More history…this, about how genes gradually
came to be understood. It’s an amazing story—scientific discovery at its very
best. This section sets the table for the next one, “Part X…The Homeobox,”
which is a sort of “genetic toolkit” that
first came to light in the 1980s and revolutionized our understanding of how
genes function. After that, I’ll take a look at the current state of
evolutionary theory and that will be followed by a review of origin of life
theories. Lining up my ducks….
IX. From Gemmules to Pangenes to Hox Genes
The
nonstop discoveries in developmental genetics are making it eminently clear
that we must expand our vision of evolution, and of evolutionary processes,
beyond our present scope. We will have to think differently about—and perhaps
totally reconceive—our basic notions of evolution.
Jeffrey Schwartz, Sudden Origins (1999)
There’s a
common misconception that, after the publication of On the Origin of Species, Darwin’s concept of evolution by natural selection
gained enthusiastic acceptance—at least among scientists. His theory survived
the skeptical public’s initial response (which ranged from mild ridicule to
outrage). As for his peers: initially, Darwin’s ideas won intellectual support but almost as quickly
fell into disrepute. He was by no means at the forefront of evolutionary
studies in the years following his rise to fame and notoriety; the period from
roughly the 1880s to the time of the modern synthesis has been called the eclipse of Darwinism.[1]
This was due in part to the work of early geneticists whose findings were at
odds with his speculative theories of natural selection and adaptation. Biology was coming into its
own and was perceived as being more “scientific.”
During this
phase, there was a good deal of controversy surrounding the matter of how
species arise. Oddly, few people seem to be aware that, despite the title of
his famous book, Darwin never really tackled the issue—he merely explained how
natural selection gave rise to favorable adaptations in plants and animals
through the influence of chance variation. For those biologists and geneticists
trying to answer the question of speciation
(how species originate), Darwin’s theory proved to be of little practical use.
In fact, it had the effect of confusing subtle but important distinctions
between the processes of evolution and adaptation. Most of the major figures
involved with the disputes had their own distinctive theory of speciation, a
matter being hotly contested. The problem wasn’t solved by applying tenets of
the modern synthesis and remains controversial. Similar to the difficulties
faced in trying to comprehensively define “life,” there is still no wholesale
agreement on what constitutes a species, nor on how species arise.
Adding
to the quandaries faced by biologists, paleontologists hadn’t yet found
evidence in fossil sequences clearly showing the gradual but continuous
modification called for by natural selection (a matter Darwin recognized and
found deeply troubling). In the words of anthropologist Jeffrey Schwartz:
The perceived need for extraordinary lengths
of time over which change could accrue was one of the factors that caused
Darwin’s ideas to fall rather quickly out of favor with many in the scientific
community. In almost every new edition of his most famous book…he called for
greater and greater periods of time over which evolutionarily significant
change could be manifested. The earth could not be made older to allow for this
model of evolution.
In 1863,
William Thomson (the future Lord Kelvin) published a calculation estimating
that a primordially hot Earth was probably around 100 (but no more than 200)
million years old. Thomson, likely the most respected scientific figure of his
time, wasn’t anti-evolution so much as a staunch believer in design. A devout
Christian, he saw living things as unmistakable evidence of God’s creation and
was happy to use his estimate of Earth’s age to refute Darwin, who admitted
that 100 million years was not nearly sufficient time to allow the extremely
slow process of natural selection to do its work. Thomson kept revising his
figures downward—finally, by 1897, to a mere 20 million years. This obstacle
distressed Darwin to the end. (He referred to Thomson as his “sorest trouble.”)
Then, only 14 years after Darwin’s death, radioactivity was discovered. Lord
Kelvin hadn’t known that radioactive elements in the crust and mantle were
slowing the planet’s cooling rate considerably. Later it was learned that isotopes[2]
of several radioactive elements could be used to date rocks and by 1941, around
the time the modern synthesis was coming together, Earth’s age was estimated to
be around two billion years.[3]
The controversy faded and, in this respect, Darwin was vindicated.
Then there
were problems posed by the early geneticists. Darwin, of course, knew nothing
about genes and believed that an offspring’s traits were a blending of those of
its parents. He formulated a theory of inheritance, calling it pangenesis, whereby all parts of an
individual—tissues and organs—had distinct and independent identities. During
all stages of development they shed minute particles he called gemmules, which were capable of
reproducing themselves. Darwin hypothesized that these were disseminated throughout
a body and were to be found in all its parts at all stages of development.
