Cladograms

Summary

In this podcast, Mr. Andersen explores the concept of cladograms, tools used in biology to hypothesize relationships among organisms based on shared characteristics. He uses a playful approach, comparing the process to the Sesame Street game 'one of these things is not like the other.' Andersenii aquaticus, his hypothetical creatures, serve as a practical example to illustrate how to construct a cladogram. He explains that the simplest explanation, or Occam's razor, often provides the most plausible cladogram, emphasizing that these are hypotheses rather than definitive phylogenetic trees. The discussion also touches on the evolution from physical characteristics to genetic DNA analysis in creating more accurate cladograms.

Takeaways

• 🌿 A cladogram is a hypothesis of how organisms might be related based on shared characteristics, rather than a definitive phylogenetic tree.
• 🔍 To construct a cladogram, one must identify the outgroup, or the organism that doesn't share characteristics with the others, using a method akin to the Sesame Street game 'one of these things is not like the other'.
• 🧬 Cladograms are read by looking at branching points, which represent common ancestors and shared characteristics among organisms.
• 🕰 Time in cladograms is considered to move from the base (ancient common ancestors) towards the tips (more recent species).
• 🤔 The concept of 'relatedness' in cladograms is determined by the most recent common ancestor; organisms sharing a more recent common ancestor are considered more closely related.
• 🐙 When adding new species to a cladogram, one must consider where it fits based on shared characteristics, and the principle of parsimony (simplicity) should guide these decisions.
• 🔬 Modern cladograms often use DNA and genetic data to infer relationships, moving beyond physical characteristics.
• 📊 DNA sequencing allows for the creation of cladograms through computational analysis, providing a more accurate representation of evolutionary relationships.
• 🌐 The human species is positioned on the primate cladogram, sharing a common ancestor with other primates but having more recent common ancestors with closer relatives like bonobos and chimps.
• ✂️ The principle of parsimony, or Occam's razor, suggests that the simplest explanation, in this case the cladogram with the fewest assumptions, is often the most likely to be correct.

Q & A

What is a cladogram?

A cladogram is a diagram that represents the evolutionary relationships among various organisms based on shared characteristics. It is a hypothesis of how organisms might be related, rather than a definitive phylogenetic tree.

How does one create a cladogram?

A cladogram is created by identifying shared characteristics among organisms and then arranging them in a branching pattern that suggests their evolutionary relationships. The process involves grouping organisms with similar traits and then branching out to show how they diverge from a common ancestor.

What is the significance of the outgroup in a cladogram?

The outgroup in a cladogram is the organism that is most different from the others in the group. It is used as a reference point to help determine the evolutionary relationships among the other organisms. It is the 'one of these things is not like the others' in the context of the cladogram.

How does the concept of a common ancestor relate to a cladogram?

In a cladogram, each branching point can be thought of as representing a common ancestor for the organisms that branch out from that point. These ancestors are hypothesized based on the shared characteristics of the organisms.

What does the direction of time represent in a cladogram?

In a cladogram, time is represented by the direction from the base of the diagram (representing the most ancient common ancestor) to the tips (representing the most recent or current organisms). This direction helps in understanding the evolutionary progression from older to more recent species.

How can one determine which organisms are more closely related in a cladogram?

In a cladogram, organisms that share a common ancestor closer to the tip of the diagram are considered more closely related than those that share a common ancestor further back in time.

What is the principle of parsimony in the context of cladograms?

The principle of parsimony, also known as Occam's razor, suggests that the simplest explanation is usually the correct one. In cladograms, this means that the hypothesis with the fewest assumptions about evolutionary changes is often considered the most plausible.

Why is it important to consider the possibility of losing characteristics when constructing a cladogram?

When constructing a cladogram, it's important to consider that organisms can lose as well as gain characteristics over time. This is important for accurately representing the evolutionary history and for making parsimonious hypotheses about the relationships among organisms.

How has the advent of DNA sequencing technology changed the way cladograms are created?

With DNA sequencing technology, cladograms can now be created based on genetic similarities rather than just physical characteristics. This allows for a more accurate representation of evolutionary relationships, as DNA provides a direct link to common ancestry.

