Evolution and Its Core Principles

•      Evolution provides the only scientific account of how people and other living things came to exist

•      Multiple varying religious accounts explaining origins also exist

•      Is religion compatible with our scientific understanding of the world?

•      Many religious believers see no problem in reconciling their religious beliefs with evolution

–  It is possible to embrace both their religion and scientific findings about how life developed

•      Others feel that science and religion cannot be reconciled on this issue

–  In their minds, the scientific account must be false

•      The evidence for evolution was amassed in the same way that scientific evidence is gathered in any field

–  Rigorous measurement

–  Conducting experiments, etc.

•      Findings in cancer research (for example) are almost never questioned by nonscientists

•      Findings in evolutionary biology are constantly challenged by nonscientists

•      Why the difference?

•      Most fields of scientific research tell us about small-scale processes in the living world

•      Evolutionary biology tells us about the origins and development of the living world

–  Evolution carries considerable philosophical and even religious meaning

•      No other concept in biology has the explanatory power of evolution

–  No other factor has so strongly shaped life

–  Evolution is the mortar holding  all of the various fields of biology together

•      One cannot help but ask questions about the natural world

–  Why are redwood trees so tall?

–  Why does a peacock have such beautiful yet cumbersome plumage?

•      How do we answer these questions?

•      Through the middle of the nineteenth century, answers to questions like these were inadequate

–  “The creator gave unique forms to living things for reasons we can’t understand”

•      Modern evolutionary biologists would answer such questions very differently

–  e.g., “Redwoods are because, over millions of years, they have been in competition with one another for the resource of sunlight.  Individual trees with the genetic capacity to grow tall got this sunlight, flourished, and left relatively many offspring, which made for taller trees in successive generations.”

•      It is the Theory of Evolution that allows us to make sense of the diversity of nature

•      Two principles lie at the core of this theory

–  Common descent with modification

–  Natural selection

Common descent with modification

•      Particular groups of living things can undergo modification in successive generations

•      Such change sometimes results in the formation of new and separate species

–  One species separates into two, and these two species separate even further

Common descent with modification

•      All of the branches in the evolutionary tree of life can be traced back to a single trunk

•      All living things on Earth are ultimately descended from a single, ancient ancestor

Natural selection

•      Natural selection is the most important process shaping evolution

–  Other processes also exist

–  These processes will be discussed later

Natural selection

•      A process in which the differential adaptation of organisms to the environment selects those traits that will be passed on with greater frequency from one generation to the next

Natural selection

•      e.g., Taller trees get more sunlight

–  Better adapted to the environment

–  Leave more offspring than others of their generation

•   Offspring inherit genetic capacity to grow tall

•   These offspring will have more offspring

Natural selection

•      e.g., Taller trees get more sunlight

–  “Tallness” is being selected for transmission to larger proportions of the tree population

•   Population evolves in direction of greater tallness

•      Evolution provides us a means of understanding nature in all its complexity

–  Allows understanding of familiar things

•   e.g., Giant redwoods, peacocks, etc.

–  Allows an understanding of new natural phenomena

•   e.g., Strains of bacteria are becoming resistant to antibiotics (antibiotic resistance is simply an evolutionary adaptation in rapidly evolving bacteria)

•      The theory of evolution is very far-reaching

–  Important beyond the domain of biology

–  Alters basic assumptions about the world

•      Humans can no longer view themselves as something separate from all other living things

–  Humans, and all other living things, are descended from other species

•   We are one tiny branch of an immense evolutionary tree

•      Understanding evolution meant the end of the idea of a fixed living world

–  Life forms only appear to be fixed because the human lifespan is so short relative to evolutionary time

–  Birds have not always been birds

•   Descendants of dinosaurs

–  Whales have not always been whales

•   Descendants of land-walking mammals

–  etc.

•      Life has been shaped through natural selection

–  Natural selection shapes, but it does not design

–  No mind and no goals

–  No consciousness and no morals

•   Neither cruel nor kind, simply indifferent

Charles Darwin and the Theory of Evolution

•      The essentials of the theory of evolution were assembled by Charles Darwin

–  Nineteenth-century British naturalist

•      Darwin’s contributions were twofold

–  Developed existing ideas about descent with modification

•   Provided a large body of evidence in support of descent with modification

–  First to perceive that natural selection is the primary mechanism of evolutionary change

•      Began medical training at the University of Edinburgh at age 16

–  Darwin’s father was a physician

–  Ultimately decided against this path

•      Darwin’s father then decided that Charles should study for the ministry

–  Entered Cambridge University at age 20

–  Theology took a back seat to the study of nature

•   Darwin received a divinity degree

•   Darwin also gained a solid background in life science and earth science

•      Darwin became the resident naturalist upon the HMS Beagle

–  22 years old

–  Position arranged by one of his Cambridge professors

–  Ship was to survey coastal areas

•   Particularly the South American coast

–  Darwin spent the next five years on this research vessel

Evolutionary Thinking
before Darwin

•      Long held views of life on Earth and of the Earth itself were being challenged at this time

