some variants have advantages over others those “better adapted” variants will survive in...
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Natural Selection and Evolution
Changes in Populations over Time
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Chapter 22 Objectives1. Define evolution.2. Discuss several observations which suggest
relatedness between different species. Note observations of both fossil and living organisms.
3. Explain the fundamental principles that form the foundation of Darwin’s Theory. Including:•The relationship between population size and
famine, disease, . . .•Variability of traits within a population•Heritability of physical traits•The role of environmental factors on the utility
of traits
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Chapter 22 Objectives4. Explain how the processes of artificial
and natural selection can cause changes in a population over time.
5. Discuss the following types of evidence as they relate to the evolutionary view. Give examples:•Comparative morphology•Comparative
embryology/development•Comparative biochemistry/genetics
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Do Populations Evolve? • Species can be as much alike as wolves and coyotes,
or as different as plants and animals• And although plants and animals are superficially very
different, at the chemical and cellular level they share a number of similarities• Are the similarities superficial or are they the result of
ancestral relationships?• Are the differences distinct or are they variations on a
fundamental theme?
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PaleontologyStudies of fossils
show a number of interesting things:• Species existed that
no longer exist• Many organisms that
are extinct bear strong resemblance to species that exist today• Most species that
exist today did not exist at some time in the past
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Comments on the Scientific Method• The nature of a hypothesis is more than just a guess, it
must follow a few simple rules:• A good hypothesis must be consistent with all available data
and observations• A good hypothesis should logically extend to accurate
predictions of both past and future observations (particularly those that will be the results of controlled experiments)• In other words, a good hypothesis shouldn’t just make
sense, it should create the expectation of exactly the types of things that are being observed
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Two Alternative HypothesesSpecies are fixed and distinct:• Fixed = unchanging• Distinct =
separate/unrelated
• Biblical creationism and “intelligent design” theory are more consistent with this hypothesis
Species change over time: • Change = “evolution”
• The assumption of any ancestral relationship between species must also assume that populations have evolved from the time of the ancestor to modern time• Modern taxonomy is based
on this hypothesis
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A quick clarification:• It is possible for species to be
distinct, but to still evolve. • Changes within a population
(“microevolution”) do not necessarily mean that a particular species shares an ancestral relationship with another species (“macroevolution“)• The same logic doesn’t hold
in the other direction though
• For example, wolves could evolve, and coyotes could evolve, but that doesn’t mean coyotes and wolves necessarily evolved from a common ancestor• But if wolves and coyotes
share a common ancestor they must have evolved since then, because they aren’t the same species now
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Evaluating the Hypotheses• Large variations (at the level of Kingdom, phylum, and
class) seem to favor the “fixed and distinct” hypothesis.• The differences between prokaryotes and eukaryotes, or
between plants and animals, seem to be more than simple variations
• Variations at the lower taxonomic levels (genus and species) seem to indicate some closer level of relationship• Wolves, coyotes, foxes, and domestic dogs seem to be very
much alike - so much alike, for example, that they can hybridize in some cases
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Evaluating the Hypotheses• Remember, a good hypothesis should not only be consistent with
the data, it should also predict outcomes• The “fixed and distinct” hypothesis doesn’t demand that species
should never resemble each other, but it does nothing to explain why they ever would• It also gives us no explanation for why extinct organisms found in
fossils should have any resemblance at all to species alive today• It certainly is inconsistent with any amount of hybridization
between species• And perhaps of greatest importance, it gives us no explanation
for the presence of species today that are not represented at earlier times in the fossil record
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With that being said . . .• The “fixed and distinct” hypothesis becomes difficult
to evaluate any further.• But in order to accept the “evolution” hypothesis we
will need to: • Demonstrate a mechanism for change that predicts the
pattern of similarities and differences we can observe• Create a series of controlled experiments that result in
changes within a population over time• Devise a method for quantifying the position a population
