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
"These complex affinities and the rules for classification, receive a rational explanation on the theory of descent, combined with the principle of natural selection, which entails divergence of character and the extinction of intermediate forms. How inexplicable is the similar pattern of the hand of a man, the foot of a dog, the wing of a bat, the flipper of a seal, on the doctrine of independent acts of creation! how simply explained on the principle of the natural selection of successive slight variations in the diverging descendants from a single progenitor!" -- Charles Darwin, 1868, Variation of Animals and Plants Under Domestication, Introduction
"Nothing makes sense in biology except in the light of evolution.", Theodosius Dobzhansky, 1963. "Biology, Molecular and Organismic" American Zoologist: 4: 443-452.
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.
Two (or more) distinct variations in an ancestral population convey their own advantage against the rest of the population
Over time, these two (or more) variations will become more distinct from each other
If they diverge enough, they will no longer be able to mate with each other: will be different species
Divergence can also occur (perhaps more commonly!) if an ancestral population is divided into two or more by changes in geography: because natural selection works by chance survivals, it is unlikely that exactly the same variations of the ancestral population will survival in the two or more separated populations. Over time, if the populations meet again, the accumulation of variations may be significant enough that they are distinct species.
Closely related species are close because their common ancestor diverged relatively recently in Earth history
Other species are more distantly related because of divergences of THEIR common ancestors even farther back in time
No separate origins for different groups; instead, patterns of common ancestry and diverging descendants
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:
Sexual Selection, a variation of Natural Selection recognized by Darwin, where the variation is "being more sexy" (and thus have better than average chance of breeding, and thus passing on "sexiness", compared to other members of the population [increased reproductive success]). Explains many extravagant display structures and behaviors (such as peacock tails, bird song, lion manes, etc.)
Correlated Progression: Ancestor and descendants form a lineage (historical line). Sometimes a particular life habit favors the slight increase in multiple different traits (e.g., longer and longer legs, more compact body, more efficient heart and respiration for fast running; longer and longer necks, longer legs, better cropping teeth and/or grasping tongue for browsing in trees; more and more streamlined body profile, more paddle like legs, more dorsal nostrils, etc. in swimmers; etc.) Traits that go against the general trend will be selected against; traits that go with the general trend will be selected for. (For many people, this series of trends in adaptations represents the totality of evolution)
Adaptive Radiation: If a population evolves some significant new adaptation, or colonizes a region without competitors, or is present when competitors die off, many different variations from that common ancestral population might survive (fill new or unoccupied "niches" (ways of life) in environment). Over a geologically short period time, a common ancestor can radiate into many very different descendant lineages.
Niche Partitioning: during an adaptive radiation, the early members of the divergence will (naturally) still be relatively similar to each other (and to their common ancestor) in terms of size, shape, behavior, etc. Over time, those variations in each lineage that are least like their relatives will more likely survive, because they will have less competition. Consequently, the different species will "partition" (divide up) the niches and the resources.
Convergence: Some adaptations are mechanically advantageous and easy to produce developmentally. Different lineages of organisms can independently develop some of the same features, even though ancestors were quite different (i.e., streamlining in sharks, tunas, ichthyosaurs & dolphins).
Living Fossils: Species do not have a fixed duration, but will persist until the evolve into something else and/or go extinct. In some cases, species (or genera) may persist for extremely long periods of time with no major changes.
Exaptation: Formerly called "preadaptation", the co-option of a structure that previously had some entirely different function for a new use. Seems to be the more common pattern of evolution than the appearance of entirely novel structures. For example, the wings of birds and bats were initially arms and hands; the mouthparts of various arthropods were legs; etc.
Heterochrony: Evolution by changes in rate of development from embryo to adulthood. Two major forms of heterochrony:
Paedomorphosis: descendant populations will retain some juvenile features into adulthood
Peramorphosis: descendant populations will develop structures beyond the adult form of ancestor
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:
Kin Selection: or, "Blood is Thicker Than Water." From a gene's perspective, protecting or nurturing close relatives with the same genes as you can be as effective in spreading additional copies of that gene. So behaviors that favor cooperation between kin might be selected for if those behaviors wind up promoting the transmission of those genes (in comparison to populations which do not have the cooperative behaviors).
Reciprocal Altruism: or, "You Watch My Back, and I'll Watch Yours." In some circumstances behaviors can be selective where individuals
keep a look out for each other (or similar type of altruism (helpfulness)) so long as they get the same benefit from other members of that population.
For example, if there are "cheaters" (ones that take advantage of others looking out for them, but that don't waste their own energy keeping a lookout)
than those populations may get additional predation (because of the predators who attacked when the cheaters could have seen them and warned others.)
If there are other populations of the same species in which the reciprocal altruistic behaviors are present, those populations will have fewer losses
and so prosper relative to the populations with cheaters.
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.
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:
"A population is said to be in an evolutionarily stable state if its genetic composition is restored by selection after a disturbance, provided the disturbance is not too large. Such a population can be genetically monomorphic or polymorphic." -- Maynard Smith (1982).
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:
The origin and evolution of embryonic development
How modifications of development and developmental processes lead to the production of novel features
The role of developmental plasticity in evolution
How ecology impacts in development and evolutionary change
The developmental basis of homoplasy and homology.
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.