–  Charles Lyell

–  Jean-Baptiste de Lamarck

–  Georges Cuvier

•      Charles Lyell’s Principles of Geology

–  Published in 1830

–  The only book Darwin brought on the Beagle

–  Geological forces still operating could account for the changes geologists could see in the Earth’s surface

•   Earth had not been put in its final form at its moment of “creation”

•   The Earth must be much older than originally thought

•      Darwin’s realization

–  If slow and continual change is sufficient to bring about drastic change on the Earth itself, then perhaps such slow and continual change is also sufficient to bring about drastic change amongst the living world

•      Jean-Baptiste de Lamarck

–  Believed that populations changed form over generations

•   “Inheritance of acquired characteristics”

–  Published in 1809

•      e.g., Lamarck argued that ducks gained webbing on their feet through the act of swimming, and passed this trait to their offspring

–  Lamarck correctly understood that populations do evolve

–  The mechanism Lamarck proposed for this evolutionary change was not accurate

•      Georges Cuvier examined fossil-laden rocks of the Paris Basin

•      Provided conclusive evidence of the extinction of species on Earth

–  Radical notion

–  Many Christians believed that their creator would never allow one of his creatures to perish

–  Cuvier erroneously thought that new acts of creation followed acts of devastation

Darwin’s Insights Following
the Beagle’s Voyage

•      Darwin observed and collected specimens throughout his entire voyage

•      Four years into the voyage, the Beagle spent five weeks at the Galápagos Islands

–  Volcanic outcroppings west of Ecuador

•      Darwin saw many strange animals

–  Iguanas, tortoises, birds, etc.

–  These animals varied between the islands

Darwin’s Insights

•      Darwin collected and preserved many specimens throughout his voyage

•      Notably, he collected many birds from the Galápagos Islands

•      A bird expert at the Zoological Society of London identified the birds Darwin collected in the Galápagos Islands

•      The three varieties of mockingbirds represented three different species

•      The small birds were all separate species of finches

–  Species found nowhere but the Galápagos

–  Darwin had believed them to be blackbirds, finches, wrens, and warblers

•      In these finches, Darwin saw common descent

–  The Galápagos finches were related to finches on the South American mainland

–  Mainland finches were transported to the Galápagos Islands

–  Over time, these finches diverged into separate species

–  Galápagos tortoises, iguanas, and cactus plants displayed the same pattern

•      Before his voyage aboard the Beagle, Darwin thought species to be fixed

•      Darwin perceived common descent with modification

•      He was not immediately aware of the mechanism by which evolution occurred

•      An Essay on the Principle of Population by T. R. Malthus

–  Malthus discussed human populations competing for limited resources

–  Darwin realized that all competition for limited resources was a common feature of all populations

–  Darwin saw this “struggle for existence” as a key driving force of evolution

•      Organisms having “favorable variations” were preserved

–  Left proportionally more offspring

•       Organisms having “unfavorable variations” were destroyed

–  Left fewer or no offspring

•      Over generations, organisms evolved in the direction of the favorable variations

•      Upon realizing how evolution occurred, Darwin did not immediately publish his ideas

–  Darwin kept busy

•   Publishing on geology

•   Breeding pigeons

•   Studying variation in barnacles

–  Darwin finally began writing his key book in 1856

•   Darwin published On The Origin of Species by Means of Natural Selection in 1859

Alfred Russel Wallace

•      Alfred Russel Wallace

–  English naturalist

–  Collected bird and butterfly specimens from South America and Southeast Asia

–  Independently realized that natural selection is the mechanism of evolution

–  Recorded his ideas and sent them, ironically, to Charles Darwin

•      Wallace’s manuscript and some of Darwin’s letters were both presented at a meeting of a scientific society in 1858

–  Darwin completed his work and published it the following year

–  The entire first printing of this book sold out on the first day it was published

•      Darwin and Wallace independently determined that natural selection was the driving force behind evolution

–  Darwin formulated the theory decades before Wallace

–  Darwin supported his ideas with far more documentation than Wallace

–  Darwin received the bulk of the credit for elucidating the mechanism of evolution

The Acceptance of Evolution

•      Darwin’s ideas sparked both praise and scorn

•      Within fifteen years of publishing The Origin, virtually all naturalists accepted that evolution occurred

–  Scientists had little trouble accepting the fact of evolution

•      It took scientists longer to accept that natural selection was the means by which evolutionary change occurs

–  This wasn’t universally accepted until the middle of the 20^th century

–  The theory part of evolutionary change

Opposition to The Theory of Evolution

•      Evolution has an unusual status among major fields of biology

–  Its findings are regularly being challenged as being unproven or simply wrong

–  For the average person, these attacks make it appear that the theory of evolution is just a guess, rather than being solidly grounded in evidence

•      There is no legitimate “scientific debate” regarding the occurrence of evolution

–  Why do many people have the mistaken impression that there is such a debate?

