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Lecture Comments (9)

1 answer

Last reply by: Dr Carleen Eaton
Tue Jun 17, 2014 7:33 PM

Post by Maria Mohd Zarif on April 30, 2014

Do you think that the bacteria example is a good example for Natural Selection when the reason why bacteria becomes resistant is through mutation. And mutation is another mode for Evolution to occur. Right?

0 answers

Post by ramtin rezaei on November 16, 2013

What would be your opinion on the state of natural selection in modern human society? Are humans still experiencing natural selection in modern societies? Have the effects of natural selection been relaxed? Are we experiencing human guided selection now? Or are the traditional mechanisms of natural selection beginning to be replaced by modern day equivalents?

Any insights would be appreciated.

0 answers

Post by Ramitha Manivannan on January 24, 2013

I still don't understand Hardy Weinburg equations. Could you please explain it to me in very simple language?


1 answer

Last reply by: Dr Carleen Eaton
Thu Jan 17, 2013 6:49 PM

Post by Ramitha Manivannan on January 17, 2013

Dear Dr. Eaton,

Could I please have your email address so I can ask you questions?


1 answer

Last reply by: Dr Carleen Eaton
Mon Nov 12, 2012 6:34 PM

Post by John Weaver on October 28, 2012

are there any examples of added information in mutations?

0 answers

Post by JUNCHAO ZHANG on September 20, 2011

thank you! I understood SO MUCH better than what my teacher taught me.... :)

Natural Selection

  • While studying species during his voyage on the HMS Beagle, Darwin observed that:
    1. Individuals within a population vary in the traits that they possess.
    2. Species produce more offspring than can be supported by the environment and the offspring must compete for survival.
    3. Offspring inherit traits from their parents.
  • Darwin concluded that the offspring who possess favorable traits will have a higher probability of surviving and producing more offspring. Over many generations, these favorable traits will increase in the population.
  • Stabilizing selection selects for intermediate phenotypes and decreases the frequency of extreme phenotypes.
  • Directional selection results in an increase in the frequency of a phenotype at one extreme of a spectrum.
  • Disruptive selection results in an increase in the frequency of phenotypes at both extremes.

Natural Selection

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Background 0:09
    • Work of Other Scientists
    • Aristotle
    • Carl Linnaeus
    • George Cuvier
    • James Hutton
    • Thomas Malthus
    • Jean-Baptiste Lamark
  • Darwin's Theory of Natural Selection 7:50
    • Evolution
    • Natural Selection
    • Charles Darwin & The Galapagos Islands
  • Genetic Variation 20:37
    • Mutations
    • Independent Assortment
    • Crossing Over
    • Random Fertilization
  • Natural Selection and the Peppered Moth 26:37
    • Natural Selection and the Peppered Moth
  • Types of Natural Selection 29:52
    • Directional Selection
    • Stabilizing Selection
    • Disruptive Selection
  • Sexual Selection 36:18
    • Sexual Dimorphism
    • Intersexual Selection
    • Intrasexual Selection
  • Evidence for Evolution 40:55
    • Paleontology: Fossil Record
    • Biogeography
    • Continental Drift
    • Pangaea
    • Marsupials
  • Homologous and Analogous Structure 50:10
    • Homologous Structure
    • Analogous Structure
  • Example 1: Genetic Variation & Natural Selection 56:15
  • Example 2: Types of Natural Selection 58:07
  • Example 3: Mechanisms By Which Genetic Variation is Maintained Within a Population 1:00:12
  • Example 4: Difference Between Homologous and Analogous Structures 1:01:28

Transcription: Natural Selection

Welcome to

In this first in a series of lectures on evolution, we are going to talk about a very important mechanism of evolution, which is natural selection.0002

We are going to start with some background to give you context for Darwin's theory of natural selection.0010

Darwin lived from 1809 to 1882.0016

And his work was influenced by various scientists, thinkers and philosophers who came before him, as well as some scientists who were contemporaries of his.0020

We are going to go through the different findings and thoughts of those whose work may have0031

influenced Darwin starting way back, before we even get to Darwin- Aristotle's theory.0038

Aristotle lived in the 4th century B.C., and he had a very different view of things.0050

He stated that species were unchanging, so 4th century B.C., and he thought that species were unchanging.0055

In other words, he believed that species do not evolve.0067

He arranged organisms in a ladder of increasing complexity, and he called this scala naturae.0071

Not all philosophers agreed with Aristotle, though, and some believed that, in fact, species did change over time.0086

Let's go way forward to Carl Linnaeus, who lived in the 18th century.0094

And he actually developed the system of classification in nomenclature, so nomenclature means naming system.0106

And this system for naming of organisms is still in use today.0117

And this is the binomial system, whereby, each group of organisms has a name that is two parts using homo sapiens as an example.0125

The first part is the genus, and the second is the species, and we will talk more about this when we discuss phylogeny.0139

Linnaeus believed that the study of nature would reveal the divine order of the universe,0152

and that the similarities between species were the result of divine origins of creation.0158

So, he came at it from that perspective.0164

Georges Cuvier was a French palaeontologist in the late 18th and early 19 centuries, and he studied fossils.0168

