Evolution – Biology 4250
Review Sheet 4 -Test 4
Chapter 16: Mechanisms of Speciation – Divergence
Species Concepts: All agree on one point – a "real" species has evolutionary independence,
which, in turn, means a boundary for spread/sharing of alleles.
Morphospecies concept: main criterion (though clearly shared with the PSC) is morphological
similarity. This was the "original" species concept.
Again, as with the PSC, the problem of what constitutes significant enough difference still exists.
Application tends to be somewhat arbitrary, and what are called cryptic species may easily be
Biological Species concept: main criterion is INTRINSIC reproductive isolation.
This is a useful
criterion, as lack of interbreeding confirms the lack of gene flow.
The main problem(s) with applying this concept is (are) . . . ?
(asexuality, allopatry, extinct species, hybridization [in plants])
Phylogenetic Species concept: main criterion is monophyly; a shared
history and now evolutionary
independence from all other species.
The idea here is that populations are looked at and many characteristics are compared to
determine if two different populations are monophyletic, and more than that, monophyletic at a level
that indicates little if any differentiation between them. Comparisons of characteristics can, of course,
include molecular comparisons: DNA, protein structure, etc., If the two populations haven’t diverged
"significantly", then they may be considered the same species.
The main problem(s) with applying this concept is (are) . . . ?
(requires phylogenies for close relatives, criterion standardization, or necessary non-
standardization [not all characteristics are of equal importance]; so application becomes the main
As indicated above, the application of molecular techniques has added both protein similarities
and DNA similarities to the list of characteristics that can be compared. Although you might think that
this should illuminate everything in terms of who is most closely related to whom, and who is
DISTINCT from whom, this has been far from reality, and the Molecular Species concept has not
gotten wide support (yet). [Boy, I’ve gotta’ get me a tricorder . . .]
The different concepts have different efficacy among different groups of organisms. The
be differently applied by different evolutionary workers. As we learn more, even our own opinions
about species evolves. The same organism may be considered independent by application of one species
concept and not by application of another. End result: there will never be a single classification that is
agreed upon by all. (Don’t lose sight of the fact that the ORGANISMS don’t care about what we call
them taxonomically, and that we are trying to apply static concepts [a snapshot in time] to dynamic
situations . . .).
Applying species concepts: Examples
African elephants -- studies such as this one clearly influence conservation issues
Mechanisms of Isolation – stopping gene flow between pops.; the first step in speciation
Physical Isolation – the establishment of allopatry
Geographic (vicariance) events, Peripheral populations, Population establishment through
dispersal and colonization, dispersal on "islands" (island hopping with the founder events)
Island Founder (radiation) events: Drosophiliids (Hawaii); Darwin’s Finches (Galapagos).
Predictions: Closest relatives should be on closest islands, and younger species on younger
Vicariance events – may include mountain building, land bridge formation, canyon formation
(river cutting), habitat fragmentation (drying, lava flow, etc.).
Snapping Shrimp (Carribean & Pacific; Panama; see text); Tufted Squirrels (Kaibab and
Abert’s; Grand Canyon)
Chromosome Change –
Examples: Polyploidy and hybridization (mostly in plants); autopolyploids/allopolyploids.
Estimates of polyploidy in plants vary, though conservatively 300,000+ species surveyed,
2-4% at least showed direct evidence of polyploidy; clearly, this is a tremendously
important speciation mechanism in the history of plants. Animal examples much less
frequent (though see the discussion of fish genus Barbus on pg. 616)
Smaller changes – chromosome fragmentation/fusion. Must be careful here, however, in
claiming causative (versus after the fact) effects in speciation. Dik-diks, horse/donkey,
various butterfly species, etc.
Mechanisms of Divergence: Already discussed in great detail earlier in the course.
Definitions: Allopatric, Parapatric, Sympatric speciation.
Genetic Drift – only important in small populations; importance has probably been overstated in past.
Many small populations of many organisms established by humans have failed to show any significant
Natural Selection – one example to recall: host plant differentiation; the Apple and Hawthorn maggot
fly (Rhagoletis) story. Apples introduced to U.S. from Europe approximately 300 years ago. The flies
on the two hosts are incipient if not full species at this point (even though a bit of gene sharing is still
going on). A Sympatric speciation event. For species which are host specific (herbivores on plants,
parasites on animals), a host shift could be an important part of the speciation event (see Table 16.1).