•   There is widespread misunderstanding of the word “theory”

•   The nature of the historical evidence for evolution is widely misunderstood

Is evolution “just a theory?”

•      As stated in Chapter 1, the term “theory” has two vastly different meanings

–  In everyday speech, a theory is often little more than a hunch, often without evidence

–  A scientific theory is a related set of insights supported by evidence

•   Able to explain some aspect of nature and possessing explanatory power

What is a theory?

•      Examples

–  Cell Theory

–  The Theory of Gravity

–  The Big Bang Theory

–  Einstein’s Theory of Relativity

–  Darwin’s Theory of Evolution

What is a theory?

•      These theories have survived, and continue to withstand, the questioning of critics

–  Legitimate scientific debate does not argue that gravity, evolution, or cells do not exist

–  Legitimate scientific debate centers only on minor aspects of these theories

Can we even see that evolution happens?

•      Evolution can not be observed as directly as one can observe cells

–  The evidence for evolution is more historical than is the evidence for genetics or photosynthesis

–  Still, this evidence is compelling

Can we even see that evolution happens?

•      Arguing that one cannot see evolution is like arguing that we cannot see the ancient Roman Empire

–  In both cases, there is a record of their existence

Can we even see that evolution happens?

•      Any ancient historical record, including the fossil record, is fragmentary

–  An incomplete record leaves room for a great deal of interpretation among scientists…

Can we even see that evolution happens?

•      This interpretation has to do with the details of evolution, not with its core principles

–  e.g., Which group descended from which group, the rate of evolution, etc.

–  It does not have to do with whether evolution occurred

The Evidence for Evolution

•      What makes scientists so sure about the core principles of evolution?

–  Lines of evidence are internally consistent

–  Lines of evidence are consistent with other lines of evidence

•      Any glaring inconsistency within or between lines of evidence would call evolutionary theory into question

–  No such inconsistencies have surfaced in the 150 years since Darwin

•      Multiple lines of evidence demonstrate descent with modification

–  Radiometric dating

–  Fossil record

–  Morphology and vestigial characters

–  Evidence from molecular clocks

•      Radiometric dating

•      Evolution proceeds at a fairly leisurely pace

–  Billions of years have elapsed since the first appearance of life

–  How do we know that the Earth is 4.6 billion years old?

•   Radiometric dating can determine the age of objects

•      Radiometric dating

–  Technique for determining the age of objects

–  Measures the rate of decay of radioactive elements (isotopes)

–  Decay occurs at a constant rate

•   An isotopes “half-life” is the amount of time it takes for half of a given amount to decay into a more stable form

•      An isotopes “half-life” does not change

–  e.g., The half-life of carbon-14 is always approximately 5,000 years

–  e.g., The half-life of uranium-238 is always approximately 4.5 billion years

•      One can accurately determine the age of a rock by comparing the proportions of radioactive and non-radioactive isotopes

•      Fossils provide powerful evidence for evolutionary change

–  Similar fossils are found in rocks of similar ages throughout the world

–  A general movement toward increasing complexity is noted as one views newer and newer layers of rock

•      Scientific claims must be falsifiable

–  The fossil record presents a falsifiable claim

•   e.g., Trilobites existed in ancient oceans from 500 million until 245 million years ago

•   e.g. Primates arose 80 million years ago

–  Humans are an example of a modern primate

•      Scientific claims must be falsifiable

–  If a fossil bed containing both trilobites and humans was found, our ideas of evolutionary sequences would be called into question

•   No such incompatible pairing has been found

•      The line of fossil evidence is internally consistent

•      When fossil placement is compared to dates obtained by radiometric dating, we get excellent agreement between these two lines of evidence

•      Morphology is the study of physical forms that organisms take

•      Comparative morphology provides classic evidence for evolutionary change

–  e.g., A very diverse group of mammals display striking similarities in their forelimb structure

•   One upper bone, two intermediate bones, and five digits

•      These forelimbs are not similar because that is necessary for function

•      These forelimbs are constructed similarly due to shared evolutionary ancestry

–  All mammals evolved from a common mammalian ancestor

–  This common ancestor possessed this 1-2-5 structure

–  Adaptations to different environments modified this structure in different ways

•      These mammalian forelimbs are “homologous structures”

–  Structures similar due to inheritance from a common ancestor

•      Our outer ears are called “pinna”

–  Most mammals have mobile pinna

•   This allows the mammal to home in on sounds of predators or prey

•   Auricular muscles are responsible for this movement

–  Human pinna are not mobile

•   Our auricular muscles allow us to wiggle our ears, but not to move them significantly

•   This is not particularly helpful

•      Many organisms possess fairly useless structures

–  e.g., Human auricular muscles

–  Placement of these costly and useless structures seems strange if we considered living things to be intelligently designed

•   How do we explain such structured from an evolutionary perspective?

