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18: Evolution and the Origin of Species - Biology

18: Evolution and the Origin of Species - Biology


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18: Evolution and the Origin of Species

Chapter Summary

Evolution is the process of adaptation through mutation which allows more desirable characteristics to pass to the next generation. Over time, organisms evolve more characteristics that are beneficial to their survival. For living organisms to adapt and change to environmental pressures, genetic variation must be present. With genetic variation, individuals have differences in form and function that allow some to survive certain conditions better than others. These organisms pass their favorable traits to their offspring. Eventually, environments change, and what was once a desirable, advantageous trait may become an undesirable trait and organisms may further evolve. Evolution may be convergent with similar traits evolving in multiple species or divergent with diverse traits evolving in multiple species that came from a common ancestor. We can observe evidence of evolution by means of DNA code and the fossil record, and also by the existence of homologous and vestigial structures.

18.2 Formation of New Species

Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that occur within a shared habitat (sympatric speciation). Both pathways isolate a population reproductively in some form. Mechanisms of reproductive isolation act as barriers between closely related species, enabling them to diverge and exist as genetically independent species. Prezygotic barriers block reproduction prior to formation of a zygote whereas, postzygotic barriers block reproduction after fertilization occurs. For a new species to develop, something must introduce a reproductive barrier. Sympatric speciation can occur through errors in meiosis that form gametes with extra chromosomes (polyploidy). Autopolyploidy occurs within a single species whereas, allopolyploidy occurs between closely related species.

18.3 Reconnection and Speciation Rates

Speciation is not a precise division: overlap between closely related species can occur in areas called hybrid zones. Organisms reproduce with other similar organisms. The fitness of these hybrid offspring can affect the two species' evolutionary path. Scientists propose two models for the rate of speciation: one model illustrates how a species can change slowly over time. The other model demonstrates how change can occur quickly from a parent generation to a new species. Both models continue to follow natural selection patterns.


18: Evolution and the Origin of Species - Biology

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This text is based on Openstax Biology for AP Courses, Senior Contributing Authors Julianne Zedalis, The Bishop's School in La Jolla, CA, John Eggebrecht, Cornell University Contributing Authors Yael Avissar, Rhode Island College, Jung Choi, Georgia Institute of Technology, Jean DeSaix, University of North Carolina at Chapel Hill, Vladimir Jurukovski, Suffolk County Community College, Connie Rye, East Mississippi Community College, Robert Wise, University of Wisconsin, Oshkosh

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 Unported License, with no additional restrictions


Field Biologist

Many people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and invigorating. What if your job was to be outside in the wilderness? Field biologists by definition work outdoors in the “field.” The term field in this case refers to any location outdoors, even under water. A field biologist typically focuses research on a certain species, group of organisms, or a single habitat (Figure 18.4).

Figure 18.4 A field biologist tranquilizes a polar bear for study. (credit: Karen Rhode)

One objective of many field biologists includes discovering new species that have never been recorded. Not only do such findings expand our understanding of the natural world, but they also lead to important innovations in fields such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and nutritive knowledge. Other organisms can play key roles in ecosystems or be considered rare and in need of protection. When discovered, these important species can be used as evidence for environmental regulations and laws.

Processes and Patterns of Evolution

Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have some genetic basis otherwise, the selection will not lead to change in the next generation. This is critical because variation among individuals can be caused by non-genetic reasons such as an individual being taller because of better nutrition rather than different genes.

Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population. The genetic changes caused by mutation can have one of three outcomes on the phenotype. A mutation affects the phenotype of the organism in a way that gives it reduced fitness—lower likelihood of survival or fewer offspring. A mutation may produce a phenotype with a beneficial effect on fitness. And, many mutations will also have no effect on the fitness of the phenotype these are called neutral mutations. Mutations may also have a whole range of effect sizes on the fitness of the organism that expresses them in their phenotype, from a small effect to a great effect. Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each of the offspring.

A heritable trait that helps the survival and reproduction of an organism in its present environment is called an adaptation. Scientists describe groups of organisms becoming adapted to their environment when a change in the range of genetic variation occurs over time that increases or maintains the “fit” of the population to its environment. The webbed feet of platypuses are an adaptation for swimming. The snow leopards’ thick fur is an adaptation for living in the cold. The cheetahs’ fast speed is an adaptation for catching prey.

Whether or not a trait is favorable depends on the environmental conditions at the time. The same traits are not always selected because environmental conditions can change. For example, consider a species of plant that grew in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed, and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions.

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. When two species evolve in diverse directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants which share the same basic anatomies however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators (Figure 18.5).

Figure 18.5 Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing star (Liatrus spicata) and the (b) purple coneflower (Echinacea purpurea) vary in appearance, yet both share a similar basic morphology. (credit a: modification of work by Drew Avery credit b: modification of work by Cory Zanker)

In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous spans of time and help explain how evolution occurs. Natural selection acts on individual organisms, which in turn can shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for the genotype of an entire species to evolve. It is over these large time spans that life on earth has changed and continues to change.

