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UNIT 7: PRINCIPLES OF DEVELOPMENT AND
INHERITANCE
We use the word ‘ontogeny’ to talk about the developmental process of an individual along its life. This ontogeny is the result of multiple factors: phylogenetic (ex.: lineage), genetic (ex.: genetic inheritance), epigenetic (ex.: methylation patterns), environmental (ex.: the diet) and learning/activity of the organism, whatever the stage it is. Our ontogeny is unidirectional (PastPresentFuture) so there is a closer relationship between present and future than between past and future. According to that, our past has a long-term effect in our future while our present has a short-term effect on it.
7.1. CONCEPT OF DEVELOPMENT
Development is phenotypic changes in a responsive (plastic) phenotype due to inputs from the environment and the genome. Gene products are made from elements imported from the environment.
7.1.1. Predetermined epigenesist vs. probabilistic epigenesist
Preformationists believed that the egg (ovists) or the sperm (spermists) contained a tiny miniature baby or homunculus. All the parts of the baby were intricately in place, correctly disposed to each other, waiting only to be inflated like a compartmentalized ballon.
Epigeneticists hold the idea that new structures and functions, from molecules all the way up to whole organisms, including behavioral attributes and cognitive skills emerge or arise progressively during development. Organisms are not made, they develop.
There are two types of epigenetic views:
- Predetermined epigenesis: (related with the preformationist model) it establishes that information flow is unidirectional from the genes to the structure (nervous system) and function (behavior).
- Probabilistic epigenesist: states that individual development is characterized by an increase of complexity of organization, this is the emergence of new structural and functional properties and competencies at all levels of biological analysis.
Besides, probabilistic epigenesist states that the individual development is also a consequence of horizontal and vertical coactions among its parts, including organism-environment coactions. Developmental outcomes at any level are caused by these coactions which are inherently reciprocal.
The orthodox view of the Synthetic Theory of evolution determines that organisms have a passive status in relation to their environments because:
- The organism proposes and the environment disposes.
- The organism makes conjectures and the environment refutes them.
- The environment poses problems and the organism throws up random solutions.
- In a sense, the study of the organism is really the study of the shape of the environmental space, the organisms themselves being nothing but the passive medium through which we see the shape of the external world.
The classical picture of the organism-environment relation in evolution is represented formally as a pair of differential equations in time:
Equation (1) states that there is some change in environment over time that is entirely a function of environmental variables. Equation (2) states that the change in the organism is a function both of the current state of the organism and of environment. The solution of these two equations leads to the view that the complete history of the organism is driven entirely by the autonomous environmental process.
The alternative constructivist view of the organism-environment relation, dubbed interpenetration, can be defined as follows:
- Organisms choose which elements of the external world are put together to make their environments and what the relations are among the elements that are relevant to them.
- Organisms actively construct, in the literal sense of the word, a world around them.
- Organisms are in a constant process of altering their environment. They destroy its own environment by using resources and transforming them into something else.
- This constructivist view holds that the world (environment) is changing because the organisms are changing.
The constructivist view of the organism-environment interpenetration in evolution is represented formally as a pair of coupled differential equations in time:
So the histories about the environment and the organism are functions of both of them. The equations must be solved together as a coupled pair describing the coevolution of organism and environment in which both are acting as both causes and effects.
7.1.3. Niche construction theory The conventional model (of the Synthetic Theory of Evolution) says that the environment determines which genes (phenotypes/genotypes) pass the natural selection. It also establishes that individuals only inherit from their ancestors their genes. The niche construction theory takes into account two fundamental aspects about evolutionary process:
- The causality between the organism and the environment: individuals can modify and construct their environment.
- Organisms also inherit their environment/niche from their ancestors.
7.1.4. Theory of developing systems (See page 274)
7.2. CONCEPTS AND MYTHS ABOUT DEVELOPMENT 7.2.1. Factors that participate in development There are four different factors that are important during the development of phenotypic traits:
- Inducing factors: those that are necessary for activating new emerging functions.
periods), the experiences and the information related to the experiences are also there, but the capacity of signaling them and reacting to them is significantly lower. These periods are essential to understand one’s behavior and mental processes. It is generally thought that, especially in species that are K-selected, juvenile periods are developmental stages in which the organism is more permeable to all kinds of environmental inputs (periods of openness to beneficial and potentially harmful information).
7.2.4. Continuity and stability Both concepts refer to the same phenomenon which, in fact, states that the past in the individual history of an organism explains largely its present and predicts its future. Continuity (or stability) is defined in terms of irreversibility of early effects along life history of an individual. Some examples are: hunger, maltreatment, harassment, sexual abuse…especially if they took place during early stages. Discontinuity in the development of organisms occurs when an abrupt change (increase or decrease) is detected in the activity of an already existing function or when a new behavior or cognitive ability emerges in an individual.
It also takes place when the inter-individual differences don’t exhibit temporal stability (instability). Individuals’ personality, temperament or attachment style must be stable along his lifetime (stability).
7.2.5. Dichotomies in development’s study
Dichotomies are the pairings of antonym terms to reflect processes that work against the others. It has been used to identify causal agents and resulting effects. With this view, it is said that the biologic is the natural, and it is intrinsic, stable within generations and autonomous with regards to the environment. On the other hand, external information (culture, education, environment…) is unstable, variable (plastic) within individuals’ lifetime, acquired and learned. Nowadays, the most accepted theory is the one that claims that dichotomies must be refuted because they do not take into account the developmental basic properties of organisms (interdependence, interpenetration and inseparability between all the external factors) and the relational nature between them.
