evolution

Copyright Pearson Prentice Hall

Biology

Chapter 15 Chapter 16 Section 17-4

EVOLUTION


15-1 The Puzzle of Life's Diversity


15-1 The Puzzle of Life's Diversity

Evolution is the process by which modern organisms have descended from ancient organisms. A scientific theory is a well-supported testable explanation of phenomena that have occurred in the natural world.

Voyage of the Beagle In 1831, Darwin left England aboard the H.M.S. Beagle for a voyage around the world. Darwin went ashore and collected plant and animal specimens for his collection. He studied the specimens, read the latest scientific books, and filled many notebooks with his observations and thoughts.


Voyage of the Beagle

During his travels, Darwin made numerous observations and collected evidence that led him to propose a hypothesis about the way life changes over time.

That hypothesis has become the theory of evolution.

Darwin observed that many plants and animals were well suited to the environments they inhabited. He was impressed by the ways in which organisms survived and produced offspring.

Darwin was puzzled by where different species lived and did not live.

Grasslands in some regions were similar to one another but were inhabited by very different animals.


Living Organisms and Fossils Darwin collected the preserved remains of ancient organisms, called fossils. Some of those fossils resembled organisms that were still alive. Others looked completely unlike any creature he had ever seen.

The Galápagos Islands Darwin observed that the Galápagos Islands were close together but had very different climates.

Darwin observed that the characteristics of many animals and plants varied noticeably among the different islands of the Galápagos.


Darwin wondered if animals living on different islands had once been members of the same species.

These separate species would have evolved from an original ancestor species.


15–2 Ideas That Shaped Darwin's Thinking


Hutton and Lyell helped scientists recognize that Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present.


Hutton and Geological Change In 1795, James Hutton published a hypothesis

about the geological forces that shaped Earth. Most of these geological forces operate very

slowly, over millions of years. Hutton proposed that Earth had to be very old.

Lyell’s work explained how geological features could be built up or torn down over long periods of time.


This understanding of geology influenced Darwin:

• If the Earth could change over time, life might change as well.

• It would have taken many years for life to change in the way Lyell suggested.

This would have been possible only if the Earth were extremely old.

Lamarck's Evolution Hypotheses

Jean-Baptiste Lamarck recognized that:

•living things have changed over time.

•all species are descended from other species.

•organisms are adapted to their environments.


Population Growth

• In 1798, Thomas Malthus published a book in which he noted that babies were being born faster than people were dying.

•The only forces he observed that worked against this growth were war, famine, and disease.


Malthus reasoned that if the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone.


When Darwin read Malthus’s work, he realized that this reasoning applied to plants and animals.

If all the offspring of almost any species survived for several generations, they would overrun the world.

This information was central to Darwin’s explanation of evolutionary change.


15-3 Darwin Presents His Case


Publication of On the Origin of Species

•Darwin filled notebooks with his ideas about species diversity and the evolution process.

•Darwin was stunned and disturbed by his discoveries.

•He shelved his manuscript for years and told his wife to publish it in case he died.


In 1858, Darwin received a short essay from naturalist Alfred Wallace.

The essay summarized Darwin’s thoughts on evolutionary change.

Later that year, Wallace’s essay was presented with some of Darwin’s work.

In 1859, Darwin published his book, On the Origin of Species.


In his book, Darwin:

•proposed a mechanism for evolution called natural selection.

•presented evidence that evolution has been taking place for millions of years—and continues in all living things.


Inherited Variation and Artificial Selection

Members of each species vary from one another in important ways.

• In Darwin’s day, variations were thought to be unimportant, minor defects.

•Darwin argued that this variation mattered.


Darwin noted that

plant and animal breeders breed only the largest hogs or the cows that produced the most milk. Darwin termed this process artificial selection.


Artificial selection is the intentional breeding for certain traits.


Inherited Variation and Artificial Selection

Brussels Sprouts

KohlrabiAncestral Species

Kale

Broccoli

Cauliflower


Evolution by Natural Selection

•Darwin compared processes in nature to artificial selection.

