Essay of Biological Evolution
As the son of the nature, human being can not help praising his great mother: she
nurtures countless lives, and creates a florid biological kingdom. On the land, there
are lush forests, towering trees, endless grasslands, and so many animal inhabitations.
Below the sea, it can be depicted with magnificent coral reefs and swimming fish in
jubilant mood. And in the sky, flying birds are kind of burnishing scenery. The living
species, from tiny microbes which can not be seen with naked eye to huge whales
weighing 100 tons, exist in the same place: the earth.
Facing such a gorgeous biological world, in addition to appreciating the
masterpiece of the nature, we always speculate: what is the origin of such a biological
world, especially ourselves? And how did living species evolve to what they look like
today? These questions that puzzle mankind, in fact, are the knowledge of archebiont
and evolution.
The exploration to evolution
The earliest exploration to evolution can be traced back to 1700s. At that time,
Buffon, a French scholar, proposed that organisms could change their appearances
and function along with the changing environment in his book natural history. But the
spark of evolution was annihilated in the intervention of theology whereafter.
After Buffon, there was another noble scholar in the same country, Lamarck, who
is thought to be the creator of evolution theory, studied many fossils of invertebrate,
and noticed that there exists a series of variation between fossils and the similar
existing species. He spent long time probing the phenomenon and published his book
The Philosophy of Zoology in 1809. In the first part of the book, he discussed the
classification and the evolution process. It was the first time that evolution theory was
introduced in detail.
In the view of the dynamia and mechanism, he presumed that living beings have
natural trends that could endlessly increase their complexity and perfection of
structure. At the same time, they have the reactive potency to environmental change.
In his theory, two points of view were introduced: 1) if organs are used frequently,
they will evolve otherwise degrade; 2) inheritance of acquired characteristics. He
emphasized the era of the earth is very long and evolution is a gradual process.
Meanwhile, he realized the effect of environment and animal propensity in evolution.
However, guess took much part in his theory and it lacked convinced evidences. So at
that time, it had little impact on the public. But several years later, the theory of
Lamarck, which was called Non-Darwinism, began to prevail.
The supernatural inspiration is alien to science, and the facts of science are usually
denied by religion. As a result, it is impossible to mix them together. When Darwin’s
book On the Origin of Species was published in 1859, the war was inevitable. Briefly,
Darwin’s evolutionism is composed of 5 relatively independent theories: evolutionary
theory itself, common ancestor, gradual evolution, speciation and natural selection.
Among them, the theory of gradual evolution stired sharp arguments and is still the
hot topic up to now, for the discontinuity between species is widespread and the
geology record also indicates the same characteristics in fossil species. And natural
selection, the spirit of evolutionism, makes biological world infinitely wonderful and
perfectly harmonious, that is, the interdependence between species and the high
degree of biological adaptation to environment promote evolution. Although it is
strongly opposed to the belief of theism, and there had been a heated debate even in
natural selectionists themselves, it finally becomes the backbone of the ingredients of
Darwinism.
Compared with Lamarck’s theory, Darwinism differs in three aspects: Lamarck
emphasized the self-improvement of species and evolution proceeds the trends that
benefit themselves. But Darwin believed natural selection and the direction of
evolution was diverse. The inheritance of acquired characters, which Darwin regarded
as a less mechanism put Darwin into a dilemma. Though he tried to use the theory of
pangenesis to explain this inheritance, there was not any solid evidence, and even
Darwin himself held a suspect attitude on the theory. Actually, it is his insufficient
genetic laws that puzzle him.
Later, Weisman, a German, solved the problem in 1885. Thus pangenesis was
replaced by his theory of the germ track. According to this theory, the germ cells are
continuous, long lasting and always separate from the somatic cells, whereas the
somatic cells are not continuous, and every generation of them derives from former
germ cells. What somatic cells function is of protection and as an assistant. He denied
any form of inheritance of acquired characters and the pangenesis of Darwin. In fact,
he completely accepted the other aspects of Darwin’s theory.
Weisman's theory clarifies some of the confusion in cytology of that times with a
possibility which could form an organized framework and his genetic theories has laid
a foundation to the further development of the evolutionary theory. Because the
germ-track theory completely denies the existence of inheritance of acquired
characters and uncompromising emphasis on natural selection, this theory has been
called neo-Darwinism. It can be said that Weisman is the one who made the largest
contribution to evolution in 19th century after Darwin.