And he determined that strata - strata are layers of earth, the singular is stratum - contained different fossils.0182

And so he noticed this change from layer to layer, so the bottom layers are the oldest and then, on up.0197

And he noticed that there were differences between the type of fossils in the layers, and he attributed this to catastrophes.0202

His thought was that catastrophes such as earthquakes or floods were responsible for these differences among strata.0209

Some flood comes along, wipes out many species.0217

And then, he believed that those species that had been wiped out were replaced by species that migrated in, moved in, from other areas.0221

He did not described changes in species as being due to evolution.0232

The work of Cuvier and others studying fossils influenced Darwin.0237

However, Darwin was also influenced by the ideas of scientists and thinkers who did believe that species could change overtime.0242

James Hutton was a Scottish geologist in the 18th century, and he proposed the theory of gradualism.0250

As you can see, it is not just naturalists whose work may have influenced Darwin, it is people from many different fields0260

so James Hutton, 18th century geologist, who published and proposed work on gradualism in 1795.0267

And what gradualism is relating to geology is a theory that suggested that the earth's geologic features such as mountains0280

and valleys were the result of cumulative slow changes0287

and that these changes were actually still taking place.0291

For example, a river over a long period of time could carve out a valley.0294

And he also believed that the earth was far older than was commonly thought during the time that he lived.0299

Thomas Malthus published a very famous essay in 1798, and this was the essay on the principle of population.0308

His work discussed the tendency of plant, animal and human populations to outgrow their resources.0322

And he attributed much of human suffering through a disease and famine to the fact0328

that there is this tendency for species to multiply beyond the resources that were available.0335

Jean-Baptiste Lamarck was also a contemporary of Darwin's, and he believed that species did evolve.0346

And he proposed his own theory on why this is- a mechanism for evolution.0355

His theory had two main ideas in it. One is the idea of use and disuse. The second is the idea of the inheritance of acquired characteristics.0362

And this was later proven to be incorrect, but it is a proposal for a mechanism on evolution.0382

So we are going to talk about this inheritance of acquired characteristics.0387

First of all, use and disuse: this is the idea that if a part of the body is used, it will develop. It will change.0394

And then, the second part is that this change that has taken place through use or disuse can be passed down to one's offspring.0400

We, now, know, of course, that changes that take place because of using a body part, like if you lifted weights and your arm muscles hypertrophy0410

- became very large -you are not going, then, have children who have these great big biceps.0421

So we know that now that acquired characteristics cannot be inherited; but at the time, this was not realized.0428

The classic example that Lamarck gave is that of the giraffe's neck.0436

He postulated that as giraffes reached, long ago ancestral giraffes were reaching for leaves high in the tree, it stretched their neck.0440

And then, when they had offspring, those offsprings had longer necks.0449

Those offspring, then, could reach even higher, stretch their necks a little more.0453

And that was passed along generation upon generation until eventually giraffes had very long necks.0458

And again, we, now, know that this is not correct, but it was a proposal to help explain how evolution occurs.0463

We are going to go on now and talk about Darwin's theory of natural selection.0472

But before we do, I want to make sure that you will understand exactly what evolution is.0475

Evolution is a change in the genetic composition of a population over time.0481

Natural selection is one mechanism for this. In a later lecture, we are going to talk about a couple other mechanisms such as genetic drift and gene flow.0487

Natural selection and evolution are not synonymous. Natural selection is a mechanism for evolution.0497

But what evolution is, is the change in the genetic composition of a population over time, so a change in the allele frequency.0503

A population starts out with a certain frequency for blue-eyed alleles.0510

And over many generations, that may change to where there is a much higher frequency of, say, brown-eyed alleles through evolution.0515

Darwin's theory was theory of natural selection. What does natural selection state?0524

Let's start with that and then, go back and talk a little bit about Darwin.0530

It states that individuals with traits that are beneficial to survival will be more likely to survive and produce offspring.0534

Over many generations, these traits will increase in frequency in the population.0542

Individuals with traits favorable to survival will have an increased chance of surviving and very importantly, reproducing.0550

And then, over many generations, these traits will increase in frequency in the population.0583

Charles Darwin actually started out in medical school, so he entered medical school, realized he did not like it; and instead, he wanted to be a naturalist.0620

He wanted to just go out and study the natural world, so when he was 22, he embarked on a voyage on the HMS Beagle.0629

This ship departed from England, travelled along the coast of South America to Australia and Africa.0636

And it also visited some islands off the coast of South America called the Galapagos Islands.0642

During these travels, Darwin carefully observed the species that he saw.0652

He noticed similarities. He noticed the differences between them, and these observations inspired his theory of natural selection.0657

Focusing on the Galapagos Islands, when he was there,0665

what he noted is that while the birds he saw there had similarities, they actually had differences, as well.0668

And these differences allowed them to be extremely well-adapted to their environment.0674

For example, looking at some finches, they were finches.0679

They had a lot of similarities, but if you looked carefully, you would notice that their beaks were slightly different.0685

Their beaks were specialized to take advantage of the different food sources on the island.0690