•      Our auricular muscles are homologous to those of a cat

–  These muscles are not functional in humans

–  These muscles are functional in a cat (etc.)

•      Our auricular muscles are homologous to those of a cat

–  These organs were presumably functional in a common ancestor to both humans and cats (and many other mammals)

•   Their function has been lost during the course of evolution

•   “Vestigial characters”

•      Why would a flightless bird such as the ostrich possess wings?

–  Wings do help to balance ostriches, but why would a designer fashion wings simply for balance, when they have such a different function in other birds?

•      The ostrich descended from birds that could fly, but its own line evolved into a flightless condition

•      Comparative morphology is also evident in embryonic development

–  “Comparative embryology”

•      Fish, chickens, humans, etc. all possess pharyngeal slits during their development

–  Develop into gills in fish

–  Develop into eustacian tubes in humans

•      Why would these organisms share a common embryonic structure?

•      Pharyngeal slits were present in ancestors to all vertebrates

–  Ancestral to fish, amphibians, reptiles, mammals, and birds

•      Similarities in embryonic development are due to descent from a common ancestor

•      Every living thing on Earth has DNA as its genetic material

•      The genetic code is identical in virtually all living things

–  Triplet of DNA bases à specific amino acid

–  The very few differences are minor

•      If all life on Earth descended from a common ancestor, one would expect this type of similarity in the genetic code

•      Common ancestors possessed numerous genes

–  Many of these genes should be present in species descending from this common ancestor

–  These genes should have similar, but not identical DNA sequences in these descendant species

•   Mutations would alter this DNA sequence at a fairly constant (and very slow) rate

•      e.g., The cytochrome c oxidase gene exists in organisms as different as humans, moths, and yeasts

–  These genes have similar functions

–  These genes differ slightly in DNA sequence

•      The number of differences is an indication of how closely related different species are

•      Evolutionary theory predicts that the number of DNA base-pair differences between two species should reflect the evolutionary distance between these two species

–  e.g., There should be relatively many differences between a human and a yeast

–  e.g., There should be relatively few differences between a human and a pig

•      Mutations are like a “molecular clock”

–  Able to measure the evolutionary distance between species

•      These predictions have been borne out by the sequencing of the cytochrome c oxidase gene

–  Similar results have been found with numerous other genes as well

•      Molecular data provides additional confirmation for evolution between lines of evidence

–  Consistent with radiometric dating

–  Consistent with the fossil record

–  Consistent with comparative anatomy

Evolution at Its Smallest Scale

•      A species is a group of organisms able to successfully interbreed in nature

•      A species generally consists of several smaller groups called populations

–  All the members of a species that live in a defined geographic region at a given time

•      Populations, not individuals, are the units that evolve

•      Evolution can be defined as a change in the characteristics of a population over time

–  Populations are often isolated to some degree from other populations

–  The environments inhabited by each population is different

–  Each population faces the natural selection pressures of its own environment

–  Different populations can evolve differently

•      e.g., Two different frog populations in two different environments may both evolve coloration patterns making them less visible to predators

–  A lighter coloration may benefit one population in their particular environment, while a darker coloration pattern may benefit a second population in their (different) environment

•      Natural selection acts upon the phenotypes of individuals

–  Individuals with one phenotype may be more fit than individuals with a different phenotype

•      These phenotypes are influenced by genes

–  More specifically, different alleles of genes

•      Many genes are likely to be involved in producing a phenotype such as coloration

•      Multiple alleles exist for each of these genes

–  Each individual possessed two alleles of each gene

–  More than two different alleles for each gene can exist within the population

•      All of the alleles of all of the genes within a population form the “gene pool”

•      The gene pool is the raw material upon which evolution operates

–  Individual alleles are constantly combined in different ways, producing different phenotypes

–  Natural selection acts upon these phenotypes

Evolution as a Change in the Frequency of Alleles

•      An individual’s phenotype may make it more likely to survive longer

–  Surviving individuals may reproduce

–  Individuals that do not survive certainly cannot reproduce

•      Some individuals are more successful at breeding

–  Their alleles are passed on to the next generation in relatively greater numbers

–  Alternative alleles possessed by less successful frogs are passed on to the next generation in reduced numbers

•      The evolution of a population involves a difference in reproductive output between individuals

