AP Biology Speciation and Patterns of Evolution

Welcome to Educator.com. We are concluding the sections on evolution with the discussion of speciation and patterns of evolution.0001

Before we go on to talk about speciation, we are going to start back at the beginning and talk about some theories about how life originated on earth.0010

Earth is estimated to be about 4.6 billion years old, and the atmosphere on early earth was much different than what it is now.0021

Early earth contains primarily methane, nitrogen, ammonia, carbon dioxide and water.0032

In the 1920s, two scientists independently came up with a hypothesis that organic molecules could have formed0059

from these gases present in the early atmosphere along with some source of energy such as lightning or ultraviolet radiation.0069

So, these scientists were named Oparin and Haldane.0080

And in the 1920s, they postulated that organic molecules formed from these inorganic molecules plus the input of energy.0085

This theory is sometimes called the primordial soup theory or primitive soup theory.0108

This theory postulates that this mixture of organic molecules ended up or perhaps even began in the ocean.0119

And it is from this primordial soup of organic molecules that the precursors of life arose.0128

Abiogenesis is a term that refers to the process of organic molecules forming from inorganic molecules.0136

Abiogenesis is organic molecules formed from inorganic molecules, which is what is being postulated here.0149

In the early 1950s, Stanley Miller and Harold Urey performed some experiments in which they simulated the early conditions of earth in the laboratory.0167

What they did is they took gases such as methane, ammonia and water vapor. They put these together.0182

And they applied electricity as an energy source in the same way that an early earth, perhaps lightning was the energy source.0188

The result is the formation of organic molecules.0195

So, they found that hydrocarbons...so they performed experiments in which hydrocarbons,0199

amino acids and other molecules formed under conditions simulating those of early earth.0206

The next step in the formation of life according to this line of thinking is that macromolecules, then, needed to form from these small molecules, so polymers.0229

Here, we have the first step.0241

It would have been abiogenesis, so inorganic molecules from these getting small organic molecules,0243

then, from the organic molecules such as amino acids getting polymers, so macromolecules.0259

Now, another important step in life is separation of these macromolecules and this environment from the environment outside.0273

And that is achieved through a cell membrane.0282

Well, it is found that phospholipids can form spontaneously into lipid bilayers, and recall that lipid bilayer is the basis of the cell membrane.0286

Then, these macromolecules such as proteins or RNA became enclosed in a membrane, and eventually, this evolved into cells.0298

One theory posits that RNA was the genetic material for life on earth at first.0325

And when we talked in the earlier lecture about ribozymes, and ribozymes are enzymes made out of RNA.0334

And they are RNA molecules that are actually catalysts, and complimentary RNA can be made from an RNA template.0342

So ribozymes can make complimentary RNA from an RNA template.0358

It is possible that early forms of life or precursors to forms of life were just simple0365

membranes containing RNA and some other molecules and macromolecules.0371

And essential to life is the ability to reproduce and the ability to carry out metabolic functions.0376

This simple system may have been able to reproduce, replicate an RNA and carry out some just simple reactions, simple metabolic functions.0385

Once life was established by some means, then, what were the earliest organisms?0393

Well, it is believed to be about 3.5 billion years ago, the earliest forms of life were anaerobic prokaryotes.0400

Early earth was pretty low in oxygen, so it makes sense that early forms of life may have been anaerobic prokaryotes.0425

However, about a billion years later, about 2.5 billion years ago, the level of oxygen in the atmosphere greatly increased.0434

This created somewhat of a crisis for life on earth because oxygen was actually toxic to these early anaerobic prokaryotes.0443

So, 2.5 billion years ago, we have a big increase in the concentration of oxygen in the atmosphere.0450

It is thought that about 1.5 billion years ago, eukaryotes evolved.0462

When we talked about cell structure, prokaryotic and eukaryotic cell structure, I discussed the endosymbiosis theory.0475

Recall that the endosymbiosis theory states that eukaryotes evolved from a symbiotic0482

relationship between anaerobic prokaryotes and aerobic prokaryotes.0490

When oxygen in the atmosphere increased, it was toxic to these anaerobic prokaryotes.0495

