© 2011 Pearson Education, Inc. Ch 24 Introduction A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on

© 2011 Pearson Education, Inc.

Ch 24 Introduction

• A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of knowledge that has been repeatedly confirmed through observation and experiment.

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Ch 24 Introduction

• Scientific theories are often comprised of a natural pattern and a process that explains that pattern.

• In 1858, Charles Darwin and Alfred Russel Wallace made the claim that evolution has occurred, that species have changed through time. Then they proposed natural selection as a process to explain the pattern of evolution.

• Evolution by natural selection has become one of the best-supported and most important theories in the history of scientific research.


A SHORT HISTORY


Plato and Typological Thinking

• The Greek philosopher Plato claimed that every organism was an example of a perfect essence or type created by God and that these types were unchanging.

• Today, philosophers and biologists refer to ideas like this as typological thinking.

– the idea that species are unchanging types and that variations within species are unimportant or even misleading.


Aristotle and the Great Chain of Being

• Aristotle ordered the types of organisms into a linear scheme called the great chain of being:

– a fixed sequence based on increasing size and complexity, with humans at the top.

• In the 1700s Aristotle’s ideas were still popular in scientific and religious circles. The central claims were that:

1. Species are fixed types.

2. Some species are higher—in the sense of being more complex or “better”—than others.


Lamarck and Evolution as Change through Time

• In 1809 Jean-Baptiste de Lamarck was the first to propose a formal theory of evolution.

• He proposed that simple organisms originate at the base of the great chain of being by spontaneous generation and then evolve by moving up the chain over time.

• Lamarck suggested that the process responsible for this pattern was the inheritance of acquired characters.

– Individuals change in response to their environment and then pass on those changes to their offspring.


Darwin &Wallace—Evolution by Natural Selection

• Darwin and Wallace proposed that change in species through time does not follow a linear, progressive pattern but instead is based on variation among individuals in populations.

– A population consists of individuals of the same species that are living in the same area at the same time.

• This view utilizes population thinking rather than typological thinking.

– Darwin claimed that variation among individuals in a population was the key to understanding the nature of species.


Darwin &Wallace—Evolution by Natural Selection

• The theory of evolution by natural selection was revolutionary for several reasons:

1. It overturned the idea that species are static and unchanging.

2. It replaced typological thinking with population thinking.

3. It was scientific. It proposed a mechanism that could account for change through time and made predictions that could be tested through observation and experimentation.


THE PATTERN OF EVOLUTION


The Pattern of Evolution

• Darwin described evolution as descent with modification, meaning that change over time produced modern, modified species from ancestral species.

The pattern component of the theory of evolution by natural selection makes two claims about the nature of species:

1. Species change through time.

2. Species are related by common ancestry.


Evidence for Change through Time

• Fossils are traces of organisms that lived in the past.

• The many fossils that have been found and described in the scientific literature make up the fossil record.


The Vastness of Geologic Time

• Most fossils are found in sedimentary rocks, which form from layers of sand or mud.

• Layers of sedimentary rock are associated with different intervals in the geologic time scale—a relative time scale based upon fossil content.

– Geologic time is divided into eons, eras, periods, and epochs.

• The geologic record indicated that the Earth was much, much older than the 6000 years claimed by proponents of the theory of special creation.


The Vastness of Geologic Time

• Researchers now use radioactive isotopes to assign absolute ages to the geologic time scale.

• Geologic data show that Earth is about 4.6 billion years old.

• The earliest signs of life are found in rocks about 3.4–3.8 billion years old.


Extinction Changes the Species Present over Time

• Many fossils provide evidence for extinct species, those that are no longer living.

• Darwin interpreted extinction as evidence that species are dynamic, and that the array of species living on Earth has changed through time.

• Recent analyses of the fossil record suggest that over 99 percent of all the species that have ever lived are now extinct.

– The data also indicate that extinctions have occurred continuously throughout Earth’s history—not just in a few catastrophic events.



