Re: Macroevolution FAQ 2.1D

Macroevolution - its definition, philosophy and history

John Wilkins

Version 2.1 DELTA

To be read in conjunction with Douglas Theobold's 29+ Evidences for
Macroevolution FAQ.

Contents
========

What is Macroevolution?

* The history of the concept of macroevolution

* Confusions

* Is macroevolution reducible to microevolution?

* Species sorting/selection

* Historical constraints/bauplans

* Emergent properties

* Barriers to macroevolution

* Falsifying macroevolution

* Conclusions

* Notes

* References


What is macroevolution?
=======================

Creationists often assert that "macroevolution" is not proven, even if
"microevolution" is, and by this they seem to mean that whatever evolution is
observed is microevolution, but the rest is macroevolution. In making these
claims they are misusing authentic scientific terms; that is, they have a
non-standard definition, which they use to make science appear to be saying
something other than it is. Evolution-proponents often say that creationists
invented the terms. This is false. Both macroevolution and microevolution are
legitimate scientific terms, which have a history of changing meanings that,
in any case, fail to underpin creationism.

In science, macro at the beginning of a word just means "big", and micro at
the beginning of a word just means "small" (both from the Greek words). For
example, a macrophage means a bigger than normal cell, but it is only a few
times bigger than other cells, and not an order of magnitude bigger. Something
can be "macro" by just being bigger, or there can be a transition that makes
it something quite distinct.

In evolutionary biology today, macroevolution is used to refer to any
evolutionary change at or above the level of species. It means at least the
splitting of a species into two (speciation, or cladogenesis , from the Greek
meaning "the origin of a branch", see Fig. 1) or the change of a species over
time into another (anagenetic speciation, not nowadays generally accepted
[note 1]). Any changes that occur at higher levels, such as the evolution of
new families, phyla or genera, are also therefore macroevolution, but the term
is not restricted to those higher levels. It often also means long-term trends
or biases in evolution of higher taxonomic levels.

Microevolution refers to any evolutionary change below the level of species,
and refers to changes in the frequency within a population or a species of its
alleles (alternative genes) and their effects on the form, or phenotype, of
organisms that make up that population or species. It can also apply to
changes within species that are not genetic.

Another way to state the difference is that macroevolution is between-species
evolution and microevolution is within-species evolution. Sometimes,
macroevolution is called "supraspecific evolution" (Rensch 1959, see Hennig
1966: 223-225).

There are various views of the dynamics of macroevolution. Punctuated
equilibrium theory proposes that once species have originated, and adapted to
the new ecological niches in which they find themselves, they tend to stay
pretty much as they are for the rest of their existence. Phyletic gradualism
suggests that species continue to adapt to new challenges over the course of
their history (see Fig. 1). Species selection and species sorting theories
think that there are macroevolutionary processes going on that make it more or
less likely that certain species will exist for very long before becoming
extinct, in a kind of parallel to what happens to genes in microevolution.

Figure 1: Anagenesis and cladogenesis. Species A anagenetically changes over
time to become species B, while species B cladogenetically changes over time
by splitting into species C and D, neither of which are very different from B
or each other. The anagenesis axis represents change of form, either genetic
or phenotypic. The cladogenetic axis represents isolation of species from each
other (for example, reproductive isolation). Of course, cladogenesis and
anagenesis can often go hand-in-hand as well.



The history of the concept of macroevolution
============================================

In the "modern synthesis" of neo-Darwinism, which developed in the period from
1930 to 1950 with the reconciliation of evolution by natural selection and
modern genetics, macroevolution is thought to be the combined effects of
microevolutionary processes.

The terms macroevolution and microevolution were first coined in 1927 by the
Russian entomologist Iuri'i Filipchenko (or Philipchenko, depending on the
transliteration), in his German-language work Variabilitaet und Variation,
which was an early attempt to reconcile Mendelian genetics and evolution.
Filipchenko was an evolutionist, but as he wrote during the period when
Mendelism seemed to have made Darwinism redundant, the so-called "eclipse of
Darwinism" (Bowler 1983), he was not a Darwinian, but an orthogeneticist.
Moreover, Russian biologists of the period had a history of rejecting Darwin's
Malthusian mechanism of evolution by competition (Todes 1989).