Another possible example of sympatric speciation involves temporal separation – the Callosamia
story (another Georgia example).
Sexual selection – back to the Hawaiian Drosophila; males fight in locations where there are leks,
and novel traits (broader head) that help you "win" should be selected for (Fig. 16.10)
Possible outcomes: populations fuse (no significant divergence); populations treat each other as
completely separate species (species level divergence); hybridization followed by fusion; hybridization
with reinforcement; or even hybridization with resultant new populations/species (again, most frequent in
Reinforcement – occurs in the hybrid zone; involves prezygotic (premating or postmating) or
postzygotic isolating mechanisms. Will give example of pre- and postzygotic isolating mechanisms in class
which you will be responsible for!
In the long run, which should be selected for, in all cases?
As might be expected: prezygotic isolation evolves much faster in sympatric species pairs than
allopatric species pairs in the Drosophila.
Hybridization – reinforcement should occur when hybrid offspring have reduced fitness. But what
should happen if hybrids survive and reproduce well?
What determines how wide, how long-lasting, etc. hybrid zones are? Fitness of the parental and
hybrid individuals, which in turn also helps determines the eventual outcome (reinforcement with complete
divergence, fusion, etc.). See Table 16.3.
The Genetics of Speciation (Isolation and Differentiation)
Genetic models (and real organisms) have shown that large-scale changes are unnecessary for
divergence and speciation to occur.
Examples: Leopard Frogs, Three-spined Sticklebacks
Chapter 17: Origin of Life and Precambrian Evolution
The origins of life were somewhere between 3.7 – 4 bya.
No direct evidence for origin of life – no physical record exists; as such, the beginning of life on
Earth can only be studied using indirect evidence alone (where's a time machine when you
So, important questions arise:
1. What was the first living thing?
2. Where did the first living thing come from?
3. What was the environment like when life first appeared on the planet?
4. What was the last common ancestor (cenancestor or LUCA) of all living things?
At Earth’s formation (4.5 – 4.6 bya), conditions too hot for life. An
early impact with another large body
added to the Earth's size, but kept Earth uninhabitable. This early impact did form our moon, and, more
importantly, gave Earth its stable spin on its axis, which would later, of course, shape our day length and
the evolutionary effects of this on organisms. As smaller planetismals finished colliding to form earth, and
collisions with other "rocks" slowed, earth’s crust cooled. Water vapor released from planet’s interior
condensed during cooling with LOTS of rain to form oceans. The timing of these events is not completely
worked out, but most evidence gathered suggests the above did happen during the first few hmy.
Discovery of "ribozymes" – Altman and Cech (won a Nobel Prize for this). The original ribozymes
discovered could break/reform nucleotide bonds. This discovery, indicated that RNA, which can
store information, could also do biological work – RNA, in essence, can possess both genotype AND
phenotype. It is possible, indeed likely, that RNA formation preceded formation of both proteins and DNA
in terms of its original formation (and with enough stability, since RNA can fold back on itself and give it a
characteristic shape and, of course, its functionality as a ribozyme; see Fig. 17.2).
We now consider the ability to evolve an important characteristic of life – the ability to record and
make alterations (function of the genotype) and a way of distinguishing valuable changes from detrimental
ones (function of the phenotype). In this respect, RNA can be considered to "have" this characteristic of
life -- the ability to EVOLVE.
Besides the existence of ribozymes (which can make more RNA), the ubiquitous ribosomes
(themselves made of RNA [and protein]) are an extremely highly conservative structure in cells. (What do
ribosomes do again, and how do they do it? . . . ) RNA is intimately involved in the most basic function of
protein synthesis. And, the main ENERGY currency of cells (that would be . . . .?) is based on an RNA
nucleotide, not to mention other important molecules (like NAD and FAD). So there is plenty of evidence
that RNA may be truly ancient. And, don’t forget retroviruses, that inject RNA, complete with a transcript
to make reverse transcriptase in the host.
