Author : Wahid Ahmad
Jean-Baptiste
Lamarck, predating Charles Darwin, proposed an early theory of evolution. He
suggested that organisms could acquire traits during their lifetime in response
to the environment, and these acquired traits could be inherited by their
offspring. For example, in a population of giraffes with short necks,
continuous neck stretching to reach leaves on tall trees could lead to longer
necks over generations. However, modern genetics has revealed that traits
acquired during an individual's life are not typically passed on through
genetic material. Lamarck's theory, while influential, lacked empirical support
and was eventually replaced by Darwin's theory of natural selection.
This
theory was revised by Charles Darwin. Darwin's theory of evolution, a
groundbreaking concept in the field of biology, introduced the idea that the
all diversity of life can be traced to a common ancestor, through a process
called natural selection. The core principles of Darwin's theory include variation
within populations, competition for limited resources, and the differential
survival and reproduction of individuals with advantageous traits. Let's
explore an example featuring a population of rabbits in a meadow.
Consider
a population of rabbits with variations in fur color. Some rabbits are brown,
while others are lighter in color.
The
meadow has various predators, including birds of prey. The brown fur provides
better camouflage in the grassy environment, making it harder for predators to
spot those rabbits.
Over
time, the brown-furred rabbits are more likely to survive because they can hide
more effectively from predators. They have a better chance of reaching
reproductive age and having offspring.
The
advantageous trait of brown fur is passed on to the next generation.
As
generations go by, the majority of the rabbit population becomes brown, as this
trait offers a survival advantage in the meadow. This change in fur color is an
evolutionary adaptation driven by natural selection.
Charles
Darwin conceived of evolution by natural selection without knowing that genes
exist. Now mainstream evolutionary theory has come to focus almost exclusively
on genetic inheritance and processes that change gene frequencies.
Back in
the 1930s and 1940s, scientists came up with the main ideas of how evolution
works. They mixed concepts like natural selection and genetics to create what's
known as the modern synthesis. New variants of traits called alleles are
created through random genetic changes call mutations in the DNA. Natural
selection will determine the suitability of the variants which in turn will
determine the frequency of alleles as they are passed down to future
generation. Mathematics is used to explain how the frequency of different
alleles or variants in a group of living things changes over time. Evolution is
simply defined as changes in allele frequencies.
Since
1960’s, evolutionary biology has added some new ideas, but it mostly sticks to
the same basics from the modern synthesis. This view, called the Standard
Evolutionary theory.
However,
this gene-centric perspective overlooks crucial aspects of evolution. The Extended
Evolutionary Synthesis argues that other processes play significant roles in
directing evolution. These include how an organism's physical development
influences variation called developmental bias, how the environment directly
affects traits called plasticity, how organisms modify their environments
called niche construction, and how traits more than genes are transmitted
across generations called extra-genetic inheritance.
The
concept of developmental bias helps us understand how organisms adapt to their
environments and evolve into different species. Take, for instance, cichlid
fishes in Lake Malawi. Although they are more closely related to other cichlids
in the same lake than those in Lake Tanganyika, both lakes' cichlids share
remarkably similar body shapes.
Rather
than attributing these similarities to coincidences through convergent
evolution, where similar traits emerge due to similar environmental pressures,
a more concise explanation involves the interplay of developmental bias and
natural selection.
One key
developmental bias in human evolution is related to the development of the
neocortex, the outer layer of the brain responsible for higher cognitive
functions. The neocortex has undergone considerable expansion in humans
compared to other primates. This expansion is not solely due to the emergence
of entirely new genes but also involves the repurposing and modification of
existing genetic and developmental pathways.
The genes
involved in regulating the size and complexity of the neocortex are ancient and
conserved across many species. However, the specific ways these genes are
activated, the timing of their expression, and the interactions with other
genes and environmental factors during development play a crucial role in
determining the unique features of the human brain.
The
process of brain development in humans is biased by pre-existing developmental
constraints, which means that certain trajectories are more likely to be
followed due to the inherent properties of the developmental system. The
complexity and size of the human brain, with its specific cognitive abilities,
can be seen as an outcome of these biased developmental processes.
This idea
helps explain how organisms adapt to their environments and diversify into
various species. It suggests that rather than being entirely random, natural
selection is guided along specific routes opened up by developmental processes,
working in tandem with selection.
Genes are
not the only factors involved in passing traits from one generation to the
next. The Extended Evolutionary Synthesis suggests that other processes, like
plasticity, play a crucial role. Plasticity refers to how living things can
change their shape or form in response to their environment. While the Standard
Evolutionary Theory sees this as minor adjustments or even random changes, the Extended
Evolutionary Synthesis views plasticity as the initial step in adaptive
evolution.
Plasticity
allows organisms not only to survive in new environments but also to develop
traits that are well-suited to those conditions. Traits can precede genes in
adaptation, meaning that the trait comes first, and the genes supporting it
follow in later generations. Studies of fish, birds, amphibians, and insects
suggest that adaptations induced by the environment can lead to the
colonization of new areas and the formation of new species.