Science often develops in the heated academic debate, in a way of abandoning the
concept with mistakes and bias, and integration of different reasonable hypothesises.
There is no exception in evolution as well. It took more than 10 years for the
Mendel’s law of classical genetics to truly be integrated into the evolution. Since then,
Darwinism had a period of rapid development again. Eventually it formed a
comprehensive evolutionary theory which is based on a gradual natural selection and
the view about population. At the same time, it is consistent with the genetic
mechanisms, in addition to taking into account the effect of environmental factors.
The most important achievement of comprehensive evolutionary theory is the
formation of unified view of the genetic mutation in real terms. According to the
theory, extensive mutations and gradual, subtle variation can be explained with the
same genetic mechanism. It reinforces the view of Darwin’s natural selection and
offers a better understanding about the main opinion of the bion as a whole in natural
selection. Meanwhile, it also emphasizes the importance of geographical environment
on the gradual formation of new species. It can be said that the formation of
comprehensive evolutionary theory is one of the most important scientific events
since the advent of On the Origin of Species and it signifies the born of modern
Darwinism.
At the same time, the thinking of evolution is extended to all branches of biology,
and is integrated into the theoretical framework of modern biology of all areas as far
as possible. Moreover, there are also a lot of disciplines based on evolution. It is just
as what Dobzhansky, a well-known modern Darwinist, said in the 1970s: Nothing in
biology makes sense except in the light of evolution.
Modern Darwinism has developed to such a degree that Darwinists believed that
natural selection has functions on all the gene variation. In fact, however, these
conclusions are based on the observations on morphology, whereas what changed the
internal structure of the gene, the genetic material in evolution, has not been
considered yet. Meanwhile, in the development of evolutionary theory, it seems that
the non-Darwinism is only a compensation for Darwinism. However, as the
achievement of molecular study accumulated, the new theory arose. In 1968, the
Japanese scientist Kimura Motoo published a paper entitled Evolutionary Rate at the
Molecular Level in Nature, proposed the theory of neutral mutation and random
drift.
The core of the theory is that most mutations that contribute to the genetic structure
of species and the molecular evolution in the sense of natural selection are neutral or
near-neutral, and therefore fail to be screened for these mutations. The fate of these
neutral or near-neutral mutations is determined by random factors. Therefore, the
neutral theory can be seen as a lucky to survive theory.
Neutral mutation and random drift theory is important to molecular evolution and it
has a significant impact on the whole evolutionary biology. It is strongly opposed by
Darwinists. Therefore, intense controversy is risen between the two camps of
evolution, and continues to the present.
Meanwhile, Darwinists encounter other impact of intermittent balance theory,
which is another non-Darwinism and proposed in 1972. That is, biological evolution
is actually an intermittent balance. The extant species may be at a standstill phase in a
long time without great change. But after it does so, the process will execute spurtly.
Frankly, the fossil evidence signifies the rationality of the theory. Living fossils, the
witness of evolution, change little even they have passed millions of years. It is
another proof to intermittent balance.
The intermittent balance theory denies the consistency of evolutionary and the
insufficiency of fossil record is no longer the reason that there lacks transitional
species. The process of speciation is of very short duration. Obviously, it topples the
theory of progressive evolution. Moreover, it emphasizes the significance of
speciation in evolution, and facilitates the study of speciation mechanism. Because it
destabilizes deep-rooted view of Darwinists, divergence is unavoidable.
The range of evolutionary theories of both Darwinism and non-Darwinism is very
wide, so it is difficult to give them a simple and comprehensive definition, but it is
very easy to be distinguished between on where the focus lays:
1 The function and importance of natural selection in biological evolution. Is it
necessary that natural selection produces and maintains stocks of biological
polymorphism? Is it true that natural selection is a major driving force of evolution?
2 The direction and nature of biological evolution. Is it evolution that makes
organism adapt to the environment more easily? Does evolution born with direction
and endogeny dynamia? Why can organism evolve from simple to complex? What is
the essence of evolution of this development?
