Macroevolutionary Patterns
In this lecture we will examine the tempo and mode of evolution, in the words of George Gaylord Simpson (1902-1984).
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Simpson devoted himself to understanding whether macroevolutionary patterns arise from the microevolutionary processes studied by population geneticists.
Simpson showed that major evolutionary developments in the fossil record took place in the irregular and undirected manner expected under Darwinian evolution. |
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Following Darwin, the prevailing view of evolution by natural selection held that evolution is gradual.
Expectation that macroevolutionary changes (= large changes in morphology that define higher taxonomic divisions) accumulate over long periods of time by gradual microevolutionary processes.
| Nevertheless, the fossil record does not always show continuous and gradual changes. |
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Simpson (1944) noted that higher taxa (e.g. orders of mammals) appear suddenly in the fossil record, describing this pattern as "quantum evolution".
Major morphological innovations sometimes appear suddenly in the fossil record, often preceded and followed by periods of relative stasis.
Interpreted as inaccuracy of the fossil record.
Eldredge and Gould (1972) argued otherwise.
(1) The pattern was real
(2) The pattern reflected a process whereby most evolutionary change happens around speciation events.
Punctuated equilibrium model of evolution
This was an extremely controversial interpretation.
[Eldredge and Gould did not argue for instantaneous evolutionary change but rather a concentration of gradual evolutionary change near a speciation event.]
(1) Is the pattern real?
Example: Punctuated change in Bryozoans
Cheetham (1986) examined 1000 fossil specimens from the Bryozoan genus Metrarabdotus, an aquatic invertebrate.
Using 46 morphological characters, Cheetham drew a phylogenetic tree connecting the specimens:
Relatively little change occurred within a morphospecies, while large shifts were observed between morphospecies.
Almost no intermediates were found in the fossil record between these morphospecies.
[Interestingly, Jackson and Cheetham (1990,1994) examined 7 living Bryozoan species from this genus and confirmed that the morphospecies identified differed significantly from one another at a number of allozyme loci.]
Nevertheless, other examples exist of fairly gradual evolutionary change.
For example, Sheldon (19) studied 3458 specimens from eight trilobite lineages.
These lineages showed gradual change of a sufficiently pronounced nature that the specimens at the beginning and end of each lineage would be classified as different species (and in one case a different genus).
Such examples illustrate that punctuated and gradualist processes can both occur.
Reviewing 58 such studies, Erwin and Anstey (1995) conclude:
"Paleontological evidence overwhelmingly supports a view that speciation is sometimes gradual and sometimes punctuated, and that no one mode characterizes this very complicated process."
Eldredge and Gould also argue that "stasis is data", which should play a more prominent role in evolutionary explanations.
(2) What explains punctuated evolution?
Why might morphological evolution be rapid around speciation events?
Why might morphological evolution be relatively static during other periods of time?
Eldredge and Gould's (1972) explanation (following Mayr): Peripatric speciation of a small isolated population might lead to rapid changes in a daughter population (drift), whereas large parental populations remain relatively unchanged.
Gould and Eldredge's (1993) explanation (following Futuyma): Populations are constantly changing, but genetic mixture across populations prevents sustained differences from accumulating. Speciation "locks up" the changes that a population has undergone.
Alternative explanations??
"It is absurd to talk of one animal being higher than another...We consider those, where the intellectual faculties most developed, as highest. -- A bee doubtless would [use]...instincts."Charles Darwin's Notebooks 1833-1844 (B46, 74)
"Progress" is a thorny concept in evolution, since it implies that there is a goal towards which evolution proceeds.
Natural selection and mutation are "myopic" processes: they act in the present and have no foresight.
Nevertheless, change does occur and often follows a particular trend (with exceptions).
A directional trend has been argued to occur along the following axes:
Cope's rule: Body size increases within a lineage over evolutionary time.
This rule has often been explained by the potential advantages of being large: increased defense, mating success, foraging success, improved homeostasis (= sustaining a constant state in a changing environment).
However, we tend to focus on extreme cases where increased body size has clearly increased. Is Cope's rule generally true?
Jablonski (1996) examined 191 bivalve and gastropod lineages over a 16 MY period, in the most extensive study of Cope's rule.
[Figure from Futuyma (1998).]
The body size of the largest species within a genus often increased (top half) but also decreased 36% of the time.
Interestingly, the body size of the smallest species within a genus decreased (left half) more often than it increased (36% of the time).
This suggests that the most prevalent pattern is one of increased variability rather than a trend towards larger size.
How might complexity be measured??
A cautionary note: There is a definite risk in defining complexity that we are simply defining "most human-like".
1. Genome size
| Species | Genome Size* |
| Escherichia coli (bacteria) | 0.005 |
| Saccharomyces cerevisiae (yeast) | 0.009 |
| Drosophila melanogaster | 0.18 |
| Arabidopsis thaliana (a weed) | 0.2 |
| Homo sapiens | 3.5 |
| Triturus cristatus (a newt) | 19 |
| Fritillaria assyriaca (a monocot plant) | 127 |
| Protopterus aethiopicus (a lungfish) | 142 |
[*Haploid genome size, measured in picograms (1 pg ~ 109 base pairs) from Maynard Smith (1989)]
What may account for these differences?
Assuming that the common ancestor to all living organisms had a small genome size (~bacterial in size), it would be easier for mutations and selection to increase genome size than the reverse.
Junk DNA may accumulate as the result of transposable elements (or other repeat elements) copying themselves throughout the genome.
There are more coding sequences in some organisms than others: E. coli have ~4000 genes, yeast ~6000 genes, Drosophila ~10,000 genes, humans ~100,000 genes.
Does the complexity of an organism double if it becomes tetraploid but otherwise looks the same?
2. Number of cell types
The total number of recognizably different cell types is much larger in vertebrates than in invertebrates, plants, fungi, etc.
Nevertheless, there is no evidence that the number of cell types has increased within any of these phyla since the Cambrian (Futuyma, 1998).
Most of the net trend toward an increased number of cell types was established early in evolution (before the Cambrian).
Again, assuming that the common ancestor to all living organisms had one cell, the only direction in which evolution could proceed is up.
"Our strong and biased predilection for focusing on extremes...generates all manner of deep and stubborn errors. Most notable of these misconceptions is the false and self-serving notion that evolution displays a central and general thrust towards increasing complexity, when life, in fact, has been dominated by its persistent bacterial mode for all 3.5 billion years of its history on Earth."-- Stephen J. Gould (1997, Nature 385: 199-200)
It may be frustrating that we cannot draw broad and sweeping generalizations about evolutionary processes.
Evolution may occur rapidly...or slowly.
Evolution may increase size...or decrease it.
Evolution may lead to greater complexity...or greater simplicity.
Yet the resulting view that evolution is a complex process leading to a richness in the forms and varieties of life is, in its own way, satisfying.
"It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms...have all been produced by laws acting around us....There is grandeur in this view of life."-- C. Darwin (Origin of Species, 6th edition)
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