Transitional Features

• Early scientists observed that extinct fossil species are typically succeeded, in the same region, by similar living species.

– This pattern became known as the “law of succession.”

• Darwin interpreted this as evidence that extinct and living forms are related and represent ancestors and descendants.


Transitional Features

• As the fossil record improved, many transitional forms have been discovered with traits that are intermediate between earlier and later species.

– These transitional forms provide strong evidence for change through time.

• Data like these are consistent with predictions from the theory of evolution: If the traits observed in more recent species evolved from traits in more ancient species, then intermediate forms are expected to occur.


Vestigial Traits

• A vestigial trait is a reduced or incompletely developed structure in an organism that has no function or reduced function, but is clearly similar to functioning organs or structures in closely related species.

• Vestigial traits are evidence that the characteristics of species have changed over time.


Current Examples of Change through Time

• Hundreds of contemporary populations have been documented undergoing evolutionary changes. For example:

– Bacteria have evolved resistance to drugs.

– Insects have evolved resistance to pesticides.

– Weedy plants have evolved resistance to herbicides.


Geographic Relationships

• There are often striking similarities among island species.

– For example, Darwin collected mockingbirds from the Galápagos islands. The mockingbirds were superficially similar, but different islands had distinct species.

• Darwin proposed that the mockingbirds were similar because they had descended from a common ancestor.

• The mockingbird species are part of a phylogeny, a family tree of populations or species.

• The relationship between different species can be shown on a phylogenetic tree.



Homology

• Another line of evidence comes from homologies.

– Homology is a similarity that exists in species descended from a common ancestor.

– Homology can be recognized and studied at three interacting levels: genetic, developmental, and structural.


Genetic Homology

• Genetic homology is a similarity in the DNA sequences of different species.

– For example, the eyeless gene in fruit flies and the Aniridia gene in humans are so similar that their protein products are 90 percent identical in amino acid sequence.


Developmental Homology

• Developmental homology is seen in embryos of different species.

– For example, tails and gill pouches are found in the embryos of all chordates, including chickens, humans, and cats. One explanation for these embryonic similarities is that the common vertebrate ancestor of these species had gill pouches and a tail.


Structural Homology

• Structural homology is a similarity in adult morphology.

– For example, most vertebrates have a common structural plan in the limb bones.


Homologies Interact

• The three levels of homology interact. Genetic homologies cause the developmental homologies observed in embryos, which then lead to the structural homologies recognized in adults.

• The most fundamental homology is probably the genetic code; nearly all living organisms use the same code for translating the language of DNA into the language of proteins.


Homologies Interact

• Many hypotheses about homology can be tested experimentally.

– For example, a fruit fly embryo received a mouse gene that signals where eyes should form; a fruit fly eye formed in the location where the mouse gene was expressed.

• The theory of evolution by natural selection predicts that homologies will occur. If species were created independently of one another, as the theory of special creation claims, these types of similarities would not occur.


Current Examples of Common Ancestry

• Biologists have documented dozens of contemporary populations that are undergoing speciation—a process that results in one species splitting into two or more descendant species.

• This is powerful evidence that extant species are the descendants of extinct species.

– This supports the claim that all organisms are related by descent from a common ancestor.


The Importance of Independent Data Sets

• The data drawn from numerous sources support the idea that species have descended, with modification, from a common ancestor.

• Perhaps the most powerful evidence for any scientific theory, including evolution by natural selection, is what scientists call “internal consistency.” This is the observation that data from independent sources agree in supporting predictions made by a theory.

– The evolution of the cetaceans illustrates the idea of internal consistency.


The Evolution of Cetaceans

• Several lines of evidence support the hypothesis that whales evolved from a terrestrial ancestor:

– Some fossil cetaceans resemble extant terrestrial mammals, others resemble extant aquatic mammals, and others are intermediate.

– A phylogeny, supported by relative and absolute dating, of fossil cetaceans indicates a gradual transition between terrestrial and aquatic, whalelike forms.

– Molecular comparisons indicate that hippos are the closest living relative of cetaceans and that they share a semiaquatic ancestor.

