Different behaviors may have also led to the emergence of two Galápagos finch species that live in the same space. The two species are separated by behavioral barriers such as mating signals. In this case, members of each group select mates according to different beak structures and bird calls. They do not need physical barriers, because behavioral differences do enough to keep the groups separated.
Allopatric speciation and sympatric speciation are both forms of reproductive isolation. Allopatric speciation is due to geographic isolation. Sympatric speciation is due to behavioral isolation, or isolation due to different mating seasons, which is also known as temporal isolation.
Rates of Evolution
How fast is evolution? How long did it take for the giraffe to develop a long neck? How long did it take for the Galápagos finches to evolve? How long did it take for whales to evolve from land mammals? These and other questions about the rate of evolution are difficult to answer, but evidence does exist in the fossil record.
The rate of evolution is a measurement of the speed of evolution. Genetically speaking, evolution is how much an organism’s genotype (the genes that make up an individual) changes over a set period of time. Evolution is usually so gradual that we do not see the change for many, many generations. Humans took millions of years to evolve from a mammal that is now extinct.
Not all organisms evolve at the same rate. It would be difficult to measure evolution in your family because you are only looking at a small population over a few generations. However there are organisms that are evolving so fast that you may be able to observe evolution! Many scientists use bacteria or other species that reproduce frequently to study evolution. Species with short life cycles and that reproduce frequently evolve much faster than others. Bacteria evolve hundreds (or thousands or more) times faster than humans do. Bacteria go through many generations in a few days, so that we can actually witness evolution. A human takes about 22 years to go through one generation. But some bacteria go through over a thousand generations in less than two months.
Evolutionary Trees
Charles Darwin came up with the idea of an evolutionary tree to represent the relationships between different species and their common ancestors (Figure below). The base of the tree represents the ancient ancestors of all life. The separation into large branches shows where these original species evolved into increasingly different populations that would not come back together again. The branches keep splitting into smaller and smaller branches as species continue to evolve into more and more species. Some species are represented by short twigs spurting out of the tree, then stopping. These are species that went extinct before evolving into new species. Other “Trees of Life” have been created by other scientists (Figure below).
Figure 7.29
Darwin drew this version of the Tree of Life to represent how species evolve and diverge into separate directions. Each point on the tree where one branch splits off from another represents the common ancestor of the species on the separate branches.
Figure 7.30
Scientists have drawn many different versions of the Tree of Life to show different features of evolution. This Tree of Life was made by Ernst Haeckel in 1879.
Theory?
Darwin's Theory of Evolution by Natural Selection is supported by well over 150 years of scientific evidence, ranging from fossil evidence to DNA evidence. By definition, this is a well tested scientific theory. An abundance of scientific evidence supports this theory. The world is very old and has undergone some dramatic changes. Life has been on the planet for most of that time. As you will see in the next lesson, life started as single celled organisms and has evolved over billions of years into complex plants and animals. But this journey has not been easy. Most species that have ever lived are now extinct. There have been a number of mass extinctions, where many species vanished all at once. The tremendous diversity of species has allowed some to adapt to whatever changes nature throws in its path, from small changes to major environmental disturbances. So it is nature that selects - hence Natural Selection - which species adapts, survives and evolves.
Lesson Summary
Microevolution results from evolutionary changes that are small and do not lead to the creation of a new species.
Macroevolution refers to large evolutionary changes that result in new species.
Macroevolution may happen when many microevolution steps lead to the creation of a new species.
Macroevolution may happen as a result of a major environmental change, such as volcanic eruptions, earthquakes or an asteroid hitting Earth, which changes the environment so much that natural selection leads to large changes in the traits of a species
The creation of a new species is called speciation.
Natural selection causes beneficial heritable traits to become more common in a population, and unfavorable heritable traits become less common.
Artificial selection is when humans select which plants or animals to breed to pass specific traits on to the next generation.
Allopatric speciation occurs when groups from the same species are geographically isolated physically for long periods.
Sympatric speciation occurs when groups from the same species stop interbreeding, because of something other than physical separation, such as behavior.
Allopatric speciation and sympatric speciation are both forms of reproductive isolation.
The rate of evolution is a measurement of the speed of evolution. Genetically speaking, evolution is how much an organism’s genotype changes over a set period of time.
Not all organisms evolve at the same rate.
Evolutionary trees are used to represent the relationships between different species and their common ancestors.