–  This is often the result of a difference in survival between individuals

•   Surviving individuals may reproduce

•   Individuals that do not survive certainly cannot reproduce

•       “Microevolution” is a change in allele frequencies in a population over a relatively short period of time

–  This is evolution within a population

•      “Macroevolution” is evolutionary change that results in the formation of new species

–  Larger changes resulting in the same way as microevolution

Five Agents of Microevolution

•      There are five “agents” of microevolution that can alter allele frequencies in populations

–  Mutation

–  Gene flow

–  Genetic drift

–  Sexual selection

–  Natural selection

•      A “mutation” is any permanent alteration in an organism’s DNA

–  Mutations can be small

•   e.g., “Point mutations”

–  A change in a single base pair

–  Mutations can be large

•   e.g., Addition or deletion of whole chromosomes or parts of chromosomes

•      Some mutations are heritable

•      The rate of mutation is very low in most organisms

–  Perhaps just one base pair per billion

•      Very few of these mutations are beneficial

–  Most are neutral, some are harmful

–  Beneficial mutations are rare, but are very important

•      Mutations are the means by which new genetic variation arises

–  New proteins are produced, which can modify the form or capabilities of the organism

•      The evolution of eyes, wings, and other structures depends upon many mutations

–  Many mutations, followed by rounds of genetic shuffling and natural selection are required

–  This process may take millions of years

•      Allele frequencies can change as a result of migration

–  Arrival of members from a different population

•   “Immigration”

–  Loss of individuals to a different population

•   “Emigration”

•      This movement may involve individuals, or may involve gametes

–  e.g., Movement of pollen

•      Allele frequencies can change simply as a matter of chance

–  Random fluctuations in allele frequencies are termed “genetic drift”

•   Alleles can increase or decrease in frequency

•   Alleles that are removed can only be replaced through new mutations or through migration

–  These fluctuations have the greatest impact on smaller populations

•      Two common scenarios produce the small populations that are most strongly affected by genetic drift

–  The bottleneck effect

–  The founder effect

•      The “bottleneck effect” is a change in a population’s allele frequencies due to chance following a sharp reduction in population size

–  e.g., Northern elephant seals were hunted very heavily in the 1890s

–  Only 50 individuals remained

–  Protective measures have increased these numbers somewhat

•      All members of the current population of northern elephant seals have descended from this few individuals that survived the bottleneck

–  Many alleles were removed from the population during the bottleneck

–  All of the members of today’s population are genetically very similar

•      Pull a dozen M&Ms from a two pound bag

–  Will all of the colors be represented?

–  Will the colors be represented in the same proportions as in the original bag?

–  If these dozen M&Ms “reproduce” to fill a new two pound bag, what proportions might you expect?

•      Pull 120 M&Ms from a two pound bag

–  Will all of the colors be represented?

–  Will the colors be represented in the same proportions as in the original bag?

–  If these dozen M&Ms “reproduce” to fill a new two pound bag, what proportions might you expect?

–  How do your answers differ from the previous scenario?

•      The “founder effect” occurs when a small population migrates to a new area to start a new population

–  The allele frequencies in this small population will not precisely mirror those of the larger population

–  The effects of genetic drift can be profound in such a small population

•       “Sexual selection” involves differential reproductive success  based on success in obtaining mating partners

–  A form of natural selection

–  This mating is based on phenotype

•      e.g., Female peacocks generally choose male peacocks with particularly nice plumage for mating partners

–  Males that mate more frequently generally leave more offspring

–  Tail size increases through generations

•      Differential mating success among members of one sex is generally based on choices made by members of the opposite sex

–  Females generally do the choosing

–  Can also be based upon the combative abilities of males

•       “Natural selection” is a process in which the differential adaptation of organisms to their environment selects those traits that will be passed on with greater frequency from one generation to the next

–  Natural selection is the means of adaptive evolution

•   Through natural selection, populations become better adapted to their current environment

 “Since there is a struggle for existence among individuals, and since these individuals are not all alike, some of the variations among them will be advantageous in the struggle for survival, others unfavorable.  Consequently, a higher proportion of individuals with favorable variations will on the average survive, a higher proportion of those with unfavorable variations will die or fail to reproduce themselves”

–  Julian Huxley

•      Through natural selection

–  Traits of individuals more successful at reproducing will become more widespread in a population

–  The alleles that bring about these traits will increase in frequency from one generation to the next

•       “Adaptation” is a modification in the structure or behavior of organisms over generations in response to environmental change

–  A particular environment may change

•   e.g., Streams drying up, etc.