3 The model, mechanisms and processes of biological evolution. Is evolution a
continuous or intermittent process? Is the formation of new species, the emergence of
new types of life, and other large-span of evolution an accumulation of small variation
of a gradual process, or not continuous, leaping process?
It seems that the answers to the above arguments between Darwinism and
non-Darwinism oppose each other with equal harshness.
Darwinism focuses on the tiny variation of individuals and polymorphism of
population. It is essential that natural selection maintains the polymorphism of
biological species, and plays an innovative role by selectively retaining some of the
variation. It is this biological role that makes evolution a gradual approach. Natural
selection determines the continuity of species and is the major mechanism from which
a new species derives. The direction of biological evolution is basically denied by
Darwinism and even in the course of a certain direction, it is just a trend that organism
continues to increase the adaptability to environment.
Non-Darwinism emphasizes discontinuity between modern species. Meanwhile, it
lays stress on the problem of the origin of life and early evolution more than that the
Darwinism depicts. That natural selection in biological evolution just plays a minor
role. Because of the presence of neutral mutation, the polymorphism of the population
is not maintained by natural selection. Biological evolution is subject to some of the
basic laws of nature, including physical and chemical laws, which have their internal
principle and direction.
In the process of biological evolution, organisms maintain a balanceable state with
environment, rather than more and more adapting to it. Discontinuity between the
species is determined by species formation mechanism, thus the large span of
evolution such as species formation is a discontinuous, leaping process.
It is common that there are debates in science, and free, objective dispute often
promotes the development of science. Two kinds of results lay out:1) one wins
completely, the other is taken place of；2) the two or more sides unify to form a new
theory, and they are symbiotic. The evolutionary theory continuously develops from
its birth. Every theory of evolution contributes to the development of evolution theory,
especially the Darwinism and non-Darwinism. Because discoveries are unceasing in
the territory of biological evolution, in the view of science, new evolutionary theory
will be proposed inevitably.
Looking to the future, if a new theory of evolution is formed, the dissension
between Darwinism and non-Darwinism will be the first problem to solve. Moreover,
the new theory of biological evolution should not be confined to the field of organism
evolution, but be taken as a way of all material development. The characteristics and
laws of biological evolution should be depicted as a broad sense evolution.
Genes and genomes：The stage of macromolecule
The evolution of genes and genomes is the most essential aspect of biological
evolution, and it is the key to solve the problem of evolutional model and process.
The external shape, structure and function are determined by genetic material. And
the evolution of genetic material is of the evolution of genes and genomes, mainly
their composition and structure. Therefore, all facets of biological evolution can be
attributed to the evolution of genes and genomes.
The origin and evolution of genome The size and function are different among
nuclear (nucleoid), mitochondrial and chloroplast genome. These three genomes both
have two types of corresponding evolutionary pathway small genome and large
genome. From the view of evolution, all the modern genome originated from a
primitive genome. Therefore, in addition to the above two evolutionary pathways,
how the genome evolve largely depends on the composition of the original genome.
And this involves the origin of the primitive genome.
The origin and evolution of genome, the process of that the primitive genome rises
from the original biomacromolecules and evolves, is just like that a person climb up
to high places from ground. If it is a kind of stair, you can stop and rest at anywhere
on the steps, and relatively easy to get to the destination. Even fall off, it is hard to fall
back to the ground. However, if it is a very steep slope, you should be careful in all
aspects. Even a slight slip will make you fall back to the ground. Thus it may be
impossible to get to the destination. The origin of genome is something like this.
Nature must have taken a strategy of “moving carefully step by step on the way”, to
make complex and sophisticated genome from the primitive soup.
Well, what is the primitive genome like? Imagine the natural forms of one or more
self-reproductive biomacromolecules produced through chemical evolution. These
primitive biomacromolecules may be RNA, and have a certain size. Initially there
might be only one kind of original biomacromolecule, and a large number of the same
kind of biomacromolecules were generated through its duplication. If there were two
or more kinds of original biomacromolecules, competitive exclusion will arise. So
only one kind of primitive biomacromolecule can expand its number successfully.
Similarly, when a large number of copies were duplicated from an original biological
macromolecule, it would be very difficult for other different macromolecules to
survive. In the sense of origin, the original biological macromolecule is the very
primitive gene.