One finch may have had a longer, pointier, narrower beak that would allow it to capture insects.0695

Another might have a blunter, stronger, larger beak that would allow it to crack seeds open.0702

So, he noted that there were variations among the species that allowed them to be very well-adapted to their environment.0708

Darwin developed a theory based on this, and he called this theory descent with modification.0719

It should be noted that Alfred Russell Wallace, who was also a British naturalist, developed on his own a theory of natural selection, as well.0738

And Wallace's findings were presented in a paper in 1858.0748

But in 1859, Darwin published a much more extensive discussion of the theory of natural0752

selection in his book titled "On the Origin of Species by Means of Natural Selection".0759

Commonly, it is just called "The Origin of Species", so his book "On the Origin of Species by Means of Natural Selection".0765

Let's think or talk more about Darwin's observations. Darwin observed several things.0782

He observed the individuals within a population vary in the traits that they possessed.0805

Looking around at the human population, short people, taller people, curly hair, straight hair,0823

different hair colors, different eye colors, it is just an endless array of traits within the population.0829

And the same is true of any species that you would look at- just about.0834

His second observation is that species produce more offspring than can be supported by the environment0839

and that the offspring must compete for survival, and you can see Malthus' influence here.0846

Summing this up, species produce more offspring than the environment can support.0852

And therefore, these individuals must compete for survival. I will just put "must compete".0870

If there is only so much food available in a habitat, then, the organisms there are going to have to compete for that food or other resources.0876

Finally, Darwin observed that offspring inherit traits from their parents.0888

From these observations, Darwin concluded that offspring who possess traits favorable to0911

survival will have a higher probability of living, surviving and producing more offspring.0917

These offspring will possess that traits they receive from their parents, as well as what we now know with modern genetics are the alleles and genes.0924

And they will pass those traits on to those offspring.0933

A higher chance of survival with favorable traits, so there is variation within a population.0937

Certain individuals will have a survival advantage through those certain traits. They will have a better chance of living longer, mating, producing offspring.0941

The offspring will have those traits. Over many, many generations, these favorable traits will increase in frequency in a population.0949

Importantly, the result of natural selection is that organisms become better adapted to their environments over time.0959

So, while there are other mechanisms of evolution, natural selection is the only one that allows population to become better adapted to their environments.0965

And I want to be clear that evolution and natural selection is something that occurs in populations, not individuals.0973

An individual has whatever traits she has, whatever genes she has.0982

It is the population that changes over time, so it is populations that are affected by natural selection; and it is populations that evolve, not individuals.0989

Let's go back to Darwin's finches. Darwin observed that finches that eat insects have more of a longer, narrow, pointed-type beak.0999

It makes sense that if there are insects available as a food source, those individuals out of the finch1010

population that happened to have a pointier beak, might be able to catch a beetle better or something.1016

Then, if they have food, they are going to survive.1022

They are going to have offspring, and those birds that are their offspring are going to tend to have the pointier beaks like their parents.1025

And if this continues to happen over many, many generations, the finch population will have longer, pointier, narrower beaks.1033

Maybe a bird with a flatter beak, then, cannot feed as well, cannot catch the insects. They do not survive as long.1043

They do not pass along their genes, and the flatter beak does not survive.1049

However, let's say that on this island, there is a disease going through the insect population, and the number of insects available decreases.1056

Well, there is still some variation of population, and perhaps then, those individuals who do have somewhat of a flatter,1067

stronger beak, those individuals, then, might be able to utilize another food source better like, say, seeds.1076

Then, selection can move the population towards having flatter, stronger beaks.1084

As those individuals eat seeds, they survive better than the insect-eating ones. Now, the food source has changed.1091

The flatter-beaked birds leave more offspring. Those offspring carry that trait of flatter beaks and so on.1097

So, you can see how variation is extremely important and essential to natural selection1104

because if there is no variation in traits, there cannot be differential survival.1108

Looking, now, at the giraffe example from a natural selection perspective,1116

so Lamarck’s theory would say that the giraffes had to reach up high for leaves that stretch their necks.1120

They pass along these longer necks to offspring.1129

Well, we, now, know that that is not how it occurred. How would natural selection explain the evolution of longer necks in giraffes?1131

Well, there must have been some survival advantage for longer necks.1139

It may have been that those giraffes with longer necks could reach the leaves better, and therefore, they were more likely to survive.1143

They left more offsprings who had genes and alleles that coded for longer necks, so their offspring tended to have longer necks.1154

Those offspring tended to survive.1163

They passed along their alleles, and over many, many generations, the frequency of the alleles for longer necks increased in the population.1166

Actually, scientists are not in agreement if longer necks are selected for due to the fact that giraffes could reach these leaves that are high out.1175

There may be other reasons.1186

But in any case, whatever the reason was, it probably had to do with increased survival or leaving more offspring, and thus, selecting for those individuals.1191

As I mentioned, in order for this to occur, there has to be variation in traits.1202

There needs to be genetic variation among the population- some giraffes with shorter necks, some with longer,1206

some birds with narrower beaks, some with wider, flatter beaks and differential survival and number of offspring.1211