Experimental evolution of RNA predated discovery of ribozymes by about 15 years – a bacteriophage
(virus) had its RNA replicated in a test tube by replicase enzymes from the virus. Seeding fresh test tubes
with replicated RNA and continuing to treat with replicase resulted in independently different RNA strands
(due to miscopy by replicase), and with differing abilities to infect bacteria. The most abundant versions of
RNA ended up being those that were most quickly replicated by the enzyme – natural selection of
molecules. With the discovery of ribozymes, it was further demonstrated that RNA molecules can have a
particular "fitness" (how well they catalyze and in turn "make" more), and that the fitness is a phenotypically
expressed function – one ribozyme with poor DNA manipulative (cleavage and attachment) capability,
when amplified with enzymes (which in turn allowed for mutations) resulted in a version with mutation at
four loci that "manipulated" DNA >100 times faster. Indeed, ribozymes with improved or entirely new
functions have evolved many times in test tube environs. When we go back to the "four assumptions" of
Natural Selection, the first two are about the genotype, the fourth about the phenotype, and the third
(overproduction of offspring) is about being replicative. So, RNA can fit 1,2 and 4; what about 3?
Self-Replication – can RNA do this?
One problem with an "RNA First" world – in the above experiments, protein enzymes were use for
the replication of RNA strands. Ooops. So far experiments have failed to produce a truly self-replicating
RNA, though short sequences within RNA molecules appear selectively advantageous in certain experiments.
IF we can discover/select for an RNA that is truly self-replicating, then we could move on to the OTHER
remaining important questions about origin of CELLS:
1. Can a self-replicating RNA evolve a sister (more stable) DNA molecule with replicative and
2. Could the DNA and RNA in turn work together to form proteins (what about them ribosomes?)?
3. Could this machinery, in turn "make" or "take over" cellular structures and cells?
Still a LOT of unanswered questions of HOW to get from one step to the next. That doesn’t mean
it couldn’t have happened (though it can’t happen again in nature on the current Earth – Why?)
The Original Biomolecular Evolution – the Organic from Abiotic?
If we assume organics came from the Earth’s abiotic environment, then:
1. There must be some way of making these organics (without living organisms).
2. There must be a source of energy to build these larger molecules.
3. These larger organics must be able to polymerize.
4. These larger organics must be stabilized – they’d have to persist in a harsh environment.
Two major alternative hypotheses exist on the source of original simple organics:
1. Abiotic C, H, O & N components of early atmosphere combined somehow – debate rages
over the contents of the atmosphere and water early in Earth’s history. After all, conditions were
extremely harsh, and it is unclear what the pressure, temperature and early atmospheric components
2. Earth was "seeded" with organics from space; cometary dust, meteorites (still leaves the
question of where and how these organics formed). For example, the 1969 Murchison, Australia
meteorite contained several amino acids (glycine, alanine, valine, proline, etc.; pg. 651). Since life
produces almost exclusively L-stereoisomers of A. A.'s, and the meteorite containedapprox. equal
amounts of L- and D-stereoisomers, then the source of the A. A.'s was unlikely biological.
Experiments: Miller (1953, 1992); Fox 1972 – ammonia, methane, and H. Provide sparks
(lightning)/UV radiation – tremendous number of organics (A.A.’s, sugars, nucleotides). However,
better understanding of the geochemical processes suggest early atmosphere contained – CO2 and
N2, much less conducive to organics formation.
Still, the Oparin-Haldane Model was formulated, and can at least be used as a null hypothesis
against which to test new findings. The model basically approaches the three questions (and four
statements listed above): 1. – biotic from abiotic; 2. – polymers assembled that can a) store information
and b) catalyze reactions; 3. – add membranes (and energy sources) to get a living organism (Fig. 17.12).