occupies at any given time so that we can re-evaluate the same population to identify any changes that may have occurred• Apply that method to populations “in the field” to look for
natural changes
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A preview of things to come• Darwin proposed a mechanism of change we will call
Natural Selection• We will also consider Artificial Selection, and see how it
serves as a controlled experiment to evaluate population change (Objectives #3 & 4)• We will become familiar with the Hardy-Weinberg
hypothesis and the calculations it utilizes to evaluate stability and change within a population (Objectives #6 & 7)• We will evaluate our student population using the Hardy-
Weinberg equations and perform simulations of populations exposed to different conditions
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Mechanisms of Change - Darwin• Charles Darwin proposed a mechanism for evolution
by “Natural Selection”• You are responsible for making yourself familiar with
the historical background and basis for Darwin’s ideas by reading sections 22.1 and 22.2 in the textbook• You are also expected to be familiar with an
alternative mechanism for evolution as proposed by Jean Baptiste de Lamarck (22.1)• Here’s a video that can help in that regard:• http://www.youtube.com/watch?v=V8KIvICfGEMhttp://
www.youtube.com/watch?v=V8KIvICfGEM
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The Foundation of Natural Selection
Premise #1There is variation within populations with regard to inherited traits
• Variability within a population results from a variety of factors, including:• Conjugation, Transformation and Transduction (in
microorganisms)• Sexual Reproduction, Independent Assortment
and Crossing Over• Mutations• Variable expression of genes• Variable post-transcriptional gene processing (in
eukaryotes)• http://
www.youtube.com/watch?v=UjMn4oHfYL4&index=44&list=PLFCE4D99C4124A27A
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The Foundation of Natural Selection
Premise #2Populations experience a Struggle for Existence• This struggle is largely imposed upon
the population by environmental stresses• Availability of food and other resources• Predators• Communicable Disease• Harsh climatic conditions
• Some of these environmental stresses become more significant as populations become more dense • Note the influence of Thomas Malthus
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Some variants have advantages
over others
• Those “better adapted” variants will survive in greater numbers
Reproduction is a function of survival
• So the survivors will produce larger numbers of offspring than those more “poorly adapted”
Traits are
inherited
• Descendants will resemble the survivors, who were “well adapted”
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Natural Selection• Is natural because natural forces (environmental
factors) select (choose) which individuals in a population will survive and reproduce• Traits which interfere with survival will quickly become
extinct within that population• They will be “selected against”
• Traits which provide advantages will quickly become the predominant characteristics within that population• They will be “selected for”
• http://www.youtube.com/watch?annotation_id=annotation_3856240253&feature=iv&src_vid=UuGrBhK2c7U&v=hOfRN0KihOU#t=10shttp://www.youtube.com/watch?annotation_id=annotation_3856240253&feature=iv&src_vid=UuGrBhK2c7U&v=hOfRN0KihOU#t=10s
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Some more Video• http://
www.youtube.com/watch?v=R6La6_kIr9g&index=3&list=PLFCE4D99C4124A27A• http://
www.youtube.com/watch?v=S7EhExhXOPQ&index=4&list=PLFCE4D99C4124A27A
• And a simulation• http://
www.biologycorner.com/worksheets/pepperedmoth.html
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But I digress . . . • Richard Dawkins, in his book The Selfish
Gene, applied the concept of natural selection to any “self-replicating agent”. Basically, he said that any replicator that replicates more than others will tend to appear more frequently in the larger pool of replicators.• Biologically speaking, the fundamental
unit of replication is a gene. So any gene that successfully promotes its own replication will tend to increase its frequency in the “gene pool”
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But I digress . . . • But Dawkins extended
this out to include things like ideas, social norms, and such things. He termed non-biological replicators “memes”• So internet “memes” are
traceable back to Dawkins, and through Dawkins back to Darwin
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Logical Extensions of the Hypothesis• The same population, exposed to a different set of
environmental conditions, will evolve based on the stresses imposed by that environment
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Logical Extensions of the Hypothesis• Segments of populations that migrate to new areas will evolve differently than the populations they leave behind
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Logical Extensions of the Hypothesis•Organisms living in the same environment, but employing different strategies (“ecological niches”) will evolve to be well adapted to the needs and stresses of that particular niche
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Logical Extensions of the Hypothesis• Populations
exposed to rapid environmental change will evolve rapidly, while those exposed to gradual change will evolve gradually