They may have engulfed smaller aerobic prokaryotic cells.0500

The larger bacteria that were anaerobic could provide protection and other benefits for the cells they engulfed.0504

Those smaller cells, in turn, were aerobic, and they could utilize the oxygen to produce energy in the form of ATP.0511

This relationship was mutually beneficial. It was symbiotic.0519

So, rather than digesting the bacteria they ingest, these larger bacteria may have evolved to live closely with the smaller bacteria.0522

And eventually, the smaller bacteria evolved into organelles particularly the mitochondria in chloroplast.0530

And recall that mitochondria in chloroplast shows some similarities to bacteria.0539

They have their own genome, but it is a circular genome; and it lacks histones, so it is similar to that of bacteria.0544

They both have double membranes, and this double membrane might be a result from an outer membrane that came from as they were engulfed0550

by the larger bacteria and then, an inner membrane that was their own original membrane from the smaller bacteria, say, that double membrane.0559

They also have their own ribosomes that are similar in size to the ribosomes of bacteria.0567

So, there is some pretty good support for the theory of endosymbiosis.0571

Here, we started out with possibly anaerobic prokaryotes,0576

then, increase in oxygen in the environment and the evolution about 1.5 billion years ago of eukaryotes.0580

Life became, then, increasingly complex, and we had the development of multicellular eukaryotic organisms.0589

During the Cambrian explosion, there was an enormous increase in the diversity of life.0599

And this is about 540 million years ago, so the Cambrian explosion.0605

During this time, there were many, many new phyla arose, and this is evidenced by the fossil record.0621

We can see a great increase in the diversity of organisms in the fossil record around this time.0626

And that was during a period of about 10 million years that this occurred.0631

Prior to the Cambrian explosion, most of the organisms on earth were small.0635

They had soft bodies, and there is different theories on why life exploded in diversity at this point.0639

One of them states that there was an increase in the oxygen in the atmosphere around this period.0646

Another possibility is what is called the Snowball Earth hypothesis that states that the Cambrian period marked the end of a series of ice ages.0655

And because those ice ages were occurring, it really limited where organisms could survive.0665

Once earth was done going through those multiple ice ages, life could really take hold.0670

Another possibility is that the breakup of those single large land mass, Pangaea,0678

created some shallow coastal areas around these new continents that could have contributed.0684

It could be a variety of mechanisms. This gives some background and ideas about how life may have evolved on earth.0690

And now, what we are going to talk about is a couple different theories about how evolution occurs and how new species developed.0700

There are two lines of thought about the general pattern of evolution, and these series are gradualism and punctuated equilibrium.0708

Gradualism describes evolution as a slow steady process of continuous change.0719

Here, we start from the ancestral species. Change occurs.0737

We have branching into a separate species, and that species continues to evolve and change until eventually, it becomes extinct; or it could continue on.0740

Here, we have evolution continuing some changes and then, splitting off into different species,0751

splitting again and then, continuing on changing and either continuing or become extinct, so that is gradualism.0759

This was once the predominant theory. However, now, there is a lot more support for another theory, which is punctuated equilibrium.0767

Looking at the fossil records, scientists have noted that there are long periods where species appear to be static or in equilibrium.0777

And these are punctuated by periods of rapid change, so long periods of stasis or equilibrium and then, rapid change.0784

And this rapid change primarily occurs during the early existence of the species, so as the species is forming. Rapid, of course, is relative.0791

If a species is in existence for, say, 10 million years, according to punctuated equilibrium,0802

a species like this might take 100,000 years to break away from its ancestral species at the beginning.0807

It is evolving relatively rapidly over 100,000 years, becomes a new species and then, just, sort of, stays the same for a long, long period of time.0814

There were two paleontologists, Aldridge and Gold, who published a paper in the0826

early 70s that really developed this theory of punctuated equilibrium more fully.0838

And if you look at this diagram of how punctuated equilibrium would occur, here, you see rapid change and then, stasis.0844

And then, we have some change, and then, maybe a new species will develop due to this rapid change.0858

Maybe not, but it could and then, just stasis, rapid change and then, may be cut off, may be continued, may be went to another period of equilibrium,0864

so, periods of rapid change that are relatively brief and towards the beginning of the existence a species during species formation0879

followed by periods of stasis versus slow, steady, continuous change, so periods of rapid change followed by long periods of stasis.0886