1. Biotic from abiotic: So far, experiments have not produced large amounts of one stereoisomer
only of nucleotides or amino acids. Nor have large amounts of ribose (as opposed to other sugars) been
made. Nucleotides can attach at several places to ribose (and do so). And without a chemical energy
source, no polymerization takes place. Getting to specific, self-replicating RNA molecules (with
appropriate chirality) seems difficult to say the least – perhaps other self-replicating systems evolved first
(3rd paragraph, pg. 657) that in turn promoted development of RNA chains. Certainly, we don’t know
2. Polymerization: in water, although monomers can easily be polymerized (with an energy source),
they also hydrolyze rapidly. Need some stability – how? Ferris, et al, have demonstrated that the
aluminum-silicate clay mineral Montmorillonite slows hydrolyzing decay, and short polynucleotides have
been formed (and remained) on clay surfaces; with repeated application of more nucleotides, the clay
catalyzes the reaction and the polynucleotides can be made longer, now up to 50 nucleotides long.
Ferris and others have similarly assembled polypeptides up to 55 A.A.’s long. So, certainly more
experimental support than for step one. Interestingly, the oldest known exposed rocks on Earth (3.7
byo) already contain evidence of life. Older rocks with evidence for life may be very difficult to find
due to erosive/tectonic/volcanic disruptions, and the conditions were unlikely conducive to life on the
planet much earlier than that.
3. Life becomes cellular -- the ancestor of all extant organisms: it seems pretty clear that,
regardless of how many times life may have "started" on the planet, that eventually one main
interbreeding lineage may have become THE one – the ancestor of all living things on the planet. We
all use the same coding mechanism, the same A.A.’s, etc., and this machinery is housed in a membrane
– a cell.
Many experiments, particularly by Fox, et al, have generated protein containing "membranes",
with, characteristics of cells (pick up other molecules, "division", etc.). As you already know,
phospholipids thrown in water spontaneously organize into bilayers; these protein/phospholipid
membranes can fuse. Still, even if "taken up" by these "cells", how do DNA/RNA take control? Still
unanswered by anything experimental.
Who is/are the Common Ancestor(s)
Direct fossil evidence of life begins between 3.2 and 3.5 bya, and appears to be similar to extant
cyanobacteria (photosynthetic); this is already too advanced to give us a view of the early common
ancestor(s) (cenancestor). Fossils can’t help us here.
Phylogenetic reconstruction, however, can help out – we look for the synapomorphies to assemble
the Tree of Life. However, morphological characteristics of the primitive prokaryotes lacked enough
diversity to assemble the tree. DNA/RNA/ Protein sequencing techniques give us much more character
diversity – similar (but not identical sequences – why?) for the same gene. The more the differences, the
longer the lineages have been apart (straightforward) – a sort of "molecular clock". Of course, if you
want to do this for all life, you need a gene that codes for the same, and an essential, functioning molecule
in ALL living things – the gene for small subunit ribosomal RNA is such a molecule.
The phylogeny intimated by this rRNA, which is counter to the five kingdom classification of life, is
shown in Fig. 17.18, 17.21, and 17.23-24 (but see fig. 17.22). Prokaryotes have two VERY distinctive
separate evolutionary lines, one including virtually all the commonly known types, the other represented
by "extremists", living in various harsh conditions (Archaea). Interestingly, these Archaea (formerly
Archaebacteria) are apparently more closely related to Eukaryotes than the "typical" Bacteria. Eukarya
are relative newcomers, with less than 10% of the variation seen in the rRNA small subunit gene.
Interestingly, in analyzing some genes, some Archaea actually have what appear to be Bacterial
genes – why? Because they ARE bacterial, picked up by the organism from another. This can happen
with these prokaryotic forms (lateral gene transfer). As for the common ancestor, all evidence points
to a rather sophisticated, evolved organism, or SET of organisms, not unlike modern bacteria, sometime
as early as perhaps 4 to as recently as 2 bya. It may very well be that extant life's history does NOT
originate with a single life form, but an interconnected set of roots to the tree, a community of interacting
species readily swapping genes (which, needless to say, makes for an interesting base to the phylogeny
-- see Fig. 17.26
(Read pages 673 – 675 for date of oldest known eukaryotes and cyanobacteria [almost identifiable
to extant forms, and apparently largely unchanged for 2 by])
Hypotheses for the evolution of the "Tree of Life" -- the descent from the "common" ancestor
1. The Universal Gene-Exchange Pool -- three ancestors of todays domains emerged from the
pool at different times, with later emergence of Archaea and Eucarya making them appear more
more closely related.
