GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2020
Macroevolution in the Fossil Record

Detail of figure of beaks of Darwin's ground finches (Geospiza spp.) from the Galápagos. Photograph by Andrew Hendry, from blogpost ""The Living Dead: Darwin's finches and museums"" on the blog Eco-Evo Evo-Eco

Key Points:
•Macroevolution is term for evolutionary changes recognized above the species level.
•One major macroevolutionary pattern testable in the fossil record is whether species-level change is mostly gradual (phyletic gradualism) or in short bursts separated by long period of stasis (punctuated equilibrium).
•Under phyletic gradualism, then species don't have real beginnings or ends, and are just "chronospecies" with anagenetic origins; under the latter, the case can be made that species are "real" rather than purely arbitrary.
•In recent decade it has been recognized that evolution is highly controlled by development: this is represented by the new study of "evo-devo".

"Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against the theory." -- Chapter 9 "On the Imperfection of the Geological Record", On the Origin of Species by Means of Natural Selection (1859), Charles Darwin

"Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists -- whether through design or stupidity, I do not know -- as admitting that the fossil record includes no transitional forms. The punctuations occur at the level of species; directional trends (on the staircase model) are rife at the higher level of transitions within major groups." --The Panda's Thumb (1980), Stephen Jay Gould

"A plausible argument could be made that evolution is the control of development by ecology. Oddly, neither area has figured importantly in evolutionary theory since Darwin, who contributed much to each." --"Festschrift" (1973), Leigh Van Valen

Macroevolutionary Patterns
With the discovery of evolution by natural selection, biologists from Darwin and Wallace's time onward have documented many different patterns and processes in evolution. Sometimes they refer to "microevolution" (changes within an species) and "macroevolution" (patterns on the larger scale; changes from one species to another, or between different lineages of ancestors and descendants). It is important to remember that "micro-" vs "macro-" is just a matter of scale and perception: at the level of individuals and populations, there is just variability, heritability, and superfecundity.

The most important pattern: the Tree of Life. Darwin and Wallace demonstrated the reality of Divergence through Time and Common Ancestry:

Thus, the basic pattern of the history of living things is a Tree of Life, where the trunk and stems are lineages of ancestors, the branching points representing divergences between lineages, and the tips of the branches living species (or extinct species that died without descendants).

Other important patterns and processes:

History of Evolutionary Models
Non-natural selection evolutionary models in the early 1900s:

Following these early attempted modifications of Darwinism, the rest of the 20th Century onward stayed largely within a Darwinian model. However, there were different major schools of thought. Many of these differences hinged on views of microevolution (evolutionary change within a species) and macroevolution (evolutionary change above the species level). While most agreed that the ultimate processes in macroevolution were ultimately microevolutionary, there were disagreement whether the patterns produced were actually reducible to microevolutionary changes.

First major school: New Synthesis (also called Modern Synthesis and Neo-Darwinism): product of collaboration of work by geneticists, statisticians, and traditional organismal biologists (including paleontologists) (but notably NOT embryologists!!)

Primarily paleontologically-generated challenge to New Synthesis: Punctuated Equilibrium:

What are the Relative Effects of Biotic vs. Abiotic Factors in Macroevolution?

Are the New Synthesis and Punc Eq Really That Different?
Evolutionary Stable Strategies (ESS): Combination of game theory and behavioral ecology. An ESS is a strategy which, if adopted by a population of players, cannot be invaded by any alternative strategy. A Nash equilibrium which is "evolutionarily" stable meaning that once it is fixed in a population, natural selection alone is sufficient to prevent alternative (mutant) strategies from successfully invading:

Although ESS proponents (many of them behavioral ecologically-oriented neontologists) and Punc Eq proponents (mostly paleontologists) often argued (especially in the 1970s-1990s), ESS provides a likely explanation for stasis: during punctuation events, the environmental changes are so extreme that one or more mutants actually does do better than the previous ecologically stable state

Evo-Devo and the Source of Evolutionary Novelties
Since the mid-1990s, emphasis on the interrelationships between development (as studied in embryology), genetics, and whole organism biology (esp. paleontology as record of Life's changes). Name given to this field (at conference at UMCP!): Evo-Devo (evolution & development).

Following Wikipedia's definition (!), it compares the developmental processes of different organisms in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. Evo-devo addresses:

Evo-devo reflects the discovery that there are developmenetal genetic toolkits (such as the HOX genes of animals) that control the timing, sequence, rate, and duration of embryological changes. Modification of these genes (first seen in homeotic mutants) can produce both minor and major morphological variations that can be acted on by natural selection.

Evo-devo also reveals deep homologies: while some organs may be the product of convergent evolution in different lineages (classic example is the eye) and are thus analogous structures, the tissues from which the eyes are made, and the developmental genes that control this, are often homologous at a much more ancient level.

To Syllabus.

Last modified: 2 September 2020

Evolution of smaller body size on the way to birds, by Davide Bonadonna, from Lee et al. (2014) doi: 10.1126/science.1252243