Evidence of Evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species, to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.

Fossils provide solid evidence that organisms from the past are not the same as those found today, and fossils show a progression of evolution. Scientists determine the age of fossils and categorize them from all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years (Figure 18.6). For example, scientists have recovered highly detailed records showing the evolution of humans and horses (Figure 18.6). The whale flipper shares a similar morphology to appendages of birds and mammals (Figure 18.7) indicating that these species share a common ancestor.

Figure 18.6 In this (a) display, fossil hominids are arranged from oldest (bottom) to newest (top). As hominids evolved, the shape of the skull changed. An artist’s rendition of (b) extinct species of the genus Equus reveals that these ancient species resembled the modern horse (Equus ferus) but varied in size.

Anatomy and Embryology

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in the appendages of a human, dog, bird, and whale all share the same overall construction (Figure 18.7) resulting from their origin in the appendages of a common ancestor. Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures.

Figure 18.7 The similar construction of these appendages indicates that these organisms share a common ancestor.

Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales.


PSA.1 sp18 - Chapter 21 Objectives: The Origin and Evolutionary History of Life Chapter 18

Complete the vocabulary-matching sections and at least the first three objectives for each chapter. Doing so will prepare you for recitations and for the quizzes. You are highly encouraged to complete the rest of the objectives to keep you on track and so you can ask for clarification during recitation. All answers can be found in the textbook, even if a section has not yet been covered in lecture. Please print, complete, and bring to recitation.

Chapter 21 Objectives: The Origin and Evolutionary History of Life

Gasses such as CO2, N2 and H2S were abundantly present. For the formation of organic molecules to for you need a reactive surface such as pyrite or clay. The enzyme like features attract monomers that will spontaneously polymerize. These organic precursors formed neat thermal vents where colonies of tube worms now live today.

  1. Describe the Miller-Urey experimental model and explain how it could be used to investigate the synthesis of organic molecules (drawing helpful!)

The Miller-Urey experiment was meant to model the conditions on primitive earth. This was replicated by using a close system that represented the water cycle, also including a spark chamber, water, methane, hydrogen, and ammonia. Over time amino acids formed.

  1. Compare and contrast protobionts, microspheres, and coacervates, and discuss their relationship to the hypothesis of “pre-cell life” All are contained in a sphere. Coacervates are the closest structure to a cell and are held together by electrostatic forces. All of these are basic cells. Protobions are abiotically produced molecules self-assemble

D_ an organism that lives in or on another _B a specific type of protobiont containing enzymes used for more complex synthesis _C single membrane organelles originated by budding off the internal surface of the plasma membrane E_ double membrane organelles arose from a symbiotic relationship in which the endosymbiont living inside the cell lost its autonomy and became incorporated as an organelle within that cell _G one type of protobiont produced by adding water to abiotically formed polypeptides _F an organism not capable of producing its own organic molecules from inorganic materials (will be a consumer) _H a vesicle of abiotically produced polymers _A an organism capable of producing its own organic compounds from inorganic materials (photosynthesis for example) _I a column of prokaryote cells that become fossilized (living ones are extremely rare)

A. Autotroph B. Coacervate C. Endo- membranous Theory D. Endosymbiont E. Endosymbiosis Theory F. Heterotroph G. Microsphere H. Protobiont I. Stromatolite

into a sphere that contains water and is a precursor to cells. Microspheres are proteinoid from the sphere and is enclosed in water and inorganic material.

Describe how naturally occurring surfaces may have contributed to early chemical reactions and compare and contrast prebiotic soup hypothesis with iron-sulfur hypothesis for the evolution of protobionts and cells Pyrite and clay have enzyme-like features that attract monomers that can spontaneously polymerize. The iron- sulfur hypotheses- energy rich molecules and precursors of biological molecules. Prebiotic soup- water was a “sea of organic soup”.

Define the terms associated with the evolution of early life (anaerobe, aerobe, heterotroph, autotroph) Anaerobic- survives in absence of oxygen, Aerobe- requires oxygen, Heterotroph- ingests previously forms material, Autotroph- synthesize own organic nutrients from inorganic molecules

Describe the requirements preceding the origin of cells and life Four requirements for chemical evolution- little of no free oxygen so atmosphere was reducing environment, energy source was lightning cosmic and ultraviolet radiation, chemical building blocks including water dissolved inorganic molecules and atmospheric gases were present, time

Outline the major steps hypothesized to have occurred in the origin of cells and discuss which order might be correct (RNA first, DNA first, metabolism first, DNA/RNA/Protein) We already know that they can spontaneously form on clay. Proteins first would allow organized and directed synthesis of RNA and DNA. RNA-first hypothesis: self-replicating RNA arose first making ribozymes and RNA replication and eventually chemical reactions in cells, the role for proteins synthesized by ribosomes. Most likely RNA came first.