Adaptive radiation refers to the development of new species through evolution of an ancestral species to fill niches in the environment.0909

The result of adaptive radiation is a huge increase in the number of new species.0918

There is a couple times in particular when this occurs. One is after mass extinction events.0924

For example, thinking about the extinction of the dinosaurs about 65 million years ago,0939

prior to that, mammals were fairly small and not very diverse group of organisms.0944

But when the dinosaur is gone, it is possible that a lot of competition or predators were eliminated.0949

And because of not being preyed upon, mammals could branch out, evolve,0956

fill different niches for the land, sea, up in trees, different places to feed and then, branch out into many, many different species.0964

This is a pretty large scale adaptive radiation that I am describing that would occur after a mass extinction event. It can occur in a smaller scale, as well.0973

The other time when adaptive radiation might occur is when a group of organisms settles in a new location that is lightly occupied.0984

So there is relatively little competition in a new area with little competition.1003

It is pretty open to settlement.1013

Through evolutionary mechanisms, that new species will develop from the ancestral species that will be adapted to different niches in the environment.1016

Some species might be adapted to live in a drier area or a wetter area or to eat a particular type of plant.1025

An example of this, we talked earlier about Darwin and the species of finches1033

that he observed on the Galapagos Islands off of the coast of South America.1038

Well, these islands were originally settled by one species of finch about 1 to 3 million years ago.1042

But through evolution, so Galapagos Islands...one species settled the island about 1 to 3 million years ago. Then, through evolution, 13 species formed.1050

And as I mentioned in the previous lecture, although there are similarities between the finch species,1074

there is important differences, as well like if you look at their beaks,1080

some beaks are narrower, larger, rounder, flatter, more pointed, less pointed.1083

And the different beak shapes allow finches to eat different sources of food for example insects versus seeds.1089

These ancestral species radiated out to fill different niches in the environment of this island.1101

Talking about a few patterns of evolution now, convergent evolution, divergent evolution and coevolution,1111

convergent evolution refers to species that are unrelated but evolved characteristics that are similar, so convergent evolution.1120

Species that are unrelated, so no recent common ancestor, they evolve to have similar characteristics.1133

And this would be due to facing similar selective pressures.1153

They might live in a similar environment. For example, bats, birds and pterodactyls are not closely related, but they can all fly.1158

And we talked before about analogous structures, and bats, birds and pterodactyls and insects all have wings. These are analogous structures.1174

They fulfil a similar function, but they are structurally different; and they are not the result of evolution from a common ancestor.1182

Marsupial animals evolved in isolation in Australia while most of the rest of the world is occupied by placental mammals.1193

One major difference with marsupials is that their offspring are born as embryos1205

and finish their development in a pouch rather than in utero-like placental mammals.1209

But marsupials evolved to develop similar niches as did placental mammals.1214

So, if you looked at some animals in Australia like the Tasmanian wolf, and then,1223

if you looked at a placental wolf here in the US, some wolf species here in the US, you might see a lot of similarities, but these are not actually closely related.1231

Or another classic example is the flying squirrel in North America and the sugar glider in Australia, also a lot of similarities.1247

But the marsupials are actually more closely related to each other than1254

they are to their placental mammalian counterparts that occupy similar niches elsewhere.1259

Whales and fish are another example of convergent evolution.1270

They both have made adaptations to life in the water.1274

Yet, they are not closely related to one another and just evolved similar mechanisms of coping with the environment that they are in.1279

That is convergent evolution. The second type is divergent evolution.1290

In conversion evolution, the animals begin from different places but converge1293

to have these similar traits such as wings, and this could be bat wings versus bird wings.1300

They did not have a common ancestor, but they both came to the same place of having wings. They converge or came together.1310

Divergent evolution is the opposite. We have an ancestral species, a common ancestor.1317

And then, due to different selection pressures, we get two different species that have differences, structural differences, functional differences.1330

But they started out from the same point.1342

So, divergent evolution is when a population evolves in a different direction than the other members of its species.1346

If this evolution goes far enough, is different enough, then, what can end up happening is speciation.1381