When the copy of the original biological macromolecule accumulates to a certain
number, only did they organize themselves into a transitional primary structure,
would they be possible to develop further. The primary structure is composed of
repeated sequences which belong to the same biomacromolecule.
By forming repetitive sequences, the biological macromolecule not only increases
its size in hundredfold and is likely to form a more stable structure, but also it
acquires greater potence. As long as the main repeat unit maintains the ability to
duplicate, the whole repetitive sequence can continues the process. Therefore, a small
number of structural units in the repetitive sequence can change relatively freely until
gain of a new function. As the differences among those changed in repeats are
unconspicuous from the original one, it is still with the equivalence of a biological
macromolecule, so exclusion of competition doesn’t exist. Even it does, it can be
ignored. In this sense, the primitive genome is composed of repetitive sequences with
ability of self replication, and it is the way genome evolution must follow.
In such a way, on the one hand, a wide variety of repetitive sequences are generated
in genome by the next evolution. On the other hand, and more importantly, it can
form the miniature of split gene directly. Split gene, contained in primitive genome, is
most likely the product of evolution of which is composed of repetitive sequences.
Early in the process of genomic evolution, because of lack of enzyme catalysis, that
the repeat units can copy themselves in the primitive genome is essential to the self
replication and organization of subsequent genome.
Furthermore, these constitutional units must maintain in a form of interlaced array,
so that it ensures the self-replication of entire genome. In addition, the successful
mutation is also limited. Therefore, only a small part of units change relatively in the
large genome, which evolved from the primitive genome directly, and got a new
function, the other parts change less. The constitutional units that change a lot and
have new function become the primitive exons, and each exon can have one or more
constitutional units. Other units that changed less become the primitive introns. It is
the miniature form of the split gene. The regions that don’t change were the interval
between the primitive split genes in genome. And they are essential to the duplication
of the whole original genome. After that, with the continuous improvement of
biosystem, exons are unceasingly specialized, while the introns differentiate in
sequence and length continuously. During it evolves into the modern form of split
gene, repeats and introns are no longer essential for the genome. For this reason,
according to the view, the original genome is composed of repetitive sequences and
original split gene. Repetitive sequences exist in the primitive genome, and then split
genes (with introns) emerge.
An increasing number of facts now support the above point of view:
1. The spontaneous formation of biological macromolecular polymer is not a
random process, thus the type of sequence is limited. Coupling with the exclusion of
competition, it made the type of original macromolecule, which could copy
themselves, quite lone in the period of anchebiosis. In this way, repetitive sequences
can be produced through chemical link between them.
2. In some more detailed studies of the modern genome, it is found that many of the
so-called single copy sequences are actually the differentiated or fossil repetitive
sequences, thus repeats play more important role than people estimated before.
3. The analysis of modern DNA sequence showed that some repetitive
oligonucleotide sequences in genes may be the original coding sequences on the earth.
In addition, analysis of large number of nucleotide sequences in modern genome find
that there are a large-scale self-similar or related nucleotide sequences. They may be
the relicts of the original genome in modern ones.
4. In split genes, the link sequences between introns and exons have the specific
repetitive mode. It means that they are identical in the corresponding boundary region
of exons and introns, whose origin is related to repetitive sequences. It is also a
vestige of primitive repetitive sequences.
5. Some molecular evidences show that the introns had been existing before
prokaryote and eukaryote divided. Moreover, the modern genome contains some
introns with the properties of self-splicing and catalysis: they are kind of original
structure.
6. As the important indirect evidences prove that that original genome contains
repetitive sequences and introns, they are found in prokaryotic, mitochondria and
chloroplast genome.
For repetitive sequences and introns in the original genome (including the ancestral
genome of mitochondria and chloroplast), if the pressure of efficient selection is
heavy and the space is limited, they will led to the small genome. The nonessential
repetitive sequences and introns are gradually lost during the evolutionary process.
But if both the pressure of efficient selection and space constraint are smaller, the
direction of evolution will tend to the large genome. This makes the repeats and
introns coexist with necessary components and result in the genome of great size.