This
theory treats environment as a background condition that triggers or modifies
selection but is not an active part of the evolutionary process. It doesn't
distinguish between how organisms adapt to different situations, such as
termites building mounds or responding to volcanic eruptions. In contrast, the
EES sees these cases as fundamentally different.
Volcanic
eruptions are unpredictable events unrelated to organisms' actions, while
organisms like termites actively construct and regulate their homes in a
consistent manner shaped by past experiences and influencing future ones. This
concept, known as "niche construction," suggests that organisms play
a role in directing their own evolution by systematically changing environments
and biasing selection. In simpler terms, living things are not just reacting to
their surroundings; they are actively shaping their evolution.
Beyond
just genes, there's a way living things pass on traits, and it's not just about
parents giving their kids genes. The Extended Evolutionary Synthesis acknowledges
that parents also pass on things from their own experiences, like the
environment they grew up in. This broader idea is called 'extra-genetic
inheritance.'
Extra-genetic
inheritance includes passing on chemical marks on DNA, called epigenetic marks,that
can influence things like fertility and disease resistance in different
species. It also includes behaviors in animals that are learned and passed on
socially, like chimpanzees cracking nuts or fish following migration patterns. Additionally,
it involves the things living things create or change in their environment,
such as beavers building dams or worms altering the soil they live in. Over the
past decade, research has shown that this kind of inheritance is widespread and
should be part of our general understanding of how traits are passed down.
When
scientists use mathematical models to study how evolution works and include
this extra-genetic inheritance, they get different predictions compared to
models that only consider genes. These inclusive models help explain puzzling
things in nature, like how certain birds quickly colonized North America or how
plants with low genetic diversity can adapt. It even sheds light on how new
species and ecosystems evolve over millions of years, with evidence suggesting
that certain organisms, like sponges, have played a big role in creating
opportunities for other life forms. So, it's not just about genes; it's about
the broader impact of living things on their environment and how that shapes the
future of life on Earth.
The above
insights derive from different fields, but fit together with surprising
coherence. They show that variation is not random, that there is more to
inheritance than genes, and that there are multiple routes to the fit between
organisms and environments. Importantly, they demonstrate that development is a
direct cause of why and how adaptation and speciation occur, and of the rates
and patterns of evolutionary change.
SET
consistently frames these phenomena in a way that undermines their
significance. For instance, developmental bias is generally taken to impose
‘constraints’ on what selection can achieve — a hindrance that explains only
the absence of adaptation. By contrast, the EES recognizes developmental processes
as a creative element, demarcating which forms and features evolve, and hence
accounting for why organisms possess the characters that they do.
Researchers
in fields from physiology and ecology to anthropology are running up against
the limiting assumptions of the standard evolutionary framework without
realizing that others are doing the same. We believe that a plurality of
perspectives in science encourages development of alternative hypotheses, and
stimulates empirical work. No longer a protest movement, the EES is now a
credible framework inspiring useful work by bringing diverse researchers under
one theoretical roof to effect conceptual change in evolutionary biology.
Not at all
Charles
Darwin's last book, published in 1881, focused on earthworms because they
showcased an interesting cycle: the worms modified their environment and were
adapted to thrive in it. Darwin's genius lay in drawing insights from various
fields, shaping evolutionary thinking that emphasized evidence and synthesis
from diverse areas.
Some
argue that concepts like niche construction and phenotypic plasticity,
highlighted in the 'extended evolutionary synthesis,' are not novel but have
long been integral to evolutionary biology. These ideas are firmly established
in the field and don't necessitate a distinct label. Their current prominence
results from their proven explanatory power, not a lack of attention.
Evolutionary biology continually explores various topics, including epistasis,
cryptic genetic variation, extinction, climate change adaptation, and
behavioral evolution, expanding the field's scope.
Contrary
to portraying evolutionary theory as static, the discipline is dynamic,
creative, and rapidly growing. Evolutionary biologists actively engage with
diverse fields such as genomics, medicine, ecology, artificial intelligence,
and robotics. Far from resisting new ideas, the field remains vibrant,
inclusive, and progressive—an affirmation of ongoing evolution in evolutionary
biology.
The focus
on genes in evolutionary theory is crucial, as genetic changes strongly predict
adaptation and speciation. Many adaptations, like antibiotic resistance and
camouflage coloration, are directly tied to genetic alterations. While
non-genetic processes, such as phenotypic plasticity, contribute to
adaptability, the primary driver of evolutionary change is heritable
differences in traits offering a selective advantage. The roles of plasticity,
developmental bias, and epigenetic modification in adaptive traits lack robust
evidence, necessitating further research. These factors, alongside niche
construction and inclusive inheritance, influence evolutionary change but are
not essential for evolution.
The call
is for a broader perspective rather than divisions. Strengthening evidence for
phenomena like phenotypic plasticity, inclusive inheritance, niche
construction, and developmental bias is the most effective way to elevate their
importance in evolutionary theory. Darwin's emphasis on rigorous evidence
before making significant claims underscores the importance of robust research
in the field of evolutionary biology.