In Dobzhansky's founding work of the Modern Synthesis, Genetics and the Origin
of Species, he began by saying that "we are compelled at the present level of
knowledge reluctantly to put a sign of equality between the mechanisms of
macro- and microevolution" (1937: 12), thereby introducing the terms into the
English-speaking biological community (Alexandrov 1994). Dobzhansky had been
Filipchenko's student and regarded him as his mentor. In science as in all
academic disciplines, it is difficult to deny a major tenet of one's teachers
due to filial loyalty, and Dobzhansky, who effectively started the modern
Darwinian synthesis with this book, found it disagreeable to have to deny his
teacher's views (Burian 1994).

The term fell into limited disfavour when it was taken over by such writers as
the geneticist Richard Goldschmidt (1940) and the paleontologist Otto
Schindewolf to describe their orthogenetic theories. As a result, apart from
Dobzhansky, Bernhardt Rensch and Ernst Mayr, very few neo-Darwinian writers
used the term, preferring instead to talk of evolution as changes in allele
frequencies without mention of the level of the changes (above species level
or below). Those who did were generally working within the continental
European traditions (as Dobzhansky, Mayr, Rensch, Goldschmidt, and Schindewolf
are) and those who didn't were generally working within the Anglo-American
tradition (such as John Maynard Smith and Richard Dawkins). Hence, use of the
term "macroevolution" is sometimes wrongly used as a litmus test of whether
the writer is "properly" neo-Darwinian or not (Eldredge 1995: 126-127).

The term was revived by a number of mainly paleontological authors such as
Stephen Jay Gould and Niles Eldredge, the authors of punctuated equilibrium
theory (see Eldredge 1992), who argued that something other than
within-species processes are causing macroevolution, although they disavow the
orthogenetic view that evolution is progressive. Many paleontologists have
held that what happens in evolution beyond the species level is due to
processes that operate beyond the level of populations - for example, the
notion of species selection (the idea that species themselves get selected
similarly to the way alleles get selected within populations, see Grantham
1995, Rice 1995, and Stidd and Wade 1995 for reviews and discussions.

The idea that the origin of higher taxa, such as genera (canines versus
felines, for example), requires something special is often based on the
misunderstanding of the way in which new lineages arise. The two species that
are the origin of canines and felines probably differed very little from their
common ancestral species and each other. But once they were taxonomically
isolated from each other, they evolved more and more differences that they
shared internally but that other lineages didn't. This is true of all lineages
back to the first eukaryotic (nuclear) cell. Even the changes in the Cambrian
explosion are of this kind, although some (e.g., Gould 1989) think that the
genomes (gene structures) of these early animals were not as tightly regulated
as modern animals, and therefore had more freedom to change.

Confusions
==========

The meaning modern authors give to the terms "macroevolution" and
"microevolution" is often confusing, and varies according to what it is they
are discussing. This is particularly the case when "large-scale" evolutionary
processes are being discussed. For example, R. L. Carroll, in his
undergraduate textbook (1997: 10) defines microevolution as "involving
phenomena at the level of populations and species" and macroevolution as
"evolutionary patterns expressed over millions and hundreds of millions of
years". Eldredge says, "Macroevolution, however it is precisely defined,
always connotes "large-scale evolutionary change" (1989: vii) and throughout
his book speaks of macroevolution as roughly equivalent to the evolution of
taxa that are of a higher rank than species, such as genera, orders, families
and the like. In his book Evolution, Mark Ridley defines the terms thus (2004:
227):

<bq>Macroevolution means evolution on the grand scale, and it is mainly
studied in the fossil record. It is contrasted with microevolution, the study
of evolution over short time periods, such as that of a human lifetime or
less. Microevolution therefore refers to changes in gene frequency within a
population .... Macroevolutionary events events are much more likely to take
millions of years. Macroevolution refers to things like the trends in horse
evolution ... or the origin of major groups, or mass extinctions, or the
Cambrian explosion .... Speciation is the traditional dividing line between
micro- and macroevolution.<eq>

There are many papers published that use the term in this "higher category"
way; why is that?

