From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: macroevolution (t.o. FAQ -- part two)
Date: 23 Jul 92 21:11:17 GMT
Organization: animal -- coelomate -- deuterostome

AN INTRODUCTION TO EVOLUTIONARY BIOLOGY -- BY CHRIS COLBY
*********************************************************
 
Part two

EVOLUTION AMONG LINEAGES (CLADOGENESIS)
***************************************

The following sections deal with how single populations ramify to
become multiple populations and eventually separate species - this is
called cladogenesis. The overall pattern of macroevolution is also
discussed.

SPECIATION -- INCREASING BIOLOGICAL DIVERSITY
---------------------------------------------

Speciation is the process of a single species becoming two or more
distinct species. Many biologists feel speciation is key to
understanding evolution. These biologists believe certain evolutionary
phenomena apply only at speciation and macroevolutionary change cannot
occur without speciation.

Other biologists think major evolutionary change can occur without
speciation. Changes between lineages are only an extension of the
changes within each lineage. In general, paleontologists fall into the
former category and geneticists in the latter.

MODES OF SPECIATION

Biologists recognize two types of speciation: allopatric and sympatric
speciation. The two differ in geographical distribution of the
populations in question.

Allopatric speciation is thought to be the most common form of
speciation. It occurs when a population is split into two (or more)
subdivisions that organisms cannot bridge. The two populations are
geographically isolated; organisms from subdivision A can only breed
with organisms from subdivision A and B organisms can only breed with B
organisms. Eventually, the two populations' gene pools change 
independently until they could not interbreed even if they were
brought back together. In other words, they have speciated.

Sympatric speciation occurs when two subpopulations become
reproductively isolated without first becoming geographically
isolated.  Monophytophagous insects (insects that live on a single host
plant) provide a model for sympatric speciation.  If a group of insects
switched host plants they would not breed with other members of their
species still living on their former host plant. The two subpopulations
could diverge and speciate.  Some biologists call sympatric speciation
microallopatric speciation to emphasize that the subpopulations are
still physically separate not at a geographic level, but on an
ecological level.

Biologists know little about the genetic mechanisms of speciation. Some
think series of small changes in each subdivision gradually lead to
speciation; others think there may be a few key genes that could change
and confer reproductive isolation. One famous biologist thinks most
speciation events are caused by changes in internal symbionts. Most
doubt this, however.

Populations of organisms are very complicated. It is likely that there
are many ways speciation can occur. Thus, all of the above ideas may be
correct, each in different circumstances.

OBSERVED SPECIATIONS

It comes as a surprise to some to hear that speciation has been
observed. In the genus _Tragopogon_ (a plant genus consisting mostly
of diploids), two new species (_T._ _mirus_ and _T._ _miscellus_)
have evolved. This occured within the past 50-60 years. The new
species are allopolyploid descendants of two separate diploid
parent species. 

Here is how this speciation occured. The new species were formed
when one diploid species fertilized a different diploid species and
produced a tetraploid offspring. This tetraploid offspring could
not fertilize or be fertilized by either of it's two parent
species types. It is reproductively isolated, the definition of
a species. 

Two other plant species have also arizen within the past 110
years in this manner, _Senecio_ _cambrensis_ and _Spartina_
_townsendii_.

MACROEVOLUTION VS. MICROEVOLUTION

Evolution is not linear progress. The popular notion that evolution can
be represented as progress from simple cells through complex life forms
to humans (the pinnacle of evolution), can be traced to the concept of
the scale of nature. This view is incorrect.

Evolution is better viewed as a branching tree or bush, with the tips
of each branch representing currently living species.  Populations or
species of organisms split and become two or more species as time goes
on. No living organisms today are our ancestors.  Every species we see
today is as fully modern as we are; each has its own unique
evolutionary history. No extant species are "lower life forms",
atavistic stepping stones paving the road to humanity. 

A related fallacy about evolution is that humans evolved from apes.
This is not the case -- humans and apes share a common ancestor.
Both humans and apes are fully modern species; the ancestor we
evolved from is now extinct and was not the same as present day
apes (or humans for that matter). Our (human's) closest relative is 
the chimpanzee (and the pygmy chimp). Our next nearest relative is
the gorilla.

EXTINCTION -- DECREASING BIOLOGICAL DIVERSITY
---------------------------------------------

"ORDINARY" EXTINCTION

Extinction is the ultimate fate of all species. The reasons for
extinctions are numerous. A species can be outcompeted by a
closely related species. The habitat a species lives in can 
dissappear and/or the organisms that the species eat could come
up with an unbeatable defense. 

Some species enjoy a long tenure on the planet while others are
short-lived. Some biologists believe species are "programmed" to
go extinct in a manner analogous to organisms being destined to die.
Most, however, disagree with this. The majority believe that if the
environment stays fairly constant, a well adapted species could 
continue to survive indefinately.

MASS EXTINCTION

Mass extinctions shape the overall pattern of macroevolution. If 
you view evolution as a branching tree, it's best to picture it as one
that has been severely pruned a few times in its life.  The history of
life on this earth includes many episodes of mass extinction in which
many taxa (groups of organisms) were wiped off the face of the planet.
Mass extinctions are followed by periods of radiation where new species
evolve to fill the empty niches left behind. It is probable that
surviving a mass extinction is largely a function of luck. Thus
contingency plays a large role in patterns of macroevolution.

