GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:
The Fossil Record of Vanished Worlds of the Prehistoric Past

Spring Semester 2012

Darwin & Beyond: Tempo and Mode in Evolution

Major Patterns of Macroevolution
Macroevolution: term for evolutionary patterns at and above species level. Since "species-level" is a difficult thing to define in a consistent way that actually applies to Nature, macroevolution can be thought of as higher-level effects of evolutionary change.

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:

Extinction, Mass Extinction, & the Game of Life
Extinction is the termination of a lineage. (If a species "dies out" by evolving into another species, this is more properly called a pseudoextinction). Extinctions occur throughout Earth History. What is more remarkable is mass extinction: the geologically-sudden disappearance of many diverse groups of organisms, which are not immediately replaced by ecological equivalents. Some mass extinction events seem to correlate with major disruptions of Earth's environment.

We will explore mass extinctions in far more detail later this semester. But for now, it is worth pointing out that mass extinctions may be bad for the taxa that die out, they are great opportunity for the survivors. Certain niches (apex predators, reef makers, etc.) have been occupied by very different groups at different phases of Earth's history.

We can think of the large scale interactions within the evolving biosphere as a series of different Games of Life. Each particular Game represents some extremely broad set of interactions (rules). Introduction of a new Game into the "game room" (i.e., the totality of Earth's ecosphere) requires some major change: perhaps a new adaptation for interacting in a novel way, or the development of an entirely new way of life. In each game there are different roles: i.e., different ecological niches such as "apex predator" or "largest forest tree" or "reef framework builder." The organisms in the different roles interact with each other according to the rules of the game. But the players in each role change through time: sometimes one species may inherit its role from its direct ancestor, but sometimes the new player might come from an ancestor in a very different role. Mass extinctions represent times when many roles might be "up for grabs"; many niches are vacated, and entirely different players might take up that role. (Similarly, when new games start, it is an opportunity for players from very different ancestors to take their spot.).

The Selfish Gene: Evolution from the Gene's Eye View
When discussing evolution, we often concentrate on physical attributes (traits), or the populations, or the lineages of organisms, or even ecosystems and the like. But evolutionary biologist Richard Dawkins is right in pointing out that the only thing that is actually passed on from one generation (and thus the "material" that is being selected for or against with each selection event), is really just the gene (or coalitions of genes). By shifting emphasis from the macroscopic scale to the gene itself, natural selection be be considered to favor those traits which maximize copies of that allele into the next generation. This became the main focus of his book The Selfish Gene (which he regrets not calling "The Immortal Gene", which gets the point across better: genes out living the body of each generation.)

Selfish gene approaches -- that is, trying to understanding evolutionary events from the point of view of alleles in competition rather than bodies -- gives an important set of explanations for certain aspects of animal behavior. For example, why should animals have gregarious behaviors: that is, live together cooperatively in groups? After all, individuals within the same species have the greatest amount of overlap in requirements for resources, and would this be each others greatest competitors. So why (and when) would natural selection favor living together cooperatively? Two main reasons that--in some circumstances--cooperative group living might be favored:

And what about sex? Sexual strategies of the two sexes are very different: male and female animals have different priorities in terms of reproduction. Males can in principle fertilize many many individuals, while females typically have fewer sex cells (eggs) available at any given time. With less cells to use, females often are "choosier" in terms of mates. So many species evolve displays in which males somehow "show off" (in terms of physical features, ritual motions, combat between rivals, etc.) and females evaluate the display.

For example:

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:

Evo-Devo & Building Bodies
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 a conference at the University of Maryland!): Evo-Devo (evolution & development).

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 developmental 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.

Many of the phenomena discussed above have been studied in fossil as well as living organisms. In fact, mass extinctions in particular are a paleontological field! We will move on to look out what insights the fossil record provides for understanding evolution.

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Last modified: 12 February 2012