–  A population may migrate to a different environment

•      Natural selection is the only agent of microevolution that consistently works to adapt organisms to their environment

–  Mutation simply generates variation

–  Genetic drift is random

–  Gene flow is not related to environment

–  Sexual selection deals with mate choice

Natural Selection and Evolutionary Fitness

•      Natural selection does not work to fashion perfect organisms

–  It fashions populations that are better adapted to their current environment

–  This involves “differential reproductive success”

•   Some individuals reproduce more than others

•       “Fitness” means the success of an organism in passing on its genes to offspring relative to other members of the population

–  Fitness is a measure of impact on a population’s allele frequencies

–  Fitness is a relative term

•   An individual is not “fit”

•   An individual is “more fit” or “less fit” relative to other members of the population

•      Charles Darwin visited the Galápagos Islands in 1835

–  He collected 13 species of finches found only in the Galápagos Islands

•   “Darwin’s finches”

•   All evolved from a single ancestral species from the South American mainland

•   These finches have been studied since the 1970s by Peter and Rosemary Grant

•      In 1977, a tiny Galápagos Island suffered a severe drought

–  Daphne Major

–  This drought had a major impact on the island’s two finch species

•   1,300 finches à fewer than 300 finches

•   85% of the Geospiza fortis population died

•      Geospiza fortis

–  Medium-sized ground finch

–  Eats plant seeds

–  Beak size is somewhat variable in this species

•      The G. fortis population that survived the drought had a larger average beak size than the pre-drought population

–  6% larger beak size

•      Their offspring had a larger average beak size than the pre-drought population

–  4 – 5% larger

•      The drought had preferentially preserved the alleles that brought about deeper beaks

•      A slightly larger beak enables a finch to get into large, tough seeds

–  Finches with larger beaks were more likely to survive the drought

–  Their offspring, on average, had larger beaks than the original population

•      In 1984 and 1985, there was excessive rain on this island

–  There was an abundance of small seeds

–  Finches with smaller beaks survived this event in disproportionate numbers

–  The average beak size in the following generation decreased slightly

•      Where is the “fittest” bird in all of this?

–  There isn’t any

–  Evolution was not marching toward the production of some generally superior bird

–  Different traits were favored under different environmental conditions

Three Modes of
Natural Selection

•      Many traits are polygenic

–  e.g., Human height

–  Display continuous variation

•      When natural selection acts upon these traits, it can proceed in any of three ways

–  Stabilizing selection

–  Directional selection

–  Disruptive selection

•      Stabilizing selection

•      Intermediate forms are favored over extreme forms

–  e.g., Human birth weights

•   Bell curve distribution of human birth weights

•   Infant mortality is highest for very small and very large infants

•   Infants of intermediate size are most likely to survive

•   6 – 7 pound average weight is maintained

•      Directional selection

•      Natural selection moves a character toward one extreme

–  e.g., Cranial capacity in hominids

•   Humans and their closest relatives

•      Disruptive selection

•      Natural selection moves a character toward both of its extremes

–  Less common than the other two modes

–  e.g., Pyrenestes ostrinus, a species of finch from West Africa

•      Pyrenestes ostrinus

–  Beaks size is either large or small, without intermediate values

•   Finches with large beaks specialize in cracking large seeds

•   Finches with small beaks specialize in cracking small seeds

•   Finches with intermediate-sized beaks are less efficient in cracking both large and small beaks

•      Stabilizing selection stabilizes a given trait within a population

•      Both disruptive and directional selection can serve as the basis of “speciation”

–  The transformation of a single species into two or more different species

–  Speciation will be the subject of the next chapter

What Is a Species?

•      The Preble’s jumping mouse has been listed as “threatened” by the U.S. Fish and Wildlife Service

–  Based on classification as a separate subspecies

•   Zapus hudsonius preblei

–  This listing brought restrictions on the development of property near the mouse’s habitat

•      Using DNA comparisons, researchers concluded in 2003 that this mouse is not a legitimate subspecies

–  They classified it with the more common bear lodge mouse

•   Zapus hudsonius campestris

•      This mouse was removed from the endangered species list in 2005

–  Development restrictions were removed

•      The northern spotted owl is an endangered species protected by federal law

–  Strix occidentalis

•      The closely related barred owl is not endangered

–  Strix varia

•      The Earth is the home for numerous species

–  Likely between 10 and 25 million species

•   4 million species is the lowest estimate

–  These species are just the survivors

•   99% of all species that have ever lived on Earth are now extinct

•      All of the species on Earth share a common ancestor ~3.8 billion years ago

–  Initial type of organisms branched into two types of organisms

•   New “species”

–  These species branched further

–  This process continued, ultimately producing all of the species that have ever lived on the planet

•      New species are formed after populations of a single species stop interbreeding with each other

–  How does “speciation” occur?

–  How do microevolutionary mechanisms ultimately result in speciation?

