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53.10: Behavior and Evolution

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    Evolutionary (ultimate) explanations

    1. Function (adaptation)

    Darwin's theory of evolution by natural selection is the only scientific explanation for why an animal's behavior is usually well adapted for survival and reproduction in its environment. However, claiming that a particular mechanism is well suited to the present environment is different from claiming that this mechanism was selected for in the past due to its history of being adaptive.[4] The literature conceptualizes the relationship between function and evolution in two ways. On the one hand, function and evolution are often presented as separate and distinct explanations of behaviour.[4]

    On the other hand, the common definition of adaptation, a central concept in evolution, is a trait that was functional to the reproductive success of the organism and that is thus now present due to being selected for; that is, function and evolution are inseparable. However a trait can have a current function that is adaptive without being an adaptation in this sense, if for instance the environment has changed. Imagine an environment in which having a small body suddenly conferred benefit on an organism when previously body size had had no effect on survival.[4]

    A small body's function in the environment would then be adaptive, but it wouldn't become an adaptation until enough generations had passed in which small bodies were advantageous to reproduction for small bodies to be selected for. Given this, it is best to understand that presently functional traits might not all have been produced by natural selection.[4] The term "function" is preferable to "adaptation", because adaptation is often construed as implying that it was selected for due to past function.

    2. Phylogeny (evolution)

    Evolution captures both the history of an organism via its phylogeny, and the history of natural selection working on function to produce adaptations.[5] There are several reasons why natural selection may fail to achieve optimal design. One entails random processes such as mutation and environmental events acting on small populations. Another entails the constraints resulting from early evolutionary development. Each organism harbors traits, both anatomical and behavioral, of previous phylogenetic stages, since many traits are retained as populations evolve.

    Reconstructing the phylogeny of a species often makes it possible to understand the "uniqueness" of recent characteristics: Earlier phylogenetic stages and (pre-) conditions which persist often also determine the form of more modern characteristics. For instance, the vertebrate eye (including the human eye) has a blind spot, whereas octopus eyes do not. In those two lineages, the eye was originally constructed one way or the other. Once the vertebrate eye was constructed, there were no intermediate forms that were both adaptive and would have enabled it to evolve without a blind spot.

    Proximate explanations

    3. Mechanism (causation)

    In examining living organisms, biologists are confronted with diverse levels of complexity (e.g. chemical, physiological, psychological, social). They therefore investigate causal and functional relations within and between these levels. A biochemist might examine, for instance, the influence of social and ecological conditions on the release of certain neurotransmitters and hormones, and the effects of such releases on behaviour, e.g. stress during birth has a tocolytic (contraction-suppressing) effect.

    Some prominent classes of causal mechanisms include:

    • The brain: Broca's area, a small section of the human brain, has a critical role in linguistic capability.
    • Hormones: chemicals used to communicate among cells of an individual organism. Testosterone, for instance, stimulates aggressive behaviour in a number of species.
    • Pheromones: chemicals used to communicate among members of the same species. Some species (e.g., dogs and some moths) use pheromones to attract mates.

    4. Ontogeny

    Ontogeny is the process of development of an individual organism from the zygote through the embryo to the adult form.

    In the latter half of the twentieth century, social scientists debated whether human behaviour was the product of nature (genes) or nurture (environment in the developmental period, including culture). Many forms of developmental learning have a critical period, for instance, for imprinting among geese and language acquisition among humans. In such cases, genes determine the timing of the environmental impact.

    Causal relationships


    Figure \(\PageIndex{3}\): Diagrammatic explanation of behavioral ecology: the interaction between an organisms behavior, its genes, the environment, and evolutionary forces (W. Pete Welch; public domain); Adopted from Tinbergen (1963).

    The figure shows the causal relationships among the categories of explanations. The left-hand side represents the evolutionary explanations at the species level; the right-hand side represents the proximate explanations at the individual level. In the middle are those processes' end products—genes (i.e., genome) and behaviour, both of which can be analyzed at both levels.

    Evolution, which is determined by both function and phylogeny, results in the genes of a population. The genes of an individual interact with its developmental environment, resulting in mechanisms, such as a nervous system. A mechanism (which is also an end-product in its own right) interacts with the individual's immediate environment, resulting in its behaviour.

    Here we return to the population level. Over many generations, the success of the species' behaviour in its ancestral environment—or more technically, the environment of evolutionary adaptedness may result in evolution as measured by a change in its genes.

    In sum, there are two processes—one at the population level and one at the individual level—which are influenced by environments in three time periods.


    Let's look at a couple of examples of explaining a trait or behavior using Tinbergen's four questions:

    Visual perception

    • Function: to find food and avoid danger.
    • Phylogeny: the vertebrate eye initially developed with a blind spot, but the lack of adaptive intermediate forms prevented the loss of the blind spot.
    • Causation: the lens of the eye focuses light on the retina.
    • Development: neurons need the stimulation of light to wire the eye to the brain (Moore, 2001:98–99).

    Sleep (Bode & Kuula, 2021):[7]

    • Function: energy restoration, metabolic regulation, thermoregulation, boosting immune system, detoxification, brain maturation, circuit reorganization, synaptic optimization, avoiding danger.
    • Phylogeny: sleep exists in invertebrates, lower vertebrates, and higher vertebrates. NREM and REM sleep exist in eutheria, marsupialiformes, and also evolved in birds.
    • Mechanisms: mechanisms regulate wakefulness, sleep onset, and sleep. Specific mechanisms involve neurotransmitters, genes, neural structures, and the circadian rhythm.
    • Ontogeny: sleep manifests differently in babies, infants, children, adolescents, adults, and older adults. Differences include the stages of sleep, sleep duration, and sex differences.

    Contributors and Attributions 

    Modified by Dan Wetzel (University of Pittsburgh) from the following sources:


    1. MacDougall-Shackleton, Scott A. (2011-07-27). "The levels of analysis revisited". Philosophical Transactions of the Royal Society B: Biological Sciences. 366 (1574): 2076–2085. doi:10.1098/rstb.2010.0363. PMC 3130367. PMID 21690126.
    2. Daly, M. & Wilson, M. (1983). Sex, evolution, and behaviour. Brooks-Cole.
    3. p411 in Tinbergen, Niko (1963) "On Aims and Methods in Ethology," Zeitschrift für Tierpsychologie, 20: 410–433.
    4. Nikolaas Tinbergen, ethology, Cartwright 2000:10; Buss 2004:12)
    5. "Phylogeny" often emphasizes the evolutionary genealogical relationships among species (Alcock 2001:492; Mayr, 2001:289) as distinct from the categories of explanations. Although the categories are more relevant in a conceptual discussion, the traditional term is retained here.
    6. Bode, Adam; Kuula, Liisa (September 2021). "Romantic Love and Sleep Variations: Potential Proximate Mechanisms and Evolutionary Functions". Biology. 10 (9): 923. doi:10.3390/biology10090923.

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