Natural Selection and Evolution

Changes in Populations over Time

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

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

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?

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

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

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

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

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

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

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

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

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

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

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

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”

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

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

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”

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

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

Logical Extensions of the Hypothesis• Segments of populations that migrate to new areas will evolve differently than the populations they leave behind

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

Logical Extensions of the Hypothesis• Populations

exposed to rapid environmental change will evolve rapidly, while those exposed to gradual change will evolve gradually Phyletic

GradualismPunctuated Equilibrium

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)

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/watch?v=_rHZCtwl-hw

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

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

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

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)

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

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

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

Skeletal Homology

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

Analogy in WingsAll of the structures shown function for flight, but each is built differently, derived from different tissues

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

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

Vestigial StructuresPersistent but Diminished

The human coccyx is a vestigial tail

Whales have a vestigial pelvic girdle

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

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

Comparative BiochemistryAmino Acid Sequences

Comparative Biochemistry & GeneticsNucleotide Base Sequences

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

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.

 

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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?

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?

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

Directional Selection

Disruptive Selection

Stabilizing Selection

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.

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.

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)

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

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

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

Habitat IsolationSmallmouth Bass spawn on gravel

Largemouth Bass spawn on mud

https://www.youtube.com/watch?v=-V53X9Gys18https://www.youtube.com/watch?v=yccBO2468Po

Temporal IsolationMaple trees flower in early spring

Ash trees flower later in the spring

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

Mechanical IsolationStructural variations in reproductive organs prevent successful fertilization

Barriers to Reproduction

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

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

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.

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

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

Grand Canyon Antelope Squirrels

Patterns of Divergence

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

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

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

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

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