Biological species concept

•      Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups

•      Some separate species may be able to interbreed in captivity, but do not do so in nature

–  e.g., Lion ♂  +  tiger ♀  à liger

–  e.g., Lion ♀  +  tiger ♂  à tigon

–  Since this interbreeding does not occur in nature, lions and tigers are separate species

•      The biological species concept is not always sufficient in defining species

–  Many bacteria reproduce asexually, not sexually

–  There is sometimes limited very limited gene flow between two species

How Do New Species Arise?

•      “Speciation” is the development of new species through evolution

–  A new species often branches from a parent species, while the parent species continues to exist

–  Speciation results from the same processes operating in microevolution

•      Evolution within a population involves a change in the population’s allele frequencies

–  Two interbreeding populations will share any changes in allele frequencies

•   These populations will evolve together and remain a single species

–  Two populations that do not interbreed will not share changes in allele frequencies

•   Changes will add up over time

•   Ultimately, a new species could be formed

Allopatric speciation

•      Geographic separation can restrict gene flow between populations

–  Glaciers can move into an area

–  A river can change course

–  Ponds can dry up

–  Part of a population may migrate into a remote area (e.g., Galápagos Islands, Hawaiian Islands, etc.)

Allopatric speciation

•      Restricted gene flow between two populations can ultimately result in the formation of a new species

–  “Allopatric speciation”

•      During their geographic separation, allele frequencies of two populations will change differently

–  These populations will evolve differently

–  Physical or behavioral changes may result

•      When two geographically separated populations are reunited, they may or may not be able to interbreed

–  If not, then speciation has occurred

–  Mechanisms preventing interbreeding are central to speciation

•   Mountains and rivers are extrinsic isolating mechanisms

•   Characteristics of the organisms are intrinsic isolating mechanisms

•      Intrinsic reproductive isolating mechanisms

–  Any factor that prevents interbreeding of individuals of the same or closely related species

•      Allopatric speciation involves extrinsic isolation (geographic separation) followed by the development of intrinsic isolating mechanisms

•      Intrinsic reproductive isolating mechanisms

–  Ecological isolation

–  Temporal isolation

–  Behavioral isolation

–  Mechanical isolation

–  Gametic isolation

–  Hybrid inviability or infertility

•      Ecological isolation

–  Two species may feed, mate, and grow in different habitats within a common area

•   e.g., Ranges of lions and tigers overlapped

–  Lions preferred the open grasslands

–  Tigers preferred the deep forests

–  No interbreeding occurred

•      Temporal isolation

–  Two populations sharing the same habitat may mate at different times

•   e.g., Some closely related species of flowering plants release pollen at different times

•      Behavioral isolation

–  Individuals choose their mating partners

–  This choice is often dependent upon courtship rituals

–  Closely related species may have incompatible courtship rituals

•   e.g., Songs of birds and crickets, fiddler crab claw waving, etc.

•      Mechanical isolation

–  Reproductive organs of two closely related species may have incompatible sizes or shapes

•   e.g., Different butterfly species have genitalia that differ in shape

•      Gametic isolation

–  Mating may occur, but the sperm is incompatible with either the egg or the female reproductive system

•      Gametic isolation

•   e.g., Sperm in pollen of one plant species cannot reach egg of related species

•   e.g., Sperm of one animal species is killed in reproductive system of related species

•   e.g., Sperm of one species cannot bind to receptors on egg of related species

•      Hybrid inviability or infertility

–  Offspring resulting from a mating between closely related species may be unhealthy

–  Offspring resulting from a mating between closely related species may be infertile

•   e.g., Horse + donkey à mule

•   Mules are healthy, but infertile hybrids

Sympatric Speciation

•      Allopatric speciation occurs following the geographic separation of two populations

–  Intrinsic reproductive isolating mechanisms develop while populations are physically separated

•      Speciation can also occur in the absence of geographic separation

–  “Sympatric speciation”

•      Intrinsic reproductive barriers can arise fairly abruptly

–  “Sympatric speciation”

–  e.g., Chromosomal alterations such as polyploidy

–  e.g., Certain mutations affecting an organism’s life cycle

•      The fruit fly Rhagoletis pomonella provides one of the best-studied examples of sympatric speciation

•      R. pomonella

–  Originally existed solely on hawthorn trees

•   “Haw flies”

–  Some moved to apple trees newly introduced from Europe

–  Flies colonizing apple trees are becoming a new species

•   “Apple flies”

•      Apple and haw fly populations overlap, but rarely interbreed

–  Only 6% interbreed

•      How did this sympatric separation of populations occur?