Phyletic Gradualism
Punctuated Equilibrium
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Sounds good, but does it work?• We’re scientists, so we can’t just take a hypothesis at
face value. We need to test the hypothesis with a controlled experiment.• A controlled experiment would require: • Simulating the conditions of natural selection
• Begin with a population that has variability for some inherited trait• Create a circumstance that results in differential rates of
survival/reproduction
• Manipulating the independent variable• Impose a selecting pressure on the population
• Observing the results in the dependent variable• See if the population changes over time (generations)
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It’s called Artificial Selection• Farmers have been doing it with crops and livestock for as long as
there have been farmers. Every domestic crop and domesticated animal is the result of Artificial Selection (“selective breeding”)
• We can come up with any number of examples. So do that. Now. • Super Cow Breeders:
• http://www.youtube.com/watch?v=rW54_vM9SF0
• Dog Breeding: • http://www.youtube.com/watch?v=tukxnJ8Gnbghttp://
www.youtube.com/watch?v=tukxnJ8Gnbg• http://www.youtube.com/watch?v=kPWDusnGgP8&feature=related
• Horse Breeding• http://www.youtube.com/watch?v=_rHZCtwl-hwhttp://www.youtube.com/w
atch?v=_rHZCtwl-hw
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But does it really happen in Nature? • The Peppered Moth
• http://www.youtube.com/watch?v=LyRA807djLc
• Antibiotic Resistant Bacteria• http://www.youtube.com/watch?v=v-Peboq0AqA
• The Evolution of Speed• http://www.youtube.com/watch?v=Gm0uCPmB0Ng
• Jaws• http://
www.youtube.com/watch?v=PP3HmDGs8G4&list=PLNtcr2FLR4dfstWMcH25YEllfPbc3VmS3
• Eyes• http://www.youtube.com/watch?v=qV_TKi266bE
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How do we evaluate past Evolution• Comparative Morphology• Morphology is body form• If species share common ancestry, they should have
similarities in body form
• Comparative Embryology/Development• Ancestry is more evident in embryonic development than it
is in adulthood
• Comparative Biochemistry/Genetics• At its most fundamental, it isn’t traits that are inherited, it is
genes• Genes are located on chromosomes, and code for the
production of proteins
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RememberNatural selection acts on variable traits that already exist in the population• You can’t use something
that you don’t already have• What you can do is use
something you do have, but use it in a different way or for a different purpose
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Comparative Morphology• If this really happens, we should see organisms with
the same basic anatomical features, but adapting them to different purposes (Homologous structures)• We should also see features with no value to their
owners become diminished (Vestigial structures)• And where a particular way of life is especially
effective, we should see a wide variety of unrelated organisms adapting to that way of life, using whatever anatomical features they have at their disposal (Analogous structures)
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Homology in Finch Beaks• The birds shown are all finches• In each, the beak is adapted to
eating a different type of food• These finches are descendants
of a common ancestor from the South American mainland• An ancestral population
migrated to the Galapagos Islands where they diverged by evolving to different environments
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Homology in Plants• All of these
structures are derived from leaves• Their
development is derived from the same embryonic tissues• Each is modified
to a different specialized function
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Homology in Vertebrate Forelimbs
Each of these vertebrate forelimbs consists of the same bones, but they vary in the rate of development and the way that they are used
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Skeletal Homology
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Homologous vs AnalagousThis paw contains the same bones as the human hand, but the paw is adapted to walking and the hand is adapted to grasping
The human hand and the panda hand both have a thumb, but the pandas thumb is derived from a wrist bone
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Analogy in WingsAll of the structures shown function for flight, but each is built differently, derived from different tissues
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Analogy in Marine Vertebrates• Dolphins• http://www.youtube.com/watch?v=AGyuO7LaOco
• Sharks• http://www.youtube.com/watch?v=o81WIfjrt5I
• Penguins• http://www.youtube.com/watch?v=A9mbCNs47FI
• Seals• http://www.youtube.com/watch?v=cQu4M--MRbc
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Vestigial StructuresPersistant but Diminished
Snakes have hips and leg bones, but they don’t function for support or movement
Modern humans have smaller jaws than primitive humans, but have retained the same number of molars. The “wisdom teeth” are vestigial
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Vestigial StructuresPersistent but Diminished
The human coccyx is a vestigial tail
Whales have a vestigial pelvic girdle
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Vestigial Structures• Why persistent?
