular sort of food to survive, inhabits a narrowly defined habitat, and cannot exist without it. Maynard Smith writes, “An animal or plant is developmentally flexible if when it is raised in or transferred to new conditions, it changes in structure so that it is better fitted to survive in the new environment.” (Maynard Smith, 1993). 

Acclimatization per se is not an adaptation, but the capacity to do so is. 

Learning suggests that behavioral performance can improve with time. Learning can alter behavior, but not all alterations happen via learning. Learning does not include behavioral changes brought on by trauma, aging (maturation), trauma, drugs, or illness. These do not qualify as learning. Learning changes the animal’s makeup and is likely to alter its fitness. We can expect the consequences of learning to be adaptive and to increase the animal’s fitness. There are two basic requirements for the evolution of a trait by natural selection. First, the trait must be at least partly heritable; genotypic variation must produce phenotypic variation in the trait. Second, variation in the trait must affect reproductive success. Learning meets both of these requirements (Abrantes, 2014).

Fitness

Adaptation and fitness are related. Often, two or more species co-adapt and co-evolve, developing adaptations that suit the adaptations of other species, e.g., flowering plants and pollinating insects. Mimicry is also an example of co-adaptation. Species evolve features to resemble other species. Sometimes, traits that evolved for one purpose turn out to be useful for another. For example, the dinosaurs’ feathers kept them warm, which led to the development of bird wings that could fly. 

• Relative fitness (or Darwinian fitness) describes a genotype’s average contribution to the next generation in relation to the contributions of other genotypes in the same population (Futuyama, 1986).
• Absolute fitness (or the Malthusian parameter when we apply it to a population) describes a genotype’s absolute contribution to the next generation (Fisher, 1930). 
• Adaptedness is a measure of how well a phenotype fits its ecological niche (Bennett, 2018; Burns, 1992).

Philosophical Aspects

Adaptation is a major topic in the philosophy of biology, as it concerns function and purpose (teleology). Some evolutionary biologists, including this author, try to avoid words that imply purpose in adaptation, partly because they imply a creator’s intentions. Others point out that adaptation is invariably purposeful (Sober, 1993). 

Aristotle introduced teleology to describe the adaptedness of organisms, but without accepting the supernatural intention built into Plato’s thinking, which Aristotle rejected (Nagel, 1977). 

In a sense, adaptation is purposeful: natural selection chooses what works and eliminates what does not. However, since natural selection is only an algorithm and has no awareness, most biologists reject the idea that evolution has a goal. 

Ernst Mayr writes, “Features that contribute to the adaptedness of an organism are in the philosophical literature usually referred to as teleological or functional systems. Both of these designations are potentially misleading.” He states that “adaptedness is a posteriori result rather than an a priori goal-seeking. For this reason, the word “teleological” is misleading when applied to adapted features.” We can only determine whether something is an adaptation after the event (Mayr, 1992).

The bumblebee, bombus, and the flowers they polinate are good examples of co-evolution. Picture by Alvares Gaspar via Wikipedia.

Müllerian mimicry

Müllerian mimicry (named after naturalist Fritz Müller) is the process in which two or more species evolve very similar features, which function as anti-predation signals, and both have the anti-predation attributes, e.g., unpalatability (Müller, 1878). Both mimic and model profit from this process since both gain safety from the predator confusing them.

Co-evolution is the process in which adaptations of one species are followed by adaptations of another species whose existence depends on the first species.

Adaptations serve survival, and they are a compromise between increasing the chances of survival in one area at the cost of other features or abilities. It is all a question of the particular selective pressures at any given time and environment. Sometimes, adaptations serving different functions are mutually destructive. Since the phenotype as a whole, not the genotype, is the target of selection, it is impossible to improve all aspects of the phenotype simultaneously.

Successive adaptations contribute to the fitness and, ultimately, survival of individuals. Organisms encounter a succession of environmental challenges during their lifetime as they grow and develop. The adaptive plasticity of their phenotypes (anatomical, physiological, and behavioral characteristics) develops in response to the prevalent conditions.

Nature shows two distinct strategies. Some animals are born with the ability to perform certain behaviors without specific training. They are experts in dealing with the environmental challenges of their habitat at a particular time, but they are not very resistant to sudden changes. Others do not show the same natural abilities and rely on subsequent experience and learning. They are not innate experts like the former, but they are better at dealing with changes in their environments.

Adaptation and speciation contribute to explaining the diversity of life forms.

A and B are real wasps. The other are four imperfect and palatable mimics of the wasp species. (A) Dolichovespula media; (B) Polistes spec.; (C) Eupeodes spec.; (D) Syrphus spec; (E) Helophilus pendulus; (F) Clytus arietes (all species European). Photos by: (A, C, E, and F) Rob Knell; (B and D) Tom Ings. 

Extinction

Extinction of a population or even species happens when the population or species cannot adapt quickly enough. A population risks extinction when the death rate of a population is higher than its birth rate for a prolonged period (Van Valen, 1973).