One group goes this way, and another group breaks off and goes this way.1393

And eventually, the two may become so isolated reproductively that even if they came together, they cannot have offspring.1397

And speciation has occurred, and we are going to talk in a minute in detail about speciation in the different types.1403

So, convergent: two species start out form a different place but end up with some1409

similar characteristics due to evolving in response to similar selection pressures.1413

Divergent evolution: species start off in the same place but, then, end up different even being separate species due to different selection pressures.1419

Coevolution: this is evolution of one species in response to change in another species through evolution,1429

so evolution of one species in response to changes in another.1441

Several examples: one is a predator and its prey.1452

A predator needs to be large. It needs to be quick, strong in order to catch its prey, so let's say a predator evolves to become very strong.1462

Prey, then, are going to have selection pressure on them, and they might respond by evolving a defense such as a hard shell,1473

a stinger or coloration, where they mimic another animal that is poisonous or a smell that repels the predator.1481

The predator, then, might in turn evolve again to be able to get past these defenses.1490

Plants, same idea. If a plant is fed on by an animal, then, over time that plant population could develop or evolve a defense like toxins or thorns.1497

Then, the insects or animals that feed on that plant can evolve to become1509

resistant to the toxins or have very thick tough skins so that they can get past the thorns.1514

Now, right here, these examples I have given, predator, prey and plants and the animals that feed on them, are examples of a fight for survival.1520

However, you can actually have mutualistic coevolution.1536

A classic example of this is plants and the insects or birds that pollinate them, so plants and their pollinators.1542

So, the plant, the flower might become shaped in such a way that a particular insect or bird species can access it very easily.1553

And the bird might in turn evolve a type of beak that fits perfectly into that flower to get the nectar.1560

So, a hummingbird, certain beak shape, and then, the flowers that it feed on might be shaped very well to fit that bird.1567

And there is a benefit for both the plant and the animal that is pollinating it.1575

Now that we talked about how diversion evolution can lead to speciation, let's first define what a species is.1583

There is different definitions of species, but the commonly used one is what we call the biological species or the biological species concept.1590

I am just going to say species, but we mean the biological species because there can be other definitions such as morphological species.1599

They are based on other similarities, but what we are going to talk about is a biological species.1606

This is a group of organisms that can interbreed and produce offspring that are viable and fertile.1610

A horse and a donkey can breed. They can mate, and they can produce an offspring.1621

This offspring is a mule. A mule is viable.1632

It can live. It can live a long life.1635

It is functional. However, it is not fertile.1637

Mules are not fertile. Therefore, horses and donkeys are separate species because they cannot mate and produce offspring that is viable and fertile.1640

New species may evolve when one population becomes reproductively isolated from other members of its species.1652

And if that goes long enough, and the two or one of the groups evolves in a different direction,1660

they may eventually not be able to come back together and reproduce.1667

Reproductive isolation can occur in various ways, but these can be divided up into two broad categories.1672

One is prezygotic, and the other is postzygotic isolation.1681

Prezygotic is just what it sound like, before a zygote formation, so this is prior to fertilization, so reproductive isolation prior to fertilization.1689

Some ways that this can occur, behavioral isolation, so example, behavioral isolation.1703

There might be a particular set of sounds that a bird makes or a mating dance, a mating ritual that is performed.1713

And if that ritual is not performed, or the sounds are not heard, mating will not occur.1720

So, there is a barrier to mating that would be a prezygotic mechanism of reproductive isolation.1725

Habit isolation can occur, so even though the two might do the same mating call, they do not come into contact with one another.1736

And this could be because one group spends most of its time in the water.1747

Another spends most of its time in land or winds up in the trees more, so they do not come into contact; and they do not end up mating.1752

Other prezygotic barriers can be physical differences such as difference is size that would prevent mating.1761

Now, mating has occurred. Fertilization has occurred.1769

There can still be barriers to reproduction, so this is postzygotic barriers.1772

The offspring may not be viable, so that is a barred reproduction.1778

Mating occurs. Fertilization occurs, but the offspring do not survive until birth.1784

They may be stillborn, or they are born.1789

But they only live a short time, or they are very fragile or infertile, fragile, infertile or stillborn, not survive at all.1791

