Natural selection happens when some organisms have traits that make them better suited (they have better accommodation) to live in a certain environment than others. They are more likely to survive, reproduce and pass their traits on to future generations than those without the special traits. The process of natural selection helps us understand how organisms appear to be so well suited or adapted to their environments. Every plant and animal depends on its traits to survive. Survival may include getting food, building homes, and attracting mates. Most of these traits have been changed through natural selection so they allow a plant, animal, or bacteria to survive and reproduce relatively well in their environments. These traits are called adaptations. As environments have changed considerably over time, organisms must constantly adapt to those environments. It is the great diversity of species that increases the chance that at least some organisms adapt and survive any major changes in the environment.
Imagine how in winter dark fur makes a rabbit easy for fox to spot and catch in the snow. Natural selection suggests that white-fur is an advantageous trait that improves the chance that a rabbit will survive, reproduce and pass the trait of white fur on to future generations (Figure below). Dark fur rabbits will become uncommon.
Figure 7.12
In winter, the fur of Arctic Hares turns white. The camouflage may make it more difficult for fox and other predators to locate hares against the white snow.
Polygenic Inheritance and Natural Selection
But natural selection leading to evolution does not just select for certain individuals, it selects for groups. More than one individual must adapt to the environment to maintain a population. Natural selection determines which groups of organisms survive, based on their traits, and which do not, that is, natural selection determines the differential survival of groups of organisms.
Figure 7.13
Natural selection determines the survival of groups of organisms. Flight as shown in these geese is an evolutionary step that probably aided in the survival of many birds.
Although some traits are determined by a single gene, many are influenced by more than one gene (polygenic). The result of polygenic inheritance is a continuous spectrum of phenotypic values which often show a bell curve pattern of distribution.
Given this pattern of phenotypic variability, natural selection can take three forms (Figure below). We will use the hypothetical color distribution in this figure to illustrate the three types of selection. Directional selection shifts the frequency curve away from the average by favoring individuals with an extreme form of the variation. The curve would still be bell-shaped, but it would have shifted to the left or right, in the direction of the lighter or darker alleles. Stabilizing selection selects for a group of phenotypically average individuals, with individuals with either extreme phenotype selected against. Disruptive selection selects for groups of individuals with extreme phenotypes, selecting against individuals with the average phenotype.
Figure 7.14
Three types of selection can alter allele frequencies, causing microevolution. The effect of stabilizing selection (1) is to select for the average phenotype, reducing variation. Disruptive selection (2) results in two different populations, which may eventually be isolated from one another. Directional selection (3) selects for a group of individuals with a single characteristic.
Lesson Summary
Evolution is change in species over multiple generations.
Natural selection is how evolution occurs.
Adaptations are the result of natural selection.
Charles Darwin is credited with developing the Theory of Evolution by Natural Selection
Darwin collected much of his evidence on a five year voyage around the world, with much of his data collected on the Galápagos Islands.
The work of many others contributed to Darwin's theory.
Review Questions
What is biological evolution?
What is natural selection?
What is adaptation?
What is the difference between an inherited trait and an acquired trait?
What was the name of the ship that Darwin traveled on?
What is the name of the islands where Darwin studied evolution?
A giraffe’s long neck allows the giraffe to eat leaves from high in the tree. This is an example of an _____________.
Who proposed a theory of evolution by natural selection that was similar to Darwin’s theory?
Further Reading / Supplemental Links
Stein, Sara, The Evolution Book, Workman, N.Y., 1986.
Yeh, Jennifer, Modern Synthesis, (From Animal Sciences).
Darwin, Charles, Origin of the Species, 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
acquired trait
A feature that an organism gets during its lifetime in response to the environment (not from genes); not passed on to future generations through genes.
adaptation
Beneficial traits that help an organism survive in its environment. Organisms with beneficial traits are more likely to survive, reproduce and pass their traits on to future generations than those without the special traits. These traits are called adaptations.
artificial selection
Selection in which people choose specific traits to pass to the next generation, such as with horse or dog breeding.
evolution
A process in which something passes by degrees to a different stage, such as a living organism turning into a more advanced or mature organism; the change of the inherited traits of a group of organisms over many generations.
evolution by natural selection
The changes in the inherited traits of a population from one generation to the next; due to a process in which organisms that are best suited to their environments have greater survival and reproductive success.