Thus, sex cells containing the adults’ sets of gemmules would be blended to
produce their offspring’s traits. Gemmules could be active or dormant and these
latter could be expressed in future generations. Active gemmules might be
repressed, which explained how features of one parent (but not both) could be
present in their offspring. And: gemmules’ ability to multiply and express the
organ or tissue from which they arose could account for, say, how a lizard can
regenerate its lost tail, a new plant can grow from a cutting, or even how
wounds heal. Aside from having no factual basis, it was a neat and elegant
theory. Few scientists gave it much credence since there was no experimental
evidence indicating that gemmules existed. The theory was finally refuted in
the late 19th century when August Weismann demonstrated that
whatever was responsible for the inheritance of traits was confined to the
nuclei of sex cells.
In coming
years, scientists continued struggling with the mysteries of inheritance. There
were various theories of orthogenesis
which, in one way or another, were predicated on the notion that an organism’s
evolution is shaped more by innate internal factors than by outside influences
(such as natural selection). There was a revival of the 18th century
ideas of Jean-Baptiste Lamarck—known as neo-Lamarckism—that
were based on the inheritance of acquired characteristics. Finally, there were
the saltationists—those who believed
that evolution proceeded by means of sudden, radical mutations and not via the
plodding gradualism of natural selection.
One of the
latter was Hugo de Vries, the Dutch plant breeder who (along with Karl Correns)
later brought Mendel’s work to light. In 1889 he published his own theory of intracellular pangenesis. It was based
in part on Darwin’s ideas which, by that time, had been rejected. He renamed
Darwin’s hypothetical gemmules, calling them pangens. Unlike Darwin, de Vries believed variation was not the
slow, continuous process that natural selection demanded but occurred in sudden
jumps, or saltations.[4]
His work with a group of plants prone to
sudden and conspicuous mutations (a not uncommon phenomenon in the plant world)
led him to believe that novel characteristics could not only arise
spontaneously but were heritable as well. De Vries’ views on evolution were
more palatable to scientists from many disciplines. (His theory, for instance,
didn’t call for an unrealistically ancient Earth, which satisfied those in
accord with Lord Kelvin’s position.) In 1900, the year Mendel’s work was
rediscovered, de Vries published a book introducing his controversial mutation theory.
British
biologist William Bateson was a disciple of Mendel and enthusiastic supporter
of de Vries’ theory.[5]
In the late 1800s Bateson, after forming comparable views on sudden variation,
explored a long-recognized pattern: the tendency of many animals to be made of
repeated parts and these parts, in turn, to be constructed of repetitive units.
(Arthropods have distinct body segments, vertebrates are organized along
segmented spines with paired ribs…limbs had repeated elements, with digits in a
wide variety of forms.) Bateson formally defined some of these themes, providing
what has proved an extremely useful framework for considering the logic of
modular design in animals. He was particularly interested in those malformed sports (“sports of nature”) from his
earlier researches, specifically those with body parts—be they legs, teeth,
tentacles, or antennae—occurring in atypical numbers or missing altogether. He
derived many important ideas from these studies and perhaps Bateson’s greatest
insight was realizing that these mutations might well divulge the nature of
specific developmental patterns.
In the early
years of the 20th century there was a good deal of controversy about
the role of chromosomes in heredity. As their behavior during cellular division
became better understood, it was realized that chromosomes might be responsible
for conveying those units of inheritance that would explain Mendel’s
inferences. One obstacle to the idea’s acceptance was that, since there were
only a small number of chromosomes, perhaps some as yet unknown, invisible
particulate matter was actually the “stuff of inheritance.” Nonetheless, the
idea showed much promise and quickly gained traction.
Ironically,
one of the early detractors of the new chromosome theory (as well as Mendelian
inheritance and Darwinism in general) would turn out to be one of its greatest
advocates. Around 1900, American zoologist Thomas Hunt Morgan was working in
embryology. Continuously exasperated by finding many of the outstanding
problems related to ontogeny leading
to dead ends, Morgan admitted defeat. He decided to look into the new-found
field of genetics (as did a number of other embryologists around that time)
hoping it might shed light on those stubborn dilemmas.
Morgan firmly
believed that the key to understanding evolution was to be discovered through genetics
but entered the arena quite skeptical (unlike Bateson) about the significance
of Mendelian inheritance and its relevance to evolutionary trends. He also
rejected Darwin’s heavy emphasis on natural selection, believing its only
relevance to the origin of adaptation was among individuals of a species, not
to the origin of that species itself.
Morgan had little use for Darwin’s ideas on sexual selection,[6]
his views on adaptation—in the sense that adaptation resulting from random variation was the main driver of
evolutionary change—and the notion of a violent
and incessant struggle for existence. Similar to Bateson and de Vries, Morgan
rejected much of Darwin’s theory (but still held the man in great esteem for
having brought to the forefront the central questions of evolution). Around the
turn of the century he had what proved to be an influential meeting with de
Vries. Morgan, a skilled experimentalist, was already aligned with those
favoring large-scale mutations (the saltationists) and de Vries convinced him
that his mutation theory could be tested.