So, let's go ahead and review some mechanisms of genetic variation.1220

To cover this in detail, you should go back and watch the lectures on molecular genetics, Mendelian genetics,1224

where we talked in great detail about how this all works but for now, just really reviewing mechanisms of genetic variation.1231

It is important to remember that mutations are the ultimate source of genetic variation.1240

That is how new traits could be introduced into a population.1244

Recall that mutations are changes in the DNA sequence.1249

And ultimately, that is where all these differences and traits in alleles come from and where brand new variation on the trait could come from.1256

But what are some ways in which these genes get mixed and matched, and we have organisms with a huge variety of phenotypes and mixture of traits?1265

Well, recall the law of independent assortment, and we talked about this in Mendelian genetics.1278

The law of independent assortment states that alleles for a particular trait assort independently of alleles for other traits.1284

Alleles for traits assort independently of one another, of alleles for other traits, so let's start with that.1293

This is a review, here, of meiosis, and the purple chromosomes are going to be the ones that are maternally derived.1314

The green are going to be paternally derived.1325

We have an individual here, and we will say that this is a male and that gamete formation - so in this case, sperm formation - is occurring.1328

And this individual is diploid. They have two copies of each chromosome.1338

Here, we have chromosome no. 1 from this individual's mother, chromosome no. 1 from this individual's father,1343

chromosome 2 and chromosome 2 from the mother and from the father.1352

Now, let's say that height is carried on chromosome 1.1357

Let's say that that carries the genes for height, and let's say that on chromosome 2, eye color is carried.1365

OK, now, let's say this individual's mother gave this individual a gene to be tall, and the father gave this individual the allele to be short.1377

The mother gave blue eyes, the blue eye color allele. The father gave the brown eye color allele.1391

Now, in meiosis, in that first division, meiosis 1, homologous pairs of chromosomes separate.1400

And here, we see this cell got chromosome 1 that was maternally derived and chromosome 2 that was paternally derived.1410

This one got chromosome 1 paternally derived and chromosome 2 maternally derived.1420

Independent assortment states that this just as easily could have happened the other way around.1426

It was completely random that the purple chromosome 1 ended up here with the green chromosome 2.1430

It just as is it could have been that chromosome 1 and 2, the maternally derived stayed together.1435

So, where tall and short go have nothing to do with where brown and blue go, so this is independent assortment.1442

And that means that the gametes have all sorts of mixtures.1448

This individual will have the tall allele with brown eyes, tall, brown, and even here, we have short with blue; but it could have been the other way around.1452

It could have been that tall was with blue.1476

Now, this simplifies it a bit because what it does not show is crossing over.1481

And now, we are going to talk about that because that is another mechanism for variation.1488

Recall that during prophase 1 of meiosis, crossing over occurs. Homologous chromosomes pair up and exchange homologous segments of DNA.1493

Actually, this tall gene could have ended up being swapped and ended up on the paternal chromosome.1506

And the short could have ended up on the maternal chromosome.1513

And there is all these other paternally derived alleles, but now, they are with the tall from the mother, from the maternally derived chromosome.1515

Crossing over is another source of genetic variation.1522

Finally, sexual reproduction and random fertilization is a mechanism for genetic variation offspring.1526

So, the fact that the chromosomes from two individuals are coming together is going to create a unique mix of traits.1536

Here, we are saying that this is a male, so these are sperm. These sperm can randomly unite with an egg.1546

So, we have sperm that contain - if you put them together - literally millions of combinations of alleles and then,1554

eggs that contain millions of combinations of alleles and mixes of traits.1560

Those two are united, and that is why siblings do not all look the same.1566

They each have a different mix depending on which alleles they got from the mother, which they got from the father and how those came together.1572

There is just an enormous diversity of individuals with different traits, and some of those traits may be more favorable in a particular environment.1578

And those individuals have a greater chance of surviving and producing offspring and passing on their traits, their alleles.1587

Natural selection can best be understood through example, so this is a classic example.1598

It is the case of the peppered moth, and prior to the mid-1800s, looking in England, almost all peppered moths were light-colored with dark flaps.1604

So, this is the light-colored peppered moth that you see, and the reason it is called "peppered" is there are some of these dark flaps.1617

And you can see how this moth could blend in well, with, say, bark on a tree or a rock, being this color.1622

Well, an interesting happened during the mid-1800s in England.1634

This was the industrial revolution, and cities became extremely polluted.1641

There was not a pollution control like we have now, so the cities were just full of soot, and this soot, sort of, layered everything and made it dark.1646

And what was noticed is that there were very few of these light-colored peppered moths left.1656

Instead, a dark variant, which prior to this had been rare, came to predominate- greatly increased.1662

When the soot covered the trees, if a moth would land on the tree and it was light-colored, a bird could see it.1670

A predator could see it. It would eat it.1676

This moth would not survive.1678

There was occasionally some dark-variant moths in the population.1681

Those individuals would blend in with that tree, would not be eaten by a bird, would reproduce, and their offspring will be more likely to be dark-colored.1687

