GENETICS TUTORIAL ‑ PART
II
"Thus the earliest
vertebrates, like the earliest amphibia, the earliest mammals, and the earliest
primates, were small predators. Over and over again in evolution, the
originators of new modes of life were small predators, and the key innovations
at each stage conferred a selective advantage in predation." John Morgan Allman,
Evolving Brains, 1999, p. 73.
In this chapter I continue describing some basic
principles of evolution that apply to all living things. It will serve as a
foundation for the more speculative and interesting evolutionary results found
in human nature.
Pre‑Adaptation
Some genes are "pre‑adapted" for new
environments. A gene is pre‑adapted if there was a negligible reward for its
presence in the genome at the time some new environmental challenge appears for
the first time, and for which the gene then confers a significant genetic
benefit.
Modern society provides many examples. Computers
didn't exist before the mid-20th Century, yet we find that many individuals are
naturally talented for computer programming, design, networks, and other
aspects of computer use. These people have genes that are pre‑adapted for the
computer environment.
Pre‑adaptations are always present, as the
following thought experiment illustrates. Imagine any task, and a procedure for
reliably measuring performance of that task. The task could be jumping as high
as possible, or remembering a sequence of numbers ‑ any task will do provided
performance can be measured objectively, producing a continuous range of scores
(a binary result, such as pass/fail, does not meet this "continuous range
of scores" criterion). After two people have performed the task, there
will invariably be a "better" and "worse" performance. After
many people have performed the task, the test scores may form something
resembling the Gaussian, or "bell curve" distribution, with many
scoring near a middle region, and fewer scoring really well and poorly. The top
scorers can be described as "pre‑adapted" for the task (provided the
task is novel or evolutionarily "new").
In real‑world situations, whatever the change in
environment, whatever the change in job opportunities, whatever new sporting
games are invented, there will always be new consistent winners and losers. Winners
in the new task might have been mediocre performers in the old ones (and old
winners may become the new losers).
"We are what we're good at," and the
forces of selection measure us by what we're good at in the context of our
times. Whereas the computer whiz is pre‑adapted to this era, so might a “nobody”
of today be pre‑adapted to some future era. We should be careful in judging
others, for they might have shined outstandingly in past settings, or be
examples of a type that will shine in future ones. Faceless nobodies of times
past might have rivaled the best of today's stars, if only given the chance by
a change of environment. Chance is everything!
Pre‑Maladaptation
But if the winners are pre‑adapted, what are the
losers, the "pre‑maladapted." It may seem "unfair" to a
civilized mentality to believe that "pre‑ordained" winners and losers
will exist when new opportunities appear, but every species has been molded by
this unfair rewarding of individuals through abrupt environmental change. We
may not like it, but this is the way things work.
Anyone feeling gratitude for evolutionary
accomplishments should also feel thankful for the diversity of individual
performance. Thanks to "inequality" evolution proceeds! But while we
celebrate inequality, and rewards for the pre‑adapted, let us also have
compassion for the pre‑maladapted, the world's ill‑fated losers, for they
cannot be responsible for the changes that doom them.
Over and over, in this book, we shall encounter
repugnant examples in nature. Our lesson is to accept that Nature doesn’t take care
about individuals, only the genes! And the genes have no qualms about wasting
individuals for their sake. Fish lay thousands and millions of eggs, so that on
average one or two will survive. Several insect species produce male brains
that are programmed to allow the female to make a nutritious meal of him after
copulation ‑ to postpone her mating with a competitor or for nourishing his
offspring! The historical record shows that humans will send legions of young
men to battle, like fodder, who in the prime of their life become maimed or
killed. The victors in battle rape the vanquished men's women, then march home
as heroes, with greater rights for domestic breeding. In all these settings,
the individual is "sacrificed," for he is engaged in risky behaviors
with benefits that accrue reliably to only the genes.