7.2.6. DNA as a blueprint or developmental programme
DNA is referred to as a blueprint because it contains the entire set of instructions that specify the inherited characteristics and traits that an organism will possess after birth. DNA (as a blueprint) can be compared to an engineer’s or architect’s plans that contain all the details on how to construct a house. As an architect’s blueprint, DNA also contains all the information for building a complete functional organism (being able to go from the final product to the blueprint). This is called ‘top-down programme’. Therefore, DNA is like a blueprint of biological guidelines that a living organism must follow to exist and remain functioning (preformationism).
Nevertheless, this view ignores the fundamental differences between the DNA and the blueprints in the nature of the mapping from low order information to the high order object (‘bottom-up programe’). This process from genotype to phenotype does not seem very straightforward but indeed very complex. Thus, from an animal’s body, its DNA can’t be reconstructed (meaning that information cannot be passed back to inheritable genetic information). Bodies develop and grow from the very beginning from a single
- Most adaptive evolution is genetic accommodation of environmentally induced phenotypic accommodation. In this regards, genes are seen as followers, not necessarily leaders, in phenotypic evolution.
- Genetic accommodation involves reorganization of pre-existing genetic architecture.
- Phenotypic accommodation is adaptive change without genetic change.
The basic takehome lesson of the two-legged goat effect is that through this effect we can conclude that changes in one’s phenotype would be driven by environment and not genotype and that the changes in the genotype are driven by changes in the phenotype. The environment makes you accommodate your phenotype and as a consequence you will expect a genetic accommodation.
If the cause is that environment makes you accommodate a phenotype, the consequence will be that you will expect a genetic accommodation: genes are followers, not leaders in this process.
7.3.3. Life history theory (LH) LH (or r-K theory) describes the ways organisms allocate resources among different components of fitness, given the environmental constraints under which they survive and reproduce. LH strategies have traditionally been characterized as r-selected and K-selected:
- R-selected: comprises species exhibiting very fast life histories, with r denoting the maximum population reproductive rate of a species. They have high mating effort strategy that represents a fast life history owing to the tendency for enhanced reproduction to be associated with more rapid maturation and to come at the expense of longevity.
- K-selected: encompasses species exhibiting very slow life histories, which saturate their environment to carrying capacity (K). They show high somatic effort and parental effort strategy, representing a slow life history and enhancing longevity.
(See slides 45, 46, 47 and 48)
7.3.4. Interspecies and inter-sexual differences in longevity The variable that shows variation in longevity between species is protection, and protection could also include body size. Meaning larger species tend to live longer. Body size is a good predictor of extrinsic mortality, especially in terrestrial animals.
There are also inter-sexual differences in longevity, because in a lot of species females tend to live longer than males due to different causes: genetic and immune systems, neuroendrocrine system and behavior.
7.3.5. Ageing: proximate and ultimate causes
either dying earlier or living longer as far as the fitness of one’s kin is concerned.
- Ageing and the individual. Darwin was the first to make the statement that longevity is generally related to…the amount of expenditure in reproduction and in general activity. And these conditions have been largely determined through natural selection. There are three individual-based theories, and at least some of them embrace the idea of the breeding vs survival tradeoff:
- Consequence of benign neglect (Medawar’s mutation accumulation theory): mutations that can cause harmful effects in later life are not as effectively purged from a population over the course of many generations-natural selection doesn’t care very much about oldies. Example: Huntington’s disease.
- The price one has to pay (Williams’ antagonistic pleiotropy theory): it proposes that ageing is a consequence of the actions of genes that favors early survival and reproduction over late survival and reproduction. This is a trade-off, in which early reproductive success is actively purchased at the cost of future reproductive success.
- Consequence of a balancing act (Kirkwood’s disposable soma theory): the energy available to an organism may be used either to preserve the soma or to reproduce the germ line. Because of the cumulative risk of extrinsic
mortality, natural selection favors investment in reproduction over somatic repair once sexual maturation has been achieved.
7.4. INHERITANCE
Hard (genetic) vs. soft (non-genetic) inheritance
- Hard inheritance: model of heredity based on the transmission from parents to offspring of a set of factors whose nature is unaffected by the environment or phenotype of the parents. It believes that parents transmit a developmental blueprint to offspring at the moment of conception.
- Soft inheritance: proposes gradual and direct changes to DNA sequences. It believes that parents transmit their features to their offspring, including features acquired during their lifetime, and that transmission can occur not just at conception but also via subsequent interactions between parent and offspring.
- Mendelian/hard/DNA-centric model of inheritance: inheritance is mediated by the transmission (blue arrow) of gene alleles, unaffected by environment (hard), from parents (Fo) to offspring (F1).
- Lamarckian/soft/genetic encoding model of inheritance: inheritance is mediated by the transmission of gene alleles, but those alleles are subject to modification in the germ-line by factors originating in the soma (green arrow), thereby allowing for the inheritance of acquired traits. DNA is the only thing that can be passed onto generations, and that environmental conditions are contained in this information and can therefore also be passed on.
- (^) Pluralistic/extended/inclusive model of inheritance: inheritance is mediated by the transmission of gene alleles (‘genetic inheritance’, blue arrow) alongside a set of other factors (‘non-genetic inheritance’, green arrow).