With these reproductive barriers, the two gene pools become separated.1806

And then, if evolution occurs on one or both of the populations,1812

they may continue to diverge from each other until they are completely separate species.1816

There are two general types of speciation we are going to learn about. One is allopatric speciation, and the other is sympatric speciation.1821

Allopatric speciation occurs following geographic isolation of populations from each other, so geographic isolation.1829

This is thought to be the more common type of speciation.1838

For example, let's say there is a population of lizards, and they live in an area.1842

And a volcano erupts, and it becomes impossible for wild to cross that area where the eruption has occurred.1847

And then, later, if you crossed it, there is no food source there, so there is this wide swath of area that separates the two lizard populations.1855

Once that separation has occurred, if there is different selective pressures on one side of the barrier compared to the other,1864

those two populations may evolve in different directions.1871

So, if that barrier remains long enough or evolution starts to occur, then, habit isolation, different behaviors,1874

eventually, even if you brought the two populations back together, they may not be able to mate anymore and produce viable offspring.1883

Genetic drift can actually contribute to this. Remember that genetic drift refers to a change in allelic frequencies by random chance.1892

And if this volcano erupts, and it splits of just a small group of lizards; and that group of lizards does not have an allelic frequency,1902

that is representative of their frequencies of alleles in the larger population.1910

That can contribute to evolution because you have already got this start with different allele frequencies.1914

The smaller and less mobile an organism is, the easier it is for allopatric speciation to occur.1920

You could see how it would be a lot easier to split off ant populations than lion populations.1927

Or if something is very mobile like a bird, even if a volcano erupts, the bird can just fly over it.1932

So a smaller, less mobile species is more likely to be divided geographically and have the setup for allopatric speciation.1937

Another way for this to occur would not just be some disaster or change in the land like the river rising.1948

It could also be a group of individuals splitting off and settling elsewhere.1955

Butterflies could fly to a distant island, land there. They make it there.1962

It is very far away, but other butterflies from the parent population do not come over. These butterflies do not go back.1966

There is not mixing between the two populations, so you also may have founder effect.1973

Again, genetic drift can continue, and this environment on the island might have different selection pressures.1978

So the butterflies, then, begin to evolve.1984

Eventually, these butterflies might end up with very different coloration, wing shape, size than their original population.1988

On the Hawaiian Islands, there is endemic species of birds and insects there that are not found anywhere else.1999

And this is probably a result of some ancestral species, a few seeds to a plant or a few birds or butterflies that mated over and then,2007

evolved into this certain direction to adapt to life on these islands.2018

A couple of mechanisms for geographic isolation could be a change in geography.2023

This could be due to a river rising or the level of the river rising or a volcanic eruption or something else2030

or a population or group of organisms settles elsewhere. Either way, there is a geographic barrier.2036

Sympatric speciation is less common. It refers to speciation that occurs without a geographic barrier.2051

So even though the populations are near each other, they become separate species.2058

How would this occur? Well, we talked about reproductive isolation occurring, and that can occur even though species are not near each other.2062

There is one example that occurs mainly in plants, and this is the mechanism of polyploidy.2070

In polyploidy, there are more than two sets of chromosomes, so remember that diploid refers to having two sets of chromosomes; so that is 2n.2083

Haploid is 1n. Triploid would be 3n, and tetraploid, 4n.2094

Speciation occurs in plants due to polyploidy, and there can be two different types of polyploidy.2112

The first is autopolyploidy.2119

And this occurs when an organism ends up with an extra set of chromosomes, but the chromosomes are all from the same species.2126

So, autopolyploidy, extra set of chromosomes from the same species. How could this occur?2132

Well, let's say that we start out with a cell, and it is diploid; so we have two chromosome 1s and two chromosome 2s, and this cell is 2n.2144

And on some part of the plant, somehow there is a failure of cell division.2160

So then, we end up with this cell does not divide properly, so it duplicates its DNA getting ready for cell division; but it does not divide correctly.2166

There is nondisjunction, or something occurs. Something goes wrong.2177

Now, instead of 2n, I have got 4n, so gamete formation occurs. During gamete formation, we get meiosis, and we get gamete formation.2183

And these gametes are going to have half the chromosome number as the parent cell, so the gametes will be 2n.2209

