Macroevolution • The origin of taxonomic groups higher than the species level • Concerned with major events in the history of life as found in the fossil record • Includes the origin of new design features such as feathers and wings in birds, upright posture of humans • Examines large scale evolutionary changes
Macroevolutionmajor questions of macroevolution • How do major novel features arise? • What accounts for apparently progressive trends found in the fossil record? • How has macroevolution been affected by global geological changes? • What explains the major fluctuations in biological diversity seen in the fossil record?
Fossils • Sedimentary rocks are the richest source • Formed from deposits of sand (compressed into sandstone) or silt (compressed into shale) • Usually form from mineral rich hard parts of organisms • Petrification – minerals dissolved in the groundwater seep into the tissues of the dead organism and replace organic matter • Occasionally fossils retain organic matter (DNA)
Limitations of the Fossil Record • A fossil represents a sequence of improbable events • A large fraction of species that have lived probably left no fossils • Most fossils that were formed have probably been destroyed • Only a fraction of the existing fossils have been discovered • So… the fossil record is comprised primarily of species that lived a long time, were abundant and widespread, and had shells or hard skeletons
Early Earth Video • Early Earth Video
Geological time scale movie 25T-01-GeologicTimeScale.mov
Dating • Relative Dating • Absolute Dating • Radiometric dating (error of less than 10%) Half-life = Number of years it takes for 50% of the original sample to decay Carbon-14 (half-life = 5600 years) Best for dating material less than 50,000 yrs old Uranium-238 (half-life 4.5 billion years)
Evolutionary novelties (1) how do new designs evolve? • Higher taxonomic groups such as families and classes are defined by evolutionary novelties (such as wings in birds) • Mechanism is a gradual refinement of existing structures for new functions • Structures may have an evolutionary plasticity that makes alternative functions possible
Evolutionary novelties (2) how do new designs evolve? • Preadaptation • When a structure evolved in one context and becomes co-opted for another function • Natural selection can not anticipate the future, but can improve on an existing structure • Example: feathers in birds
Evolutionary novelties (3) how do new designs evolve? • Genes that control development play a major role • A slight alteration in development becomes compounded in its effect on the adult • Allometric growth • Differences in relative rates of growth of various body parts. A slight change in these realtive growth rates may yield a substantial change in the adult
Genes controlling development • Regulatory genes can effect hundreds of structural genes, so changes here have a great impact • Paedomorphosis = Retention of features in the adult that were juvenile in ancestral species.
Paedomorphosis movie 24-21-PaedomorphosisAnim.mov
Paedomorphosis in axolotl (a salamander which retains some larval (tadpole) characteristics)
Genes controlling development • Heterochrony = evolutionary changes in the timing or rate of development.
Heterochrony and the evolution of salamander feet among closely related species Feet are shorter with more webbing, better for climbing up a vertical surface. Possible influence of an evolutionary change in a regulatory gene which switches off foot growth earlier in tree-dwelling species
Genes controlling development • Homeosis = alteration in the placement of different body parts
Evolutionary trends (1) • At times it appears that there are trends in the fossil record (toward greater size, more feathers, etc.) • A trend does not mean macroevolution is goal-oriented • No intrinsic drive toward a preordained state of being is indicated by the presence of an evolutionary trend
Evolutionary trends (2) • Species Selection • Species that exist the longest and generate the greatest number of new species determine the direction of major evolutionary trends • Differential speciation may play a role in macroevolution similar to the role of differential reproduction (natural selection) in microevolution
Evolutionary trends (3) • A trend may cease or reverse itself under changing environmental conditions. • Conditions in the Mesozoic era favored giant reptiles, but by the end of that era the smaller species prevailed
The branched evolution of horses Hyracotherium to modern horses. Smooth progressive trend toward increased size, less toes and grazing teeth??? Not a straight line. It is just that Equus is the only survivor of a much more complicated evolutionary tree.
Earth’s crustal plates and plate tectonics (geologic processes resulting from plate movements)
History of continental drift PANGEA Ghana and Brazil are separated by 3000 km of ocean, but matching fossils in both areas show the areas were once connected.
Mass Extinctions and Adaptive Radiations • Mass extinctions were followed by extensive diversification of some of the taxonomic groups that survived extinction. • Surviving species are able to undergo new adaptive radiations into the vacated habitats and produce new diversity
Adaptive radiations • Examples • Flying Insects • Mammals
Mass Extinctions • Why? Habitat destruction? Unfavorable environmental conditions? • Permian Extinctions • About 250 million years ago • 90% of species were eliminated • Cretaceous Extinctions • About 65 million years ago • Over 50% of species eliminated
Trauma for planet Earth and its Cretaceous lifeThe Asteroid Impact Hypothesis Immediate effect-Cloud of hot vapor and debris that could have killed most plants and animals in N. America in minutes?
Mass Extinction Video Mass Extinction Video
The Sixth Extinction The Earth may be on the brink of a sixth mass extinction on a par with the five previous episodes This time it appears that the cause is the activities of a growing human population. Rate of species extinction estimated from the fossil record is about 10-100 per year. In tropical habitats alone the current rate may be 27,000 per year.
Systematics • Phylogeny = the evolutionary history of a species or group • Systematics = the study of biological diversity in an evolutionary context. • Taxonomy = identification and classification of species
Homology/Analogy • Homology • Likeness attributed to a shared ancestry • Forelimbs of mammals are homologous structures • Analogy • Similarities due to convergent evolution, not common ancestry • Insect wings and bird wings are analogous structures