•By doing so, he developed a scientific hypothesis to explain how evolution occurs.

The Struggle for Existence

Darwin realized that high birth rates and a shortage of resources would force organisms to compete.


The struggle for existence means that members of each species compete regularly to obtain food, living space, and other necessities of life. The struggle for existence was central to Darwin's theory of evolution.


•The ability of an individual to survive and reproduce in its specific environment is fitness.

•Darwin proposed that fitness is the result of adaptations.

•An adaptation is any inherited characteristic that increases an organism's chance of survival.


Successful adaptations enable organisms to become better suited to their environment and better able to survive and reproduce.


Individuals with characteristics that are not well suited to their environment either die or leave few offspring.

Individuals that are better suited to their environment survive and reproduce most successfully.

Darwin called this process survival of the fittest.


Because of its similarities to artificial selection, Darwin referred to the survival of the fittest as natural selection. In natural selection, the traits being selected contribute to an organism's fitness in its environment.

Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species' fitness in its environment.


•Natural selection produces organisms that have different structures, establish different niches, or occupy different habitats.

•Each living species has descended, with changes, from other species over time.

•Darwin referred to this principle as descent with modification.


Descent with modification implies that all living organisms are related to one another, a principle known as common descent.


Evidence of Evolution

Darwin argued that living things have been evolving on Earth for millions of years. Evidence for this process could be found in the fossil record, the geographical distribution of living species, homologous structures of living organisms, and similarities in early development, or embryology.


The Fossil Record • Darwin saw fossils as a record of the history of life on Earth.

•By comparing fossils from older rock layers with fossils from younger layers, scientists could document that life on Earth has changed over time.


Geographic Distribution of Living Species •Darwin decided that all Galápagos

finches could have descended with modification from a common mainland ancestor.

•Darwin’s theory was that species now living on different continents had each descended from different ancestors.


However, because some animals on each continent were living under similar ecological conditions, they were exposed to similar pressures of natural selection.

Because of these similar selection pressures, different animals ended up evolving certain features in common.



Similar, But Unrelated Species



Homologous Body Structures

•Structures that have different mature forms but develop from the same embryonic tissues are called homologous structures.

•Similarities and differences in homologous structures help biologists group animals according to how recently they last shared a common ancestor.


Homologous Structures

Turtle Alligator Bird Mammal

Ancient, lobe- finned fish


Not all homologous structures serve important functions.

The organs of many animals are so reduced in size that they are just vestiges, or traces, of homologous organs in other species.

These organs are called vestigial organs.

Vestigal organs no longer work, they are remnants of organs that ancestor species once used.


Similarities in Embryology

•The early stages, or embryos, of many animals with backbones are very similar.

• The same groups of embryonic cells develop in the same order and in similar patterns to produce the tissues and organs of all vertebrates.


Summary of Darwin's Theory

Individual organisms differ, and some of this variation is heritable.

Organisms produce more offspring than can survive, and many that do survive do not reproduce.

Because more organisms are produced than can survive, they compete for limited resources.


Individuals best suited to their environment survive and reproduce most successfully.

These organisms pass their heritable traits to their offspring. Other individuals die or leave fewer offspring.

This process of natural selection causes species to change over time.


Species alive today are descended with modification from ancestral species that lived in the distant past.

This process, by which diverse species evolved from common ancestors, unites all organisms on Earth into a single tree of life.


Evolutionary Theory

•Scientific advances in many fields of biology, geology, and physics have confirmed and expanded most of Darwin’s hypotheses.

•Evolutionary theory continues to change as new data are gathered and new ways of thinking arise.

16-1 Genes and Variation16-1 Genes and Variation

How Common Is Genetic Variation? Many genes have at least two forms, or alleles. All organisms have genetic variation that is “invisible” because it involves small differences in biochemical processes. An individual organism is heterozygous for many genes.

Genetic variation is studied in populations.

A population is a group of individuals of the same species that interbreed. A gene pool consists of all genes, including all the different alleles, that are present in a population.