Therefore, the evolutionary path of the small genome means lose and reside repeats
and introns, including the genomic evolution of prokaryotic genome, animal
mitochondria and the chloroplast genome of most alga and higher plants. While the
evolutionary path of the large genome is characterized by reservation and
development of repeats and introns, including the genomic evolution of nucleus,
plant type mitochondria and Acetabularia chloroplast. In this way, the two paths lead
to the three kinds of genomes.
The evolutionary paths of the small genome and the large genome exist in three
genomes, so the evolution has certain universality and direction. On the other hand,
because the two pathways divide largely, they can be used as a criterion to classify the
species.
Repetitive sequence: the redundant DNA Because the repetitive sequences are not
necessary in modern genome and even some of them were reduced, the effect are little.
It can be called redundant DNA. Since the existence of them was determined, there
has been no agreement among many views as to their origin and function.
An epigenesis is that the repetitive sequences are generated by amplifying the
sequences which have been present in genome. The people holding the view think that
certain similarity between repetitive sequences and some of the modern genic
sequences is the mainstay. Frankly, the similarity can demonstrate there may be some
relationship with regard to their origin but fail to explain which takes the first place. It
is a subjective view that just bases on the fact of similarity in them. Meanwhile, it is
difficult for epigenesis to elucidate the function and why they can be as the important
composition in genome.
In contrast, preformation theory deems that the origin of repetitive sequences can
trace back to the earliest period of biological evolution and the most primitive genome
is from original biological macromolecules which are composed by repetitive
sequences. On this basis, a part of a repetitive sequences become genes with more
complex biological function and others remain relatively primitive form awaiting
further development. Most of the so-called modern gene is likely to be formed in this
period, because they exist in almost all organisms. For this reason, we can find some
functional genes, especially the ones with long history, are homologous with some
modern repetitive sequences. It is what repetitive sequences play early in evolution.
In addition, repetitive sequences have also played a role that continues to the
present. The repetitive sequences themselves without constituting functional genes
early in the evolution can evolve into various modern repeats in the following process.
Also, they have the roles both in structure and evolution. Benefiting from these
functions, new repetitive sequences are able to be created by amplification of
sequences in genome in different periods. Thus, some of the sequences are epigenetic.
Meanwhile, the repetitive sequences in genome that has evolved are not essential,
so in order to have a higher reproductive efficiency, some repetitive sequences are
gradually abandoned in the pathway of small genome. However, certain evolutionary
potentiality is lost following the process.
In short, repetitive sequences are very important in biological evolution, for them
assign their carriers more evolutionary possibility and greater potentiality. But, the
future role of repetitive sequences can not be used to explain the reasons they are
present. And the regulation to gene of a small number of repetitive sequences is not
the rational reason that they are widespread. Therefore, it seems that the preformation
is more reasonable.
Molecular Evolution The study of molecular evolution is an important aspect to
explore evolution of gene and genome. In addition to the creation of biological
macromolecules spontaneously in the original Earth, molecular evolution mainly
refers to the change of biological macromolecules sequence during the evolutionary
process. Because the technique of protein sequencing was produced earlier than that
of DNA sequencing, the early study on molecular evolution is by means of protein
sequencing. Early in the 1960s, the research team led by U.S. scientists Pauling had
found that hemoglobin is similar in different organisms, and only some individual
amino acids are different. It is a strong support to the view that they originate from a
common ancestor. With the progress in molecular biology, the techniques of DNA
and RNA sequencing are set up. These simpler and faster techniques make the nucleic
acid sequencing easier than the determination of protein sequence, so the study of
latter stage on the molecular evolution is analysis and comparison of DNA or RNA
sequence. It is superior for DNA research in molecular evolution. Firstly, the DNA
that codes protein can reveal more molecular variation than protein itself. Meanwhile,
it can be used to study the evolution of non-coding sequences of in genome.
Therefore, the progress of DNA and RNA research in molecular evolution is very
rapid, and the study mainly focuses on sequencing the rRNA in different organisms
and analysis of homology (that is, comparative analysis of two sequences). rRNA
gene is very suitable for study of molecular evolution. It exists in almost all organisms
except for the virus, and also exists in the mitochondria and chloroplast genome.