Beginning in
1908, the fruit fly Drosophila
melanogaster became the “organism of choice” for genetic research. The tiny
flies pass through an entire generation in only two weeks, produce many offspring,
and are easy to rear. In addition, they have only four pairs of
chromosomes—abnormally large ones—which are easily visible under a microscope,
making their physical manipulation less demanding. By 1909, Morgan (who had
taken a professorship at Columbia University) began breeding Drosophila in what became famously known
as “the Fly Room.”[7]
The only
problem was, they all looked identical. It was already known that the tiny
flies produced distinctive mutant forms; for no apparent reason, these aberrant
strains would occasionally show up. They would differ from the wild type (normal) flies, for instance,
in number of abdominal segments, in having differing wing and antenna forms, or
eye color. After two years of patiently breeding wild type flies in the hope of
finding one of these abnormal individuals, by sheer chance one of the strains
being studied by Morgan’s group mutated into a white-eyed form. With the
enthusiastic assistance of numerous long-time colleagues, Morgan began to
isolate and breed several types of mutant strains. By cross-breeding these mutants with wild types and observing their
offspring, he was able to show that genes were carried on the chromosome and
were the physical basis of heredity, thus confirming Mendel’s findings.
Having become
a converted proponent of the new chromosome theory, Morgan attempted to
demonstrate, at first employing only circumstantial evidence, that a chromosome
could carry many genes. Fortunately for him and his collaborators, variations
among different strains of fruit fly (in the size, shape, and numbers of
bristles) allowed them to begin to map which traits arose from which
chromosome. Further, using cunning methods linked to a mode of chromosomal
interaction called crossing over,
they were able to figure out the genes’ positions relative to one another.
Morgan recognized that this routine reordering of gene segments on individual
chromosomes was a critical source of introducing variation into a population.
Building on Mendel’s hereditary theories, he and his dedicated research team’s
discoveries formed the basis for the modern science of genetics. Thomas Hunt
Morgan remains a shining example of the pure scientist—one willing to
repeatedly adjust intellectual positions in the face of new data.
During the first
two decades of the 20th century, population genetics—armed with
compelling, testable experimental results—overshadowed several fields that had
originally been responsible for a broad acceptance of the modern conception of
evolution. (These included paleontology, comparative anatomy, and embryology.)
Starting around 1930, some notable embryologists began to alter that trend.
Richard
Benedict Goldschmidt, a German Jew who emigrated to America in the 1930s to
escape Nazism, was one of the finest geneticists of his day. Modern students of
evolution remember him—if at all—for the unfortunately named hopeful monsters (Goldschmidt’s
much-ridiculed notion that beneficial evolutionary change can arise instantaneously through radical, chance
mutations).[8] This
was his way of accounting for the sudden and drastic morphological changes so
apparent in the fossil record and, in general, the vast differences in form
that Goldschmidt believed couldn’t be accounted for by gradual change. He first
presented these views when neo-Darwinism was becoming the prevailing school of
thought and, partly due to a tendency to be dismissive and harshly critical of
his peers’ work, the unorthodox notions were received with an arguably
disproportionate amount of ridicule. (The regrettable name choice for his
genetic novelties didn’t help support Goldschmidt’s position.) As it
transpired, one of his novel ideas—that controlling genes responsible for early
development could produce significant effects in adults—accurately describes what
contemporary developmental biology sees as one of the main sources of
mutations.
While still in
Germany studying gypsy moths in the late 1920s, Goldschmidt varied temperatures
surrounding the larvae and found that this altered the timing of their development—and, subsequently, their final
internal structure. (Other researchers, conducting similar heat-shock experiments on a variety of organisms, were obtaining
impressive results.) He began to appreciate that timing was of crucial
importance in embryonic development. Later, working with Drosophila, Goldschmidt subjected adult populations to deleterious
conditions (such as exposing them to ether fumes for fixed periods of time)
that resulted in mutant offspring—some of which had extra sets of wings or sprouted
legs where their antennae should be. He saw in these mutations further evidence
of the consequence of altering hypothetical timing mechanisms during early
stages of ontogeny. Additionally, the freakish changes in number or placement
of parts suggested there might be—during this critical early phase—types of
special cells that could be induced to become different parts. And finally:
that this was somehow related to modular
growth patterns (which Bateson had so perceptively recognized as being central
to evolutionary change).
After the
neo-Darwinian synthesis became the dominant evolutionary paradigm,
embryological research carried on
steadily while population geneticists continued to argue over the mechanisms of
speciation and paleontologists remained puzzled by the gaps in their fossil
records. After the discovery of DNA, problems that had been lingering for
decades fell like dominoes. Some believed that the genetics had entered a
humdrum, mop-up phase. Gunther Stent, a researcher who had been in the thick of
many of the exhilarating breakthroughs, wrote of the looming decline of a
discipline that was “only yesterday an avant-garde but today definitely a
workaday field.”
