Freeman Chapter 22 BB Biological Science PDF

Biological Science Seventh Edition Chapter 22 Evolution by Natural Selection Copyright © 2020, 2017, 2014 Pearson Educ...

Biological Science Seventh Edition Chapter 22 Evolution by Natural Selection Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Chapter 22 Opening Roadmap Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Introduction to Evolution by Natural Selection (1 of 2) Evolution by natural selection: – Formulated independently by Darwin and Wallace – Theory explains: ▪ How populations have come to be adapted to environments ranging from arctic tundra to tropical wet forest ▪ Reveals 5 key attributes of life: – Populations of organisms evolve Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Introduction to Evolution by Natural Selection (2 of 2) Charles Darwin published On the Origin of Species by Means of Natural Selection in 1859 Theory: – Explains broad class of observations and is widely supported by evidence Scientific theories are often comprised of two components: 1. Pattern—observations about natural world 2. Process—mechanism that produces that pattern Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 22.1 The Rise of Evolutionary Thought Theory of evolution by natural selection is often described as revolutionary Scientific revolution replaces an existing idea about nature with radically different idea Models of the Diversity of Life Have Changed through Time Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Plato and Typological Thinking Greek philosopher Plato claimed: – Every organism was an example of a perfect essence, or type, created by God – Types were essentially unchanging Today, philosophers and biologists refer to ideas like this as typological thinking Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Aristotle and the Scale of Nature (1 of 2) Aristotle organized typological thinking into linear scheme called the great chain of being (scale of nature): – Species were fixed types – Species were organized into a sequence based on increasing size and complexity – Sequence started with minerals and lower plants – Humans were at top of the chain: ▪ Surpassed only by angels and God Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Aristotle and the Scale of Nature (2 of 2) 1700s—Aristotle’s ideas were still popular: – Central claims: ▪ Species were fixed types as Plato had proposed ▪ Some species are higher: – In the sense of being more complex or “better” than others Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Lamarck and the Idea of Evolution as Change through Time (1 of 2) 1809—Jean-Baptiste de Lamarck propose formal theory of evolution: – Simple organisms originate at base of chain by spontaneous generation (pattern of Lamarck’s theory) – Organisms evolve by moving up in chain over time – Lamarckian evolution is progressive: ▪ Producing larger, more complex or “better” species over time Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Lamarck and the Idea of Evolution as Change through Time (2 of 2) Lamarck suggested process responsible for this pattern was inheritance of acquired characters: – As individuals develop, their phenotype changes in response to environmental challenges – Phenotypic changes are passed on to offspring – Example: ▪ Giraffes develop long necks from stretching to reach food and produce offspring with long necks Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Darwin & Wallace and Evolution by Natural Selection (1 of 2) Darwin and Wallace proposed change in species through time: – Does not follow a linear, progressive pattern – Based on variation among individuals in populations: ▪ Population- group of individuals of the same species living in the same area at the same time – Individuals with certain traits produce more offspring than others without these traits – Darwin claimed that variation among individuals in a population was the key to understanding evolution Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Darwin & Wallace and Evolution by Natural Selection (2 of 2) 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 ▪ Its predictions could be tested through observation and experimentation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 22.2 the Pattern of Evolution: Have Species Changed, and Are They Related? Darwin described evolution as descent with modification: – Change over time produced modern, modified species from ancestral species Pattern component of the theory of evolution by natural selection predicts that: 1. Species change through time 2. Species are related by common ancestry Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence for Change through Time Fossils, remains or traces of organisms that lived in the past, are usually found in sedimentary rock, which appears in layers or strata The study of fossils, Paleontology, helped to lay the groundwork for Darwin’s ideas Paleontology was largely developed by Georges Cuvier, who speculated that the boundaries between strata represent catastrophic events Fossil record: – The many fossils that have been found and described in scientific literature make up the fossil record – What data from extant species support claim that they are modified forms of ancestral species? Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Evidence 1: The Vastness of Geologic Time (1 of 4) Fossils were 1) Rivers carry sediment into seas and swamps. Over time, sedimentary rock initially organized layers (strata) form under water. Some strata contain fossils. according to their relative ages Sedimentary rock Sedimentary layers (strata) rocks: 2) As water levels change and the seafloor is pushed – Form from sand upward, the strata and their fossils are exposed. or mud Younger stratum – Form in layers: with more recent fossils ▪ Younger Older stratum with older fossils layers are deposited on Formation of sedimentary strata top of olderCopyright © 2020, 2017, 2014 Pearson withInc. Education, fossils. All Rights Reserved Evidence 1: The Vastness of Geologic Time (2 of 4) Scientists created a geologic time scale: – Based on fossils’ relative positions in layers of sedimentary rock, geologic time is divided into eons, eras, periods, and epochs Geologic record indicated Earth was much, much older than 6000 yearsCopyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 1: The Vastness of Geologic Time (3 of 4) Researchers can now assign absolute ages to relative ages in geologic time scale: – Use radioactive decay – Steady rate of conversion of unstable “parent” atoms into stable “daughter” atoms Radiometric dating is based on: – Observed decay rates – Ratio of parent to daughter atoms in newly formed rocks – Ratio of parent to daughter atoms in particular rock sample Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 1: The Vastness of Geologic Time (4 of 4) Radiometric dating data indicate 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 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 3: Transitional Features Link Older and Younger Species (1 of 2) Early scientists observed that fossil species are strikingly similar to living species in the same geographic areas: – This pattern became known as “law of succession” Darwin interpreted this pattern as evidence that: – Species changed through time – Extinct and living forms were related and represented ancestors and descendants Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Evidence 2: Extinction Changes the Species Present over Time Many fossils provide evidence for extinct species (species that no longer exist) Darwin interpreted extinction as evidence that: – Species are dynamic – Array of species living on Earth has changed through time Recent analyses of fossil record suggest: – Over 99% of all species that have ever lived are now extinct – Extinctions have occurred continuously throughout Earth’s history Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Evidence 3: Transitional Features Link Older and Younger Species (2 of 2) Transitional features: – Traits in fossil species that are intermediate between ancestral and derived species – Provide strong evidence for change through time Data like these are consistent with predictions from theory of evolution: – If traits observed in more recent species evolved from Transitional Features during the traits in more ancient species, Evolution of the Tetrapod Limb then transitional forms are Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 4: Vestigial Traits Are Evidence of Change through Time (1 of 2) Vestigial trait: – A reduced or incompletely developed structure in an organism that has no (or reduced) function – Clearly similar to functioning organs or structures in closely related species Vestigial traits are: – Inconsistent with the idea of special creation – Evidence that the characteristics of species have changed over time Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Evidence 4: Vestigial Traits Are Evidence of Change through Time (2 of 2) Examples of vestigial traits include: – Some whales and snakes have tiny hip and leg bones that do not help them swim or slither – Ostriches and kiwis have reduced wings and cannot fly – Blind cave-dwelling fish have eye sockets but no eyes Vestigial Traits Are Reduced Versions – Monkeys and other primates of Traits in Other Species have long tails: ▪ But our tiny tailbone is too small to help maintain Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 5: Species Can Be Observed Changing Today Hundreds of contemporary populations have been documented undergoing evolutionary changes: – Evolution of antibiotic resistance in bacteria – Evolution of beak shape in finches Evolution can be observed on the scale of: – Days, weeks, and months – Not just millions of years Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 6: Similar Species Are Found in the Same Geographic Area (1 of 2) During five-year voyage, Darwin realized that species are related by common ancestor: – Collected plants and animals during voyage – Most famous among the birds were the Galapagos mockingbirds and finches – Distinct species collected from small geographic area: The Galapagos Finches Share a Common Ancestor ▪ Pattern made sense when interpreted in context of evolution or descent with modification Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 6: Similar Species Are Found in the Same Geographic Area (2 of 2) Phylogenetic tree: – Diagram that illustrates ancestor–descendant relationships among taxa Researchers compared finch DNA: – Placed birds on phylogenetic tree, which illustrate that they share a single common ancestor estimated to have Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 7: Related Species Share Homologies Another line of evidence comes from homologies Homology: – Similarity that exists in species descended from common ancestor Homology can be recognized and studied at three interacting levels: 1. Genetic 2. Developmental 3. Structural Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Genetic Homology