• Because the genes for those structures still exist
• Why diminished?• Because the expression of a
gene is expensive• Transcription costs energy
because each nucleotide must be tri-phosphorylated
• Translation costs raw materials because amino acids used to express that gene are tied up and not available for expression of other genes
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Comparative Embryology/Development
• Early embryonic development shows marked similarity between even very distantly related forms of life• As diverse as vertebrates
are, they all share a number of embryonic features• Cranium• Pharyngeal gill slits• Post anal tail• Segmented vertebrae
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Comparative BiochemistryAmino Acid Sequences
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Comparative Biochemistry & GeneticsNucleotide Base Sequences
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Comparative Biochemistry & GeneticsChromosomal Comparisons
• Remember that in Karyotyping, homologous chromosomes are identified by size, shape, location of the centromere, and banding patterns.• While karyotypes match
homologous chromosomes from the same cell, the same techniques can be used to search for regions of homology in chromosomes from different species• https://
www.youtube.com/watch?v=zi8FfMBYCkk
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Chapter 23 Objectives6.Define species & population.7.Use the Hardy-Weinberg principle to
evaluate stability or change in allele frequencies within a population.
8. Define Genetic Drift. Discuss how population size effects genetic drift. Note circumstances which might cause drastic changes in population size.
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Chapter 23 Objectives 9. Explain and give examples of each of
the following types of natural selection• Directional• Stabilizing• Disruptive
10. Explain how sexual selection differs from natural selection. Give examples.
11. Explain several ways in which genetic variability might arise within a population.
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Species and Populations
• Evolution is not about changes within individual organisms. It is about changes within the species
• We generally think of species in terms of their ability to interbreed successfully
A population consists of all the members of a species that live and interact in the same period of time
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The Hardy Weinberg Hypothesis• The Hardy Weinberg
hypothesis attempts to quantify the stability or change in a population by measuring the frequency of alleles in the “gene pool” of a population• It also makes
assumptions regarding factors which might drive evolution
• A population that is stable (at equilibrium) will have the same proportion of alleles in the gene pool in the future as it has during any given period of time• If a population is
evolving, the “frequency” of alleles in the gene pool will change
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Hardy-Weinberg Equilibrium• The Hardy-Weinberg hypothesis suggests that a
population will remain at genetic equilibrium if:• There are no mutations • Mating within the population is random with respect to the
gene being considered• There is no natural selection occurring (there is no
survival advantage resulting from a particular allele)• The population is statistically large enough to minimize
the effects of any random events• Alleles do not enter or leave the gene pool due to
migration or hybridization
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Some vocabulary clarification• If the alleles in question affect the likelihood of finding
a mate, we call that “sexual selection”• If the alleles in question affect the likelihood of
survival, we call that “natural selection”• If random events in a relatively small population have
a large statistical effect on allele frequencies, we call that “genetic drift”• If alleles enter or leave the gene pool due to
migration, we call that “gene flow”• We will consider each of these in greater detail
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Calculating Allele Frequencies• Hardy-Weinberg
quantifies the gene pool by effectively using a Punnett square to calculate genotype frequencies for the whole population• p = the frequency of
the dominant allele• q = the frequency of
the recessive allele
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The Hardy-Weinberg Equations•p + q = 1•p2 + 2pq + q2 = 1• p2 = the frequency of
homozygous dominant• 2pq = the frequency of
heterozygous• q2 = the frequency of
homozygous recessive
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Sexual Selection• Reproduction, more
than survival, determines allele frequencies in future populations• Traits that affect
mate choice are just as likely to be selected for as traits that affect survival
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Genetic Drift• Genetic Drift is evolution due to random events in relatively
small populations• One random event in a population of 50 individuals causes a
2% difference, but the same random event in a population of 5000 is only a .02% change• A forest fire that accidently kills the only homozygous
recessive individual in a small population drops “q” frequency significantly, even though the trait and the fire had nothing to do with each other• The concept of “endangered species” centers around the
need to keep populations big enough that random events aren’t an issue
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Genetic Drift Practicalities• So what kinds of situations are likely to cause statistically
small populations, and how would Genetic Drift manifest itself?• “Founder Effect”
• Let’s say a small flock of birds migrates from one island to another in an archipelago (like, say, the Galapagos islands)
• It’s unlikely that by pure chance that group would perfectly represent the same allele frequencies of the larger established population
• The birds that migrated will be the “founders” of the new population, and the allele frequencies would be immediately different from the parent population. Evolution would have occurred in one generation
• Some alleles, by pure luck, would have a high frequency while others that might have been common in the parent population might be absent (extinct) in the new migrant population
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How else might drift occur?• Bottleneck effect
• Some natural disaster takes a statistically large population and kills off enough of them to make random events significant
• Could be a fire. Maybe volcanic activity, or flooding, or even a meteorite strike. Maybe habitat destruction by human activity. Could be anything that drops the population below the threshold level for random events to become statistically significant
• Now you have the same issue as you did with Founder Effect. The survivors didn’t survive because of a particular allele, but some alleles are suddenly more common and others less common just because their owners managed to come out alive.