What is the significance of homologous structures in cladograms?

Homologous structures are characteristics that are similar in different species due to shared ancestry. In cladograms, the presence of homologous structures is used as evidence of common ancestry and helps in determining the evolutionary relationships among organisms.

Outlines

00:00

🌿 Understanding Cladograms

Mr. Andersen introduces the concept of a cladogram, which is a hypothesis about the evolutionary relationships between organisms based on shared characteristics. He uses the example of seven aquatic creatures named 'Andersenii aquaticus' to demonstrate how to construct a cladogram. He explains the process by identifying an outgroup and then sequentially branching out based on characteristics such as the presence of an eye. The cladogram is visualized as a tree with branches representing shared characteristics and nodes representing common ancestors. Mr. Andersen emphasizes that cladograms are not definitive but are based on the best available evidence and are subject to revision as new data emerges.

05:02

🧬 DNA and Cladograms

In the second paragraph, Mr. Andersen discusses the evolution of cladogram construction with the advent of DNA and genetic analysis. He contrasts the older method of using physical characteristics with the modern approach of DNA sequencing to infer evolutionary relationships. He provides an example of a primate cladogram, illustrating how different species like lemurs, loris, tarsiers, monkeys, apes, and humans are related based on their genetic similarities. Mr. Andersen explains that while cladograms are still hypotheses, they are now more accurately constructed using genetic data. He also introduces the concept of 'Occam's Razor' or the principle of parsimony, which suggests that the simplest explanation is often the most likely, and applies this principle to the placement of a new species, 'H', on the cladogram.

Keywords

💡Cladogram

A cladogram is a branching diagram used in biology to illustrate the evolutionary relationships among various organisms or groups of organisms. It is based on the shared characteristics of the organisms, which are used to infer their common ancestry. In the video, Mr. Andersen uses a cladogram to hypothesize how seven subspecies he discovered, named Andersenii aquaticus, might be related. The cladogram is a central tool in the video for understanding evolutionary relationships.

💡Phylogenetic tree

A phylogenetic tree is a graphical representation of the evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics. Unlike a cladogram, which is more of a hypothesis, a phylogenetic tree is a more detailed and scientifically accepted representation of evolutionary relationships. The script mentions that a cladogram is not a phylogenetic tree but a guess at how organisms might be related.

💡Out group

In the context of cladistics, the out group is a group of organisms that is used as a reference point to help infer the characteristics of the common ancestor of the in-group, which is the group of organisms being studied. In the script, Mr. Andersen uses the concept of the out group to identify the first branch in his cladogram, suggesting that the organism without a particular characteristic (in this case, an eye) is the out group.

💡Characteristics

Characteristics in biology refer to the traits or features of an organism that are used to classify and compare it with others. In the video, Mr. Andersen discusses how different characteristics of the subspecies of Andersenii aquaticus are used to create a cladogram, such as the presence or absence of an eye, teeth, or defensive spikes.

💡Cell membrane

The cell membrane is a biological membrane that separates the interior of all cells from the outside environment and regulates the movement of substances into and out of the cell. In the script, Mr. Andersen mentions that all the organisms in his cladogram have a cell membrane, which is used as a common characteristic to group them together.

💡Common ancestor

A common ancestor in evolutionary biology is a single organism from which every member of a group of organisms is descended. In the video, Mr. Andersen uses the concept of a common ancestor to explain the branching points in the cladogram, suggesting that each branch point represents a common ancestor for the organisms that branch off from that point.

💡Homologous structures

Homologous structures are anatomical structures in different organisms that have a common origin from an ancestor, often indicating a shared evolutionary history. In the video, Mr. Andersen explains that modern cladograms use homologous structures, particularly DNA sequences, to infer evolutionary relationships, moving beyond just physical characteristics.

💡DNA sequencing

DNA sequencing is the process of determining the precise order of nucleotides within a DNA molecule. It is a fundamental technique in genetics and is used to identify and compare the genetic makeup of organisms. In the script, Mr. Andersen mentions that DNA sequencing is used to create more accurate cladograms by comparing the genetic information of different species.