Galápagos Islands
A group of islands in the Pacific off South America; owned by Ecuador; known for unusual animal life. Many scientists, including Charles Darwin made many discoveries that led to the theory of evolution by natural selection while studying the plants and animals on these islands.
inherited traits
Features that are passed from one generation to the next.
natural selection
Results when some organisms have traits that make them better suited to live in a certain environment than others; they are more likely to survive, reproduce and pass their traits on to future generations than those without the special traits.
species
A group of individuals that are genetically related and can breed to produce fertile young.
trait
A feature or characteristic of an organism. For example, your height, hair color, and eye shape are physical traits.
Points to Consider
Evolution by natural selection is supported by extensive scientific evidence. What do you think this evidence consists of?
Lesson 7.2: Evidence of Evolution
Lesson Objectives
Understand that the scientific theory of biological evolution is based on extensive physical evidence and testing. This includes: differences between fossils in different layers of rock
the age
of rocks and fossils
vestigial structures
similarities between embryos of different organisms
the same DNA and RNA materials found in all organisms
similar genomes found in almost all organisms.
Check Your Understanding
Where did Charles Darwin collect evidence of evolution and what kinds of evidence did he find?
What is natural selection?
What kinds of traits change through evolution?
Introduction
Though the idea of evolution had been proposed prior to Charles Darwin, most people think of Darwin’s name when they think of evolution. Unlike others before him who based their ideas on speculation, opinions, myths, or folklore, Darwin’s theories were based on a tremendous amount of scientific evidence.
In 1859, Charles Darwin and Alfred Russel Wallace first presented several forms of evidence of evolution. Their evidence included:
fossils of extinct species from different eras
similarities between the embryos of different species
physical traits of different species
the behavior of different species
the distributions of different plant and animal species around the world.
Darwin and other 19th century scientists came to the conclusions they did without knowing anything about molecular biology. Today, even more evidence of evolution by natural selection is coming from molecular biology and genetics. Genetics is also helping explain the mechanisms of how evolution occurs.
The Fossil Record
Paleontologists are the scientists who study fossils to learn about life in the past. Fossils are the preserved remains or traces of animals, plants, and other organisms from the distant past. Examples of fossils include bones, teeth, impressions, and leaves. Paleontologists compare the features of species from different periods in history. With this information, they try to unravel how species have evolved over millions of years (Figure below). This method works better with some species than others. For example, it is difficult to track the evolution of bacteria from fossils, because their single cells do not last well as fossils.
Figure 7.15
Evolution of the horse. Fossil evidence, depicted by the skeletal fragments, demonstrates evolutionary milestones in this process. Notice the 57 million year evolution of the horse leg bones and teeth. Especially obvious is the transformation of the leg bones from having four distinct digits to the conformation of today's horse.
Until recently, fossils were the main source of evidence of evolution (Figures below and below). The location of each fossil in layers of rocks provides clues to the age of the species and how species evolved in the past. Older materials and fossils are deeper in the earth; newer fossils and materials are closer to the surface.
Figure 7.16
A fossil is the remains of a plant or animal that existed some time in the distant past. Fossils, such as this one, were found in rocks or soil that was laid down long ago.
Figure 7.17
About 40 to 60 million years ago this mosquito and fly were trapped in the gooey stuff, called resin that comes from trees. The fossils in the movie , were trapped in resin.
Fossils and the rocks they are embedded in provide evidence of how life and environmental conditions have changed throughout Earth’s history. They also help us understand how the past and present distribution of life on Earth is affected by earthquakes, volcanoes, and shifting seas, and other movements of the continents.
The Age of Rock Layers and Fossils
The many layers of sedimentary rock provide evidence of the long history of Eearth and the order of life forms whose remains are found in the rocks. The youngest layers are not always found on top, because of folding, breaking, and uplifting of layers. If the layers of earth were tilted by earthquakes or volcanoes, geologists can determine which layers came from the deepest parts of the Earth.