Meanwhile, the gene has the regions that evolve faster and very slowly (that is,
conserved region). So it can be used in the study of the evolution in species with
genetic relationship both near and far, even the evolution spanning the whole living
kingdom. In addition, some studies in aspects of molecular evolution also have been
carried out in many other genes and DNA molecules. These studies verify biological
evolution and genetic relationship that are understood at the level of morphological
structure. And they also solve some ambiguous or unsolved problems of taxology and
phylogenetics in evolutionary biology at the level of cell and morphological structure.
The investigative superiority of molecular evolution is based on that the sequence data
of biological macromolecules is accurate and specific, and it is also easy to analyze
the features in quantity. So it is very useful to solve some ambiguities in the evolution.
The research of molecular evolution in DNA and RNA also develops the concept of
molecular clock.
Through the comparative analysis of the DNA sequence in different organisms, the
existence of molecular clock was confirmed. Whereas, a number of exceptions were
also found. All in all, the precision of molecular clock was lower than it was
originally estimated.
Although it has made great progress in the research of molecular evolution, there
are some deficiencies in the use of biological macromolecule sequences to study
molecular evolution. Because of the heavy workload of sequencing, what can be done
is just analysis of individual genes or certain biological macromolecule sequence
under the current condition. Generally speaking, it is unable to study the whole
genome sequence universally, hence there are certain limitations and one-sidedness.
On the other hand, the variations of biological macromolecule sequence are often a
reflection of their polymorphism, that is the sequence evolution is just the change of
biological macromolecules along with time. Therefore, it could not explain the nature
of biological evolution, but only can be seen as a measure scale or instruction scale
of evolution. It is just like the relationship between road signs and road. Road sign is
the measure of road length, not the road itself. A kind of organism evolves into
another, it is not because (or just because) of the changes of nucleotide, but has a
more profound mechanism and process which have not yet been fully understood.
Therefore, we must carefully interpret the results of comparative genomics and study
for the various aspects of genomic evolution, it will lead us to reveal those
mechanisms and processes.
The Problems I Care about
1 RNA: The ruler of old world? In modern biosystem, the carrier of genetic
information (DNA) and those who implement the function (protein) are discrete.
However, RNA plays both the roles. Should we say that RNA is the emperor who
rules the old world?
Basic facts:
Firstly, by mimicking the condition of the condition of aechaeo-earth, it is found
that RNA (or poly-RNA) is formed easier than DNA (or poly-DNA). Even in modern
biosystem, the synthesis of nucleotide is derived from RNA, then after reduction
reaction, DNA can be shaped. Accordingly, it’s reasonable to deduce that RNA
appeared earlier than DNA.
Secondly, we are able to draw the conclusion easily from the RNA virus, in which
RNA stores genetic information that RNA is a kind of gene.
Thirdly, RNA generally exists in the form of single strand, thus can fold into
various kinds of structure, which is the fundament of multifunction. Up to now, some
RNAs still play the roles of enzymes，and some we call them coenzyme. Furthermore,
although the acquisition of plastein may be not difficult, the protein with even a
simple function, hard to be produced by means of artificial imitation. So RNA
executes the function of enzyme before protein.
In conclusion, we can speculate that the primitive organism is only composed of
RNA. If it is true, well, how does RNA transfer the function of storing genetic
information to DNA and the function of catalysis to protein? How does the RNA
biosystem (you know, it’s not so stable) evolve successfully in bad environment?
2 The origin of nuclear envelope: related to CM, ER, or both? The origin of
nuclear envelope is very important, for it does not exist in prokaryotic cells and it
must have the process of de novo. The nucleus of some low-level eukaryotes is
similar to that of prokaryotes and the main difference is that the former has nuclear
envelope.
As for the origin of nuclear envelope, there are 3 hypothesises:
1. The cell membrane (CM) infolds and surrounds the original nucleoid, at last it
became nuclear envelope. And both the outer membrane and inner membrane of
nuclear envelope are from the CM of prokaryotes. However, the nuclear envelope has
no nuclear pore at beginning, and the nucleopore could only be formed in the
subsequent evolution. Therefore, the puzzle is: how does the normal exchanges
maintain between archikaryon and cytoplasm?
