Is Natural Selection Necessary

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Is Natural Selection Necessary?

• Hook in the title word necessary.
• What is natural selection and what did Darwin
  hope to accomplish with it?
• Is there more to the evolutionary process than
  Darwinian selection? If so, why do we still call
  it natural selection?

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The hook in the title refers to two criteria
for initial evaluation of a hypothesis 1) Is
it sufficient? Mayr (1982) provides several
pages of review of early opponents of Darwins
theory, all of which either call into question
its sufficiency or are themselves not scientific.
This is by far the most common objection raised
to the hypothesis of natural selection. 2) Is it
necessary? Mayr also reviews various
alternatives to natural selection, which he lumps
under saltationist theories. He dismisses all
these as either non-selectionist or not
fundamentally different from Darwinian natural
selection. In a nutshell, todays talk
addresses the validity of his dismissal.
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What is Natural Selection?

• It is a mechanism to accomplish evolution, not
  the same thing as evolution. The way evolution is
  typically taught doesnt adequately make the
  distinction.
• It pays also to think about what it isnt what
  Darwin wanted it specifically not to be.
• The most obvious thing he didnt want it to be
  was similar to J.B. Lamarcks evolutionary
  mechanism(s).

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Lamarck and Inheritance of Acquired Characters

• Lamarck is often credited with the first
  published theory of evolution.
• He hypothesized that use of a character could
  improve it, and that disuse could lead it to
  atrophy.
• The new state of the character could then be
  passed along to offspring, who could further
  improve upon it, thus changing the character of
  the species. Lamarck was the first to realize
  the importance of the hereditary aspect of
  evolution.
• Otherwise, there is no similarity to Darwins
  Natural Selection, and Darwin would not have
  insulted us by supposing we might confuse them.

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What exactly did Darwin object to in Lamarck?
2. Each lineage is generated with an innate
tendency to climb the scala natura.

• Lamarcks evolutionary theory required a
  hierarchy of processes operating at three levels
  creation of lineages, intrinsic scaling of the
  ladder, and localized adaptation to environment.

3. Inheritance of acquired characters tweaks
individuals to fit into local environments
1. New lineages originate independently (by
spontaneous generation), all beginning at the
simplest level of infusorians.
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What did Darwin want from Natural Selection?

• a materialistic explanation for the changes in
  the fossil record (fossil succession) and the
  similarities apparent in extant organisms
  (taxonomic hierarchy).
• the creation of diversity by the evolution of new
  branches resulting from natural selection.
• a non-hierarchical mechanism, creating not just
  local adaptation, but also major changes in form,
  and even new lineages.
• Darwin often hints that he thinks natural
  selection will eventually explain the origin of
  life as well.

In other words, Darwin wanted to cover all of
Lamarcks ground with one, non-hierarchical
mechanism!
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How well did he do?
3. Creation of a new lineage occurs when a
sub-population of a species begins to trend in a
new direction.
1. The ascent up a branch is simply a projection
of one lineages adaptive trajectory.
2. Any lineage can drift far from its ancestral
form
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When we teach evolution the way we do, focusing
on natural selection, we focus in on a tiny part
of the story, at the expense of all the best
parts.
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How did Darwin Imagine Natural Selection Working?

• There is a struggle for existence among members
  of all species because reproductive rates
  invariably outstrip resources. This leads
  automatically to a death rate that equals the
  birth rate once carrying capacity of the
  environmentis reached. (Note that this is an
  observation, not a conjecture.)
• Variation in form is ubiquitous, and traits are
  heritable through family lineages. (Also an
  observation)
• Some traits may appear in lineages that enhance
  the probability of survival of their bearers over
  bearers of alternative traits. (Darwins first
  an only (quite logical) conjecture.)
• Because bearers of these traits are favored to
  survive to sexual maturity, and because the
  traits will be passed to their offspring, those
  traits favoring survival will accrue in
  subsequent generations at the expense of the
  alternatives. (The logical conclusion, given
  3.)
• Over many generations the form of the entire
  species can be changed by this process. (Also
  following inescapably from the argument.)