They would survive, pass on these dark alleles, and within just a few generations, this dark allele came to predominate.1696

And almost every moth in these polluted, industrial areas were dark-colored.1704

But if you went outside, you went out to the countryside and looked for peppered moths, what you would see is that the vast majority were light-colored1711

because out in the country where the trees were not blackened, a dark moth like this would stand out against this background.1718

This is an example of natural selection, selecting for coloration on a moth depending on the environment.1724

Another example and an example that is a huge issue in modern times is that of drug-resistance.1733

As antibiotics have become increasingly commonly used, bacteria that carry genes for drug-resistance have been selected for.1738

They survive. They pass on their DNA to their offspring.1747

Those offspring are resistant, so now, we might try a medication for, say, an ear infection.1750

And all those bacteria in the person's ear that are susceptible to the antibiotic will die off.1758

But those few that are resisting can survive, and then, sometimes, we have to try a second antibiotic that will kill those remaining bacteria,1764

but then, we are risking now, we are selecting for some bacteria to be resistant to that second line drug.1774

So, we have to keep coming up with more and more drugs to stay ahead of this natural selection that is occurring in the bacterial population.1779

OK, right now, we have been talking natural selection in general, but you can break down natural selection into different subtypes.1788

Here, you can see three subtypes. The first that I am going to talk about is stabilizing selection here in the middle.1795

And the Y axis represents, the number of individuals with that particular type of trait.1812

Here, we have the trait or phenotype, and it can be quantified different ways depending on what trait we are talking about.1822

With stabilizing selection, the example I am going to use is human birth weight.1830

So the phenotype here is going to be birth weight; and it is going to be from 0 to, let's say, 10 pounds.1834

Stabilizing selection, first of all, favors intermediate phenotypes and decreases the frequency of extreme phenotypes.1849

Stabilizing selection favors intermediate phenotypes, so it maintains the status quo. It stabilizes the situation.1856

It turns out that babies who have a relatively low birth weight, and those who have a1873

relatively high birth weight are at an increased...they have a greater mortality rate.1877

So, the best chance of survival for a human new born is to have an intermediate birth weight.1884

Intermediate is defined as from 6.5 to about 8.5 pounds.1890

Now, if you look at the gold line, that would be the population before selection has acted on it, and you see a broader spread here of birth weights.1898

What happens is, though, if babies that are 6.5 to 8.5 pounds at birth have a favorable survival,1910

that phenotype will increase in the population as those individuals are selected for over many generations.1918

So, what we see now in the blue line, this is before. It is the gold line, and then, after selection, we see an increase in the number of individuals.1925

The frequency of this phenotype, who were in this range of 6.5, actually 8.5, 6.5 to 8.5 would be right here at this peak.1936

Those individuals are selected for. Individuals at either end, either extreme low or high birth weight, are selected against.1950

So, stabilizing selection favors the intermediate phenotypes.1958

Directional selection is at the top. This is selection in favor of a particular phenotype with the result that alleles for that phenotype will increase in frequency,1964

so selection in favor of a phenotype, I will say, at one extreme. That is what they mean "in one direction".1979

Here, we could use the example of the moths that we just talked about.1996

Here, we are going to have the phenotype that we are looking at, the moth color, and we can quantify this however we want.1999

And I am just going to say the light is here, and then, dark is here.2008

And a very, very light moth that is bright white would probably stand out, not have a great chance of survival.2015

But before the soot and that selection pressure, the peak might have been here, which is the classic light-colored pepper moth.2021

Then, when the soot occurred in the environment, these darker moths got selected for2029

as you can see a shift in the population towards increased frequency of the darker phenotypes, so it is in one direction.2035

Directional selection could occur in the opposite.2043

If the environment become colder and snowier, then, individuals who are almost white or white, who can blend in with the snow, could then, be selected for.2046

So, directional selection could have worked in either selection.2058

Disruptive selection is the third type. It is the third pattern of selection.2062

This is, kind of, an interesting one because it favors phenotypes at both extremes, so how could this work?2069

Well, let's say that a species of squirrel could be either very light brown, medium brown or dark brown.2087

OK, so we have color, and we have, again, light to dark; and this is going to be squirrel coat color.2096

But this squirrel lives in an area where two types of trees predominate: light brown and very dark brown trees.2104

But there are not many of intermediate color.2114

What could happen is, then, the very light-colored squirrels could be camouflaged against these trees.2117

The very dark-colored squirrels could be camouflaged against that trees.2124

But those squirrels with the intermediate coloration will not blend in as well and get eaten by predators.2128

In this case, we see what is called disruptive selection, where there is two peaks.2133

And these are at the extreme; and the intermediate phenotype becomes less common.2138

This type of selection, then, favors genetic variation.2144

The thing about disruptive selection is it increases the genetic variation in a population.2151

Three types of selection: stabilizing that favors the intermediate phenotype; directional that favors selection for one the extremes in one direction;2163

and disruptive, where there is two peaks, the two extreme phenotypes are selected for.2173

We are going to talk about another type of natural selection, and this is sexual selection.2180