Normally, a gamete from this type of plant would be 1n, would be haploid.2224

But here, since this cell is 4n, when the chromosome number is halved, we end up with gametes that are 2n.2229

This plant could self-fertilize. These gametes come together and, now, form an offspring that is 4n.2238

These gametes would not be able to join with the gamete from the parent species2259

because here, if we have gamete formation, this gamete is going to be haploid.2264

It is going to be n, and if you unite a haploid gamete with a diploid gamete, you are going to end up with a triploid zygote.2276

And that is not going to be viable because there would be segregation of the chromosomes would not be balanced because you have an odd number.2284

While this tetraploid plant is viable, it can produce offspring.2294

It has to mate with a plant either self-fertilization or cross-fertilization with another plant that is also tetraploid.2300

So, you see how quickly this plant became isolated from the parent species due to polyploidy.2309

And this is actually a very common mechanism in plant speciation.2319

By some estimates, more than 70% of angiosperms, which are flowering plants, are polyploids.2322

And it is not just four sets of chromosomes. Some are hexaploid, so they have six sets of chromosomes.2328

So, I have been mentioning plants because this is far more common in plants than in other types or organisms.2338

But there are some insects that are polyploid, fish.2342

And there is even a discovery of a species of rat that is polyploid.2346

One mechanism by which isolation can occur without a geographic barrier is polyploidy. A second mechanism that we talked about is habit isolation.2353

We talked about that earlier when we talked about reproductive isolation.2364

An example that is in the works right now is what appears to be the formation of separate species from a single species of fly.2368

And this is the case of the apple maggot fly, or it is also called the Hawthorn fly.2379

Apple maggot flies in North America originally fed on hawthorns, which are thorn apples.2386

However, regular apple seeds were brought to the US in the 17th century.2393

And around maybe the 18th century, one population of apple maggot flies started diverging from the other population.2399

And what happened is some of the apple maggot flies started feeding on regular apples.2414

So, traditionally apple maggot flies fed on hawthorns or thorn apples, but now, we have a population that feeds on apples.2419

The females lay their eggs on the fruit that they tend to eat.2437

And the females tend to mate with males that are feeding on those same food source, so either hawthorns or apples.2440

There is not a lot of mating between the two groups, and scientists have studied these two groups2455

and believed that they are on their way to becoming separate species that some of the loci have started to diverge.2462

And eventually, these could become two different species that will not be capable of interbreeding, and you see that this is due to habitat isolation.2470

It started out all feeding on thorn apples. Now, one group feeds on regular apples, and eventually, the two may be completely separate.2478

Temporal isolation, that is another type of isolation that can occur, if one group is more active during the day, another group is more active at night.2488

Sexual selection that we talked about in a previous lecture can also lead to reproductive isolation.2501

There may be selection for certain characteristics that increase the chances of attracting a mate for example in bird species.2513

I had mentioned that male birds like peacocks have brightly-colored feathers, and females may be attracted to a particular feather color or display.2521

And if two populations somehow become isolated, even though they are physically capable of interbreeding,2532

if they are not selecting each other as mates, they can become reproductively isolated.2537

Sympatric speciation occurs despite a geographic barrier because there are other barriers.2544

It could be genetic like polyploidy, habitat isolation, temporal or sexual selection.2550

And if these barriers persist long enough, and the two groups evolve in different directions, they may not be able to come back together2557

even if they were put together, mate and produce viable offspring, and by that time, that means speciation has occurred.2567

Example one: what pattern of evolution is being described in each of the examples below?2574

One: eyes in octopuses and humans serve a similar function. However, they evolved independently in these two groups of organisms.2581

When you see a structure with similar function but different structure, and it is not due to evolution from a common ancestor,2589

what you are looking at is convergent evolution- a solution to a similar problem, similar selection pressures, such as wings in insects and wings in birds.2603

They both evolved for flight, so they are going to have the same function, but they came from two different evolutionary paths.2617

That is convergent- starting from a different place and converging on this similar structure2625

of eyes or similar function and some structural similarities, as well, but similar function.2632

The flippers of whales and the legs of a dog evolved from the limbs of a common ancestor but are used for different purposes.2639