Genetic homology: – Similarity in DNA nucleotide sequences, RNA nucleotide sequences, or amino acid sequences – Genetic code—most fundamental homology: ▪ With minor exceptions, all organisms use the same rules for transferring information coded in DNA into proteins Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Developmental Homology Developmental homology: – Similarity in developmental structures or processes – Example: ▪ Early chick, human, and cat embryos have tails and structures called pharyngeal pouches: – Product of similar development processes Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Structural Homology Structural homology: – Similarity in adult morphology – For example, most vertebrates have a common structural plan in the limb bones Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evidence 8: Formation of New Species from Preexisting Species Can Be Observed Today Biologists have documented dozens of contemporary populations that are undergoing speciation Speciation: – Process that results in one species splitting into two or more descendant species Killer whales appear to be undergoing speciation: – Some populations are so different in prey choice and social behavior: ▪ They no longer interbreed with other populations Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Anatomical and Molecular Homologies Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor They are a product of divergent evolution: – an accumulation of changes in two closely related populations derived from the same ancestor (same species) leading to speciation  A Different Cause of Resemblance: Convergent Evolution Convergent evolution is the evolution of similar, or analogous, features in distantly related groups Analogous traits arise when groups independently adapt to similar environments in similar ways Convergent evolution does not provide information about ancestry Evolution’s “Internal Conspiracy”— The Importance of Independent Data Sets (1 of 3) Multiple data sets support idea that species have descended, with modification, from common ancestor Internal consistency: – Observation that data from independent sources agree in supporting predictions made by a theory Evidence for the evolution of cetaceans—whales and dolphins—illustrates idea of internal consistency: – The fossil record: cetaceans are identified by unique ear bones – Phylogeny of the fossil cetaceans: ▪ Indicates a gradual transition between terrestrial and aquatic forms Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evolution’s “Internal Conspiracy”— The Importance of Independent Data Sets (2 of 4) Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Evolution’s “Internal Conspiracy”— The Importance of Independent Data Sets (2 of 3) Relative dating both support order of species indicated in phylogeny Phylogeny of living whales and dolphins: – Estimated from similar similarities and differences in DNA sequences – Indicate that hippos are closest relative to cetaceans Data on Evolution from Independent Sources A Consistent Vestigial hip and hindlimb bones are found in some adult whales and dolphin Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evolution’s “Internal Conspiracy”— The Importance of Independent Data Sets (3 of 3) Data from many different sources are much more consistent with evolution than with special creation Descent with modification: – More successful and powerful scientific theory – Explains observations—such as vestigial traits and close relationships among species on neighboring islands Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 22.3 The Process of Evolution: How Does Natural Selection Work? Many researchers had proposed the fact of evolution: – Darwin’s contribution was describing a process, natural selection, that could explain the pattern of descent with modification Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Darwin’s Inspiration (1 of 3) Darwin based his idea of natural selection on: – Model system – Work of Thomas Robert Malthus Artificial selection in pigeon breeding: – Darwin crossbred pigeons and observed how Diversity of Pigeon Breeds in Captivity characteristics were passed on to offspring – He concluded that diverse pigeon breeds all descended from wild Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Darwin’s Inspiration (2 of 3) Thomas Robert Malthus wrote the book An Essay on the Principle of Population: – Described a “struggle for existence:” ▪ Many more individuals are born than can survive ▪ Thus, people compete for resources Darwin combined several ideas to arrive at his concept of natural selection: – His observations of artificial selection – The notion of struggle for existence in natural populations – Variation in natural populations Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Darwin’s Inspiration (3 of 3) Both Darwin and Wallace arrived at the same idea, but Darwin’s name is widely associated with concept of natural selection: – He thought of it first – He provided extensive evidence in On the Origin of Species Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Darwin’s Four Postulates (1 of 2) Darwin broke process of evolution by natural selection into four criteria, or postulates: 1. Variation exists among individual organisms that make up a population 2. Some of the trait differences are heritable 3. Survival and reproductive success are highly variable 4. Subset of individuals that survive best and produce the most offspring is not a random sample of the population Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Darwin’s Four Postulates (2 of 2) 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:  Heritable variation leads to differential reproductive success Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Biological Definitions of Fitness, Adaptation, and Selection Biological fitness: – Ability of an individual to produce surviving, fertile offspring relative to that ability in other individuals in population Adaptation: – Heritable trait that increases an individual’s fitness in particular environment relative to individuals lacking that trait Selection: – Differential reproduction as result of heritable variation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 22.4 Evolution in Action: Measuring Natural Selection in Populations Today Theory of evolution by natural selection is testable: – Drug resistance in bacteria – Beak size and shape and body size changes in the Galápagos finches Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Case Study 1: How Did Mycobacterium tuberculosis Become Resistance to Antibiotics? The bacterium Mycobacterium tuberculosis causes tuberculosis (TB): – Infects lungs, causes fever, coughing, sweats, weight loss and often death TB receded in early 1900s due to: – Improved nutrition – Development of antibiotics However: – In late 1980s, rates of TB surged – In 1993, the WHO declared TB a global health emergency – Strains of bacteria responsible for increase were resistant to once-effective antibiotics Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved A Patient History Young man from Baltimore diagnosed with active TB infection: – Given several antibiotics for 6 weeks and doses of antibiotic rifampin for an additional 33 weeks – Tests showed the infection was cleared after 10 months – Two months later, he was readmitted to the hospital with recurrence of symptoms – He died 10 days later, despite additional antibiotic treatment Tests showed that M. tuberculosis bacteria in his lungs were completely resistant to rifampin Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved A Mutation in a Bacterial Gene Confers Resistance DNA from rifampin-resistant bacteria was found to have single point mutation in gene called rpo B: – rpo B codes for component of RNA polymerase – Mutation caused change in amino acid sequence resulting in change in enzyme shape Rifampin works by binding with M. tuberculosis RNA Alleles That Confer Drug Resistance polymerase and interfering with Increase in Frequency When Drugs Are Used transcription: Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Testing Darwin’s Postulates (1 of 2) Variation existed in the population: – Due to mutation, both resistant and nonresistant strains of T B were present before 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 © Copyright drug-resistant allele 2020, 2017, 2014 Pearson had Education, higher Inc. All Rights Reserved Testing Darwin’s Postulates (2 of 2) This example shows how natural selection acts on individuals because individuals experience differential success It also shows how only populations evolve, as allele frequencies change in populations, not individuals Understanding evolution by natural selection requires population thinking Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Web Activity: Natural Selection for Antibiotic Resistance Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved The Rise of Drug Resistance Resistance to wide variety of antibiotics is skyrocketing: – Physicians have no effective antibiotics available to treat infections – WHO announced that the world could soon Like Antibiotic Resistance, Pesticide and enter a “post-antibiotic Herbicide Resistance Are on the Rise era” – Drug resistant strains have killed 700,000 people annually Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Case Study 2: Why Do Beak Sizes and Shapes Vary in Galápagos Finches? Peter and Rosemary Grant conducted long- term research on population of medium ground finches found on Isle Daphne Major of the Galápagos Islands: – Found that beak form and body size are heritable in Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Selection during Drought Conditions (1 of 2) During Grants’ research, major drought led 84% of ground finch population to die of starvation Research team realized that die-off was a natural experiment: – Natural experiments allow researchers to compare treatment groups created by an unplanned change in conditions – Grants could test whether natural selection occurred by comparing population before and after the drought Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Selection during Drought Conditions (2 of 2) In only one generation, natural selection led to measurable change in characteristics of population: – Finch population’s average beak depth had increased: ▪ Deeper beaks were adaptive due to food availability during the drought ▪ Large, deep beaks were an adaptation for cracking large fruits and seeds – Alleles that led to development of Changes in a Medium Ground deep beaks had increased in Finch Population in Response to frequency Drought Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Continued Changes in the Environment, Continued Selection, and Continued Evolution 1983—Galapagos environment changed again: – In just seven months, more than 10 times the average annual rainfall occurred – Altered relative plant abundance – Small individuals with small, pointed beaks had exceptionally high reproductive Beak Depth in Medium Ground Finches Fluctuated