Evolution 15.2

Objectives Describe how evolution can refine existing adaptations. Explain how existing structures can take on new functions through evolution. Explain the role of developmental biology in understanding evolutionary change

Figure 15-11 These diagrams show the range of complexity in the structure of eyes among various species of mollusks living today.                                                                                                                                                                            

Evolution and Development Embryology is the study of the processes of multicellular organisms as they develop from fertilized eggs to fully formed organisms. One important focus of embryology are genes that control the development of an organism as it begins to take shape.

Figure 15-15 Compared to the ground-dwelling salamanders' feet (right), the feet of the tree-dwelling salamanders (left) have shorter toes and more webbing—an adaptation in the tree salamander's climbing life style                                                                                 

Evolution 15.3 Objectives Explain how fossils can form. Describe the geologic time scale. Summarize methods used to determine the ages of fossils. Describe how continental drift and mass extinctions relate to macroevolution

The geologic time scale (Figure 15-18) organizes Earth's history into four distinct ages known as the Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. These eras are divided into shorter time spans called periods. Periods are divided into epochs.

The boundaries between eras are marked in the fossil record by a major change (or turnover) in the forms of life. For example, the beginning of the Paleozoic Era (the start of the Cambrian period) is marked by the appearance of a diversity of multicellular animals with hard parts. Fossils of these animals are absent in rocks of the Precambrian Era. The boundaries between eras and between some periods are also marked by widespread extinctions. For example, many of the animals that lived during the late Paleozoic Era became extinct at the end of that era.

CAMBRIAN PERIOD Named for Cambria, the Latin word for Wales Period from 540 to 500 million years ago Previously believed to be the first major time period containing life.

LANDSCAPE: At the beginning of the Cambrian, there was one major landmass called Rodinia. This landmass existed primarily below the equator. During the Cambrian period, Rodinia broke into Laurentia (most of present day North America) and Gondwana (most of present day southern hemisphere).

The Cambrian period began and ended with ice ages which caused mass extinctions of many animal and plant species. These ice ages may have lowered sea level.

LIFE IN THE CAMBRIAN Plants as such did not exist during the Cambrian period. Photosynthesis was carried out by bacteria and algae. During the Cambrian, oxygen in the atmosphere mixed with ocean water. This allowed many animal species to exist in the oceans.

During the Cambrian, invertebrates ruled the Earth During the Cambrian, invertebrates ruled the Earth. Many species that have a hard shell (and many that don’t) predominated. In particular: sponges echinoderms (spiked star shaped animals colony forming animals (coral, bryozoans)

Trilobites first appeared during this time period. The only vertebrates that existed were jawless fish. All life existed in the water. Not enough ozone had accumulated in the atmosphere to block ultraviolet rays.

The fossil record reveals that Earth's history has long periods of relative stability broken by comparatively brief episodes of great species loss known as mass extinctions. For example, at the end of the Cretaceous period, about 65 million years ago, the world lost an enormous number of species. Before then, dinosaurs had thrived on Earth for 150 million years. Less than 10 million years later—a brief period in geologic time—all the dinosaurs were gone.

Figure 15-18 This diagram of geologic time indicates some key events in the history of life on Earth. Note the relative time spans of the eras, depicted in the lower left corner.                                                  

MEASURING ABSOLUTE TIME Tree rings: The theory is that during each growing season, a tree’s width increases. Large tree cells are associated with spring/summer (white rings) and small cells are associated with fall/winter (dark rings).

Advantage:. Gives an idea of how old a. region is in absolute time Advantage: Gives an idea of how old a region is in absolute time. Thick light rings indicate wet seasons, thick dark rings indicate dry seasons. Disadvantages: Variable climate conditions may produce more than one ring in a year. You can’t be sure what part of the tree you are looking at. Branches tend to underestimate the age of the tree.

Radioactive dating: Radioactivity is the process of matter losing protons or neutrons spontaneously. Radioactive compounds have a half life: The amount of time it takes for half of the mass of the element to disappear. Example: radioactive Chromium has a half life of 30 days.

Day 0: 100 grams Cr Day 30: 50 grams Cr Day 60: 25 grams Cr Day 90: 12.5 grams Cr Day 120: 6.25 grams Cr Carbon 14: half life = 5700 years Living things contain a known ration of carbon 14 to carbon 12. When the living thing dies, all the carbon 14 decays to carbon 12

By measuring the ratio of carbon 14:carbon 12, we can tell how long ago something died within about 300 years This method only works for items less than 7000 years old.

Figure 15-20 From the time an organism dies, decay of half the carbon-14 present in its body takes 5,730 years. Half of that remainder decays in another 5,730 years, and so on.                                                                                                   

Radiometric dating is based on the measurement of certain radioactive isotopes in objects. It is the method most often used to determine the absolute ages of rocks and fossils. Every radioactive isotope has a fixed rate of decay. An isotope's half-life is the number of years it takes for 50 percent of the original sample to decay. The half-life is unaffected by temperature, pressure, and other environmental conditions.

Figure 15-19 If the ages of two volcanic rock layers are measured using radioactive isotopes, the data can be used to estimate the age of fossils found in the sedimentary rock between the volcanic layers                                                                                                                                                                            

Continental Drift and Macroevolution Earth's continents are not locked in place. They move about the planet's surface like passengers on great plates of crust, floating on the hot mantle. Landmasses on different plates change position relative to each other as a result of movement known as continental drift. North America and Europe, for example, are presently drifting apart at a rate of about 2 centimeters per year.

                                                                                                   Figure 15-21 About 180 million years ago, Pangaea split into northern and southern landmasses that later separated into the modern continents. India collided with Eurasia just 40–50 million years ago, forming the Himalaya mountain range. The continents continue to drift today.