In this case, we are talking about a common ancestor and then, evolution in different directions to serve different functions.2648

But there is some underlying structural similarities that are evidence of this common ancestor.2657

So, here, we started out different places and come together to a similar function.2662

Here, we started out the same and diverged into different functions. This is divergent evolution.2666

Three: a reptile evolves a hard shell as protection against a mammal that preys on it.2676

The mammalian species evolved to have large strong jaws, which allow it to break the shell.2682

The reptile is evolving in response to this mammal that preys on it. The mammal, then, evolves in response to evolution in the prey.2687

So, this evolution in response to each other is called coevolution, and we see this in host-parasite relationships,2699

as well as predator-prey, in a neutralistic relationships like the evolution of flowering plants with their pollinators.2707

Example two: in sympatric speciation, a new species forms after a group of organisms2717

becomes isolated from the rest of the species despite the lack of a geographic barrier.2723

What are two mechanisms by which such isolation can occur? Name and describe each.2730

If we have a group of organisms is all together, and then, something happens to separate them; but it is not a geographic separation,2737

it is a different type of isolation, speciation can eventually occur. How could this occur?2745

One mechanism especially in plants is polyploidy, and what is this? This is more than two sets of chromosomes.2750

A cell may start out as 2n, and then, cell division fails. It is not completed.2766

These become 4n, tetraploid individuals. They are gametes.2775

Gamete formation, so the gametes form, and these are 2n; and these 2n gametes, fertilization can occur producing 4n offspring.2783

This offspring cannot mate with the parent species, but it can mate with other 4n individuals.2800

That is polyploidy. That is one mechanism.2811

Another mechanism of isolation is habitat isolation like with the case of the apple maggot flies.2814

One has a habitat in apple trees, the other in thorn apple trees, or one may spend more time in the water. One may spend more time in the land.2825

The two are separated. They may evolve differently and eventually become completely reproductively isolated from one another.2836

Temporal isolation: one group may be more active at one time of day and then, not encounter a group that is more active, say, at night.2845

And one group is active during the day.2853

They asked for two. I am going to give you four, but all you needed is two correct.2856

Sexual selection, that individuals select mates based on certain characteristics.2861

And even though a whole group of organisms may be near each other geographically,2869

they do not mate because the females may only pick males that have a certain coloration or make a certain mating call.2877

So they are isolated due to sexual selection.2884

Example three: some areas have an isolated island, receive a large amount of rainfall while other areas are much drier.2889

They type of soil and nutrients available on the soil also vary greatly throughout the island.2895

Genetic testing of multiple plant species on the island reveals that many species evolved from a single2902

ancestral species that arrived on the island shortly after its formation. What is this called?2907

So, we started out with an ancestral species of plant, and eventually, through evolution, many species formed.2912

One species might be equipped to survive in a drier area. One may live better in a wetter area.2923

One might be able to tolerate greater salinity in the soil. One might evolved to utilize a particular type of nutrient in the soil.2930

So, we have species finding different niches, and what this is called is adaptive radiation.2940

What is happening is that that ancestral species, through evolution, radiated out to fill many niches in the environment.2949

And this is particularly likely to occur after mass extinction or when a group of organisms settles in a new area where there is minimal competition.2956

Example four: a major discovery of fossils reveals that multiple species that were2969

thought to have remained unchanged after their initial formation actually underwent slow changes2975

to their morphology over tens of millions of years until the species eventually became extinct.2981

Does this provide support for the punctuated equilibrium theory of evolution or gradualism and why?2987

What this is saying is that a species...at first, they thought it formed and then, remained unchanged.2994

But then, this new discovery showed that this species actually underwent3004

slow changes over a long period of time and then, eventually became extinct.3009

This is described being gradualism. Gradualism describes evolution as a series of slow steady changes throughout a species' existence.3014

So, the species formed, and they continue to evolve and evolve.3027

Punctuated equilibrium describes evolution as periods of a rapid change.3032

It could be early on when a species is forming followed by long periods or stasis or equilibrium.3037

And punctuated equilibrium is now like the predominant theory right now that is accepted and supported.3043

But if this discovery were made, it would actually support gradualism.3053

That concludes this lecture, and thanks for visiting Educator.com3058