MACROEVOLUTION, THE FOSSIL RECORD, AND SYSTEMATICS.

Macroevoultion- is the origin of taxonomic groups higher than the species level. That is, macroevolutionary change is substantial enough that we view its products as new genera, new families, even new phyla.

In broader terms, macroevolution is the story of the major events in the history of life that are revealed by the fossil record.

The fossil record documents macroevolution.

Fossils are collected and interpreted by specialists called paleontologists (Gr. palaios, “old”)

A fossil is any preserved remnant or impression left by an organism that lived in the past (L. fossilis, “dug up”). Sedimentary rocks are the richest sources of fossils. Aquatic organisms, and terrestrial ones swept into the seas and swamps, settle when they die, along with the sediments. A tiny fraction of them is then preserved as fossils.

The organic substances of dead organisms buried in sediments usually decay rapidly. However, hard parts that are rich in minerals, such as bones and teeth of vertebrates and the shells of many invertebrates and protists, remain as fossils. Under the right conditions, minerals dissolved in ground water seep into the tissues of a dead organism and replace organic material. The plant or animal turns to stone. Rarer than mineralized fossils are those that retain organic material. They are sometimes discovered as thin films pressed between layers of sandstone or shale.

The fossils that paleontologists find in many or their digs are not actual remnants of organisms at all, but rocks that form as replicas of the organism. These fossils result when a dead organism captured in sediments decays and leaves an empty mold that becomes filled with minerals dissolved in water. The minerals may subsequently crystallize, forming a cast in the shape of organism.

Casts called trace fossils form in footprints, animal burrows, or other impressions left in sediments by the activities of animals. These rocks are fossilized behaviour: they tell paleontologists something about how the animal lived.

The discovery of a fossil is the culmination of a sequence of improbable coincidences. First, the organism had to die in the right place and at the right time for burial conditions to favor fossilization. The fossil then had to survive the geological processes that distort and alter rock, such as erosion. Then the fossil would have to be exposed on the surface, and someone would have to find it. It is no wonder that the fossil record is incomplete. A substantial fraction of species that have lived probably left no fossils, most fossilized forms have been destroyed, and only a fraction of the existing fossils have been discovered. However, the fossil record is a remarkably detailed document of macroevolution over the vast scale of geological time.

Dating Fossils

Fossils are reliable historical data only if we can determine their ages.

Relative Dating.

As sediments are laid down the rock strata are formed. Layers of rock that are deeper were laid down at an earlier period that were layers closer to the surface. Different locations would have a different geological history. That is, while one location was covered with a shallow sea, and sedimentation was occurring, another location could have been exposed the air and erosion could have been removing existing layers. Therefore different locations often have a different pattern of strata. The strata at one location can often be correlated with the strata at another location by the presence of similar fossils, known as index fossils. These are quite often the shells of sea animals that were wide spread.

By studying many different sites, geologist have established a geological time scale with a consistent sequence of historical periods. These periods are grouped into four eras: the Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. Each represents a distinct age in the history of the earth and its life; the boundaries are marked in the fossil record by explosive radiations of many new forms of life following mass extinctions.

The record of rocks is a serial that chronicles the relative ages of fossils; it tells us the order in which groups of species present in a sequence of strata evolved. However, the series of sedimentary rocks does not tell the absolute ages of the embedded fossils.

Absolute Dating

“Absolute” dating does not mean errorless dating, but only that age is given in years instead of relative terms like, before and after, early and late. Radiometric dating is the method most often used to determine the ages of rocks and fossils on a scale of absolute time. Fossils contain isotopes of elements that accumulate in the organisms when they were alive. Because each radioactive isotope has a fixed rate of decay, it can be used to date a specimen. An isotope's half life, the number of years it takes for 50% of the original sample to decay, is unaffected by temperature, pressure, and other environmental variables. Carbon-14 has a half life of 5600 years a reliable rate of decay that can be used to date relatively young fossils. Paleontologist use radioactive isotopes with longer half-lives to date older fossils.

What are the major questions about macroevolution?

What processes actually cause the large-scale evolutionary changes that we see in the fossil record?

How, for example, do the novel feature that define taxonomic groups above the species level, such as flight adaptations in birds, arise?

How have global geological changes affected macroevolution?

Some evolutionary novelties are modified versions of older structures.

Most biological structures have an evolutionary plasticity that makes alternate functions possible. From a retrospective vantage point, evolutionists use the term preadaptation for a structure that evolved in one context and became co-opted for another function. Natural selection cannot predict the future and can only improve a structure in the context of its current utility.

For example: the light, honeycombed bones of birds could not have evolved in earth-bound reptilian ancestors as an adaptation for upcoming flights. If these honeycombed bones predated flight, as clearly indicated by the fossil record, then they must have had some function on the ground. The probable ancestors of birds were agile, bipedal dinosaurs that also would have benefited from a light frame.