In this case, characteristics that increase an individual's chance of mating and, thus, producing offspring are selected for.2185

And remember that natural selection increase in the frequency of those traits that allow an individual to survive and produce more offspring.2193

It is the producing more offspring that allows those alleles to be passed on.2205

So one way to ensure that an individual produces more offspring is that that individual survives in the first place.2209

They are better at catching food. They are bigger and stronger, and they can evade predators.2216

So, just merely surviving will increase one's chance of reproducing.2220

Selection, though, can also work on traits that just focus on the chances of mating and reproducing and not surviving.2224

You need both, but sexual selection really focuses on the mating and reproduction aspect.2232

Sexual selection, then, refers to selection of characteristics that increase the chance of attracting a mate.2242

The term sexual dimorphism refers to a difference in appearance between males and females in a species.2250

For some species, you cannot tell the males and the females apart, but for many, you can.2260

If you look at peacocks, the male peacock has large, brightly-colored feathers.2266

The female peacock is much more drab looking, blends into the background better, same with a lot of species of birds.2271

If we talk about intersexual selection, we are talking about one sex choosing a mate of the opposite sex based on certain characteristics.2278

So one sex chooses a mate based on certain characteristics.2294

Usually, the females are selecting the mate, and for example, a male bird who has brighter feathers might be more likely to attract a mate.2307

He will, then, pass along that trait of brighter feather color. Those offspring will have a brighter feather color.2321

They will be more successful attracting mates, and you can see how over many, many generations, male birds could evolve to have brighter feather colors.2328

So, it does not help them to survive per se, to catch food or anything, and in fact, it can be a disadvantage because they could be targeted by predators.2337

But they have an advantage in selection because they are more likely to mate and produce offspring, so this is intersexual selection.2345

It involves one sex choosing a mate of the other sex. There can also be sexual selection that involves intrasexual selection, and this is selection within a sex.2356

And this really has to do with competition also for mates.2377

Lions live in prides where there is usually, say, three or four females per one male.2386

A small pride will just have one male and a few females. A larger pride could have two males, but then, there is going to be, say, six or eight females.2391

Now, once the male offspring come to maturity, they get kicked out of the pride, and they need to go take over a pride from another lion.2400

And in order to take over a pride, the male lion needs to be large.2408

Therefore, a larger, stronger male is going to have a better chance of taking over a pride,2413

mating with the females in that pride and passing along his traits including larger size.2420

So, this time, it is not a female choosing a male.2425

It is males competing against males, but it still has to do with chances of mating, so this is intrasexual selection versus intersexual selection.2428

Again, sexual selection is just a subtype of natural selection that has to do with characteristics2436

that increase an individual's chance of mating, reproducing and passing along their alleles.2442

Now, we have talked about what evolution is and the mechanism of natural selection.2449

What we are going to discuss next is some evidence for evolution.2454

So lines of evidence come from many fields: paleontology, biogeography, comparative anatomy, embryology and molecular biology.2458

One that is not listed here but that is important, as well, is observation.2469

We talked about the example of the peppered moths, which occurred relatively rapidly.2474

And experiments have been done to, sort of, repeat that and see what happens.2479

And we have been able to observe natural selection at work, evolution at work, so observation also provided evidence.2484

Now, we are going to talk about paleontology and studying the fossil record.2491

The fossil record provides some good evidence for evolution, but to understand that evidence, we need to talk about how fossils are dated.2502

Radioactive or radiometric - so radioactive also called radiometric dating - is a method used to determine how old fossils are.2514

Recall in the lecture on chemistry at the beginning of this course, we talked about half-life.2528

And half-life is the amount of time required for half of an isotope to decay.2534

We are going to focus on carbon dating, and carbon dating is a type of radioactive dating that is based on the half-life of carbon.2541

And it can be used to date organisms that were once alive.2547

C-14/carbon-14 is a radioisotope, so it is a radioactive form of carbon, whereas, C-12 and C-13 are stable. They are stable isotopes.2551

And recall that radioisotopes spontaneously decay. They are unstable.2568

They give off energy, and carbon-14 decays to nitrogen.2572

What scientists do is they can look at the ratio of C-14 to C-12, and use that information to determine2582

how old something that they have found that was once living, how old that is, how old that fossil is.2595

The amount of C-14 is going to decline over time as it decays into nitrogen, and that allows us to determine the age of a fossil.2604

Carbon has a half-life of about 5730 years. That means that in 5730 years, half of the C-14 will be gone.2613

Wait another 5700 years, half of that remaining amount will be gone and so on.2628

In about 10 half-lives, the C-14 will be almost gone, and that means that we cannot use carbon dating for fossils that are very, very old.2635

We need to use something that has an older half-life, but this gives you an idea of how radiometric dating is done.2646

And based on radiometric dating, the earth is believed to be about 4.6 billion years old.2657

Now, we talked about how fossils are dated, how does the fossil record support evolution?2670

Well, even though the fossil record is incomplete, it does give us some good evidence.2676

What we see in the fossil record is the appearance of new types of organisms and the disappearance of other types of organisms such as the dinosaurs.2679