Humans who ponder the consequences of what I'm
calling a pre‑maladaptation have grounds for bemoaning their bad luck. I like
the thought that each person "has their time," a time when they would
have some maximum of pre‑adaptation, and since people are born into times
"at random," they most often are "out of their preferred
time." Imagine how frustrating it would have been for Beethoven to have
been born before pianos existed, and before orchestras. Or for Einstein to have
lived before the preliminaries of 19th Century physical theory had
been set. Delay Darwin's birth a century;
would he have become the giant we know today? Bring to the 21st Century
such notables as H. G. Wells, Lucretius, Democritus, Shakespeare, Homer, and
others; what would become of them? We cannot know how fortuitously attuned to
their age these giants were, or what nobodies they might have been had the
"roll of their genes" occurred at some other time.
Species Shaping Forces
Pre‑adaptation is a useful concept calling
attention to the fact that whatever an organism's make‑up it will have some kind
of “match” to every hypothetical new environment, and the match of some
individuals will be adaptive. It might be adaptive whether or not the
environment in question has ever existed before, and whether or not any of the
organism's ancestors have been exposed to that environment. In such cases, we
should not say that the organism has become adapted to the environment in
question, just because it fares better than some of its cohorts. Rather, it is adapted due to a pre‑adaptation.
Most organisms will be pre‑maladapted to
the new environment. Thus, most individuals will fall behind, watching a
minority of pre‑adapted individuals leap forward. The greater the number of
changes to the environment, the greater will be the disparity in relative
rewards between the pre‑adapted and pre‑maladapted. A species should evolve
"faster" at such times.
When I refer to changes in the
"environment" I mean to include not only the climate for a region,
and the disappearance of a food staple (plant or animal), or appearance of new
foods, but also the appearance of a new predator, the invasion of a new
parasite, or the adoption of a new element of "culture" (in the case
of humans, and perhaps chimpanzees).
There is a special case, unique to humans, in
which culture has created an entirely new environmental condition: the removal of most of the natural threats to
survival.
An advanced civilization shields people from
diseases, animal predators, and, in some cases, the need to work. It even
shields people from each other to a great extent, by reducing the frequency of outbreaks
of "tribal warfare." In this environment genes that in harsher,
unforgiving environments would be maladaptive would now be neutral. Only
the most severe genetic defect will be eliminated from a human genome shielded
this way.
Under these conditions we might want to think in
terms of "potential pre‑adaptation" and "potential pre-maladaptation."
Today's genome is accumulating a large reservoir of potential pre‑maladapted
genes, carried unknowingly by individuals who may be reproductively successful
only because they are not subjected to selective forces.
At the risk of getting ahead of my story, I
believe that such genes will become apparent only after natural forces of
evolution are restored, and put "the squeeze" on our burgeoning
global population. Winners and losers in this new environment will not be close‑call
winners and losers, they'll be clear‑cut winners and losers. The disparity
between those now destined to win and those destined to lose is greater than
ever, and growing faster than ever. The complexion of Humanity could change
dramatically apre le deluge.
To understand a species we must consider the
selective forces that have "shaped" it. In other words, we must learn
what kills individuals before they reach reproductive age, what factors
determine which individuals reproduce after reaching maturity, what foods are
eaten, and how precarious is the supply.
For example, if our ancestors 5 million years
ago were eaten by lions the survivors would have been good at avoiding lions.
This might have rewarded the evolution of bipedality, which would have enabled
standing tall and running fast. It might also have rewarded the capacity for
social cooperative strategies, a precursor to intelligence.
Another theory speculates that our ancestors had
to learn how to find and store root plants that would have grown on the
grasslands (Wrangham and Peterson, 1996). This would have rewarded the creation
of digging tools, and the ability to carry extra roots to a storage place at a
home base, which in turn would have rewarded bipedalism and a self‑control that
provisioning requires.
Whichever environment accompanied the branching
of bipedal chimpanzees from their jungle‑dwelling forebears 5 million years
ago, we can be sure that the forces of selection rewarded individuals carrying
genes for dealing with whatever were the causes of mortality in their new
environment, whether they were escaping from lions or digging and storing
roots.
Perhaps 500,000 years ago some humans migrated
to the edge of constantly‑moving glaciers. Mortality in this new setting would
have been climate‑related, such as cold and hunger. We may presume that genes
for planning and foresight were rewarded. To the extent that large animals were
hunted, and meat became an essential food source, genes for a strategic type of
cooperative hunting would also have been rewarded.