• Again, you have new allele frequencies. Evolution occurring in a single generation
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Some other consequences . . .• One of the precepts of the natural selection hypothesis
is Malthus “struggle for existence” – As populations become large and dense, competition for limited resources increases• A small population may have little or no competition.
Suddenly scarce resources are plentiful. Maybe diseases that were common in the native habitat are absent in the new homeland. Maybe the disaster that created the bottleneck in your population extincted your natural predators.• Alleles that might have been detrimental before are
suddenly neutral. Everyone has a chance to survive and reproduce
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Ewwww, that’s disgusting . . .• In a small population you’re going to run into another
problem. It’s going to get difficult to find a mate that isn’t related to you. Some amount of inbreeding is going to happen.• The big issue with inbreeding is the presence of
recessive mutations within a population. There will be some, and two things used to be true:• They exist only in the descendants of the original “mutant”• They aren’t expressed because they are recessive
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But doesn’t inbreeding cause birth defects?• If inbreeding never occurred, recessive mutations would never
be expressed. There would be carriers, but not homozygous• With inbreeding, you get a ¼ chance of homozygous recessive
occurring. If that causes a birth “defect” it will likely be fatal• But not all recessive traits are “defects”, and with limited
competition this new trait may persist and become fixed in the population• It may even be selected for, either naturally or sexually• When you see isolated variations in coloration or appearance
that occur in certain populations but not others – they probably got started this way
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Variability• One precept of natural
selection is variability. Natural selection can’t create new types, it can only affect the success of existing variations• We can think of
variations in a population in terms of a “normal distribution”, the traditional bell shaped curve
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Evaluating variability in a trait
Very commonly, traits in a population exist in a continuum. Most of the population looks sort of average, and as you approach the extremes of the range you find fewer and fewer that fit that profile
• Average: Mean, Median, Mode
• Range: from least to most
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A sample normal distribution• It appears most men
average just above 70” in height, with women averaging about 65”• So 70” for a man would
be an “intermediate” phenotype, and over 80” would be an “extreme” phenotype• So what would happen
if survival or mate choice depended upon height?
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Selection for and Selection Against• Under what
circumstances might one height be advantageous over another?• Who would live?• Who wouldn’t?• What would be the
result?
• What are your options if you are “selected against”? Is extinction inevitable?
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Patterns of Natural Selection• Natural Selection is
categorized into 3 patterns, based on• which segment of the
population is selected for, • which segment is
selected against, • and the evolution that
results from it
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Directional Selection
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Disruptive Selection
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Stabilizing Selection
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Chapter 24 Objectives1.Explain the biological species
concept. Contrast this concept with other methods of defining species
2.Contrast gene flow with reproductive isolation. Discuss the effects of isolation on the gene pool of a population.
3.Contrast prezygotic and postzygotic barriers to reproduction
4.Discuss several mechanisms of reproductive isolation.
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Chapter 24 Objectives5.Define speciation.6.Discuss circumstances which might
logically result in speciation.7.Distinguish between allopatric,
sympatric, and parapatric speciation.
8.Make and interpret diagrams representing patterns of evolution and speciation.
9.Distinguish between gradual and punctuated speciation.