The fossils and the order in which fossils appear is called the fossil record. This record provides important records of how species have evolved, divided and gone extinct. Methods used to date the age of rocks and fossils make it possible to determine when these events occurred.
Geologists use a method called radiometric dating to determine the age of rocks and fossils in each layer of rock. This technique measures the decay rate of radioactive materials in each rock layer (Figure below).
Figure 7.18
This device, called a spectrophotometer can be used to measure the level of radioactive decay of certain elements in rocks and fossils to determine their age.
Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4 and 5 billion years old. The oldest fossils are between 3 and 4 billion years old.
Vestigial Structures
Millions of species of animals, plants and microorganisms are alive today. Even though two different species may not look similar, they may have similar internal structures, and chemical processes that indicate they can have a common ancestor.
Some of the most interesting kinds of evidence for evolution are body parts that have lost their use through evolution (Figure below). Most birds need their wings to fly. But the wings on an ostrich have lost their original use. These are called vestigial structures. Penguins do not use their wings to fly in the air; however they do use them to "fly" in the water. A whale’s pelvic bones-which were once attached to legs- are also vestigial structures (Figure below).
Figure 7.19
Mole rats live under ground where they do not need eyes to find their way around. This moles eyes are covered by skin. Body parts that do not serve any function are vestigial structures.
Figure 7.20
The bones in your arms and hands have the same bone pattern as those in the wings, legs, and feet of the animals pictured above. How have the bones adapted for different uses in each animal?
If you look at an x-ray of the bones in your back (called vertebrae), you will see several vertebrae that come under your hips. These are called your coccyx, or tailbone. We do not use these small vertebrae; they are further evidence of our evolution.
Embryological Evidence
Some of the oldest evidence of evolution comes from embryology, the study of how organisms develop. An embryo is an animal or plant in its earliest stages of development, before it is born or hatched.
Centuries ago, people recognized that the embryos of many different species have similar appearances (Figure below). The embryos of some species are even difficult to tell apart. Many of these animals do not differ much in appearance until they develop further. Many traits of one type of animal appear in the embryo of another type of animal. For example, fish embryos and human embryos both have gill slits. In fish they develop into gills, but in humans they disappear before birth (Figure below).
The similarities between embryos suggests that these animals are related and have common ancestors. For example, humans did not evolve from chimpanzees. But the similarities between the embryos of both species may be due to our development from an ancestor we have in common with chimpanzees. As our common ancestor evolved, humans and chimpanzees diverged and developed different traits.
Figure 7.21
This drawing was made to show the similarities between the embryos of many species. Embryos of many different kinds of animals: mammals, birds, reptiles, fish, etc. look very similar.
Figure 7.22
This is a six week old human embryo. Notice the similarities between this embryo and those of the other animals in figure 3.
Similarities Between Molecules and Genomes
Molecular Clocks
Arguably, some of the most significant evidence of evolution comes from examining the molecules and DNA found in all organisms (Figure below). The field of molecular biology did not emerge until the 1940s and has since confirmed and extended the conclusions about evolution drawn from other forms of evidence. Molecular clocks are used in molecular evolution to relate the time that two species d
iverged to the number of differences measured between the species' DNA sequences or protein amino acid sequences. These clocks are sometimes called gene clocks or evolutionary clocks. The fewer the differences the less time since the divergence of the species. For example, a chicken and a gorilla will have more differences between their DNA and protein amino acid sequences then a gorilla and an orangutan. This provides additional evidence that the gorilla and orangutan are evolutionally closer related than the gorilla and the chicken.
Molecular clocks, combined with other forms of evidence, such as evidence from the fossil record, have provided considerable evidence to estimate how long ago various groups of organisms diverged evolutionarily from one another.
Figure 7.23
Almost all organisms are made from DNA with the same building blocks. The genomes (all of the genes in an organism) of all mammals are almost identical.
Molecular Genetics
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