– Some cetaceans have vestigial limbs as adults or embryos.



How Does Natural Selection Work?

• While many researchers had proposed the fact of evolution, Darwin’s contribution was describing a pattern, natural selection, that could explain the process of descent with modification.

• Darwin broke the process of evolution by natural selection down into four steps, or postulates.


Darwin’s Four Postulates

1. Individuals in a population vary in their traits.

2. Some of these differences are heritable; they are passed on to offspring.

3. In each generation, many more offspring are produced than can survive; of these, only some will survive long enough to reproduce, and some will produce more offspring than others.

4. Individuals with certain heritable traits are more likely to survive and reproduce. Natural selection occurs when individuals with certain traits produce more offspring than do individuals without those traits.


Darwin’s Four Postulates

• The selected traits will increase in frequency in the population from one generation to the next, causing evolution—a change in the genetic characteristics of a population over time.

• Evolution is thus a logical outcome of the four postulates.

• Modern biologists condense Darwin’s four steps into two statements: Evolution by natural selection occurs when:

1. Heritable variation leads to

2. Differential success in survival and reproduction.


Biological Definitions of Fitness and Adaptation

• Biological fitness is the ability of an individual to produce offspring, relative to that ability in other individuals in the population.

In biology, an adaptation is a heritable trait that increases an individual's Darwinian fitness in a particular environment relative to individuals lacking that trait. Adaptations increase fitness—the ability to produce offspring.


Recent Research on Natural Selection

• The theory of evolution by natural selection is both observable and testable, with numerous examples available.

• One example is drug resistance in bacteria. Another is beak size and shape and body size changes in the Galápagos finches.


Resistance to Antibiotics: M. tuberculosis

• The bacterium Mycobacterium tuberculosis causes tuberculosis (TB), a disease that was once as great a public health issue as cancer is now.

• Sanitation, nutrition, and antibiotics such as rifampin greatly reduced deaths due to TB in industrialized nations between 1950 and about 1990.

• However, in the late 1980s, rates of TB started to surge due to the evolution of drug-resistant strains.

– In 1993 the World Health Organization declared TB a global health emergency.



• DNA from rifampin-resistant bacteria was found to have a single point mutation in a gene called rpoB.

• Rifampin works by interfering with RNA polymerase and transcription, but the mutation prevents rifampin from binding.

• Under normal conditions, mutant forms of RNA polymerase do not work as well as the normal form.

– However, during antibiotic therapy, cells with normal RNA polymerase grow more slowly or die, while those with mutant RNA polymerase proliferate.


Testing Darwin’s Postulates

• Variation existed in the population. Due to mutation, both resistant and nonresistant strains of TB were present prior to administration of the drug.

• The variation was heritable. The variation in the phenotypes of the two strains was due to variation in their genotypes.

• There was variation in reproductive success. Only a tiny fraction of M. tuberculosis cells survived the first round of antibiotics long enough to reproduce.

• Selection occurred. The cells with the drug-resistant allele had higher reproductive success.



• This example shows how natural selection acts on individuals (the human), because individuals experience differential success.

• It also shows how only populations (the bacteria) evolve, as allele frequencies change in populations, not individuals.


Drug Resistance Is a Widespread Problem

• Resistance to a wide variety of insecticides, fungicides, antibiotics, antiviral drugs, and herbicides has evolved in hundreds of insects, fungi, bacteria, viruses, and plants.

• In every case, evolution has occurred because individuals with the heritable ability to resist some chemical compound were present in the original population.

– As the susceptible individuals die from the pesticide, herbicide, or drug, the resistance alleles increase in frequency.


Morphological Changes in Galápagos Finches

• Peter and Rosemary Grant have done long-term research on the population of medium ground finches found on Isle Daphne Major of the Galápagos Islands.

• They found that beak form and body size are heritable in these birds.


Selection during Drought Conditions

• During the Grants’ research, a major drought led to 84 percent of the ground finches dying of starvation.