After the last glacial cold period, that peaked
19,000 years ago, humans had to adapt to an ever‑warming climate. For some,
this meant adopting an agricultural lifestyle. Those who were pre‑adapted for
farming would have prospered, provided they could also band together for mutual
protection from raiders. Others remained nomadic, and targeted the new farmers.
Thus, the main killer of Man became other men (it probably has been "other
men" for the past 100,000 years, at least). As farming achieved
unprecedented success, urban living became possible, sometime after 3000 BC. This
created opportunities for microbes, which competed with Man as the main killer
of men. Our ancestors are the ones with immune systems that afforded protection
against "urban" diseases.
In every step of this evolution toward modern
Man, the change in what killed people was a principal selective
"force."
H. G. Wells made the point, 100 years ago, that
long‑lived life forms cannot adapt to fast changes of conditions, unlike short‑lived
forms, that can adapt. This leads more often to the demise and replacement of
long‑lived large creatures by other large creatures, both of whom are competing
with small, short‑lived creatures. He warns that humans, with a long life span,
are vulnerable on this account.
The looming threat to Humanity posed by viruses
and bacteria may become a classic example of this evolutionary dynamic. How
ironic if our demise, or loss of greatness, which is most often portrayed in
terms of dramatic events, such as global thermonuclear war, instead is dealt by
tiny viruses!
If in fact viruses produce large‑scale human die‑offs
during the 21st century (Garrett, 1995), the survivors will be those
with pre‑adapted immune systems; not the physically strongest or most
intelligent. This possibility illustrates in dramatic fashion the principle that
"a species is shaped by what kills its members."
How Many Genes Can Compete?
Human tribes are supposed to have numbered 50 to
100 individuals throughout much of our prehistory. The number of adults in such
a tribe would have been about half this number, half of whom would have been
adult males (12 to 25 in number). It is tempting to think that fewer than this
number of genes can be evolving in the tribal genome. But such an assumption is
erroneous, as I will illustrate.
When a man goes into the world "to be
measured," it is his phenotype
that is being measured. And his phenotype could be the result of many genes
(interacting with each other to produce a unique phenotype). Consider the
extreme case where each of the men in a tribe differs from "an average"
by just one allele. Consider 4‑year intervals, during which each woman of child‑bearing
age bears one baby. During each 4‑year interval, if only one man prospers and
is accorded sole breeding status, then every 4 years one allele can be declared
a winner over it's competitor(s). In a lifetime, 10 alleles can be declared
winners (where we imagine the environment places great importance upon
different aspects of phenotype each 4‑year period). After 80 years, 20 alleles
could be declared winners, etc.
Even though this is a thought experiment, it
proves that there is no fundamental, mathematical reason forbidding the number
of gene sites for allelic competition to exceed the number of adult males in a
self‑sufficient tribe, or the total number of tribe members. (A "multiple
regression" statistical argument is also possible, and more persuasive for
me, but I shall spare my readers of this daunting argument.)
Every individual is a carrier for many genes
that are competing with allele counterparts. The number could be 50, or 500;
and it doesn't matter if the individual is a member of a tribe with only 50 or
100 members.
Migration, New Gene Competition, and Pace of
Evolution
The number of gene loci hosting allelic
competitions has undoubtedly increased in number since the advent of
urbanization, and the more recent globalization of our species. A tribe of
Africans may be homozygous for genes influencing skin color, but if they were
captured and brought to America as slaves their
ancestors would find that those same genes influencing skin color had become a
factor in determining individual welfare in the non‑African society. The same
argument would apply to many genes.
The inescapable conclusion is that the more
diverse a population becomes, due to migration, the more genes there are in
competition with each other. Does this mean human evolution is progressing
faster today, or slower? If a person's "measure" is affected by more
genes, it must take longer for all genes to have their measure taken. Stated
another way, when more genes enter the fray of competition, those already in
competition may feel a decrease of selective pressure influencing their fate.