Describe stromatolites and discuss their significance in the evolution of early cells Stromatolites are microfossils, minute layers of prokaryotic cells, fossil evidence of early cells, some still living. These were thought to be the first cells.

Describe the first cells and the transformation from an anaerobic to an aerobic environment

Chapter 18 Objectives: Introduction to Darwinian Evolution

  1. Name several historical figures and describe their contribution to views on classification and evolution

Leonardo da Vince- recognized fossils as extinct animals/organisms Hutton- gradualism, could find intermediates Cuvier- punctuated equilibrium, caused by mass extinction Lamarck- acquired characteristics, use vs. disuse, first indication of theory of evolution, natural selecion

Name and explain Darwin’s four premises of evolution by natural selection

Varation- individuals in a population exhibit variation in traits, some improve chances of survival and reproductive success.

Overproduction- each generation can produce more than can survive.

Limits on population growth- competition for limited resources, not all survive to reproduce.

Differential reproductive success- survival of the fittest, individuals with most favorable combination of characteristics more likely to survive and reproduce

Describe the modern synthesis and how it impacts views on evolution Modern synthesis is the fusion of mendelian and Darwinian theory of evolution. It impacted the views of evolution because it emphasized the importance of genetics in evolution.

Define the terms population, species, and evolution Population- a group of individuals of the same species, species- a group of successfully interbreeding that also produce fertile offspring, evolution- similar organisms capable of interbreeding and producing offspring.

Compare and contrast the ideas of Darwin, Lamarck, and Wallace Darwin- variation, overproduction, limits on population growth, differential reproductive success Lamarck- acquired characteristics, use vs. disuse, first indication of theory of evolution, natural selection Wallace-

Compare and contrast the various forms of evidence supporting evolution (e.g. fossil record, homology, homoplasy, vestigial structures, and molecular and development homologies) Bias fossil record: favored-organisms that die is aquatic/marine environments such as bogs and tar pits, Not favored- dry environments, rainforests, organisms rapidly decay so they rarely fossilize

B major evolutionary changes that occur over a long period of time resulting in large phenotypic changes such as the formation of new species E__ a group of individuals of the same species __F a group of successfully interbreeding organisms that also produce fertile offspring C more-minor evolutionary changes that occur over just a few generations _G remnants of structures that were present and functional in the ancestral organisms _A organisms evolved similar characteristics as a result of exposure to similar environmental challenges (natural selection) _D an explanation of evolution that incorporates many aspects of biology such as molecular genetics, phylogeny, natural selection, mutations, etc.

A. Convergent Evolution B. Macroevolution C. Microevolution D. Modern Synthesis E. Population F. Species G. Vestigial Structure

Chapter 19 Objectives: Evolutionary Change in Populations

Define, compare and contrast, and give examples of microevolution including, nonrandom mating (inbreeding, assortative mating), mutation, genetic drift (bottleneck effect and founder effect), and gene flow Microevolution is the generation-to-generation changes within the population. Nonrandom mating is when mates seek others of similar size and textures. Within nonrandom mating, inbreeding and assertive mating occurs. With inbreeding the individuals are more closely related than if chosen randomly from the general population. Mutation can be spontaneous and produces genetic variation. Genetic drift will decrease the genetic variation within a population. An example of this is the bottleneck effect that decrease the population rapidly and randomly, and the founder effect when a few individuals found a colony. Gene flow generally increases variation within population and describes the migration of breeding individuals between two populations.

Define, compare and contrast, and give examples of natural selection and the impact on allele frequencies through mechanisms such as stabilizing selection, directional selection, and disruptive selection (drawing helpful!) Natural selection is when individuals with greater fitness are able to adapt to their environment. This causes changes in normal phenotypic distributions and will favor the alleals that are of greater fitness. Stabilizing selection is when the selective pressures do not favor the phenotypes on the ends of the curve. Directional selection will favor one side of the curve. Disruptive selection will favor colaltions at both ends of the distribution and is unfavorable to the middle of the curve.