💡Ockham's razor

Ockham's razor, also known as the law of parsimony, is a problem-solving principle that states that the simplest explanation or the one with the fewest assumptions is most likely to be correct. In the video, Mr. Andersen applies this principle to the construction of cladograms, suggesting that the simplest evolutionary explanation is often the most plausible.

💡Theory of parsimony

The theory of parsimony, also known as Occam's razor, is a principle used in various fields including cladistics, which favors the simplest explanation or the one with the fewest assumptions. In the context of the video, Mr. Andersen uses the theory of parsimony to justify why a new species, H, should be placed on the cladogram in a way that assumes the fewest evolutionary changes, such as losing an eye rather than gaining complex features like teeth and defensive spikes.

Highlights

Cladograms are a method for hypothesizing relationships between organisms based on shared characteristics.

Andersenii aquaticus is a hypothetical group of organisms used to demonstrate cladogram construction.

Cladograms are not definitive phylogenetic trees but rather educated guesses about evolutionary relationships.

The process of creating a cladogram involves identifying an outgroup and then branching inwards based on shared traits.

The concept of 'one of these things is not like the others' is used to identify the outgroup in a cladogram.

Characteristics such as the presence of a cell membrane are used to establish common ancestors in a cladogram.

The branching points in a cladogram represent the evolution of new characteristics.

Cladograms can help determine which organisms are more closely related based on shared recent ancestors.

The concept of time is integral to understanding the progression of a cladogram, with the most ancient ancestors at the base.

Adding new species to a cladogram can be challenging and involves making educated guesses about their placement.

The theory of parsimony, or Occam's razor, suggests that the simplest explanation is often the most likely.

Cladograms can be constructed using physical characteristics, but DNA and genetics provide a more accurate method.

DNA sequencing allows for the creation of cladograms that reflect genetic similarities and differences.

Modern cladograms often use homologous structures of DNA to determine evolutionary relationships.

The human species' position on a primate cladogram shows our relationship to other primates based on genetic evidence.

Cladograms are hypotheses and are subject to revision as new data and understanding emerge.

Transcripts

Hi. It's Mr. Andersen and in this podcast I'm going to talk about the cladogram.

Cladogram is essentially taking characteristics that are shared by organisms and then putting

together a hypothesis of how you think they may be related. Or a phylogenetic tree. It's

not a phylogenetic tree, it's just a guess at how that might occur. And so luckily I

was just working in some pond water yesterday and I discovered these seven creatures. And

I've named them after my self of course. They're called Andersenii aquaticus. And they're all

subspecies of that. But basically they have a lot of different characteristics. And so

if we were to put those together in a cladogram, well, when you were growing up you probably

watched Sesame Street and they used to play a game called one of these things is not like

the other. And so that's a good way to think about a cladogram. And so basically which

of these doesn't look like the others? Which of these is an out group? Well you should

be able to figure this out. First one would be C. And then try to guess before I actually

click on it. The next one would branch of according to this would be B. Because it doesn't

have an eye. But we're going to have that in G. So the next would be G. Try playing

along with me. What's going to be the next one? That's right. That's going to be D. And

then after that? A. And then after that? F. And then finally after that we have E. So

you could probably do that. Just put them in order looking at what characteristics they

have. So the out group is going to be the one that doesn't have any of these characteristics

or maybe one. And so basically a cladogram is going to look like this. It's going to

be a hypothesis of how these things came to be. And so I could look down here and I could

say well right here there's one characteristic. And that would be that there is a cell membrane.

In other words all of them have a cell membrane. And so to show you how this is read, basically

everything that branches out from this point is going to be a cell membrane. What is this

branching point? You can think of that as a common ancestor between all of the organisms

that are on here. Or if I were to go right here this would be a circle. Or if I were

to move up a little bit what's going to be the next thing that comes, the next thing

is going to be the eye. And so I could keep going all the way up. These are characteristics

that are added. And we use those to make a cladogram. Now there's some interesting things

you should know. So if we look at this cladogram and say that it's true. Who's more related?

