Author: Chris Colby (colby@bu-bio.bu.edu) Title: Some Evidence of Common Ancestry 1.] Comp
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Author: Chris Colby (colby@bu-bio.bu.edu)
Title: Some Evidence of Common Ancestry
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1.] Comparative genetic and biochemical data
DNA or protein sequence comparisons of closely related species
(as determined by morphologists) yeild similar sequences.
The genetic code is read in blocks of three nucleotides. A nucleotide
is a single "letter" of the genetic code, each block of three is called
a codon. Each codon signifies an amino acid. Amino acids are the sub-
units of proteins just as nucleotides are the subunits of DNA. When
a protein is needed by a cell, the DNA of the gene is transcribed
into RNA in the nucleus of the cell. The RNA is then transported
out into the cytoplasm where it is needed and proteins are synthesized
by cellular organelles (ribosomes) that "decode" the RNA and make the
appropriate protein.
All living organisms use DNA as their genetic material(*). In addition
the three letter code is the same for all organisms, in fact it is
called the universal genetic code.
(*) some viruses use RNA
The universal genetic code is a redundant code; that is there are
more possible codons than are needed. So, most amino acids are coded
for by several codons.
Gene sequences from closely related species show that at homologous
positions within a given gene the same codon is most often
used for amino acids. If, however, there are differences
they are usually in these "silent" sites -- nucleotide positions
that can be changed without changing the amino acid they code for.
In addition, the genome is loaded with 'dead genes' called pseudo-
genes (of which there are several classes). Pseudogenes occupy
the same location in the genome in closely related species. The
same can be said for introns, intervening sequences of DNA that
do not code for anything. Introns are spliced out of the RNA prior
to translation so they do not contribute information needed to
make the protein. They are sometimes, however, involved in
regulation transcription of the gene from DNA to RNA.
Evolutionary biologists take the above mentioned patterns of
similarity as evidence of common descent. All organisms use
DNA (and the universal genetic code) because they are descendants
of an ancestral organism that used these. The same redundant
codons are used by closely related species because their common
ancestor had it. Pseudogenes and introns are located in the
same positions in closely related species because both inherited
their gene/intron placement from their common ancestor. This
pattern of genetic variation is what one would expect if lineages
accumulated genetic changes (mutations) and occasionally split
(i.e. speciated).
2.] Comparative developmental biology --
Closely related species share similar patterns of ontogeny
(development). The first stages (for ex.) of mammal development
are extremely similar for all mammals -- differentiation occurs in
later stages.
In addition, the ontogeny of an individual organism yeilds clues
about the ancestral species it evolved from. For instance, why should mammals
(including humans) develop gill slits as a stage in their development?
Evolutionary biologists would say this is because mammals evolved
from organisms that had gills. In order for modern day mammals to
develop, they must pass through a stage where the gills begin to form
in order to get to their present form.
Present day organisms are modified descendents of their ancestors.
Traces of their ancestry cannot be erased from ontogeny because there is
no evolutionary mechanism for starting an organism from scratch. Natural
selection can only modify existing traits of an organism. So, modern
species adapt to their environment (if neccessary) by modifying their
ancestral species ontogney. Since changes early on are more likely to
be deleterious because all subsequent development is dependent on early
growth, organisms pass through stages of development that are
reminiscent of the stages of develop ment of their ancestors.
Thus, early development is highly conserved so it
appears as if mammals (for example) share an early developmental pathway
then diverge from it. Remnants of an organisms phylogenetic history that
serve no present purpose are called vestigal structures. A good
example of a vestigal structure are the rudiments of legs still seen
in whale skeletons.
3.] Comparative Morphology/Anatomy --
Groups of related organisms are 'variations on a theme' --
the same set of bones/tissues/whatever are used to construct all
mammals. The bones of the human hand grow out of the same tissue
the bones of a bat's wing or a whale's flipper does and they
share many identifying features (muscle insertion points, ridges).
The only difference is that they are scaled differently. Evolutionary
biologists say this indicates that all mammals are modified
descendents of a common ancestor.
4.] The heirarchical organization of biologic diversity
For evidence of common descent, look at the heirarchical
distribution of various biological traits. DNA sequence data is
extremely good for this sort of thing, but all the above examples
also fit. Why, for instance do only plants with other angiosperm
characteristics have flowers? If flowers are a useful character-
istic, why don't we see an occasional fern or moss with a
flower? Why don't we see any eubacteria with histone proteins?