– The research team realized that the die-off was a natural experiment.

• In only one generation, natural selection led to a measurable change in the characteristics of the population—an increase in the finch population's average beak depth.

– (Mechanism: larger seeds more drought-resistant)

– Alleles that led to the development of deep beaks had increased in frequency.






Environmental Changes, Selection and Evolution

• Later, another change in the population's characteristics occurred as a result of seven months of rain, during which small individuals with small, pointed beaks had exceptionally high reproductive success.

• Over subsequent decades, the Grants have documented continued evolution in response to continued changes in the environment.


Natural Selection and Adaptation Misconceptions

• Although natural selection appears to be a simple process, research has shown that it is often misunderstood.


A Contrast with “Lamarckian” Inheritance

• In contrast to Lamarck’s hypothesis about the inheritance of acquired characteristics, individuals do not change when they are selected, they simply produce more surviving offspring than other individuals do.

– In other words, natural selection just sorts existing variants—it doesn’t change them.


Acclimation Is Not Adaptation

• Acclimation occurs when an individual’s phenotype changes in response to changes in the environment, but an individual’s genotype remains fixed, so the changes are not passed on to offspring.

• In contrast, adaptation occurs when the allele frequencies in a population change in response to natural selection.


Evolution Is Not Goal-Directed

• Evolution by natural selection is not goal directed. It simply favors individuals that happen to be better adapted to the environment at the time.

– Adaptations do not occur because organisms want or need them.


Evolution Is Not Progressive

• Evolution is not progressive, meaning producing “better” or more complex organisms.

• In fact, complex traits are routinely lost or simplified over time as a result of evolution by natural selection.

• Species are related by a common ancestry and all have evolved equally through time. As evolution continues, species may become simpler or more complex, depending on which traits are favored by the environment.


“Higher” or “Lower” Organisms?

• Scientifically, there is no such thing as “higher” or “lower” organisms.

• A human is no higher than its tapeworm parasite; each is well adapted to its environment.

• All populations have evolved by natural selection based on their ability to gather resources and produce offspring. All organisms are adapted to their environment, and are related by common ancestry.



Organisms Do Not Act for the Good of the Species

• Individuals with self-sacrificing alleles die and do not produce offspring.

• Individuals with selfish, “cheater” alleles survive and produce offspring.

• As a result, selfish alleles increase in frequency while self-sacrificing alleles decrease in frequency.

• Thus, it is not possible for individuals to sacrifice themselves for the good of the species.


Limitations of Natural Selection

• Although organisms are often exquisitely adapted to their environment, adaptation is far from perfect.

• A long list of circumstances limits the effectiveness of natural selection.


Nonadaptive Traits

• Adaptation is not a perfect process.

– For example, vestigial traits confer no known benefit—those organisms possessing them do not have higher fitness than those without.

• Not all traits are adaptive, and the adaptations that organisms have are constrained in a variety of important ways. These include genetic constraints, fitness trade-offs, and historical constraints.


Genetic Constraints

• Selection is not able to optimize all aspects of a trait due to certain genetic constraints.

• Genetic correlation occurs when selection favoring alleles for one trait causes a correlated but suboptimal change in an allele for another trait.

– Genetic correlations occur because of pleiotropy, in which a single allele affects multiple traits.

• Lack of genetic variation can also constrain evolution, because natural selection can work only on existing variation in a population.


Fitness Trade-Offs

• A fitness trade-off is a compromise between traits, in terms of how those traits are adapted for the environment.

• Because selection acts on many traits at once, every adaptation is a compromise.

• Examples include trade-offs between the size of eggs or seeds that an individual makes and the number of offspring it can produce, between rapid growth and long life span, and between bright coloration and tendency to attract predators.


Historical Constraints

• Because all traits evolve from previously existing traits, adaptations are constrained by history.

• Not all traits are adaptive, and even adaptive traits are constrained by genetic and historical factors.

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© 2011 Pearson Education, Inc. Ch 24 Introduction A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on