•      Haw fly life cycle

–  These flies winter underground as larva

–  Adult flies emerge in the summer

–  Flies fly to their host trees, mate, and lay their eggs in the fruit

–  Adult flies live for approximately one month

•      A mutation or new combination of existing rare alleles arose in the ancestral haw flies

–  Mutant flies emerged earlier in the summer

–  These flies were attracted to apples as well as hawthorns

•   Apples mature slightly earlier than hawthorn fruit

–  These early emerging flies interbred amongst themselves to a high degree

•   Limited gene flow between these populations

•      Mating periods of “haw flies” and “apple flies” do not fully overlap

–  Temporal isolation

•      These two types of flies occupy different habitats in the same area

–  Ecological isolation

•      These two intrinsic reproductive isolating mechanisms have occurred without geographical separation

When Does Speciation Occur?

•      Some species remain relatively unchanged for long periods of time

–  e.g., Horseshoe crabs have changed little in 300 million years

•      Other species change dramatically over relatively short periods of time

–  e.g., The 13 species of Darwin’s finches arose from an ancestral species within the past 100,000 years

•      Horseshoe crabs are generalists

–  Extremely diverse diet

•   Eat plants, animals, scavenged debris

•   Shifts from one food source to another depending on availability

–  Do not adapt in response to changes in food source

•      Many of Darwin’s finches are specialists

–  Species sometimes exist on a single variety of plant seed

–  Adaptation to different food sources results in change

•   Change can ultimately result in speciation

•      The Galápagos Islands were formed by volcanic activity 5 million years ago

–  Initially barren

–  Organisms were brought to the island by air and ocean currents

•      Plants were established on the Galápagos Islands prior to arrival of finches

–  No similar birds preceded the finches

•      There was very little competition for the resources the islands offered

–  Many niches were unoccupied

–  Populations could specialize to fill one of many available niches

•      Finches could fly between the 25 islands

•      Water between the islands did represent a geographical barrier

–  Reduced gene flow between populations on different islands

–  These populations evolved into multiple species

•      Darwin’s finches exemplify an “adaptive radiation”

–  Rapid emergence of many species from a single species introduced into a new environment

•      Two conditions conducive to speciation

–  Specialization

–  Migration to a new environment

•      The Hawaiian Islands are also isolated

•      Adaptive radiations have occurred

–  Approximately 50 species of honeycreepers evolved from a single finch-like bird

–  Approximately 800 species of drosophilid flies evolved from a single ancestral species

•   Perhaps even from a single pregnant fly

The Categorization of
Earth’s Living Things

•      A taxonomic system is used to classify every known species on Earth

–  Organisms are classified into various groups based on their evolutionary relationships

–  Eight basic categories are used

•   Species, genus, family, order, class, phylum, kingdom, and domain

–  Species is the most specific grouping

–  Domain is the broadest grouping

Categorization

•      Taxonomy gives a specific (Latin) scientific name to every species

–  e.g., Homo sapiens, modern humans

–  e.g., Rhagoletis pomonella, a fruit fly species

–  e.g., Drosophila melanogaster, another species of fruit fly

•      Specific scientific names allow scientists to know which type of fruit fly (for example) they are talking about

•      Closely related species are combined in a larger group called a “genus”

–  The first word in a scientific name is actually the name of the genus

•   e.g., Canis lupus, the gray wolf

•   e.g. Canis familiaris, the domestic dog

•   Both of these species belong in the same genus (Canis)

•      Closely related genera are combined to form groups termed “families”

•      Closely related families form “orders”

•      Closely related orders form “classes”

•      Closely related classes form “phyla”

•      Closely related phyla form “kingdoms”

•      Closely related kingdoms form “domains”

•      A variety of techniques are used to construct evolutionary histories

–  Comparative morphology, etc.

–  Comparisons of DNA, RNA, and protein sequences provide the bulk of this information today

•      Evolutionary trees can be constructed

–  “Phylogenetic” trees

•      Homologous structures provide evidence for the occurrence of evolution

–  Similar structure due to common descent

•      Structures may arise independently in multiple evolutionary lineages

–  “Analogous structures” arise through “convergent evolution”

•   Similar environmental pressures lead to similar adaptations

•      The eye has evolved at least 40 separate times in various groups of animals

•      Echolocation has evolved at least four separate times

–  Bats

–  Two types of nocturnal, cave-dwelling birds

–  Toothed whales and dolphins

•      Analogous structures can be misinterpreted as homologous structures

–  One would conclude that organisms share evolutionary ancestry when in fact they do not

Taxonomy and Relatedness

•      There is a well-established system for classifying organisms

–  Evolutionary relatedness is the most important factor used in placement of organisms

–  Taxonomy sometimes recognizes other factors when placing organisms

•      Class Reptilia includes organisms such as snakes, lizards, crocodiles, and dinosaurs

•      Class Aves includes all birds

•      Dinosaurs and birds are more closely related than dinosaurs and lizards

–  Birds split off of the dinosaur lineage long after dinosaurs split from other reptiles

•      Birds are arguably different enough from modern reptiles to have their own class