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Module 3Principles of Development and Inheritance
"Nothing in Biology Makes Sense except in the Light of Evolution"
The idea that understanding the biological world and the myriad of living organisms is best
achieved by considering the process of evolution. In essence, it highlights the fundamental role of
the theory of evolution in biology. It suggests that without taking into account the concept of
evolution, many biological phenomena, structures, and behaviors might appear random, chaotic, or
disconnected. In essence, the phrase underscores the central and unifying role of evolutionary
principles in explaining the diversity and complexity of life and how all biological phenomena are
interconnected through the process of evolution. It emphasizes that to truly comprehend the
intricacies of biology, one must consider the evolutionary history and context of living organisms.
"Nothing in Biology Makes Sense except in the Light of Evolution and Development"
Extends the idea that understanding biology is deeply rooted in the principles of evolution and adds
the concept of development. Here's what it means:
Evolution: This part of the phrase reiterates the importance of the theory of evolution in
comprehending biology. It emphasizes that many aspects of living organisms and their interactions
can be best explained by considering the historical processes of evolution. Evolutionary principles
help us understand how species have changed over time, why they have certain traits, and how they
are adapted to their environments.
Development: By adding "and Development" to the phrase, it incorporates the study of how
organisms grow, develop, and undergo changes throughout their life cycles. This includes embryonic
development, growth, maturation, and aging. Developmental biology explores how genes, environment,
and other factors influence the transformation of an organism from a single cell to a complex,
functioning individual.
In summary, "Nothing in Biology Makes Sense except in the Light of Evolution and Development"
underscores that biology cannot be fully understood without considering both the evolutionary
history of species and the processes of development that shape an organism's life. Together, these
two aspects provide a comprehensive framework for making sense of the complexities of the
biological world
,What is development?
The development of an organism is the process that unfolds from the very moment it is conceived
until its death. Its life trajectory, another way of referring to the developmental process or ontogeny,
is the result of the operation of numerous factors, including phylogenetic (e.g., the lineage to which it
belongs), genetic (the alleles received from the parents), epigenetic (e.g., methylation patterns), and
environmental (e.g., nutrition), as well as the learning and activity of the organism itself, at any of its
stages. The process at hand is complex not only due to the network of mechanisms at play and their
interactions but also because change (or its stabilization) affects a multitude of phenotypic variables.
Thus, an individual's motor, behavioral, social, and psychological skills change during their life
trajectory, just as the physical and physiological processes that underpin them change.
Preformationism vs, Epigenesis
Preformationnists believed that the egg or the sperm contained a tiny miniature baby. All the parts of
the baby were intricately in place, correctly disposed to each other, waiting only to be inflated like a
compartmentalised balloon. There are several problems with this view:
• Information is inherited from the two parents, not only either the mother or the father
• For this view to be correct we have to face a Russian-doll-style infinite regress of homunculi
within homunculi… to take us back to Eve (or Adam)
• According to this view, organisms are made from preexisting, tiny parts that grow progressively
until they reach the adult shape and size
• If Adam was made, i.e., he just sprang into existence, then he didn’t have genes, he had no
embryology, he didn’t develop (didn’t go through an ordered series of stages)
Epigeneticists hold the idea that new structures and function, from molecules all the way up to whole
organisms, including behavioural attributes and cognitive skills emerge or arise progressively during
development. Organisms are not made, they develop!
,What drives development?
An organism’s life history, from conception (or birth) to death, consists of successive physiological,
behavioural and psychological phenotypes which find themselves in a succession of environments.
Understanding the feature of genes, organisms and environments during the organism’s entire lifespan
requieres identifying the information flow in each level across levels
• Information within and across levels is held to be bidirectional (information flows back and forth
between organism and its environment); there's a constant exchange of information
• The time arrow that links the past, the present and the future of an organism’s genes, phenotype,
and environment is unidirectional (the changes an organism goes through have a clear direction in
time, from the past to the present and into the future). Cause-and-effect relationships operate over
different time frames, so effects can happen immediately, in the short term, or over the long term.
A) Predetermined epigenesis:
Genetic activity —> Structural maturation —> Function, activity or experience
(Genes) —> (Nervous system) —> (Behaviour)
B) Probabilistic epigenesis
Genetic activity <—> Structural maturation <—> Function, activity or experience
(Genes) <—> (Nervous system) <—> (Behaviour)
, Probabilistic epigenesis implies the adoption of a shared causality perspective. The condition of an
individual at any stage of its development and at any level of organisation is the result of reciprocal
information flow between levels of organisation.
In contrast to a predetermined conception of epigenesis, the definition of probabilistic epigenesis states
that the individual development is characterised by an increase of complexity of organisation, i.e.,
emergence of new structural and functional properties and competencies at all levels of biological
analysis, as a consequence of horizontal and vertical coaction among its parts, including organism-
environment coactions. Developmental outcomes at any level are caused by these coactions which are
inherently reciprocal. According to this view, genomes and gene activity are no longer encapsulated; they
do not contains a blueprint or master plan for the construction of the organism. They are “simply”
contributing partners, as much as the organism and the environment are, in the developing system.
Myths on Development
1) Myth 1: Genotype determines phenotype
2) Myth 2: Organisms are passively selected by the environment
3) Myth 3: DNA as blueprint and as programme
4) Myth 4: Inheritance is only hard or genetic
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