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A few notes

• The primary type of variation is among
  individuals of a single species (or population)
  and the primary selector is inter-individual
  competition.
• Though Darwin occasionally mentioned competition
  with other species or populations, invariably, he
  realized, this could not increase diversity, but
  would do exactly the opposite. Clearly he needed
  to focus on what would create diversity.
• The origin of a new species thus follows from
  either of two possible scenarios
• 1. A small subpopulation, isolated from its
  relatives, would gradually adapt their forms to
  their new location by accumulating slight
  differences via interpersonal competition.
• 2. A subset of a continuous population would
  modify its form in this way, and wedge
  Darwins term itself beside its relatives by
  adapting in order to specialize in, and capture
  more efficiently, a portion of the available
  resources.

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Fast Forward to the later 1900s

• Ernst Mayr (1982) has shown that Darwin convinced
  most contemporary natural historians of the
  reality of organic evolution, but his explanation
  for it (natural selection) was virtually
  disregarded until the early-mid 1900s. Even
  Huxley (Darwins Bulldog) was ambivalent and
  even skeptical.
• After the 1910s, genetic work began to revive
  Darwins selectionist ideas, but in a modified
  form called Neo-Darwinism or The Modern
  Synthesis, and natural selection as the
  evolutionary mechanism became almost dogmatic.

However, in the neo-Darwinian model, all sorts of
new concepts got folded into the term natural
selection that Darwin probably would neither
have recognized nor condoned.
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The neo-Darwinian view The neo-Darwinian view
naturalists from Darwin on and the more
perceptive geneticists have always emphasized
that not genes but whole organisms potentially
interbreeding individuals are the unit of
selection. (Mayr, 1982) group selection has
led to uncertainties concerning various aspects
of selection we will not understand
selection until it is partitioned into its
components. (Mayr, 1982)
I shall argue that the fundamental unit of
selection, and therefore of self interest is
the gene, the unit of heredity. (R,. Dawkins --
The Selfish Gene, 1976) Since writing my
manifesto of genic selection, I have had second
thoughts about whether there might not also be a
kind of higher-level selection Dawkins, 2006
30th anniversary edition of The Selfish Gene.
It is quite impossible to partition selection
into two parts, one caused by the external
environment and another by internal factors of
physiology and development. There is no
internal selection. (Mayr, 1982)
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Should the term natural selection be used in
any context but that which Darwin (and Wallace)
intended? The question is not trivial. The
state of Georgia has recently been considering,
and may actually now have adopted a set of
standards for teaching Earth Science in public
schools that includes a provision to Describe
how fossils provide a record of shared ancestry,
evolution, and extinction that is best explained
by the mechanism of natural selection.
Emphasis mine The Historical Geology book we
use at GSW does a fair job of outlining some of
the evidence that species have evolved, then
proposes natural selection as the mechanism,
and explains natural selection in precisely the
words that Darwin himself might have chosen, as
if the 20th century never happened. So what
exactly are we teaching our students, and what
exactly will the public school teachers of the
state be teaching?
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Holocene Pleistocene
Pliocene Miocene
0.25 my
1.81 my
M. nimbosa
?
5.33 my
Macrocallista maculata
23.3 my
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M. nimbosa is usually about 1.4x longer than M.
maculata at a given height.
The color patterns are obviously different.
Could a simple elongation of M. maculata produce
a shape like that of M. nimbosa?
Other structures, such as the teeth, also have
different forms.
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Does this satisfy Darwins definition of natural
selection?

• Suppose that a few, or even one, gene change can
  alter the growth gradients of the anterior and
  posterior portions of one shell (M. maculata) and
  create a new, elongated morphology (M. nimbosa).
• The morphological change would be quick, and the
  new form would make an apparently instantaneous
  appearance in the fossil record.
• Each morphology (ancestor and descendent) would
  be adaptive, and so would persist for some time
  essentially unchanged.
• This is precisely what we see in the fossil
  record of these two species.

Natura facit saltum
Natura non facit momen
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Does this satisfy Darwins definition of natural
selection?