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The Biological Species Concept
The Biological Species concept works from the premise that members of the same species can interbreed successfully and produce viable offspring (like the Dog/Wolf hybrids shown above)
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Ability to Interbreed & Gene Flow• Populations that can
interbreed together can have “gene flow”• Alleles from one population
can “flow” from one population to the other• Gene flow will maintain a
degree of similarity between the two populations• Consider these “grolar”
bears, grizzly/polar hybrids
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Reproductive Isolation• Populations that are
reproductively isolated are somehow prevented from interbreeding successfully.• There are a large number of
circumstances that might result in isolation• The simplest, geographic
isolation, is a matter of location. They don’t breed because they never come in contact with each other
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Geographic Isolation• Lions and Tigers can
hybridize, but their offspring are sterile• In nature these
animals would never encounter each other. Lions live in Africa. Tigers live in Asia.• https://
www.youtube.com/watch?v=vybfyhYK-m0
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Habitat IsolationSmallmouth Bass spawn on gravel
Largemouth Bass spawn on mud
https://www.youtube.com/watch?v=-V53X9Gys18https://www.youtube.com/watch?v=yccBO2468Po
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Temporal IsolationMaple trees flower in early spring
Ash trees flower later in the spring
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Behavioral Isolation – Mating Rituals• Blue Footed Booby
• https://www.youtube.com/watch?v=LLdC-8nqPog&list=RDoYmzdvMoUUA&index=6
• Peacock • https://
www.youtube.com/watch?v=jTBHiZtnCsA
• Wild Turkey• https://
www.youtube.com/watch?v=uGOMJqv8BOY
• Prairie Chicken• https://
www.youtube.com/watch?v=lw773cr_jXk
• Elk Bugling• https://
www.youtube.com/watch?v=pYzWmKlZtrU
• Bighorn Sheep• https://
www.youtube.com/watch?v=E6Fx3CaJhgk
• Deer• https://
www.youtube.com/watch?v=mpTcu_h1xuE
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Mechanical IsolationStructural variations in reproductive organs prevent successful fertilization
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Barriers to Reproduction
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Post-zygotic Barriers to Reproduction• With some related species
hybridization is possible, but can’t be sustained• Reduced hybrid viability
• Offspring are produced, but aren’t likely to survive
• Reduced hybrid fertility• Offspring are sterile
• Hybrid breakdown• Viability of hybrid offspring
breaks down in later generations
The mule is a sterile hybrid of horses with donkeys
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Speciation• Darwin’s hypothesis suggests that different species form by
the divergence of ancestral populations• Different segments of a population exposed to different
selection pressures (different environments or different survival strategies) would evolve differently• Isolation would prevent gene flow, so any mutations that
accumulated in one population would be absent in the other, compounding the differences between them• Eventually the differences would become great enough
that interbreeding would no longer be possible, and without interbreeding the populations would be different Species
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Speciation in Galapagos Finches• A small flock of finches
migrates from the South American mainland to the Galapagos Islands• The island population grows,
and gradually the birds have to resort to different ways of life and different food sources, and to migrate to other habitats on different islands• With each migration, genetic
drift and founder effect result in rapid evolution.
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Speciation in Galapagos Finches• Geographic isolation
minimizes contact between the different groups of birds• Variations in habitat,
appearance and behavior influence mate selection. Differences that have accumulated reinforce reproductive isolation• The result is a wide variety of
finches that share a common ancestor
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Speciation: Allopatric vs. Sympatric
All speciation requires an isolation mechanism• Allopatric speciation
• The isolation is due to geography
• Populations habitats do not overlap
• Sympatric speciation• Habitats overlap• Isolation results from a
different mechanism
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Grand Canyon Antelope Squirrels
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Patterns of Divergence
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Gradualism• Darwin believed that evolution
was a slow, steady process of consistent, gradual change• If that is the case, we should
find evidence of steady change in the fossil record• That would predict
“transitional forms” in the fossil record that are intermediate in phenotype between ancient and modern forms
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Punctuated Equilibrium• Niles Eldredge and Steven J.
Gould proposed that is more likely to consist of very rapid change followed by long periods of stability• The logic is that small
populations must adapt to changing circumstances, and once adapted they reach equilibrium• Punctuated equilibrium would
predict very few transitional forms, and only in very constricted time periods https://www.youtube.com/watch?v=_
YpAG3miURY
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Chapter 25 Objectives10. Discuss the conditions of the early earth11. Discuss several hypotheses regarding
the chemistry of formation of macromolecules on the early earth
12. Discuss circumstances which favor the formation of fossils. Recognize several different types of fossils.
13. State the Law of Superposition and use it to infer the relative age of fossils.
14. Explain the process of radiometric dating
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Chapter 25 Objectives15. Discuss the general pattern that appears
in the fossil record. Relate this pattern to Geological Time.
16. Explain the concept of plate tectonics. Discuss the condition of the modern earth from the perspective of plate tectonics.
17. Describe several mass extinction events that appear in the fossil record
18. Define adaptive radiation. Explain the role extinctions play in the radiation of new species
19. Discuss the role of developmental genes in evolution
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Chapter 26 Objectives20. Explain the modern taxonomic
system from the perspective of divergent evolution.
21. Explain how homologous characters
are used in the cladistic approach to classification
22. Explain how molecular clocks can be
used to make inferences regarding evolutionary history