It also shows changes that organisms have undergone over time.2689

And the fossil record can show us links between modern organisms and their ancient ancestors.2693

Horse evolution, we have a particularly good record of that.2699

A lot of intermediate fossils have been found to show links between the ancient ancestor,2705

a species from the genus hyracotherium and the modern horse genus equus.2711

We have found in the fossil record many links between these two that helped us see how evolution may have occurred.2726

That was paleontology. The next area that can provide support for evolution is biogeography.2736

Biogeography is a field that dedicates itself to the study of geographic distribution of organisms.2744

Continental drift refers to the slow movement of continents over a very long period of time.2767

At one time, the earth had only a single continent called Pangaea, so this was a single large continent.2789

Approximately 200 million years ago, this continent separated out into multiple continents, and the result is the separate continents that we have today.2801

Now, we would expect, if evolution is correct, that if a species evolved after the separation of the continents, that species would only be present2814

on a particular continent rather than being distributed evenly all over the face of the earth, and in fact, we do have many examples to that.2823

Marsupials - when we talk about the diversity of life, we will talk about marsupials - are nearly only found in Australia, and Australia lacks placental mammals.2832

So, they have marsupial mammals.2844

And these are different placental mammals because in marsupials, offspring are born as embryos and then, finish their development in the pouch, like in a kangaroo.2846

And in Australia, there is a huge diversity of marsupials that have filled different niches in the environment, but no natural population of placental mammals.2858

And this makes sense given that Australia has been isolated from the other continents for many millions of years, and marsupials evolved there.2869

In other isolated areas such as the Hawaiian islands, we also see species such as bird species that are not found anywhere else.2880

And actually the Hawaiian islands lacked large land mammals prior to the arrival of humans.2889

The species that are unique to an area like this and that evolved there are called endemic species.2895

These species are unique to that area, and instead of seeing that "OK, similar environments have the same species". We do not see this.2902

In fact, two tropical islands far apart do not have the same species.2910

Even though they have the same climate, they may have species that have some superficially similar features,2914

which we will talk about in a minute, but they are not closely-related species.2923

They are not the same species.2926

Often, what you will see is that species found on an island are more closely-related to animals2929

on the nearby mainland than they are to far away but similar environment type islands.2934

Again, this is some evidence that these populations evolved separately from each other because they were isolated from each other.2944

So, that is paleontology and biogeography we covered. Now, we are going to jump over to embryology.2953

We will do comparative anatomy in a minute.2959

When we compare the developing embryos of different organisms, we can find some evidence,2961

some support for the idea that species evolved from a common ancestor.2967

For example, if you look at a fish embryo, a human and a bird, at some point of development, they all have gill slits and a tail.2972

These go on to develop into different structures, into different species.2980

But as they are going through that development, you can see this possible evidence of a common ancestor.2984

Finally, molecular biology, if we look at the DNA sequence in different organisms,2992

the more closely-related two species are, the more similar their DNA sequence is.2999

Comparative anatomy is going to be our next topic under evidence of evolution.3005

Well, first of all, anatomy is the study of the structure of organisms.3014

Comparative anatomy is the study of similarities and differences among the anatomy of organisms.3020

Homologous structures are structures that are similar, but they have similarities in underlying structure but different functions.3028

So, why would they even have similarities in structure? Because these organisms evolved from a common ancestor.3041

Homologous structures: similar due to evolution to evolving from a common ancestor.3049

These are structures with different functions, but similar underlying structure due to evolution from a common ancestor.3069

A classic example: if we look at the arm of a human, the flipper of a whale, the wing of a bat and the leg of a dog, there are similarities in structure.3077

Here, we have the large bone, the humerus. We can see the counterpart here in the whale fin, the bat wing and the dog leg.3091

Next, are two bones, the radius and ulna right here, here. The bat, this bone here actually forks so right there and the dog.3099

Next, we see the series of smaller bones at the wrist, and continuing on down, you can see the structural similarity in these digits.3111

Why would there be similarities between structures with such different functions?3125

Well, one possibility is that all these individuals evolved from a common ancestor, but due to selection pressure, they developed differences to survive.3129

It is believed that whales evolved from an ancestor that was on land, so whales had selection pressure to be able to survive in the ocean,3139

to be able to swim well, whereas, a bat faced different selection pressures in its environment; and these all diverged into different structures.3148

So, this is an example of homologous structures from species that evolved from a common ancestor but came to live in different environments.3156

Now, analogous structures have the same function but a different structure,3170

so homologous, same structure, different function, common ancestor, not exactly the same but similarities in structure.3177

But I will just put same structure, different function, common ancestor. That is homologous.3190

Analogous would it is a different function. Now, in analogous, we have same function, different structure, no recent common ancestor.3202

I will just put "no common ancestor", but no recent common ancestor. It might be a little more precise- no common ancestor.3223

Example would be the wings of birds and wings of insects.3231

These two types of organisms do not have a recent common ancestor. Yet, they both have wings.3247

Those wings have the same function.3254

They allow for flight, but they evolved completely separate paths and got to a similar place but through different paths.3256