Aspects of a person which were important in the tribal setting suddenly recede
in importance, as other genes, which had been firmly established many
generations earlier, resume their competition with alleles they had never
before encountered, or had encountered long ago and had triumphed over. The
coming together of ethnicities must introduce major changes to the set of genes
that are subject to evolutionary forces, in terms both of which genes are in
competition and the relative strength of selective forces upon specific genes. These
considerations suggest that the pace of evolution has slowed in modern times.
Another slowing influence is the declining rate
of infant and childhood mortality.
Unless something important has been overlooked, these arguments suggest
that the pace of evolution has slowed in modern times (Kondrashov, 1988).
Conservation of Selective Pressures: Pleiotropy
and Polygenes
When selective forces suddenly reward a new
capability the species undergoes a quick disintegration in other, more recently‑acquired
capabilities. This is due to the random, unintended deleterious effects that
any mutation produces, which places a brake on the speed with which new
capabilities can be acquired.
To understand this, recall that a gene has many
effects, referred to as pleiotropy. This
is most dramatically illustrated when a mutation occurs that has no redeeming
consequences. For example, one mutation causes its carrier to have 6 fingers,
short stature and heart murmurs (Ellis‑van Creveld syndrome). These phenotypic
effects are seemingly unrelated, yet they are caused by just one allele.
Mutations that are adaptive, judged by the fact that they have been selected
during the course of evolution, will also have many effects, with perhaps just
one of them being adaptive to a far greater extent than the numerous,
small negative effects. Thus, whenever a
mutation occurs and confers an increment of adaptive advantage, its future in
the gene pool will depend not only on how well it performs its adaptive task,
but also upon how many unintended, deleterious effects come with it.
Assuming for the moment that there are only
40,000 genes in the Human genome, since there are more than 40,000 properties
defining a human, each gene must have more than one beneficial effect. This
implies that after a gene is "in place" it can be modified over time
to produce more desired effects. An "old gene" may thus have several
beneficial effects, in addition to a few small negative ones. The selection of
a modified, dual‑purpose (or multi‑purpose) gene must occur with "painful
slowness" since the original function of the gene should not be disturbed
appreciably, and since every mutation is likely to produce other unwanted
effects. To get things "just right" must require many generations and
many small compromises.
Whenever a new selective force becomes
important, the other selective forces must lose importance, else the population
will drop to dangerous levels. This "partitioning" of selective
pressure leads to a more conservative behavior of our genome, causing already
established gene alleles to remain longer than otherwise.
By the same reasoning, a recently‑established
gene allele is more likely to be disrupted with deleterious effects than a long‑established
gene allele. This "genetic entrenchment" is due in part because of
the rewards of redundancy for genes that are important enough to respond to
selective forces for long periods of time. A task that must be done by a gene
will be less vulnerable to mutation to that gene if it has been exposed to
mutations and selection for a long time. In addition, genes that exist for a
long time may become "depended upon" by other genes that are selected
after the first gene and which in some way depend upon the presence of the
first gene for its new effect to be expressed properly. When two or more genes
must be present to produce a specific phenotypic trait that has adaptive value,
those two or more genes are referred to as a "polygene" group. Genes
that are members of a polygene are more difficult to get rid of, provided they
have not become harmful. New genes have not had this opportunity to achieve
robustness, or become entrenched, and they are thus more likely to be lost by
random mutations because they are likely to have small phenotypic consequences.
The concept of "genetic entrenchment," and a culturgen counterpart to
this concept, is treated at greater length in Chapter 16.
Brain
Genes
For humans it has been estimated that at least
20% of the genome influences the brain. This is not to say that 20% of human
genes are present exclusively for brain wiring, since many genes will exist
mainly for other purposes which have "acquired" brain wiring roles.
If one of these genes mutates it is more likely to affect its new brain task
than the older, original task. Undoubtedly some genes are mostly brain‑related,
and probably some genes are exclusively brain‑related. Whether a gene is
partly, mostly, or exclusively brain‑related, if it recently acquired this role
it is likely to be more vulnerable to random mutation than the other parts of
the genome, or to older genes that mostly affect anatomy or physiology.