The grouping of biological traits only makes sense when you
assume all organisms share a common ancestor, and subgroups of
organisms share common ancestors and so on. Traits are thus
"linked" via common ancestry.
It's not just that similar species share similar traits (although
that is evidence in and of itself); when you look at large groups
of organisms, biological traits are heirarchically distributed.
for example:
KINGDOM ANIMALIA
/ \
Parazoa Eumetazoa
| | \
| Radiata Bilateria
| | |\ \
| | | \ \
| | | \ \
| | | \ \
| | Acoelomates Pseudocoelomates Coelomates
| | | | | \
| | | | Protostomes Deuterostomes
| | | | | |
sponges cnidarinas flatworms nemotodes arthropods echinoderms
ctenophores probiscids rotifers annelids chordates
mollusks
________________________________________________________________
(all the above animals have a more complex devlopmental plan
than sponges)
____________________________________________________
(all the above animals develop three germ layers
in early development, they are also (with a few
derived exceptions) bilaterally symmetric)
________________________________________
(all the above animals have a body
cavity)
________________________
(all the above animals
have a body cavity
lined with mesoderm)
________ _________
blastopore blastopore
becomes becomes
mouth anus
..... ......
spiral, radial,
determinate indeterminate
cleavage cleavage
Note: not all animal phyla are represented above, just the more
well known ones. Heirarchical distributions of biological traits
is seen at all levels you care to look at (even within species).
It is due to common ancestry of groups and groups within groups
and groups within groups within groups and etc.
Another way of looking this is, if you see an organism with one
trait -- you can specify several others traits it will (probably)
have. From the diagram above you can see that if an organism undergoes
spiral cleavage, it will also be determinate; the animal will
follow the deuterostome developmental pathway and it will have
a true coelom (mesoderm lined body cavity) and so on.
I should point out that a heirarchical pattern to biological
traits does not neccessarily mean the mechanisms of evolution
need to be heirarchical to produce them.
This pattern of diversity (which is seen both within and among
species) is predicted if species share a common ancestor and
lineages split and diverge with time.
5.] Evolution is a successful predictive theory. The hierarchy
explained above is not only an observation, but a prediction.
Any new traits surveyed should fall with previously established
hierarchies. Recent advances in molecular biology have shown
what a solid, predictive theory evolution is. Phylogenies used
to be based primarily on morphological data. With the advent of
molecular biology (esp DNA sequencing) another character was
available to systematists. Phylogenies drawn from DNA sequences
match morphological trees in almost all cases. In the studies
where they do not, usually there has been some argument about
classification before the sequencing was done.
[A later clarification:]
> when you look at large groups
>of organisms, biological traits are hierarchically distributed.
>
>sponges cnidarinas flatworms nemotodes arthropods echinoderms
> ctenophores probiscids rotifers annelids chordates
> mollusks
> ________________________________________________________________
> (all the above animals have a more complex devlopmental plan
> than sponges)
> ----------------------------------------------------
> _______________________________________
> ________________________
[deletions]
In my response to McGrath, I included the above (now edited) diagram
intended to show a hierarchy. In it I only followed the subdivisions
down one "path". The end result was protostomes (arthropods and etc.)
and deuterostomes (echinoderms and etc.) had the most subdivisions.
I should point out that this is only a consequence of following that
particular path -- I did not mean to imply that outgroups (where I did
not fill in any further information) were more primitive or less
evolved. I could have followed any (or all) lineages and showed
different subdivisions (and therefore a hierarchy), but I singled
out the path leading to more familiar organisms because I thought
the characteristics might be a bit more, well... familiar.
I could have just as easily (if I knew anything about sponge
taxonomy) started out with "sponges" and "other animals" and
continued to divide up the sponge phylum (Porifera). All
lineages of organisms have equally long evolutionary histories.
Within any phylum you can find species with highly derived
characters along with less derived characters. In fact,
all species are a mix of derived and ancestral characters, that's
where the hierarchy bit comes from. In the diagram I simply
chose to mention increasingly derived characters in the path
leading to chordates.
So, in any case, I didn't mean to imply an _overall_ more/less
specialized direction to the diagram. If I was a sponge or a
jellyfish, or writing for an audience of sponges or jellyfish,
I would have filled in the diagram with different information.
(I got some email telling me I was implying "higher" and "lower"
so I thought I'd try to clear that up.)
E-Mail Fredric L. Rice / The Skeptic Tank