H_ a change in allele frequencies from one generation to the next _F when a small group of individuals starts a new colony and the new population arises from that original group as a result, the group exhibits little genetic variation _G works to preserve balanced polymorphism occurs when the heterozygote has a higher level of fitness than either homozygote _M natural selection selects against one of the phenotypic extremes and favors the intermediates and other phenotypic extreme I_ genetic variation among individuals of a population _B an event that rapidly, randomly, and dramatically decreases the size of a population _K works to preserve balanced polymorphism occurs when the frequency of a phenotype in a population determines the fitness of that trait _L mating of genetically similar or genetically close individuals _C gradual change in a species phenotype and genotype through a series of geographically separate populations of the same species D natural selection selects against phenotypic extremes and favors intermediate phenotypes _E natural selection selects against the intermediates and favors the phenotypic extremes _J difference in genotype and phenotype frequencies in a population as a result of an environmental gradient (altitude for example) A a type of genetic polymorphism in which two or more alleles persist in a population as a result of natural selection

A. Balanced Polymorphism B. Bottleneck Effect C. Cline D. Directional Selection E. Disruptive Selection F. Founder Effect G. Frequency Dependent Selection H. Genetic Drift I. Genetic Polymorphism J. Geographic variation (cline) K. Heterozygote Advantage L. Inbreeding M. Stabilizing Selection

Chapter 20 Objectives: Speciation and Macroevolution

  1. Compare and contrast and give examples of prezygotic and postzygotic isolating mechanisms and barriers for reproductive isolation (e.g. temporal, habitat, behavioral, mechanical, and gametic isolation hybrid isolating mechanisms)

Prezygotic barriers are those that happen before zygote formation, whereas postzygotic barriers are those that occur following zygote formation. An example of a prezygotic barrier is mechanical isolation, where there are structural differences in reproductive organs that prevent mating. An example of a postzygotic barrier is hybrid inviability, where the embryo of interspecific hybrid spontaneously aborts.

  1. Define, describe and discuss macroevolution in the context of novel features, including preadaptations, allometric growth, and paedomorphosis Preadaptations is when new structures suddenly appear. Allometric growth is when growth rate of a particular body part changes over time. Paedomorphosis is the retention of juvenile characteristics as an adult.

A evolution of several species from one or a few ancestral species occurs in relatively short time frame H an area of overlap between closely related species or subspecies in which interbreeding occurs K retention of juvenile features in the adult body form O evolution proceeds with period of little or no change and then rapid changes occur over a relatively brief period of time C__ formation of two new species following the physical separation of individuals of a single population N something that occurs after fertilization (formation of a zygote) that prevents a hybrid from living long enough to form a new species F gametes of interspecies hybrid are not normal and able to produce a zygote G the hybrid is unable to reproduce successfully F1 and F2 generations may be produced J small-scale changes that occur within a species as a result of changes in the allele or genotype frequencies B growth of different body parts at different rates L a characteristic that functioned in one way originally but later changed in a way that was adaptive to the structure having a different role I large-scale changes over long time periods resulting in phenotypic changes that warrant placement of the organism into a new taxonomic group at or above the species level P formation of two new species within the geographic region of the parent population no physical barrier is present but reproductive isolating mechanisms are M something that prevents fertilization from occurring (prevents formation of a zygote) prevents hybrid formation D evolution occurs as a result of slow steady changes over time E egg and sperm of two different species are genetically incapable of producing a viable zygote and embryo

A. Adaptive Radiation B. Allometric Growth C. Allopatric Speciation D. Gradualism E. Hybrid Inviability F. Hybrid Sterility G. Hybrid Breakdown H. Hybrid Zone I. Macroevolution J. Microevolution K. Paedomorphosis L. Preadaptation M. Prezygotic Barrier N. Postzygotic Barrier O. Punctuated Equilibrium P. Sympatric Speciation

There are three hybrid zones- reinforcement zone, fusion zone and stability zone. Reinforcement zone is when overtime the hybrid is less fit than either the parents and the hybrid is no longer produced. Fusion zone is when over time differences in parental species weaken and hybrid numbers increase. Stability zone is where over time hybrids stabilize as a new species.

Define and describe the biological species concept of speciation and the associated problems A species is reproductively isolated and fertile offspring are produced. This only included sexual reproductions, whereas sometimes successful inbreeding can occur.

Define, compare and contrast and give examples of allopatric and sympatric speciation Allopatric speciation occurs when there is a physical separation of breeding members in the population. This can result in reproductive isolation of the two groups, potentially allowing a new species to evolve. Sympatric speciation is when breeding members of the population become reproductively isolates. This can occur in two ways: change in ploidy or change in ecology. This can cause two new species to evolve and maintain.

Name, define, compare and contrast the types of rate and pattern of speciation Punctuated equilibrium is long periods of stats followed by a short period of rapid speciation. This causes the species to have no intermediates and a rapid formation of new ones. Gradualism is the continual evolution of a new species over a long period of time. This makes it so there are intermediate changes in between.

Define, describe and discuss the macroevolutionary significance of adaptive radiation and extinction Macroevolution is the change in basic features from an existing organisms. A large-scale phenotypic change can result in a new taxon name. An example is jointed limbs in the evolution of arthropods or the appearance of feathers in birds.


Watch the video: Origin of species by Charles Darwin Darwins theory of evolution Edited by Ehtesham (October 2022).