Co-extinction is the extinction of a population (or species) because of the extinction of another. A break in some link of a food chain may cause the extinction of several species. Plants may become extinct if they lose their pollinator insect, and parasites depend highly on their hosts.

 The famous Galapagos finches, from Darwin’s “The Origin of species.” Darwin explains that the different beaks evolved by means of natural selection to suit the different foods available for the birds.

Flexibility, Specialization and Exaptations

Flexibility, the ability of an organism to survive under various and different conditions, increases the probability of survival. Acclimatization and learning are good examples of the flexibility of some individuals. Even though these traits are individual and, as such, not heritable, the propensity to show them may pass on to the next generation.

Specialization confers significant advantages in stable environments but is fatal in changeable conditions. Flexibility, on the other side, sets an individual at some disadvantage when competing with specialists but makes it more robust to changes in the long term. Nothing is perfect. Survival is the name of the game, and many strategies exist to further it. Natural selection produces adaptations and evolution and ensures that the surviving individuals, and consequently populations and species are the fittest at any given time—but evolution has no goal. It is not perfect; it is a compromise.

Exaptations, although resembling adaptations, are a different process. An exaptation occurs when a trait, initially adapted for one function, coincidentally becomes beneficial for some other function. Human fingers did not develop specifically to operate a computer but became useful for that operation. Should the ability to operate a computer become a selective factor, we may (in one million years) think that was the decisive factor for the evolution and development of fingers, which is not true. Gould and Vrba proposed the term “exaptation” as a replacement for ‘pre-adaptation,’ a teleologically loaded term. (Gould & Vrba, 1982).

Without variation, there would be no natural selection because there would be nothing to choose between. However, there would be no evolution without heredity because chosen traits would not affect subsequent generations. 

Evolution is only possible because changes occur in heritable traits. Genes control inherited traits and pass from one generation to the next by way of DNA, which contains encoded genetic information. 

Darwin did not know about genes (which were first discovered by Mendel, ironically still in Darwin’s time, but without his knowledge), so he coined the term ‘units of inheritance.’

Watch movie: “Galapagos Islands: Origin and Life” by Terra Films TV.

References

Abrantes, R. (2014) Animal Learning. Wakan Tanka Publishers.

Bennett, K. (2018). Environment of Evolutionary Adaptedness (EEA). In: Zeigler-Hill, V., Shackelford, T. (eds) Encyclopedia of Personality and Individual Differences. Springer, Cham. https://doi.org/10.1007/978-3-319-28099-8_1627-1.

Burns, T. P. (1992). Adaptedness, evolution and a hierarchical concept of fitness. J Theor Biol. 1992 Jan 21;154(2):219-37. doi: 10.1016/s0022-5193(05)80404-7. PMID: 1573906

Dobzhansky, T. (1968). On Some Fundamental Concepts of Darwinian Biology. In Dobzhansky, Theodosius; Hecht, Max K.; Steere, William C. (eds.). Evolutionary Biology. Vol. 2. Appleton-Century-Crofts. pp. 1–34. doi:10.1007/978-1-4684-8094-8_1. ISBN 978-1-4684-8096-2. OCLC 24875357.

Dobzhansky, Theodosius (1970). Genetics of the Evolutionary Process. Columbia University Press. ISBN 978-0-231-02837-0. OCLC 97663.

Fisher, R. A. (1930). The Genetical Theory of Natural Selection. The Clarendon Press. OCLC 493745635.

Futuyma, D. J. (1986). Evolutionary Biology (2nd ed.). Sinauer Associates. ISBN 978-0-87893-188-0. OCLC 13822044.

Gould, S. J.; Vrba, E. S. (1982). “Exaptation — a missing term in the science of form” (PDF). Paleobiology. 8 (1): 4–15. doi:10.1017/S0094837300004310. JSTOR 2400563.

Maynard Smith, J. (1993). The Theory of Evolution (Canto ed.). Cambridge University Press. ISBN 978-0-521-45128-4. OCLC 27676642.

Müller, F. (1878). “Ueber die Vortheile der Mimicry bei Schmetterlingen”. Zoologischer Anzeiger. 1: 54–55.

Mayr, E. W. (1992). “The idea of teleology” Journal of the History of Ideas, 53, 117–135.

Nagel, Ernest (May 1977). “Functional Explanations in Biology”. The Journal of Philosophy. 74 (5): 280–301. doi:10.2307/2025746. JSTOR 2025746. Teleology Revisisted: The Dewy Lectures 1977 (second lecture).

Price, Trevor D.; Qvarnström, Anna; Irwin, Darren E. (July 2003). The role of phenotypic plasticity in driving genetic evolution. Proceedings of the Royal Society B. 270 (1523): 1433–1440. doi:10.1098/rspb.2003.2372. PMC 1691402. PMID 12965006.

Sober, E. (1993). Philosophy of Biology. Dimensions of Philosophy Series. Westview Press. ISBN 978-0-8133-0785-5. OCLC 26974492.

Van Valen, L. (July 1973). A New Evolutionary Law (PDF). Evolutionary Theory. 1: 1–30.