The first
major shake-up in decades came in the early 1970s when two graduate students in
invertebrate paleontology at Columbia, Niles Eldredge and Stephen Jay Gould,
proposed their controversial model of punctuated
equilibria. Their own studies focused on hard-shelled creatures preserved
in large numbers and thus leaving reasonably intact fossil records. The pattern
they saw was not the slow-but-steady transition from older species to new (as
specified by Darwin’s theory). Instead, they found that species tended to
remain virtually unchanged for long periods, only to suddenly disappear and be
replaced just as suddenly by entirely new varieties during occasional periods
of rapid morphological change. The two co-wrote and published their landmark
paper in 1972.[9]
There was an immediate and vocal reaction.
Eldredge and Gould never suggested that new species
arose instantaneously (as Goldschmidt believed) or even over a few generations
but, in geological time, did so far more
abruptly than would occur during the slow transition called for by natural
selection. Nonetheless, their hypothesis is still frequently construed this way
and remains controversial. (Creationists take advantage of this
misunderstanding to further the notion that evolutionists are conflicted and
their “theory” in disarray—a tactic that rankles their adversaries no end.)
Much of the opposition from scientists falls along party lines, with a
predictably dogmatic response from staunch neo-Darwinists who all along have
insisted that Eldredge and Gould’s observations reflect nothing more than
normal gaps to be expected in the fossil record (an argument first employed,
somewhat reluctantly, by Darwin himself). However, hard-shelled marine
organisms leave behind a much more accurate record than do land animals.
Paleontologists, for their part, had long been well aware of the pattern and
Gould famously wrote in 1980, “The extreme rarity of transitional forms in the
fossil record persists as the trade secret of paleontology.”[10]
The piecing together of the
complex narrative of heredity and development has been one of science’s grand triumphs. The
characters involved were brilliant and often colorful, their methods models of
empirical logic. Still, loose ends remained. Then: a huge step forward for both
embryology and evolutionary theory occurred in the early 1980s with the
discovery of something that is often likened to a “genetic tool kit.”
©2016 by Tim Forsell 15 May 2016
[1] The term was coined by Julian Huxley (1887–1975),
brother of Aldous, grandson of Thomas (“Darwin’s bulldog”) and early proponent
of the modern synthesis.
[2] Isotopes are forms of elements with different atomic weights; they share the same
number of protons and electrons but have differing numbers of neutrons in the
nucleus. Some isotopes are inherently unstable (radioactive) and spontaneously decay at known rates, thus
altering—through passing time—the minute quantities naturally found in rock,
water, ice, or gases. Certain radioactive isotopes occur in various types of
rocks and organic substances, making possible the radiometric dating methods that have proven invaluable for
establishing absolute ages of geologic formations and carbon-bearing historic
artifacts.
[3] Less than half the current figure of 4.54 billion
years. Creationists, of course, deny the validity of radiometric dating methods
and still cite Lord Kelvin as the
leading authority on Earth’s age.
[5] Bateson is perhaps best known among modern geneticists
for having the works of Mendel translated and introducing them to English-speaking
scientists. He coined the term genetics
in 1905 (before Swedish botanist Wilhelm Johannsen introduced the word gene in 1909). Bateson is also
considered “the father of population genetics.”
[6] Darwin proposed sexual
selection as a way to explain many conspicuous features of animals
(typically males) that couldn’t be attributed to natural selection as he
envisioned it. These are characteristics more likely to reduce rather than enhance
an animal’s chances of survival. The classic example is a male peacock’s extravagant
nuptial train: while being attractive to potential mates and thus possibly
helping pass on the male’s genes, it makes the bird much more vulnerable to
predators.
[7] Many researchers unexpectedly developed a genuine
fondness for their tiny subjects and practically lived with them in the cramped
space. An early photo shows a typical crowded lab, shelves lined with scores of
glass beakers (containing the flies) and other equipment…with an enormous bunch
of bananas hanging prominently on one wall.
[8] In that era, plants and animals with severe mutations
were typically called either “sports” or “monstrosities.”
[9] The paper, entitled “Punctuated Equilibria: An
Alternative to Phyletic Gradualism,” was first presented at the Annual Meeting
of the Geological Society of America in 1971.
[10] A reappraisal of the fossil record in recent times has
vindicated the claims made by Eldredge and Gould in this regard. (As one
example, see Lyne & Howe, in Harris, p 73: “[R]e-analysis of existing
fossil data has shown…that Eldredge and Gould were correct in identifying
periods of evolutionary stasis which are interrupted by much shorter periods of
evolutionary change.”)