Parts of the modern human brain evolved during
the past 100,000 to 200,000 years, and some people speculate that for the past
40,000 years little has changed. I will argue later that brain genes continue
to evolve in response to changing social conditions, which add in subtle ways
to the repertoire of human behaviors. For now, I merely claim that behaviors
which are uniquely human, and which are recently evolved, are most vulnerable
to disruption by the appearance of new selective forces.
If a new adaptation has been selected for
strongly, it might acquire robustness even in a relatively short time. Human
language, which may have appeared 200,000 years ago, is a candidate example. Language
played such a crucial role during its evolution that the genes that code for it
are probably already robust.
The capacities for reading and writing have a
briefer evolutionary history, and the genes that code for these abilities are
more vulnerable. Until recently, few people engaged in reading and writing. These
genes provided a niche to only a small fraction of the population during the
past 4000 years. It is therefore not surprising that dyslexia affects several
percent of the population, whereas verbal language impairment is virtually
unknown.
Unintended Deleterious Effects
I suffer from occasional 20‑minute blind spells,
called "scintillating scotoma." It is an impairment produced by a
gene that in women produce migraine headaches. As I type with difficulty
through a flashing zig‑zag blind‑spell pattern, it occurs to me that I am
paying a penalty for some genetic mutation that is doing good somewhere
else. Every mutation does many small bad
things for every big good one, and the sum of bad ones found in most people
must be worth their penalty; otherwise the gene allele would not have evolved.
In the case of my blind‑spells a dilated blood
vessel is putting pressure on a nerve fiber carrying signals from my eyes to my
brain's occipital lobe. What if the dilation occurred elsewhere within my
brain? I might not know that it was occurring since I could not see it. But it
might nevertheless have subtle effects upon mood or thought. There must be
people, probably many people, who do indeed experience mild mental afflictions,
lasting 20 minutes for example, which are counterparts to my scintillating
scotoma. We should be prepared, then, for the possibility that a certain amount
of irrational human behavior is caused by genes that are conferring a greater
adaptation benefit in some other behavioral realm, with the unintended side
effect that behavior is mildly irrational in a different realm.
I frequently think about the penalties that are
paid when evolutionary pressures for one trait rise above the others. Sure, you
can quickly evolve skin color in response to latitude migrations, but you'll
pay with other unintended defects that accumulate, until the new skin has been
achieved and a better balance of evolutionary forces has been established.
Only 12,000 years ago, just after the climate
warmed but before the glaciers had melted enough to raise the world's sea
level, people in Siberia migrated across Beringia to the new world. As they
moved south, generation after generation they would have lost their need for
light skin. Central American Indians are dark‑skinned, and this must have been
achieved in less than 10,000 years. But those who continued their migration
southward, past the equator, they would have needed to re‑achieve light skin. Perhaps
at each migration juncture those who were best adapted to the latitude stayed
behind and the others continued the migration. This could have minimized the
risks of unintended deleterious mutations, but it is more likely that the
southward migration was so hurried that skin color played no role.
Such fast adaptations must have produced defects
in other aspects of the American Indian. Perhaps they lost the ability to
metabolize alcohol; we shall probably never know what compromises the genes had
to make to adapt quickly to the need for a different skin color.
Cancer may afflict humans more than most other
species because we have recently undergone a rapid evolution under strong
selective forces that rewarded brain re‑wiring (to accommodate behavioral
adaptations) and immune system enhancements (to fight pathogens seizing the opportunities
offered them by the newly evolved super‑tribe human lifestyle). To achieve
these new traits, genes must have been selected that would normally not be
acceptable because of their unintended deleterious effects, and a defense
against cancer may have been one such compromised ability.
The Dangers of Fast Evolution
Species evolve at different rates. Even a given
species may remain genetically static many generations, then respond to an
abrupt change in climate by evolving fast. Rates of change must vary by orders
of magnitude, with long eras of equilibrium punctuated by short periods of
disruptive change. Mammals lived throughout most of the dinosaur era, and
flourished only after the meteorite impact of 65 million years ago (which
killed the dinosaurs because of a brief, disruptive climate change lasting
several years).
The equilibrium periods are available for
"clean up" of unintended deleterious effects created during the fast
evolving times.