The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

Relative frequency is often expressed as a percentage.

Gene Pool for Fur Color in Mice

Sample Population Frequency of Alleles

allele for brown fur

allele for black fur

Evolution is any change in the relative frequency of alleles in a population.

The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

A mutation is any change in a sequence of DNA.

Mutations occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment. Mutations do not always affect an organism’s phenotype.

Most heritable differences are due to gene shuffling. Crossing-over increases the number of genotypes that can appear in offspring.

Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.

Single-Gene and Polygenic Traits

The number of phenotypes produced for a given trait depends on how many genes control the trait.

A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.

The allele for a widow’s peak is dominant over the allele for a hairline with no peak.

However, the presence of a widow’s peak may be less common in a population.

In real populations, phenotypic ratios are determined by the frequency of alleles as well as by whether the alleles are dominant or recessive.

Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes.

Height in humans is a polygenic trait.

A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution.

16-2 Evolution as Genetic Change16-2 Evolution as Genetic Change

16-2 Evolution as Genetic Change

Natural selection affects which individuals survive and reproduce and which do not.

If an individual dies without reproducing, its alleles are removed from the population’s gene pool. If an individual produces many offspring, its alleles may increase in frequency.


Evolution is any change over time in the relative frequencies of alleles in a population.

Populations, not individual organisms, can evolve over time.


Natural selection can lead to changes in allele frequencies and thus to evolution.

Organisms of one color may produce fewer offspring than organisms of other colors.

For example, a lizard population is normally brown, but has mutations that produce red and black forms.

Red lizards are more visible to predators, so they will be less likely to survive and reproduce. Therefore, the allele for red color will become rare.

Black lizards may warm up faster on cold days. This may give them energy to avoid predators. In turn, they may produce more offspring.

The allele for black color will increase in relative frequency.

Natural selection can affect the distributions of phenotypes in any of three ways: •directional selection

•stabilizing selection

•disruptive selection

Directional Selection

When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.

The range of phenotypes shifts as some individuals survive and reproduce while others do not.

In this case, birds with larger beaks have higher fitness. Therefore, the average beak size increases.

Stabilizing Selection

When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.

This keeps the center of the curve at its current position, but it narrows the overall graph.

Human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average.

Disruptive Selection

When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.

If the pressure of natural selection is strong enough and long enough, the curve will split, creating two distinct phenotypes.

If average-sized seeds become scarce, a bird population will split into two groups: one that eats small seeds and one that eats large seeds.

A random change in allele frequency is called genetic drift.

In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance.

Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

Genetic drift may occur when a small group of individuals colonizes a new habitat.

Individuals may carry alleles in different relative frequencies than did the larger population from which they came.

The new population will be genetically different from the parent population.

Genetic Drift

Descendants

Population A Population B

When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect.

Evolution Versus Genetic Equilibrium

The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

When allele frequencies remain constant it is called genetic equilibrium.

Five conditions are required to maintain genetic equilibrium from generation to generation: • there must be random mating, • the population must be very large, • there can be no movement into or

out of the population, • there can be no mutations, and • there can be no natural selection.


Random Mating

Random mating ensures that each individual has an equal chance of passing on its alleles to offspring. In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits.

Large Population

Genetic drift has less effect on large populations than on small ones. Allele frequencies of large populations are less likely to be changed through the process of genetic drift.


No Movement Into or Out of the Population

Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population. The population's gene pool must be kept together and kept separate from the gene pools of other populations.


No Mutations

If genes mutate, new alleles may be introduced into the population, and allele frequencies will change.


No Natural Selection

All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another. There can be no natural selection operating on the population.


16-3 The Process of Speciation16-3 The Process of Speciation

16-3 The Process of Speciation

Natural selection and chance events can change the relative frequencies of alleles in a population and lead to speciation.

Speciation is the formation of new species.

A species is a group of organisms that breed with one another and produce fertile offspring.

Isolating Mechanisms

What factors are involved in the formation of new species?

The gene pools of two populations must become separated for them to become new species.



As new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.