• There is, in fact, a slight variant, but it is a
  genetic variant, not a phenotypic one. Darwin
  insisted upon selection upon individual
  phenotypic variants.
• Mayr (and the other neo-Darwinians) translated
  the idea of morphologic variation into genetic
  variation, but without justification. Indeed,
  Mayr insisted that he had done no such thing.
• In any case, the rapid morphological change
  flies in the face of Darwins insistence upon
  individual competition leading to a gradual
  improvement in the form. No gradual accrual of
  slight change is evident to the environment,
  which cannot directly touch the genetic change,
  and which sees only the major morphologic
  shift.
• Most importantly, there is no competition among
  individuals. Selection is 1) internal and 2)
  dependent upon the availability of an appropriate
  niche for the new morph.

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The story gets worse (for natural selection) if
we propose an environmental reason for the
morphologic change.
Stanley (1970) indicates that elongated shell
form is an adaptation to rapid burrowing, and
further reports that M. nimbosa is a rapid
burrower. He did not study M. maculata, but
other venerids of similar equant shape are
generally slow burrowers using his burrowing
rate index
M. nimbosa is reported from Inlet influenced
sands (Andrews, FL shells) Tidal channels
(Stanley, 1970) Low Tide to 65 (Norris, et al.,
NC seashells)
M. maculata is reported from Offshore
(Andrews, FL shells) 50-120 (Norris, et al., NC
seashells)
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Physical Environments of the Two
Species (Cross-section of onshore/offshore
transect)
No sediment movement below this depth, except
during storms.
Greater wave/tide energy greater stirring of
sea-floor sediment
Wave Base
lower probability of shell excavation
By inhabiting deeper settings (50-120 off NC) M.
maculata avoids frequent exhumation, so burrowing
rate need not be very high. Shallower, tidally
influenced settings inhabited by M. nimbosa
subject it to more frequent exhumation,
necessitating more frequent re-burrowing. Thus a
rapid burrowing rate is favored. Stanley (1970)
indicated that these tidally-dominated settings
are difficult for bivalves to inhabit, and that
diversity is low here.
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Evolution Occurs When a Variant Morphology
Manages to Survive and Establish a Viable
Population in an Environment
Proposition During the Middle Pliocene, an
ancestral, offshore population of M. maculata
produces a (?one-gene) mutation that allows its
bearers to burrow more rapidly.
ISLAND
TIDAL DELTA
A small sub-population is washed on storm waves
into a tidally dominated sand body, from which
their ancestors and all their relatives have been
excluded since the Middle Miocene by their
inability to burrow rapidly. But these newly
elongated individuals are preadapted to colonize
this setting.
ISLAND
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Does this hypothetical evolutionary event meet
Darwins definition of Natural Selection?
DARWIN Macrocallista Slight variants No
change in form for 4-20 my Gradual
change Instantaneous 40 increase in
length Genes? What genes? Slight genetic
alteration big effect Competition among
individuals No competition between the
forms Internal selection? What internal M.
nimbosa able to grow to adult form selection?
despite developmental change Diverge to fit
into new circumstances Able to find a place after
divergence 1 type of variation/1 selector 2
types of variation, 2 selectors
Macrocallista apparently didnt read Darwin.
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Evolution appears to be hierarchical

• Genes, individuals, and possibly even groups of
  individuals can be the target of selection. 3
  possibilities
• Selection can either be external from
  competition, external from the physical
  environment, or internal. 3 possibilities
• If natural selection is, in fact, ever a creative
  mechanism then individuals are the sole target of
  selection and inter-individual competition is the
  sole external selector, by Darwins definition.
• This has all the markings of a three-tiered
  hierarchy of variation and at least a
  three-tiered hierarchy of selectors. This
  conflicts with Darwins definition of natural
  selection, which therefore cannot be the sole
  driving mechanism of evolution.

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SINCE DARWIN INTENDED NATURAL SELECTION TO BE
NON-HIERARCHICAL, SHOULD THE EVOLUTIONARY
MECHANISM AS WE NOW UNDERSTAND IT BE CALLED
NATURAL SELECTION?
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What, exactly, does natural selection mean?
(and is that a belt or a cravat that Humpty is
wearing?)

• When I use a word, Humpty Dumpty said, in
  rather a scornful tone, it means just what I
  choose it to mean -- neither more nor less.
• The question is, said Alice, whether you can
  make words mean so many different things.
• The question is, said Humpty Dumpty, which is
  to be master -- that's all.

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