And the structures of the wings are not very similar.3265

This is due to adaptive solutions to certain environmental situations.3269

So similar environmental pressures resulted in structures that carry out the same functions.3276

Homologous: similar structure, different function, common ancestor.3281

Analogous: different structure, similar function, no common ancestor.3285

While we are talking about anatomy and structures, you should also be familiar with the vestigial structures.3292

These are a vestiges or remnants of structures from an ancestor, but these are just, like I said, remnants.3298

They are not useful. An example would be the pelvis of a whale.3307

Whales do not walk. They do not really need a pelvis, but for some reason, they have a pelvis.3312

Well, one possibility is that the pelvis is a remnant of a fully functional structure from its ancestor.3315

And that structure was useful in the ancestor; but it is just, sort of, left over from that.3323

The the pelvis in the whale, another one is in humans, the appendix.3327

In humans, the appendix does not have a function that we know of.3332

It is a little outpouching attached to the intestine, and it can cause problems and needs to be removed but does not really have an important function.3335

And it is thought, though, that in ancestral species that had a much more plant-based diet, the function of the appendix was in digestion.3346

There are some species of blind fish that have eyes.3356

Again, those eyes are not useful to them, but to ancestor of theirs, they may have been useful; and they are just remnants of vestiges.3361

Alright, so now that we have discussed natural selection and some different patterns of evolution of natural selection,3369

we are going to go ahead and do some examples.3375

Example one: why is genetic variation within a population essential for natural selection? Why do we have to have genetic variation?3377

Well, remember that natural selection is based on different rates of survival and production of offspring.3386

Therefore, in order for it to occur, there has to be traits that provide a survival advantage to some individuals in the population.3396

OK, there must be traits that provide a survival advantage.3407

If there are not, if all individuals have the exact same traits, there would be no difference in survival. There would be no way for natural selection to act.3421

Traits that increase survival and number of offspring increase in frequency in the population over many generations.3431

Without differences in traits, there would be no way for species to adapt to changes in the environment.3458

There would not be differential survival like in the case of the peppered moth.3465

If there was no variation, if there were all light-peppered moths, there were no dark moths at all,3469

when the soot came, then, all those light moths would have been killed.3475

Eventually, the species would probably have just died out completely. There would have been no way for the species as a whole to adapt.3479

Example two: a population of birds lives in an area where a new predator has recently become established.3488

The smallest birds are able to evade the predator by hiding, and the largest birds can fight off the predator.3495

Which graph below represents the type of selection that will likely take place? What is the name of this type of selection?3501

So, in this case, what we see, here is some selection graphs, and this is going to be the number of individuals on the Y axis.3509

And on the X axis, we are going to have the trait or the phenotype.3517

And what is happening is the smallest birds are going to be selected for because they can evade the predator.3521

The largest birds can fight off the predator, but those birds of intermediate size will probably get killed off, so the smallest and the largest will survive.3528

They will have offspring, and those traits will increase in frequency in the population.3538

So, what I am looking for is a type of selection that favors small and large birds but not intermediate types.3543

The gold, to remember, is the before in the graphs that we are using. The blue if after.3549

And the one that I see that is favoring extreme phenotypes, so if we are going to say size from small to large, here, small and large are being favored.3555

And recall that the name of this is disruptive selection.3570

Here, recall this is directional selection. The phenotype at one extreme is being selected for.3579

It is moving the population in this direction, just in one direction, though, not two.3586

Finally, we have stabilizing selection, and this is going to favor the intermediate phenotype.3592

So we are going to end up with individuals that are medium size being favored.3601

The correct answer is disruptive selection, which is this graph right here.3607

Example three: list three mechanisms by which genetic variation is maintained within a population.3615

Well, new mutations introduce new genotypes into the population.3622

Two: crossing over. Crossing over allows a mix between alleles, paternally derived and those that are maternally derived, during gamete formation.3631

We need three. A third could be independent assortment, whereas, the alleles for one trait3644

assort independently from the alleles of another trait unless, of course, there is linkage, which is given in detail in an early lecture.3651

You were just asked for three. Here is a fourth, though- random fertilization, so four mechanisms that maintain genetic variation within a population.3659

So natural selection is possibly pressuring for a particular phenotype with an associated genotype.3672

But yet, genetic variation can be maintained through various mechanisms.3685

OK, explain the difference between homologous and analogous structures.3688

Homologous structures: these are similar structures, so similarities in the structure but different functions.3696

And this is due to descent or evolution from a common ancestor.3719

And the example I gave was the flipper of a whale, the wing of a bat, the human arm, dog leg.3734

Those all have very different functions.3743

But they have some structural similarities that are thought to be due to the fact that all those species way back had a common ancestor.3745

Analogous structures: analogous structure, similar structure for homologous.3753

So, analogous structure, we have different underlying structure, but the same or similar function; and they evolved separately.3760

They evolved independently. Example I gave is the wings of a bat, or excuse me, the wings of a bird or a bat and the wings of an insect.3780

Those are both wings. They have similar function, very different structure.3790

And they evolved along separate lines so homologous versus analogous structures.3796

That concludes this lecture on natural selection.3803

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