2. The origin of the outer membrane and inner membrane of nuclear envelope is
different. The inner membrane is from plasma membrane and the outer membrane
from endoplasmic reticulum (ER). At first the nucleoid of primitive prokaryote is
surrounded by bilayer plasma membrane, and then the outer membrane is replaced by
ER. It is also difficult to handle the problem of exchanges between archikaryon and
cytoplasm. In addition, how does the ER replace the outer membrane?
3. The nuclear envelope does not directly originate from plasma membrane, but
from the original ER which originates from plasma membrane. The primitive ER
wrapped up the nucleoid, gives rise to the birth of the original nuclear envelope.
It seems that the third view is most likely the evolutionary pathway. However,
science is not guess, we need more evidences. Maybe there will be another theory that
convinces us?
3 Epigenetics: Overturn the foundation of evolution? As the Genetics developed, a
branch of it, which is called Epigenetics, makes gene evolution in dilemma. Though
corresponding base sequences do not change, the phenotypes of some species are
diverse. The problem is that the whole process is inheritable. Obviously, it is different
from the classic Genetics, which is the base of evolution.
People have known that environment can effect the phenotype which determined
by genetic factors, and central dogma expounds the mechanism of action, but fail to
depict the molecular mechanism how environment effects gene expression. We may
have a reasonable explanation from genomic imprinting, a kind of Epigenetics: the
changing environment can help to bring about gene epi-modification, and then it
causes gene mutation. The mutation can occur in germ cells, and deliver to its
offspring. Likewise, as to homotropic inheritance, which has been denied by almost
evolutionists, we need to recognize again: the activity of genes is inhibited by
epi-modification, and because of some effect on the modification, the gene activity is
regained.
Well, what’s the evolutionary significance as to Epigenetics? Apparently, it is not
just the change of DNA sequences behind the genomic evolution. Moreover, many
mechanisms in Epigenetics are not clear. It need to solve how Epigenetics, which is a
fact but the mechanism is not well known, integrate with evolution and the new
foundation of evolution is formed.
4 New gene: the key to evolution? Because it is impossible to directly observe the
initial molecular dynamic process of new gene’s emergence and gene’s extension and
fixation, the study on gene is helpful to understand the formation of species and the
relationship between molecular evolution and species evolution.
Although the exploration to new gene began as early as in the 1920s, it was until
1993 that the first new gene was identified. After years of research, lots of new genes
were found in primates and fruit flies. The mechanisms that have been found or
clarified as: exon shuffling, gene duplication, retroposition, mobile element, lateral
gene transfer, gene fusion/fission, de novo origination, acquisition of new functional
exon, and repetitive element-mediated recombination.
Despite so many new genes and mechanisms were discovered, the prospects are not
so optimistic. It remains unclear as for the process of population dynamics in the
production of new genes up to now. Meanwhile, just as it is aforementioned, one
organism evolved into another is not only the changes of some genes: the protein and
nucleonic acid has become an inseparable unit. In addition, the research on new gene
is a pathway of understanding molecular evolution, but it is not the only way. What
we are doing, just like reading a mutilated map, is continuing to find new clues, until
it is understood completely.
If the direction of biological evolution is from low to high, well, why do so many
organisms remain at the low-level in the extant living beings? Is it that they
accommodate to the environment and give up evolution or are inadaptable to it and
forced to evolve? In other words, the present low organisms are adapted to the
original environment, whereas the higher organisms evolve from the living beings that
failed to adapt to their environment? To be sure, adaption to the environment is not a
judgment of higher organisms.
In the view of evolution, the present organisms belong to a non-evolution
phenomenon, which seems to be widespread. For example, in cell level, there have
not been any major changes in eukaryotic nucleus from monocellular to multicellular.
Well, how to explain this phenomenon?
In the process of biological evolution, it should follow the principle of parsimony.
That is to say, under the precondition of maintenance their basic functions, they
simplify themselves as much as possible, or gain a variety of functions with the same
organ. The direction of biological evolution, just likes a box of match that spread on
the ground, tending to disorder and decreasing the pressure. They develop in the
direction of decreasing the entropy as it in Physics. Is it true? Similarly, what is the
driving force of evolution?
These are all worthy to research in the future.
Written by
Lianqing Wang
Bioscience Basics 2005,
School of Life Science,
Inner Mongolia University