To: All Msg #27, Jan2593 11:27AM Subject: Re: Has Macroevolution Occured? In article 1993J
From: Chris Colby
To: All Msg #27, Jan-25-93 11:27AM
Subject: Re: Has Macro-evolution Occured?
Organization: animal -- coelomate -- deuterostome
From: email@example.com (Chris Colby)
In article <1993Jan23.firstname.lastname@example.org> email@example.com (Russ
>I'm asking for the observations that make up the fact of
Evolution (changes in a gene pool) has been observed; so has natural
selection (the only mechanism of adaptive evolution). Likewise,
speciation has been observed. The most recently documented example
I know of is in polychaete worms. (I noticed one poster seemed to
think speciation would be really big news; it wasn't. It was a note in
the journal _Evolution_, not even a full paper.)
Common descent is inferred from several observations:
1.) Evolution occurs
2.) Natural selection occurs
3.) Comparative anatomy -- Groups of organisms are "constructed"
using the same "building blocks"; mammalian skeletal features are
often used to illustrate this in introductory texts. For example
the wing of a bat, flipper of a whale, front paw of a cat and
arm of a human are all made from the same bones -- the bones are
just scaled differently.
4.) Comparative developmental biology -- Closely related organisms
share similar developmental pathways, the differences in develop-
ment are most evident at the end. This is, again, usually illustrated
using mammalian (or sometimes vertebrate) examples. As organisms
evolve, their developmental pathway gets modified. It is easier to modify
the end of a developmental pathway than the beginning since changes
early on have a cascading effect. Therefore, organisms pass through
stages of early development that their ancestors passed through.
These stages, however, are modified because selection "sees"
all stages of an organisms life cycle. So, an organism's development
mimics its ancestors although it doesn't recreate it exactly.
5.) Comparative biochemistry -- closely related organisms have similar
genes and proteins.
6.) Biogeography -- Organisms clustered spatially are frequently also
clustered phylogenetically; this is especially true of organisms with
limited dispersal opportunities. The mammalian fuana of Australia is
often cited as an example of this; marsupial mammals fill most of the
equivalent niches that placentals fill in other ecosystems. If
all organisms descended from a common ancestor, species distribution
across the planet would be a function of 1.) site if origination 2.)
potential for dispersal and 3.) time since origination. In the case
of Australian mammals, their physical separation from sources of
placentals means potential niches were filled by a marsupial radiation
rather than a placental radiation or invasion.
7.) The fossil record -- Fossils show hard structures of organisms
less and less similar to modern organisms as you go down the strata
(layers of rocks). In addition, patterns of biogeography apply to
fossils as well as extant (living) organisms. When combined with
plate tectonics, fossils provide evidence of patterns of distributions
and dispersals of organisms. For example, South America had a very
distinct marsupial mammalian fauna until the land bridge formed
between North and South America. After that marsupials started
disappearing and placentals took their place. This is commonly
interpreted as the placentals wiping out the marsupials (but this
may be an over simplification).
8.) Evidence of "jury-rigged" design -- If all organisms descended from
a common ancestor and their phenotypic traits developed over long
periods of time due to modification with descent, organisms should
show evidence of "jury-rigged" design -- i.e. structures that were
modified enough to make do, but could have been designed much better
if a mechanism for building the structure from scratch were avail-
able. I have a file that lists some of these; I'll try to post it
later today or maybe tomorrow. (Someone else has a huge file of
this, maybe they will post theirs "hint hint" 8-) Evolution works
by diddling with what it has available; as a result of this,
suboptimal design abounds in nature.
9.) The nested pattern of biological traits -- This is IMHO, the
best evidence for common descent. If organisms share a common
ancestor and are modified with descent, derived traits should
be distributed across groups of organisms in a nested manner --
traits should not be "mixed and matched". This pattern is seen
in comparative anatomy, development and biochemistry. Although
this is the best evidence for evolution, it also requires the
most knowledge of biology and is sometimes hard to get across
to people. This will come up in my discussion with Mr. Johnson,
watch that thread for a full explanation.
10.) Current knowledge generates predictions -- In several of the
above examples I stated, closely related organisms share X. If I
define closely related as sharing X, this is a contentless statement.
It does however, provide a prediction. If two organisms share (oh
lets say) a similar anatomy (two birds, for ex.), I would then predict
that their gene sequences would be more similar than a morphologically
distinct organism (like a plant, for ex.). This has been spectacularly
borne out by the recent flood of gene sequences -- the correspondence
to trees drawn by morphological data is very high (amazingly high,
IMHO). The discrepancies are never too great and usually confined to
cases where the pattern of relationship was hotly debated. The worst
correspondence (although still pretty good) seems to be amongst
invertebrate animals (rapid radiations give less time for enough
traits to be nested to see a nice pattern, especially since rapid
radiations usually involve heavy duty modification.)
If you doubt evolution at all, check it out on your own. Read
some biology texts (ignore everything that is interpretation
-- read it, but don't believe it with out proof) and make up
your own mind. Go to the zoo and look at animals. Go to botanical
gardens and look at plants. Go to museums and look at fossils. Take
notes -- what features do the organisms have, where did they come
from, etc. Sit down and think, "If everything came from a common
ancestor, what predictions does this make?" and "Does the evidence
bear this out?" The great thing about science is, you don't have to
listen to anybody's opinion about anything; if you are curious, you
can check out the facts and make up your own mind.
In my own highly biased opinion, biology is the coolest
thing since cheeze whiz. The more you study it, the more realize
A.) How diverse and fascinating living organisms are and B.) what
an amazingly descriptive and (most importantly) predictive theory
You had a few other good questions, I'll try to get around to them
in a few days.
PS I mailed you my FAQ. If you don't get it, email me at the below
address and give me an alternate address (I seem to bounce a lot
of mail responding to people on the net.)
Chris Colby --- email: firstname.lastname@example.org ---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
--Kurt Vonnegut from "Galapagos"
E-Mail Fredric L. Rice / The Skeptic Tank