Is F more related to E? Or is it more related to A? That's a really good question. And so

basically how do we know which it's related to? And I would say the right answer is E.

You have to understand that a cladogram or a phylogenetic tree for that matter, time

is going to go in this direction. So we're going to put time on this side. In other words

this is going to be the most ancient common ancestor. And then we're going to work progressively

up. And so that means that F and E would share a common ancestor right here. But F and A

are going to share a common ancestor all the way back down here. And since this is way

farther back in time, we would say that F is more related to E. But this is a pretty

simplistic one. And again it's just a guess as to how it might occur. A tougher question

might be, well let's add a new one. Let's say I discover something like this. This is

species H. Or subspecies H. Where would you put this on this cladogram? And when I talk

to students in class there's three areas or four areas where they might want to add that.

So some people think we should add it kind of like branching off here. Some think we

should do it right here, branching off D. Some think it should be something like this.

And some think it should look something like that. And so these are all plausible. In other

words these are all guesses. And we could put way more in there than this. But what's

the quote unquote right answer? Well if I had to put this somewhere, what I would do

is I would put it right here. In other words I would have H branching off here. And the

reason why is that if I were to put it way back here before the eye evolved because we

didn't have that eye, well it could have occurred like that, but it would be odd that it would

also evolve the same exact teeth and the same defensive spikes that we find farther up.

So a more plausible guess would be that it branched off of this A and then it lost its

eye. Now how could it lose its eye? Well if we were to put one of these in an area where

there's no light, then the eye wouldn't be an advantage. And so lots of times when we're

adding a cladogram we just add characteristics that are added. But know this, that it's just

as likely to lose something that we've gained as to gain something new. And so this would

be kind of a pretty good guess as to where H goes. And what I'm talking about now is

ockham's razor. In other words I'm talking about the theory of parsimony. What the heck

does that even mean? Well it basically means this. If all things are equal, the simplest

explanation for how that occurred is probably the most right. In other words, if I come

out in the morning and there's a glass of milk that's spilled in the kitchen, I could

come up with a bunch of different hypotheses. It could be aliens. It could be big foot.

It could be a massive earthquake. Or it could be my cat. And if I were to look at all of

these, well it's most likely going to be my cat. It doesn't mean that it's right. It doesn't

mean that aliens didn't come down and spill my milk. It just means that it's more than

likely the cat that did it. And so when you're building a cladogram you kind of try to do

that. And you try to use this idea of parsimony. The idea that the simplest explanation is

the best. So let me show you an example of one of these. This would be a cladogram that's

created for primates. And if we look at this one again to go back, in general in biology

we used to us all of these characteristics to put organisms in a cladogram. Physical

characteristics. But you can imagine that's not a great way to classify life. Or to even

show its evolutionary relationships. Today we now have DNA. We have genetics. And so

you can create a cladogram like this by just sequencing the DNA in all these different

organisms. And then throwing them into computer. And it could create a plausible kind of a

cladogram. And so where are you? You're right here on the human side. And so if we were

to go across, from the left. So if we look at these two, the lemur and the loris, they

share a common ancestor right here. But they also show a distant common ancestor with all

of the primates on here all the way back here. If we were to work across to the tarsier or

the new world and old world monkeys, is kind of a weird term. New world is a South American

monkey. So an example could be the squirrel monkey. An old world monkey would be like

a baboon. And so all of these things have tails. And now we move up to the apes. The

first one would be the gibbon. That would be this first branch point. Then we have the

orangoutang, the gorilla, the chimp. And we could put bonobo branching off here. And they

we finally have the humans. And so again this is just a cladogram. It's a hypothesis of

how these things are related. And you're looking at characteristics that are shared by all

of these. But the big characteristics we're looking at now are homologous structures of

DNA. In other words where the genetics match up, that means they had a common ancestor.

Does it mean that we turned from gorillas to humans? No. It just means that we had an

ancestor way back at this point. And so those are cladograms and I hope that's helpful.