Reproductive isolation can develop in a variety of ways, including:

behavioral isolation geographic isolation temporal isolation


Behavioral Isolation

Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.


Geographic Isolation

Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains.


Geographic barriers do not guarantee the formation of new species.

If two formerly separated populations can still interbreed, they remain a single species.

Potential geographic barriers may separate certain types of organisms but not others.


Temporal Isolation

Temporal isolation occurs when two or more species reproduce at different times.

Testing Natural Selection in Nature

Speciation in Darwin's Finches

Speciation in Darwin's Finches Speciation in the Galápagos finches occurred by:

founding of a new population geographic isolation changes in new population's gene pool reproductive isolation ecological competition


Founders Arrive

A few finches—species A—travel from South America to one of the Galápagos Islands.

There, they survive and reproduce.


Geographic Isolation

Some birds from species A cross to a second island.

The two populations no longer share a gene pool.


Changes in the Gene Pool

Seed sizes on the second island favor birds with large beaks.

The population on the second island evolves into population B, with larger beaks.


Reproductive Isolation

If population B birds cross back to the first island, they will not mate with birds from population A. Populations A and B are separate species.


Ecological Competition

As species A and B compete for available seeds on the first island, they continue to evolve in a way that increases the differences between them.

A new species—C—may evolve.

Studying Evolution Since Darwin

Studying Evolution Since Darwin

Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past.

Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved.

17-4 Patterns of Evolution

Macroevolution refers to large-scale evolutionary patterns and processes that occur over long periods of time.

Six important topics in macroevolution are:

•extinction •adaptive radiation •convergent evolution •coevolution •punctuated equilibrium •changes in developmental genes

Extinction

•More than 99% of all species that have ever lived are now extinct.

• In the past, most researchers looked for a single, major cause for each mass extinction.

•Many paleontologists now think that mass extinctions were caused by several factors.

Extinction

What effects have mass extinctions had on the history of life? Mass extinctions have: •provided ecological opportunities for

organisms that survived • resulted in bursts of evolution that

produced many new species

Adaptive Radiation

•Adaptive radiation is the process by which a single species or a small group of species evolves into several different forms that live in different ways.

•For example, in the adaptive radiation of Darwin's finches, more than a dozen species evolved from a single species.

Adaptive radiations can occur on a much larger scale. The disappearance of dinosaurs then resulted in the adaptive radiation of mammals.

Adaptive Radiation of Mammals

ArtiodactylsCetaceans

Perissodactyls

Tubulidentates

HyracoidsSirenians

Proboscideans

Ancestral Mammals

Convergent Evolution •Different organisms undergo adaptive radiation in different places or at different times but in similar environments. •The process by which unrelated organisms come to resemble one another is called convergent evolution. •Convergent evolution has resulted in sharks, dolphins, seals, and penguins.

Structures that look and function similarly but are made up of parts that do not share a common evolutionary history are called analogous structures. A dolphin’s fluke and a fish’s tail fin are analogous structures.

Coevolution

•Sometimes organisms that are closely connected to one another by ecological interactions evolve together.

•The process by which two species evolve in response to changes in each other over time is called coevolution. Watch vid

Punctuated Equilibrium

•Darwin felt that biological change was slow and steady, an idea known as gradualism.

Punctuated equilibrium is a pattern of evolution in which long stable periods are interrupted by brief periods of more rapid change.

The concept of punctuated equilibrium has generated debate and is still controversial among some biologists today.

Evolution has often proceeded at different rates for different organisms at different times during the history of life on Earth.

Developmental Genes and Body Plans

• It is suspected that changes in genes for growth and differentiation during embryological development could produce changes in body shape and size.

•Small changes in the activity of control genes can affect many other genes to produce large changes in adult animals.

•Evolution of Wings in Insects

Ancient Insect Two Types of Modern Insects

Small changes in the timing of cell differentiation and gene expression can make the difference between long legs and short ones.

evolution

Education

fossils darwin

different species

galpagos islands darwin

different animals

different islands

modern organisms

geological change

shaped earth