2. The Ring of Life -- the first eukaryote arose by the fusion of an archaen and bacterium. Less
support for this hypothesis (though see discussion of mitochondria, below), as there are
significant gaps in molecular development with this theory, and no cytoskeletal control in
either makes it difficult to envision the necessary phagocytotic event
3. The Chronocyte -- an independent organism that developed a cytoskeleton and phagocytotic
capabilities. Eventually, a digestion-resistant archaen is eaten and becomes the "nucleus".
Several holes in this theory as well, including lack of evidence for such a chronocyte.
4. The Three Viruses/Three Domains -- viruses, with RNA gene for reverse transcriptase, in
turn generates host DNA from host RNA. Host, with developed resistance, in turn has more
stable DNA than RNA, and DNA becomes THE molecule.
Origin of Organelles
Mitochondria, widespread but not universal in Eukarya, and chloroplasts, more restricted in their
distribution, are therefore not defining characteristics of the Eukarya. Where did they come from? Both
have their own DNA, and analysis puts chloroplasts rRNA small subunit gene with the cyanobacteria,
and that for mitochondria within the Bacteria as well. Lynn Margulis’ endosymbiosis theory is correct!!
Chapter 18: The Cambrian Explosion and Beyond
The Nature of the Fossil Record: Understand how organic remains fossilize, and that occasionally
flesh can be preserved under very restricted conditions. Understand, also, that, like any data set,
fossils present a biased look at the history of life, for several reasons. Different organisms/organs
fossilize or don't (presenting a DISTINCT taxonomic bias in the fossil record), different conditions
at time of fossilization and after fossil formation, different access to fossils, etc. See * below.
KNOW the following:
Timeline – pages 692 & 693:
Time (mya) for beginning and end:
Proterozoic – Precambrian times
Paleozoic – with Cambrian, Ordovician, Silurian, Devonian, Carboniferous (M & P), Permian
Mesozoic – with Triassic, Jurassic, Cretaceous
Cenozoic – with Paleocene, Eocene, Oligocene, Miocene, Pliocene, Quaternary
Know Laurasia, Gondwana, Pangaea, India colliding with Asia, beginning and end of
Appalachians, Alps/Himalayas rise, formation of landbridge between North and South America
Know coelomate/acoelomate, diploblast/triploblast, protostome/deuterostome
Know times for:
First shelled organisms
First chordates and first vertebrates
First tetrapods (amphibians), reptiles, mammals, birds, placental mammals
First hominids, appearance of Homo (sapiens)
First insects, and first winged insects
First land plants
First vascular plants (ferns), first seed plants, first flowering plants
You should also understand connections, such as why land animals came after land plants, and
why insect diversification really took off after evolution of flowering plants
*Understand what we know comes largely from fossils, which have some inherent biases
(geographical [require some minimal sedimentation at least], taxonomic [organisms with hard
parts preserved better], and temporal [older fossils less numerous]).
Some famous fossil deposits:
Ediacaran Hills in south Australia
Burgess Shales in British Columbia
Chengjiang deposits in Yunnan province, China
Was Cambrian Explosion really Explosive?
Molecular clocks suggest derivation of some lineages older than fossils suggest (NOT a
surprise). Older fossil finds starting to confirm these older dates.
Even so, there DOES appear to be a very rapid evolution of morphological traits, not only
in complexity (meaning more complex developmental program) but size as well. Increasing
oxygen levels, more ecological interactions, etc. may have driven this evolution, but still there
would have had to have been some significant genetic variability available to ALLOW for this
Macroevolutionary patterns -- know the following:
Radiations (why, for instance, mammals radiated significantly after the K-T asteroid)
"Stasis" and "zigzag" evolution
Punctuated Equilibrium (stasis [in morphology] followed by rapid diversification).
Mass Extinctions: The "Big Five" – especially the K-T asteroid
Human driven Extinction (see middle paragraph page 719)
One last tidbit:
C values (page 576 and handout) -- total DNA per cell does NOT correlate with organisms
perceived morphological complexity or phylogenetic position.