Science is not always consistent in its use of terms; this is the source of
much confusion. Sometimes this is carelessness, and sometimes this is because
of the way in which terms are extended over time. When biologists and
paleontologists talk about macroevolution in the sense of "large-scale"
evolution, they are strictly speaking meaning only a part of the phenomena the
term covers, but it is the most interesting part for those specialists. That
is, they are talking about the patterns of well-above-species-level evolution
(Smith 1992).

In order to have a pattern you have to be able to compare three or more
species (Fig. 2). On its own, species A forms no patterns, and so long as the
changes within it do not result in a new species, evolution is
microevolutionary. If a new species B splits from A, then you have
macroevolution, but no patterns. For there to be a pattern, you need to be
able to say that one species is more closely related to another than a third
is (in this case, that A is closer to B than it is to C).


Figure 2: If only two species or higher taxa are identified (red set) there is
no pattern. If three or more (blue set) are included, then you are able to say
that one is more closely related, evolutionarily speaking, to another than the
third - in this case A and B are more closely related to each other than
either is to C, which split off earlier than the A/B split.

The sorts of patterns that people are interested in when discussing
macroevolution tend to involve very many species, either as a single large
group ("higher taxon") or individually. This is why many authors use the term
"macroevolution" to mean "large-scale evolution". However, just like
anagenetic speciation, "large-scale" is an arbitrary and often subjective
term, and the objective meaning of macroevolution is evolution at or above the
level of species [note 2]. Hence, Carroll's "definition" is mistaken, despite
his prominence in the field, and this sort of confusion is to be avoided.

A more considered definition is that of Levinton: "I define the process of
macroevolution to be... the sum of those processes that explain the
character-state transitions that diagnose evolutionary differences of major
taxonomic rank" (Levinto 2001:2). Here, Levinton is trying to define
macroevolution in a way that is not prejudicial for the debate. It focuses on
the characters of taxa, and is neutral about what level of taxa are involved.
He denies the "species level" definition because he thinks, I believe wrongly,
that it makes macroevolution the study of speciation. But if the "pattern"
analysis above is right, then macroevolution includes the study of speciation,
but it is hardly restricted to it. The scope of macroevolution rises very far
above that level. It's worth observing, though, that higher taxonomic levels
are artificial, constructed for convenience by systematists. Conclusions about
evolution that rely upon taxonomic levels like genera or families (e.g.,
Raup's and Sepkoski's work on extinction, Raup and Sepkoski 1986, Sepkoski
1987, Raup 1991) must be taken with a grain of salt, since the taxon levels
are not the "same" across phylogenetically distant groups, because they are
not "natural". Incidentally, the study of speciation has taken off
significantly in recent years with some solid theoretical work that suggests
many macroevolutionary effects are indeed the result of population level
processes (Gavrilets 2003, 2004, Gavrilets and Gravner 1997).

Is macroevolution reducible to microevolution?
==============================================

From a philosophical perspective, one might say macroevolution is just a
bunch of microevolution. It's also just a bunch of chemistry. And physics.
These are unhelpful answers, so we might find it worthwhile to ask how
scientific domains relate to each other. Whenever a scientist or philosopher
asks if two theories are reducible one to the other, there are several answers
that can be given. One is if the first theory being reduced A is adequately
captured by the reducing theory B. Another is that A is not entirely captured
by B. A third is that A and B each have overlapping areas, and areas only they
capture. This is called the problem of theory reduction.

Reduction has been a philosophical problem with respect to science for about
60 years. It comes in three main varieties: methodological reduction, which is
the notion that one ought to try to explain wholes in terms of the parts and
their interactions; ontological reduction, which is the notion that all the
units or entities of one theory are composed of units or entities of another;
and metaphysical reduction, which is the claim that only one kind of thing
exists. Ontological reduction includes reducing all the laws and dynamic
generalisations of the A theory to laws and dynamic generalisations of the B
theory. In philosophy of science, the case is often put in just these terms,
but increasingly philosophers are attending to the objects of scientific
theories as well as the models.

Consider atoms, as an example. At the time Dalton proposed atoms, he was
trying to explain larger things in terms of smaller things with properties
that added up to the properties of the whole. He did this because he felt it
was a good rule to follow, explaining wholes in terms of parts. So he was a
methodological reductionist, explaining things in terms of ontological
reduction. He wasn't a metaphysical reductionist, though, if he allowed that
reality comprised stuff other than atoms - such as gravity or light. A
parallel case is genetic reductionism, in which behaviours are "reduced" to
genes - it is both methodologically and ontologically reductionist in the
domain of behaviour and biology. It doesn't assert that everything is genetic,
though, because we know that how genes are expressed is affected by
non-genetic factors, such as the availability of food during crucial phases of
development.

The reductive relation between microevolution and macroevolution is hotly
debated. There are those who, like Dobzhansky, say that macroevolution reduces
to microevolution. We can break this down to three claims: within the
"universe" of biology, one might say that everything biological is best
explained by microevolution (methodological), or that all entities and
processes of macroevolution are microevolutionary (usually genetic - this is
ontological), or that everything that happens (in biology) is genetic
(metaphysical). In the metaphysical case, genes acquire an almost mystical
significance, and no serious biologist makes this claim, although opponents
accuse (particularly Dawkins) some of doing so.

The two reductive claims we will consider now are the methodological and the
ontological.

The methodological claim that macroevolution (Ma) reduces to microevolution
(Mi) is a claim that the optimal solution for investigating evolution is to
apply modelling and testing genetic techniques. And this has been very
successful. However, it has not been an unqualified success - developmental
biology is not easily reducible to genetics, nor is ecology. Cell division,
specialisation and signalling explain development, and the relationship
between genes and these processes is equivocal - that is, some genes play a
role in many developmental processes, and many genes play a role in pretty
well all processes. Moreover, there are many other things involved in
development: epigenetic factors (para-genetic inheritance and environmental
modulation of genetic effects), cytological inheritance (organelles, cell
membranes, ribosomes and enzymes from parent cells, and parent organisms). So
genes on their own are not enough to explain why evolution occurs along the
pathways that it has. One reaction to methodological reductionism in biology
has been to assert that genes are merely "bookkeeping" entities for
evolutionary investigation (Gould 2002). The methodological reduction is not
sufficient, even if genes turn out to be the only significant "players" in
evolution.

It is this assumption that antireductionists challenge in the ontological
reductionist case. There are entities and processes, they say, that affect
macroevolutionary dynamics which are not in their nature microevolutionary.
What could these be?

Well, a list that Mi-reductionists would accept includes climate change,
geomorphological processes like mountain building, tectonic isolation and
drift, vulcanism, extraterrestrial influences like bolide impacts, galactic
wobble, precession of the earth's axial rotation, and possibly even local
stars approaching and changing the impact on the earth of comets and other
bolides in a cycle averaging around 13 million years. The point the
Mi-reductionists would make, though, is that everything that these things
affect is microevolutionary - only the frequencies of genes in populations,
and so on. They serve as the environment in which genes change their frequency
(or fail to, and the species goes extinct). What the "player" is in Mi is the
population, comprising organisms, traits and genes; in short, the gene pool.
Nothing else is important.

Ma-nonreductionists will argue, however, that there are emergent processes and
entities in macroevolution that cannot be captured ontologically. There are
several candidates for these, each challenged by Mi-reductionists. The basis
for this is a view of evolution as a series of inclusive hierarchical levels,
each of which is somewhat independent of the lower levels.

Figure 3.The hierarchical relations between Macroevolution (Ma) and
Microevolution (Mi), and the Environment (E). Mi consists of Organisms (O) and
their Interactions (I) together with factors from the Environment. Examples
A-G illustrate the levels of environmental influence: A. Mutation caused by
thermal or radioactive interference. B. Heat shock on developing zygotes. C.
Local adaptation to a niche. D. Climatological change causing migration. E.
Geographical isolation. F. Environmental changes that cannot be adapted to for
historical or developmental reasons (causing extinction). G. Changes that
affect speciation rates and type.

