From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: macroevolution (a t.o. FAQ -- part two)
Date: 12 Nov 92 17:15:54 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 arisen within the past 110 years in this 
manner, _Senecio_ _cambrensis_ and _Spartina_ _townsendii_.

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 disappear 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 
indefinitely.

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- 65MYA). This 
extinction eradicated the dinosaurs. Some hypothesize that the K/T event 
was caused by environmental disruption brought on by a large impact on 
earth. Several lines of evidence point to a large collision at the time of 
the extinction, but attempts to link the two have not been convincing to 
some biologists. Following this extinction the mammalian radiation 
occurred. Mammals coexisted for a long time with the dinosaurs but 
were confined mostly to nocturnal insectivore niches. With the 
eradication of the dinosaurs, mammals radiated to fill the vacant niches.

The largest mass extinction came at the end of the Permian (250MYA); it 
is estimated that 96 percent of all species died out at this time. The sea 
animals that make up the so-called Paleozoic Fauna (among them 
crinoids, cephalopods, brachiopods and corals), suffered the worst. This 
assemblage did did not reradiate after the event and remains at the level 
of diversity it sunk to after the extinction. In contrast, the sea animals 
that make up the Modern Fauna (gastropods, bivalves, crabs, echinoids 
and bony fishes) were barely affected and continued to increase in 
diversity after the event. 

The Permian extinction coincides with the formation of Pangea II, when 
all the worlds continents were brought together by plate tectonics. A 
worldwide drop in sea level also occured at this time. Currently, human 
alteration of the ecosphere is causing a global mass extinction. 

THE PATTERN OF MACROEVOLUTION 

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 relatives are the 
chimpanzee and the pygmy chimp. Our next nearest relative is the 
gorilla.

MACROEVOLUTION

Whereas microevolution can be studied directly, macroevolution is 
studied by examining patterns in biological populations and clades 
(groups of organisms) and inferring process from pattern. Given the 
observation of microevolution and the knowledge that the earth is 
billions of years old -- macroevolution could be postulated. However, this 
extrapolation in and of itself does not really provide a compelling 
explanation of the patterns of biological diversity we see today. Evidence 
for macroevolution, or common ancestry and modification with descent, 
comes from several other fields of study. These include: comparative 
biochemical and genetic studies, comparative developmental biology, 
patterns of biogeography, comparative morphology and anatomy and the 
fossil record. These are not discussed here, see the references for further 
details.

Further strong evidence for macroevolution comes from the fact that 
suites of traits in biological entities fall into a hierarchical pattern. For 
example, plants can be divided into two broad categories, non-vascular 
(mosses) and vascular. Vascular plants can be divided into seedless 
(ferns) and seeded. Vascular seeded plants can be divided into 
gymnosperms (pines being one example) and flowering plants or 
angiosperms. And angiosperms can be divided into monocots and dicots. 
Each of these types of plants have several characters that distinguish 
them from other plants -- traits are not "mixed and matched" in groups 
of organisms. For example, flowers are only seen in plants that carry 
several other characters that distinguish them as angiosperms. This 
pattern arises due to lineages splitting (speciation), retaining ancestral 
traits and deriving new traits. Derived traits only appear in lineages 
descended from the population that first displayed the trait. This 
hierarchical pattern of diversity is what one expects to see if species 
branch into new species and are modified with descent. 

A hierarchical pattern can be produced even if the process responsible is 
not hierarchical. For example, microevolution leads to hierarchical 
patterns of genetic diversity even though it works at a single level. The 
question of hierarchical processes in evolution is still being debated. 

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

Some paleontologists believe evolution is a hierarchical 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 
stretches 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. 

They 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 abrupt changes in the fossil record are due to blindingly fast 
evolution; this is not what the theory of punctuated equilibria says. 

Some PE proponents envision the theory as a hierarchical 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 these paleontologists posit, but many disagree with the 
mechanism -- species selection. Critics would argue that species selection 
is not analogous to natural selection and therefore evolution is not 
hierarchical.

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? 

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 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, by Li and Graur, 1991, Sinauer, 
Sunderland, Mass 
A very concise introduction to this field. 

A text that deals with theories of macroevolution is: 
Macroevolutionary Dynamics, by Niles Eldredge, 1989, McGraw-Hill, New 
York

A text that documents the history of life on earth is: 
History of Life, by Richard Cowen, 1990, Blackwell Scientific, Boston 
A readable introduction to the history of our planet and especially the 
changes that have occurred in the biota. 

A popular introduction to the field that also debunks the most common 
creationist arguments is:
The Blind Watchmaker, by Richard Dawkins, 1987, Norton, New York 
Dawkins is (IMHO) a very engaging writer. 

A close look at the creation/evolution debate can be found in: 
Abusing Science, by Philip Kitcher, 1982, MIT, Cambridge, Mass 
A meticulous critique of creationism. 

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"
 

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