The most famous extinction occurred at the boundary between the
Cretaceous and Tertiary Periods (the K/T Boundary). This extinction
eradicated the dinosaurs. Following this extinction the mammalian
radiation occurred. Currently, human alteration of the ecosphere
is causing a global mass extinction. In terms of rate, this is the
largest mass extinction to date.

PUNCTUATED EQUILIBRIA
---------------------

Some paleontologists believe evolution is a heirarchical process. 
The theory of punctuated equilibria attempts to infer the process
of macroevolution from the pattern of species documented in the
fossil record.

In the fossil record, transition from one species to another is 
usually abrupt in most geographic locales -- no transitional forms 
are found. In short, it appears that species remain unchanged for
long streches of time and then are quickly replaced by new 
species. However, if wide ranges are searched, transitional forms 
that bridge the gap between the two species are sometime found in
small, localized areas.

For example, in Jurassic brachiopods of the genus _Kutchithyris_,
_K. acutiplicata_ appears below another species, _K. euryptycha_.
Both species were common and covered a wide geographical area. They
differ enough that some have argued they should be in a different
genera. In just one small locality an approximately 1.25m sedimentary
layer with these fossils is found. In the narrow (10 cm) layer that
separates the two species, both species are found along with 
transitional forms. In other localities there is a sharp transition.

Gould and Eldredge, the authors of punctuated equilibria, interpret
this in light of theories of allopatric speciation. They concluded 
that isolated populations of organisms will often speciate and then
invade the range of their ancestral species. Thus at most locations
that fossils are found, transition from one species to another will
be abrupt. This abrupt change will reflect replacement by migration
however, not evolution. In order to find the transitional fossils,
the area of speciation must be found. 

Gould and Eldredge also argue that evolution can proceed quickly in 
small populations so that the tempo of evolution is not continuous. 
This has lead to some confusion about the theory. Some popular
accounts give the impression that abrubt changes in the fossil
record are due to blindingly fast evolution; this is not what the
theory of punctuated equilibria says.

Gould and Eldredge envision their theory as a heirarchical theory
of evolution because they see speciation as analogous to mutation 
and the replacement of one species by another (which they call
species selection) as analogous to natural selection. Speciation
adds new species to the species pool just as mutation adds new 
alleles to the gene pool. Likewise, they argue that species
selection favors one species over another just as natural selection
can favor one allele over another. This is the most controversial
part of the theory. Many biologists agree with the pattern of
macroevolution Gould and Eldredge posit, but disagree with their
mechanism -- species selection. Critics would argue that species
selection is not analogous to natural selection and therefore
evolution is not heirarchical.

The theory of punctuated equilibrium was designed to replace the theory 
of phyletic gradualism. Phyletic gradualists held that a species would 
slowly transform into another species over it's entire range. Phyletic
gradualism is often associated with the assumption of a uniform rate
of evolution, but this need not be the case. 

CONCLUSION
----------

ARE WE STILL EVOLVING?

Yes, evolution is still occurring; all organisms continue to adapt to
their surroundings and "invent" new ways of better competing with
members of their own species. In addition, allele frequencies are being
changed by drift, mutation and gene flow constantly. Studying the 
process of evolution as it continues to occur is a major field of
biology today. Although evolution has been observed and all the
mechanisms have been shown to work, there is still no consensus on
the relative contribution of each of the mechanisms to the overall
pattern of evolution within a lineage. Likewise, although new species
have been seen to arise; biologists have many questions about what
influences the pattern of macroevolution. Are some groups "good" at
speciating? Who survives mass extinctions and why? These are two
questions that biologists ask.

Evolution is the unifying theory of biology. The functions of
biological entities at all levels (populations, organisms, genes) are
the product of a non-random factor (e.g. natural selection) operating
in conjunction with random factors (such as mutation and mass extinction)
within a framework of historical constraint. For centuries humans have 
asked, "Why are we here?". A question such as that probably lies
outside the realm of science. However, biologists can now provide
an elegant answer to the question, "How did we get here?"

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SOME GOOD EVOLUTION TEXTS (IMHO)

A good introductory text in evolutionary biology is:  Evolutionary
Biology, by Douglas Futuyma, 1986, Sinauer, Sunderland, Mass

The text assumes some previous knowledge of biology, but reviews most
critical background material. It contains numerous references to the
primary literature. Most of the information in this file can be found
(along with the references to the primary literature) in this text.

A good introductory text into population genetics, the field that
mathematically describes changes in the gene pool is:  Principles of
Population Genetics, by Hartl and Clark , 1989, Sinauer, Sunderland,
Mass

None of the math is very daunting (it's just an intro text after all)
but it's really critical (IMHO) to really understanding what evolution
is all about. And again, lots of refs.

A text that deals with the interface of molecular biology and evolution
is:  Fundamentals of Molecular Evolution, Li and Graur, 1991, Sinauer,
Sunderland, Mass

A very concise introduction to this field.

Chris Colby 	---	 email: colby@bu-bio.bu.edu	---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
 	--Kurt Vonnegut from "Galapagos"