Species selection/sorting
-------------------------

Elisabeth Vrba (Vrba 1985, Gould and Vrba 1993) proposed that species come
into being and go extinct in a biased way. Generalist species (eurytopes) tend
to survive longer - when one food source is unavailable, they switch to
another until it comes back, thus avoiding predator-prey cycles known as
Lotka-Volterra cycles (such as fox numbers dropping dramatically when rabbits
are over-predated). Specialist species (stenotopes), though, are sensitive to
the contingent changes forced by climate changes - even long droughts. But
specialists tend to speciate more frequently, even if they go extinct more
frequently, too, as they adapt to loss of degradation of their food resources.
Selection is a process of differential survival correlated with ecological
success, so proponents of selection of this kind consider this to be a
selection process on species. Others refer to it as a species "sorting"
process (see Grantham 1995 for a review) because species are not sufficiently
like organisms/individuals. Gould published an extensive discussion shortly
before his death (Gould 2002: 644-673). It is worth noting here that if
species are selected, it is more like asexual evolution than the evolution of
sexual organisms, as species rarely evolve by recombining lineages, or at
least animal species don't. Plant species often do (at about 5-10% of new
species), and we have insufficient evidence about other groups to generalise.

Historical constraints/bauplans
-------------------------------

Some "Non-Synthesis" or post-Synthesis evolutionists think that the processes
that cause speciation are of a different kind to those that occur within
species. That is, they admit that macroevolution occurs, but think that normal
genetic change is restricted by such proposed mechanisms as developmental
constraints. This view is associated with the names of Schmalhausen and
Waddington, who were often characterised as being non-Darwinians by the modern
synthesis theorists. However, with the recent rise of the field known as
"evo-devo", or evolutionary developmental biology, many of the ideas proposed
by Waddington and others have been revisited (Schlichting and Pigliucci 1998,
Amundson 2005, Levinton 2001).

There are several kinds of constraints upon evolution. The best known is of
course selective constraints: some forms are just not viable, one way or
another. But developmental constraints have been proposed to explain why, for
example, in centipedes the segment number is always an odd number (Arthur
2003). In these cases, the constraint is the nature of the developmental
system itself. Others (Schlichting and Pigliucci 1998) consider this as much a
case of selection as anything else; the developmental system - indeed, the
ability to evolve - is subjected to selection as well. Historical constraints
form a kind of "you can't get there from here" class. Once something has
evolved, any state that requires reversing the evolution of that trait to get
somewhere else is vanishingly unlikely. So the dynamics of the evolution of
that trait are constrained by what has evolved already.

The notion of a bauplan - a German word meaning "blueprint" or "builder's
plan" - has been applied to evolution most notably by Gould and Lewontin
(1979). Bauplans (the word takes the English plural in this context) are the
body plans of phyla, the second highest Linnaean taxonomic level. Since
Georges Cuvier named them in the early 19th century, phyla (singular phylum)
have been seen as distinct and natural groupings within animals (arguably not
in plants, where the level is Division). Bauplans have been tied into the
notion of a developmental and a historical constraint. There have been
criticisms of the notion of a bauplan as being mystical in its causal power.
Others see it as something that cannot be easily modified by the processes of
within-species (Mi) evolution.

Emergent Properties
-------------------

One of the claims made by Ma-nonreductionists is that evolution occurs on
emergent properties. An emergent property is one in which the property of a
higher level system or object cannot be reduced to the properties of its
constituent elements, but instead it "emerges" from the interactions between
them (O'Connor and Wong 2002, Mandik 2004). Emergent properties were first
proposed by, coincidentally, a friend of Darwin's, G. H. Lewes, in the field
of psychology, but the idea goes back to J. S. Mill in 1843. It is often
sloganised as "the whole is more than the sum of its parts". Emergence was
made an issue when applied, ironically enough, to evolution in the 1920s by
Jan Smuts and C. D. Broad.

In evolution, a species is considered by some Ma-nonreductionists as being a
system that has properties above the level of the individual, the kin, or the
deme (breeding population), based somewhat on Mayr's definition of a species
as being a protected breeding gene-pool (Mayr 1996). This has been challenged
on various grounds, not least being that usually species appear to have no
systematic interactions between all its parts, and that the appropriate level
is the population.