over a 40-Year Interval success: ▪ Alleles associated with small, pointed beaks increased in frequency Over subsequent decades, the Grants have documented Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Which Genes Are under Selection? Many characteristics, including beak size, are polygenic: – Many genes each exert relatively small effect – Example: Beak depth is a polygenic trait – ALX1 gene regulates beak shape – HMGA2 gene regulates beak size This illustrates connection between natural selection on phenotype and evolutionary change in genotype Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 22.5 Debunking Common Misconceptions about Natural Selection and Evolution Although natural selection appears to be a simple process, research has shown that it is often misunderstood Let’s clarify three of the most common types of confusion Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Natural Selection Does Not Change Individuals Most important point to clarify about natural selection: – During the process, individuals do not change—only population does – During the drought, individual beaks did not change: ▪ Average beak depth increased over time because deep-beaked individuals had greater reproductive success – Individual M. tuberculosis cells did not change: ▪ Frequency of rifampin-resistant allele increased in population over time Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Natural Selection Is Not “Lamarckian” Inheritance In contrast to Lamarck’s hypothesis about inheritance of acquired characteristics: – Individuals do not change when they are selected: ▪ They simply produce more surviving offspring than other individuals do – Alleles in selected individuals become more frequent in population In other words, natural selection just sorts existing variants—it doesn’t change them Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Individuals Do Not Adapt Acclimatization: – Occurs when individual’s phenotype changes in response to changes in environment – Individual’s genotype remains fixed – Changes are not passed on Adaptation Has a Different to offspring because no Meaning in Science than in Everyday alleles have changed In contrast, adaptation occurs when allele frequencies in population change in response to natural selection Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Natural Selection Is Not Goal Directed Evolution by natural selection is not goal directed—it is not purposeful Mutations do not occur to solve problems—mutations just happen Adaptations do not occur because organisms want or need them Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Evolution is Not Progressive Evolution is not progressive: – Does not produce “better” or more complex organisms In fact, complex traits are routinely lost or simplified over time as a result of evolution by natural selection 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 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Traits Are Not Always Adaptive Vestigial traits confer no known benefit: – Organisms possessing them do not have higher fitness than those without Silent mutations: – Changes in DNA sequence that do not result in change in amino acid sequence of protein encoded by gene – Extremely common – Do not change phenotype and thus cannot be acted on by natural selection and are not adaptive Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Fitness Trade-Offs Exist (1 of 2) Fitness trade-off: – Compromise between traits, in terms of how those traits perform in the environment Because selection acts on many traits at once, every adaptation is a compromise Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Fitness Trade-Offs Exist (2 of 2) Examples of fitness trade-offs include compromises between: – Size of eggs or seeds an individual makes and the number of offspring it can produce – Rapid growth and long life span – Bright coloration and tendency to attract predators Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Traits Are Genetically Constrained Genetic correlation: – Occurs when selection favoring alleles for one trait causes correlated but suboptimal change in an allele for another trait – Because of pleiotropy, in which a single allele affects multiple traits Lack of genetic variation can also constrain evolution: – Natural selection can work only on existing variation in a population Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Traits Are Historically Constrained All traits evolve from previously existing traits: – Adaptations are constrained by history Not all traits are adaptive: – Even adaptive traits are constrained by genetic and historical factors Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Traits Are Environmentally Constrained Natural selection often occurs in context of changing environment Abiotic and biotic features of environment can change over geographic range of population: – Purple loosestrife plant has spread from southern U.S. to Canada in less than a century – Plant grows large and flowers late in the south where season is long – Plant grows less and flowers early in the north Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Natural Selection Is Not the Only Process of Evolution Do not confuse evolution with natural selection Evolution: – The pattern of change through time Natural selection: – A process that causes adaptive evolution Three other processes (random with respect to fitness): – Genetic drift – Gene flow – Mutation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved