Whereas microevolution explains diversification on an individual level over relatively short periods of time, macroevolution defines changes in large populations that often entail catastrophic environmental changes.
The fossil record establishes the ancestral lineage of both plants and animals and identifies periods of punctuated equilibrium in both. Rock strata can be used to date fossils because the organisms from which the fossils were derived died and were eventually buried in the material from which the rock was made. This allows a relative dating of the fossils by assigning their age in comparison with other rock strata. Except in the case of major Earth events, such as mountain building and erosion, the youngest rock strata and the fossils they contain are closest to the earth's surface and become older the deeper they are found in the crust. Also, rock strata in neighboring areas can be reconciled with each other if they are composed of similar rock or mineral type.
The evolutionary history for an area is arrayed in rock layers that jigsaw together to trace the macroevolution or major events of the history of life on Earth. As paleontologists discover fossils in a rock layer, they can make assumptions based on present-day life-forms about the environmental conditions that existed at that time. For instance, the discovery of a fern fossil would indicate a warm or temperate climate, adequate precipitation, and perhaps shade, all of which are conditions that support the growth of modern-day ferns. It may also provide clues that link with other fossil evidence to shed light on the animal life present at that time. For instance, an environment that supports ferns would also likely support herbivores like snails or an assortment of grazing animals that might feed on ferns.
Carbon-14 analysis is effective up to 50,000 years, after which other radioactive isotopes, such as potassium-40 and uranium-238, are used because they have a much longer half-life.
Additional fossil evidence is collected using radiometric data analysis, which is a more approximate dating of once-living organisms by comparing the ratio of radioactive isotopes in their remains to that found in the atmosphere. Radiometric dating compares the ratio of the normal carbon-12 atom to the unstable, radioactive carbon-14 isotope. While alive, the ratio of carbon-12 to carbon-14 atoms in any living organism is nearly equal to their ratio in the atmosphere. Upon death, the carbon-14 is no longer added to the organism, and the existing amount begins to decay at a constant and known rate to a more stable isotope like nitrogen-14. The decay rate for all radioactive isotopes is called their half-life, which means that one-half of their mass will be converted, or decayed, into the more stable form in a known amount of time. So if you can determine the ratio in the sample and compare it to the atmosphere and then multiply by the known decay time, or half-life, you can establish the age. The half-life for carbon-14 is 5,600 years, meaning that one half of the beginning amount will decay to nitrogen-14 in 5,600 years. Radiometric analysis gives an actual age of the specimen and, when combined with rock strata data, can yield a more accurate time and location placement.
Macroevolution is often displayed on a geologic time scale, which highlights major evolutionary events in a comparative time scale. The smallest units of time on the geologic time scale are called epochs and measure in the millions of years, such as the Pleistocene epoch approximately two million years ago that included the Ice Age and the appearance of the first human fossils. Chronologically, epochs are clumped together into larger units called periods, such as Quatenary, which are combined to make eras, such as Cenozoic, which are the largest unit of macroevolution measure. The following table includes examples of macroevolution, from the oldest to the most recent.
|Precambrian||—||—||Oldest prokaryote fossils.|
|Paleozoic||Ordovician||—||Origin of plants.|
|Silurian||—||Land colonization by plantsand arthropods.|
|Cretaceous||—||Dinosaurs are extinct; flowering plants emerge.|
|Cenozoic||Tertiary||Paleocene||Radiation of mammals and birds.|
A phylogenetic tree is like a family tree or pedigree. It shows the ancestral relationships and genealogy for an organism.
Combined radiometric and rock-strata data analysis demonstrates evolutionary pathways for many plant and animal species. One of the most studied is the evolution of the modern horse, whose phylogenetic tree begins with a small, doglike creature and branches many times before reaching today's horse.
Backmapping the phylogenetic tree to establish evolutionary links between fossils establishes the science of systematics—the organized scheme of classifying all living things into their phylogenetic tree.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.