In this writer's opinion, an emergent property is simply a property that we
have trouble computing or predicting from a knowledge of the constituent
parts, but this simple dismissal is insufficient. We have to identify the
following aspects of the matter:

E: The environmental factors in which a species exists - for example,
geological and climatological changes

O: the properties of the organisms - for example their traits and capacities
severally

I: the interactions between the organisms - for example, the lineages of
heredity at the gene, haplotype, genome and developmental levels of
organization. Also, the issue of organisms changing their environment through
a process known as "niche construction" affects both E and I (Oyama et al.2000).

Ma is therefore the result of the union, in some way, of E, O and I. This can
be massively complex and give rise to "sudden" changes [note 3], or hold the
evolutionary process in a state of stasis for long periods. Whether or not one
wants to call this "repeated rounds of microevolution" or not (Erwin 2000) is
open to debate. And even if it is, we still need to know the models for how
they relate, and what Mi covers.

Barriers to macroevolution?
===========================

It is a common claim of antievolutionists that there is a limit to the amount
of change that can be made. Creationists, like Gish (1979) claim that there is
some limitation within "basic kinds", without being able to express exactly
what basic kinds might be, or why change is restricted within them. Others,
such as Johnson (1991:18) claim that the limit lies in the availability of
genetic variety, and that when that limit is reached change ceases, although
he does accept that "Darwinists" have "some points to make" - although he is
hardly fair when he says that variation "might conceivably be renewed by
mutation, but whether (and how often) this happens is not known" (p19). Of
course it is known. We have had experimental evidence of rates of mutations
since the 1910s, and modern research both mathematically and empirically
confirms that rates of mutation occur at around 0.1-1.5 per zygote, which is
to say every embryo has around 1/1000th to 1.5 mutations (Crow 1997). The
average mutation rate - that is the average rate of persisting mutations in a
population - is 2.2 x 10-9 (Kumar and Subramanian 2002). Further, genes aren't
identical in their evolution to the species; a field known as coalescence
genetics covers the ability of novel genes to persist across speciation, so
that the variability is "available" when it is selectively advantageous (Hey
and Walkley 1997). Note that this is not to say that variation is maintained
in order to be available. It's just that it is available when selective
pressures change some of the time.

Creationists often say that species cannot be evolved from each other because
chromosome numbers are different. Humans, for example, have 46 chromosomes,
while chimpanzees have 48. But the human chromosome 2 is the result of what is
called a Robertsonian fusion - the ancestral ape chromosomes 2p and 2q appear
to have fused at their ends (telomeres) to form the human chromosome 2
(Williams, not dated), and other species that have large chromosomal
differences can still interbreed (Nevo et al. 1994). DNA aligns according to
local sequence rather than large-scale chromosome structure, and this is why
inversions and translocation in parts of the sequence still allow interbreeding.

There appears to be no single amount of genetic variation common between
closely related species that prevents interbreeding. In some, only a few are
sufficient. In others, much variation, such as the large chromosomal
difference in Nevo's mole rats, fails to prevent interbreeding. Introgression,
or the leakage of genes across species boundaries, has been observed in
lizards, plants, birds, and fish.

In summary, there is no barrier to species forming. This may not be enough to
show that large-scale macroevolution occurs, though, according to writers like
Johnson and Hitching (1982), but the logic here implies some causal force
actively preventing change, rather than a problem with change occurring. For
if there is enough change to form new species, and each species is slightly
different from its ancestor, then simple addition shows that many speciation
events can cause large-scale evolution over enough time. A journey of a
thousand miles begins with a single step. Conversely, many single steps can
traverse long distances. There is no evidence of any kind of barriers to
large-scale change (Brauer and Brumbaugh 2001).

Falsifying macroevolution
=========================

Antievolutionists try to make out that macroevolution is a tautology, the way
they claim that natural selection is a tautology. The implication is that
macroevolution cannot be tested and shown to be wrong, and therefore it is not
science.

To clarify this, consider what it is that scientists test when they test a
hypothesis. Let's suppose that we are testing the idea that global warming is
caused by a rise in CO2 in the atmosphere. There are two parts to this - one
is that CO

.

Relevant Pages

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