The great diversity of human anatomy, relative
to other animals, testifies to the great potential for fast human evolution. Strong
selective forces must have superceded such things as head shape, for example.
When a species is suddenly subjected to a strong
selective pressure, a few gene sites will suddenly grow in importance. More
than two alleles may exist at each "hot" site (if only one allele
exists, it won't be a site for selective pressure). Other sites, being
relegated to lesser importance, are likely to accumulate mildly deleterious
mutations with less consequence than before the fast evolution (to use a
metaphor, it's as if no one is "minding the store" when a new one appears).
Humans, who have been evolving fast for the past 7 million years (since
separating from the chimp lineage) must have many multi‑allelic gene sites. The
more alleles that are in competition, the greater the fraction of maladaptive
offspring. Thus, the faster evolution occurs in response to some new selective
pressure, the greater the likelihood of a low offspring survival rate in order
to prevent a proliferation of the unfit.
Is it not ironic that today, after coming out of
a phase of extremely fast evolution in several traits, humans have just
achieved what must be the highest offspring survival rate ever? Does this not
mean that humans also must be exhibiting the greatest rate of survival of
maladaptive individuals? How long can this last? This topic will be returned to
in Chapters 6 and 8.
Lag and Regression
An abrupt environmental change, such as those at
the onset of an interglacial (occurring every 120,000 years, typically), must
set evolution in motion in new directions. Until a new "optimum" has
evolved, producing stasis (and genetic consolidation), there will be
"lag." Some things are easier to evolve than others, and they will
lag less. Skin color may be one example.
Because adaptation takes time, there could be a
lag in many traits after an environmental change. Present aspects of human
nature should "make sense" only in the Pleistocene context, not
necessarily in that of the Holocene (the past 12,000 years). For this purpose
it has been useful to create the term "environment of evolutionary
adaptation" (EEA), also referred to as the Ancestral Environment (AE). Common behaviors that were adaptive 20,000
years ago need not be "adaptive" today (Symons, 1979).
The Yanomamo Indians of South America appear to
be more "primitive" than their Asian stock who began migrating to the
New
World
~12,000 years ago. How can this happen? Could the forces of evolution actually
cause a population to regress? Yes! And maybe this happened with the Yanomamo. Their
regression is only in relation to what was adaptive in their former setting. By
definition, they must be better adapted to the Venezuelan jungle than were the
original Siberian stock, or even the partially modified Central American Indian
stock.
The longer the race, the greater the disparity
between the contestants – especially between winners and losers. This is
certainly true for a foot race, but is it true in evolution? Consider that our
ancestry traces back to a chimpanzee‑like animal 7 million years ago, or 2
billion years ago to a one‑celled life form, and 3 or 4 billion years ago to
strands of DNA. Things like those early DNA strands may exist today, as do many
one‑celled life forms that may resemble those in our ancestry. So
"yes," the longer the race, the greater the spread between the evolutionary
contestants (note that all extant living forms are "winners").
In human affairs there is a discernible
spreading in the quality of life of winners and losers. The most prosperous
people of today have a higher standard of living than the most prosperous of
yesterday, yet there are people living today who are no better off than the
worst off yesterday. Can there be stability in a world where the rich get
richer, and the poor stay poor? This is a topic for Chapters 11, 14 and 16.
Evolutionary Reversal
Random mutations rarely produce benefits to the
individual organism (i.e., for the
ability of the organism to stay alive, out‑compete its contemporaries and to
out-reproduce them). A mutation that alters a gene is likely to have effects on
many phenotypic traits (pleiotropy), and usually all or most traits suffer from
random mutations. For a mutation to succeed, it must confer some advantage that
outweighs damage done to many other traits. "Forward" evolutionary
change is thus difficult.
After a genetic mutation spreads throughout a
gene pool, it becomes part of a genetic setting that new mutations must deal
with. If a new mutation relies upon the presence of the first one, and if this
second mutation also spreads throughout the gene pool, then the first gene has
a more secure future. This occurs because any challenge to the first gene must
confer an advantage that outweighs the contributions of two genes ‑ the first
one and the other gene that relies upon the first one for it's proper
expression. The longer a gene stays within a genome the greater is the chance
that other genes will become dependent on it and therefore provide it with
additional security. When this happens, the gene has become
"entrenched."