Preadaptation offers one explanation for how novel designs can arise gradually through a series of intermediate stages, each of which has some function in the organism's current content.

Genes that control development play a major role in evolutionary novelty.

In some cases, relatively few changes in the genome can apparently cause major modifications of morphology. Genes that program development control the rate, timing, and spatial pattern of changes in an organism's form as it is transfigured from a zygote into an adult.

For example: allometric growth, a difference in the relative rates of growth of various parts of the body, helps shape an organism. In humans, the body proportions change during development. Change these relative rates of growth, even slightly, and you change the adult form substantially.

In addition to affecting developmental rates, genetic changes can also alter the timing of developmental events - the sequence in which different body parts start and stop developing. One such change in developmental timing results in paedomrphosis, in which a sexually mature adult retains features that were juvenile structures in its evolutionary ancestors. For example; most salamander species have a larval stage that undergoes metamorphosis to become an adult. But some species grow to adult size and become sexually mature while retaining gills and certain other larval features. Such an evolutionary alteration of developmental timing from their ancestors, even though the overall genetic change may be small.

Macroevolution has a biogeographical basis in continental drift.

On a global scale, the drifting of continents is the major geographical factor correlated with the spatial distribution of life and with such macroevolutionary episodes as mass extinctions and explosive increases of biological diversity. The continents are not fixed, but drift about Earth's surface like passengers on great plates of crust floating on the molten mantle. Unless two land masses are embedded in the same plate, their positions relative to each other change.

 

The history of life is punctuated by mass extinctions followed by adaptive radiation's of the survivors.

The fossil record reveals an episodic history of life on earth, with long, relatively quiescent periods punctuated by briefer intervals when the turnover in species competition was much more extensive. The episodes include explosive adaptive radiations of major taxonomic groups as well as mass extinctions.

Major adaptive radiations.

Many taxonomic groups have diversified prolifically early in their history after the evolution of some novel characteristic that opened a new adaptive zone, a term for a new way of life that presents many previously unexploited opportunities. For example, the development of wings enabled insects to enter an adaptive zone with many new resources, and adaptive radiation produced hundreds of thousands of variations on the basic insect body plan.

Cambrian explosion.

The boundary between the Precambrian era and the Paleozoic era is marked by a large increase in the diversity of sea animals. All the animal phyla that exist today evolved during a span of less than 10 million years during the middle Cambrian, the first period of the Paleozoic era. One key evolutionary novelty behind this remarkable diversification may have been the origin of shells and skeletons in a few phyla.

Examples of mass extinctions.

A species may become extinct because its habitat has been destroyed or the environment has changed in a direction unfavorable to the species. The average rate of extinction has been between 2.0 and 4.6 families per million years. However there have been crises in the history of life when global environmental changes have been so rapid and disruptive that a majority of species were swept away.

The Permian extinctions, which define the boundary between the Paleozoic and Mesozoic eras, claimed over 90% of the species of marine animals about 250 million years ago. Terrestrial life also crashed; for example 8 out of 27 orders of Permian insects did not survive into the Triassic, the next geological period. The mass extinctions occurred in less than 5 million years - possibly much less - an instant in the context of geological time. Several factors could have combined to cause radical environmental change during the Permian. That was about the time the continents merged to form Pangea, which disturbed many marine and terrestrial habitats and altered climate. There were also massive volcanic eruptions in what is now Siberia, creating the most extreme episode of volcanism of the past half-billion years.

Another emphatic punctuation, the Cretaceous extinctions of 65 million years ago, delineates the boundary between the Mesozoic and the Cenozoic eras. This event exterminated more that half the marine species and many of the families of terrestrial plants and animals, including the dinosaurs. The climate became cooler, and shallow seas receded from continental lowlands. Large volcanic eruptions in what is now India may have contributed to the cooling by releasing material into the atmosphere that blocked sunlight. However, the most debate about the causes of the Cretaceous extinctions centers on the impact hypothesis, which posits that an asteroid or comet struck Earth and caused mass extinction. Walter and Luis Alvarez and their colleagues at the University of California, Berkeley studied an anomalous layer of clay between the Mesozoic and Cenozoic sediments. this layer of clay contains a large amount of iridium, an element very rare on earth but common in meteorites and other extraterrestrial debris that occasionally falls to Earth. Located beneath the sediments of the Yucatan coast of Mexico lies an impact crater which could be as large as 300 km in diameter. Advocates of the hypothesis argue that the size of the impact was large enough to darken the Earth for years, not just month, and that the reduction of photosynthesis would be long enough for food chains to collapse.

Adaptive Radiations.

Whatever the cause, mass extinctions affect biological diversity profoundly. But there is a creative side to the destruction. The species that manage to survive these crises, by their adaptive qualities or by sheer luck, become the stock for new radiations that fill many of the adaptive zones vacated by the extinctions.