Review Questions
What is the difference between macroevolution and microevolution?
What conditions cause organisms to evolve and adapt?
What do the branches on the Tree of Life represent?
Which organism has a faster rate of evolution: a human or a bacterium?
How do you know if two related organisms are members of the same species?
Why do the squirrels on opposite side of the Grand Canyon look different?
How is artificial selection different from natural selection?
What, other than physical isolation, could cause a species to split into two different directions of evolution?
Further Reading / Supplemental Links
Yeh, Jennifer, Modern Synthesi, (From Animal Sciences).
Darwin, Charles, Origin of the Specie, Broadview Press (Sixth Edition), 1859.
Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature, Perennial Books, 2003.
Ridley, Matt, Genome, Harper Collins, 2000.
Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006.
Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton, 2006.
Dawkins, Richard, The Blind Watchmaker, W.W. Norton & Company, 1996.
Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989.
Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins, 2006.
Mayr, Ernst, What Evolution Is, Basic Books. 2001.
Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008.
Vocabulary
allopatric speciation
Speciation that occurs when groups from the same species are geographically isolated physically for long periods.
artificial selection
Occurs when humans select which plants or animals to breed to pass specific traits on to the next generation.
behavioral isolation
The separation of a population from the rest of its species due to some behavioral barrier, such as having different mating seasons.
evolutionary tree
Diagram used to represent the relationships between different species and their common a
ncestors.
genotype
The genes that make up an individual.
geographic isolation
The separation of a population from the rest of its species due to some physical barrier, such as a mountain range, an ocean, or great distance.
macroevolution
Big evolutionary changes that result in new species.
microevolution
Small changes in inherited traits; does not lead to the creation of a new species.
natural selection
Causes beneficial heritable traits to become more common in a population, and unfavorable heritable traits become less common.
primate
A group of related mammal species that have binocular vision, specialized hands and feet for grasping, and enlarged and differentiated brains; includes humans, chimpanzees, the apes, monkeys, and lemurs.
reproductive isolation
allopatric and sympatric speciation; isolation due to geography or behavior, resulting in the inability to reproduce.
speciation
The creation of a new species; either by natural or artificial selection.
sympatric speciation
Speciation that occurs when groups from the same species stop interbreeding, because of something other than physical separation, such as behavior.
temporal isolation
Isolation due to different mating seasons.
Points to Consider
How long do you think humans have been around?
How long do you think Earth existed before life formed?
For how much of Earth’s history have humans existed?
Lesson 7.4: History of Life on Earth
Lesson Objectives
Know that geologists and paleontologists use evidence to determine the history of Earth and life on Earth.
Know that geologists and paleontologists measure the radioactivity in certain rocks to determine the age of the earth and fossils.
Know that the earth is between four and five billion years old.
Know that scientists need to know what the environment (what chemicals were around, the temperature, etc.) was like on Earth billions of years ago to know how life formed.
Check Your Understanding
What are fossils?
How does the fossil record contribute to the evidence of evolution?
Introduction
It is no surprise that people have wondered about the age of the earth, how it was formed, and how life began on Earth for hundreds, even thousands, of years. Try to imagine how ancient philosophers tried to explain the history of the earth and life. Many people used mythology or cultural beliefs to explain elaborate stories about how and when the earth formed.
The past two to three hundred years has been an exciting time for geologists, paleontologists and other scientists who are trying to trace the history of the earth. What was once a hobby, studying land forms and fossils has become a science that is revealing the history of the earth and life on Earth.
Age of Earth
During the 1800s, geologists, paleontologists and naturalists found several forms of physical evidence that confirmed that the earth is very old, far older than the 6,000 years that some leaders had claimed. Their evidence included:
Fossils of ancient sea life on dry land far from oceans: This supported the idea that the earth changed over time and that some dry land today was once covered by oceans.
The many layers of rock: When people realized that rock layers represent the order in which rocks and fossils appeared, they were able to start to trace the history of the earth and life on Earth.
Indications that volcanic eruptions, earthquakes and erosion that happened long ago shaped much of the earth’s surface. This supported the idea of an older Earth.
Radiometric Dating
During the past one hundred years, geologists and paleontologists have been able to delve even deeper into the earth’s history with new tools of science. The most convincing method, called radiometric dating, was developed more than one hundred years ago. Rocks are made up of minerals. Scientists found that they could measure the age of rocks by measuring the radioactivity of certain minerals in rocks. Geologists and paleontologists still use variations of radiometric dating to determine the age of fossils and rocks today (Figure below).