Consider the situation of environmental change
that reverses itself at some later time.
The first change may lead to the appearance and widespread acceptance into
the genome of a mutation. Let us assume that this new gene, which has almost
completely displaced an older one, confers an adaptive advantage in the new
climate. Suppose, now, that before this new allele has time to become
entrenched, the climate changes back to the original state. The few individuals
who carry copies of the original gene allele will become a source for the quick
re‑emergence of the original allele. Evolution can be said to have
"reversed" itself.
If the second climate change occurred much
later, however, this evolutionary reversal might not be feasible. First, the
original allele may have disappeared, and second, other genes may have become
dependent upon the presence of the new allele, making it more difficult to
dislodge from its entrenched location. In theory, both difficulties for an
evolutionary reversal can be overcome, but they constitute a more
insurmountable obstacle to the reversal.
Laboratory evidence exists for "reverse
evolution" (Teotonio and Rose, 2000). Fruit flies from a standard stock
were selected for various experiments over the course of 20 years (200
generations) and were subjected to new environments to produce variant strains.
When fruit flies from these new strains were subjected to the original
environment, in every case reverse evolution was observed. In two cases, the
reversal was almost complete after only 10 generations; others required 50
generations. In some cases the amount of reverse evolution was small.
At every instant of a species evolutionary
history, the most vulnerable genes are the most recently‑acquired ones. This
concept will be returned to in later chapters.
Culture can be thought of as a collection of
"culturgens" or "memes" ‑ similar to a genome being
comprised of a collection of genes (Lumsden and Wilson, 1981). Although some similarities exist between
genetic and cultural evolution, the differences are striking. This topic will
also be dealt with in a future chapter (Chapter 16), as a unifying theory for
understanding the rise and fall of civilizations.
Mutational Load
Although the idea of "mutational load"
was described by Kondrashov (1988), we owe H. G. Wells it's first brief
expression (ironically, in the same journal, almost 100 years earlier). In 1895
Wells wrote: "Has anything arisen to show ... that where the life and
breeding of every individual of a species is about equally secure, a
degenerative process must not inevitably supervene?" (Wells, 1895).
Primitive people today produce about 7 offspring
per woman. Allowing for slightly shorter life spans in past times, about 6 offspring
per woman was normal. On average, only 2 survived to adulthood. Is it possible
that some of the 4 who died were genetically inferior? Yes, of course.
Approximately half of all conceptions fail to
produce a live birth. It is speculated that the half that die are genetically
defective due to some incompatibility between the paternal and maternal
alleles. It is a small step to suggest that there will be a residue of live
births that are also destined to fail to survive childhood due to genetic
defects. If this is true, then what would be the genetic consequences of
intervening medically to sustain all live births through childhood and into
adulthood?
If some of the 2/3 of live births that formerly
died were due to genetic defects (a fraction derived from the ratio of
childhood mortality rates in primitive and modern societies), and if all live
births now live a full and reproductive life, then surely the genetic defects
which they carry will be contributed to the gene pool in larger numbers than
would have occurred in the ancestral environment (AE). Our gene pool must
inevitably accumulate these defective genes at a higher rate than in the
past. This phenomenon is called
"genetic load" (Kondrashov, 1988).
It may be impossible for a species to average
only one offspring per adult for a long time. With no excess of births, the
downward pulling force of “genetic load” would degrade the gene pool of the
species. Therefore, the survival ratio must be kept well below one if humanity
is to maintain a healthy genetic future! We who survive without serious genetic
defects should be grateful to those less fortunate, whose deaths in the past
made us possible.
I feel sorry for the bent masses of future
people, for they will suffer from cruel disabilities that were traditionally
weeded out by the neglect of less benign times in the AE. Humanity reaps what
it sows, and it is sowing the wrong genes ever more often and preserving
defective offspring with an excess of unthinking compassion.
Compassion can be a double‑edged sword. What
seems laudable for one generation may in fact create unlaudable consequences
for many future generations. I shall return to this moral dilemma in Chapter
11