Figure 7.31
The most reliable way to figure out the earths age is to measure the radioactivity of certain minerals found in rocks (called radiometric dating). This mass spectrophotometer can also be used to measure age of fossils from the level of radiation in minerals surrounding the fossil.
Over 4 Billion Years
The earth is at least as old as its oldest rocks. The oldest rock minerals found on Earth so far are zircon crystals that are at least 4.404 billion years old. These tiny crystals were found in the Jack Hills of Western Australia. Since the earth is at least as old as the oldest minerals found on Earth, geologists estimate that the minimum age of the earth is 4.404 billion years.
Likewise, the earth cannot be any older than the solar system. The oldest possible age of the earth is 4.57 billion years old, the age of the solar system. Geologists and geophysicists based the age of the universe on the age of materials within meteorites that are formed within the solar system.
Origin of Life on Earth
There is good evidence that life has probably existed on Earth for most of Earth’s history. Some of the oldest fossils of life forms on Earth are at least 3.5 billion year old fossils of blue green algae found in Australia (Figure below).
Figure 7.32
Some of the oldest fossils on earth are stromolites, made of algae and a kind of bacteria, found along the coast of Australia.
The next step is to determine exactly how life formed billions of years ago. First, scientists need to know what the environment was like 3.5 to 4 billion years ago; they need to know what kinds of materials were available then that could have been involved in the creation of life. Scientists believe the early earth contained no oxygen gas, but did contain other gases, including nitrogen, carbon dioxide, carbon monoxide, water vapor, hydrogen sulfide and probably a few others.
Life from Random Reactions
Today, we have evidence that life on Earth came from random reactions between chemical compounds that formed molecules; in a series of random steps, these molecules created proteins and nucleic acids (RNA or DNA), and then cells. We know that the ingredients for life (the building blocks of life), were present at the beginning of Earth’s history. Some chemicals were in water and volcanic gases. Other chemicals would have come from meteorites in space. Energy to drive chemical reactions was provided by volcanic eruptions and lightening. Keep in mind that this process may have taken as much as 1 billion years. Our understanding of how life originated on Earth is developing gradually (Figure below).
Figure 7.33
Some clues to the origins of life on Earth come from studying the early life forms that developed in hot springs, such as the Grand Prismatic Spring at Yellowstone National Park. This spring is approximately 250 feet by 300 feet wide.
Geologic Time Scale
Geologists and other earth scientists use geologic time scales to describe when events occurred throughout the history of Earth. The time scales can be used to illustrate when both geologic events and events affecting plant and animal life occurred. All of the earth events we see happening today, such as earthquakes, volcanic eruptions, and erosion, have happened throughout history. Past catastrophic events, such as asteroids and comets also hit the earth long before humans evolved.
The geologic time scale in Figure below illustrates the timing of events such as:
earthquakes
volcanic eruptions
major erosion
meteorites hitting Earth
the first signs of life forms
mass exterminations
Figure 7.34
The geological time scale of Earth's past is organized according to events whi
ch took place during different periods on the time scale. Geologic time is the same as the age of the earth: between 4.04 and 4.57 billion years. Look closely for such events as the extinction of dinosaurs and many marine animals.
Evolution of Major Life Forms
Life on Earth began about 3.5 to 4 billion years ago. The first life forms were single cell organisms, prokaryotic organisms, similar to bacteria. The first multicellular organisms did not appear until about 610 million years ago in the oceans. These of course would be eukaryotic organisms. Some of the first multicellular forms included sponges, brown algae, and slime molds.
Many of the modern types of organisms we know today evolved during the next ten million years in an event called the Cambrian explosion. This sudden burst of evolution may have been triggered by some environmental changes that made the environment more suitable for a wider variety of life forms.
Plants and fungi did not appear until roughly 500 million years ago. They were soon followed by arthropods (insects and spiders). Next came the amphibians about 300 million years ago, followed by mammals around 200 million years ago and birds around 100 million years ago.
Even though large life forms have been very successful on Earth, most of the life forms on Earth today are still prokaryotes – small, single celled organisms. Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; in fact, it is estimated that 99% of the species that have lived on the earth no longer exist.
CK-12 Life Science Page 18
