How to Understand Evolutionary Trees

Summary

This tutorial elucidates the concept of evolutionary trees, or phylogenies, which illustrate the paths of evolution and relatedness among species. It explains the tree's anatomy, including roots, nodes, and branches, and clarifies misconceptions about relatedness and evolutionary advancement. The video also distinguishes between different types of groups within trees, such as monophyletic, paraphyletic, and polyphyletic, and highlights the importance of cladistics in classifying organisms based on shared characteristics and evolutionary relationships.

Takeaways

• 🌳 An evolutionary tree, or phylogeny, shows the paths of evolution and relatedness of species, helping us understand evolution on a larger scale.
• 📍 The root of the tree represents the common ancestor of all organisms in the tree, and internal nodes indicate further diversification points.
• 🔍 Terminal nodes or tips represent current organisms, illustrating relationships that might not be immediately apparent.
• 🌐 Branches on the tree represent sister taxa, groups that share a direct common ancestor.
• 🔑 The ingroup in a phylogeny shows species with common characteristics relevant to the specific tree, while the outgroup is less closely related but included for broader context.
• 🔄 Trees can be represented in various ways, but the key is the topology, or the order of branching, rather than the direction of branches.
• 📏 In cladistics, branch lengths are not significant, only the order of branching matters, whereas in a phylogram, branch lengths indicate time or divergence.
• 👨‍👩‍👧‍👦 Monophyletic groups show a common ancestor in all descendants, and the number of clades can be found by counting nodes.
• 🚫 Paraphyletic groups include a recent common ancestor but not all descendants, while polyphyletic groups include distant relatives but no common ancestor.
• ❓ Soft polytomies in a tree indicate unknown patterns of branching, representing relationships that are not fully understood.
• 🙅‍♂️ Common misconceptions include misunderstanding the relatedness indicated by the order of tips, the role of the root and tips in time of divergence, and the evolutionary status of nodes.
• 🚫 It's important to note that species do not evolve based on being hybrids of their ancestors but through natural selection based on heritable traits.

Q & A

What is an evolutionary tree also known as?

An evolutionary tree is also known as a phylogeny, which shows the paths of evolution and relatedness of species.

What does the root of a phylogenetic tree represent?

The root of a phylogenetic tree represents the common ancestor of all organisms present in the tree.

What do the internal nodes in a phylogenetic tree indicate?

The internal nodes in a phylogenetic tree indicate other common ancestors and represent diversification of species.

What are terminal nodes or tips in a phylogenetic tree?

Terminal nodes or tips in a phylogenetic tree represent current organisms.

What is the significance of sister taxa in an evolutionary tree?

Sister taxa are groups that come from the same direct common ancestor, indicating a close evolutionary relationship.

What is the role of the ingroup in a phylogenetic tree?

The ingroup shows the species with common characteristics relevant to the specific tree, highlighting their shared traits.

Why is an outgroup included in a phylogenetic tree?

An outgroup is included to provide a broader context of evolution, showing that species that may not look similar can still be related.

What is the difference between a cladogram and a phylogenetic tree in terms of branch lengths?

In a cladogram, branch lengths are not significant and only the topology matters, whereas in a phylogram, branch lengths represent the time or quantity of divergence.

What is a monotypic group in the context of phylogenetic trees?

A monotypic group, also known as a holonome, shows the common ancestor in all of its descendants within a phylogenetic tree.

What is a paraphyletic group and how is it represented in a phylogenetic tree?

A paraphyletic group includes a recent common ancestor but not all descendants, showing an incomplete evolutionary lineage.

What does a soft polytomy in a phylogenetic tree indicate?

A soft polytomy indicates that the pattern of branching is unknown, representing an evolutionary relationship that is not fully understood.

Why is it incorrect to assume that tips beside each other are more related than tips further apart in a phylogenetic tree?

The order of the tips does not show relatedness; it is the topology that matters. Closeness of tips does not necessarily indicate closer evolutionary relationships.

What misconception about phylogenetic trees is corrected by emphasizing that the root is the ancestor and the tips are the evolved species?

The misconception that the most recently evolved species are at the tips and the oldest ancestor is at the root is corrected by understanding that the root represents the common ancestor and the tips represent current, evolved species.

Why is it incorrect to view nodes in a phylogenetic tree as indicators of levels of advancement or primitiveness?

Nodes simply indicate the number of common ancestors and do not represent levels of advancement or primitiveness, as evolution is driven by natural selection, not a linear progression of advancement.

What is the misconception about outgroups undergoing no evolutionary change in a phylogenetic tree?

The misconception is that outgroups, being less closely related to other taxa, do not undergo evolutionary change. In reality, their changes are not shown in the specific tree because they do not fit the characteristics of that tree.

Why is it incorrect to assume that species arising from nodes are hybrids of their ancestors?

Species evolve based on natural selection, which is non-random and based on heritable traits that confer survival advantages. New species are not simply combinations of their ancestors but evolve independently.

Outlines

00:00

🌳 Understanding Evolutionary Trees

This paragraph introduces the concept of evolutionary trees, also known as phylogenies, which illustrate the evolutionary relationships and paths among species. It explains the structure of a phylogenetic tree, including the root as the common ancestor, internal nodes representing other common ancestors, and terminal nodes representing current species. The paragraph also discusses the significance of branches, sister taxa, ingroup and outgroup, and clarifies that the direction of branches is irrelevant, but the topology is crucial. It introduces the concept of cladistics, distinguishing between different types of groups like monophyletic, paraphyletic, and polyphyletic, and explains the concept of soft polytomy. The paragraph concludes by addressing misconceptions about the interpretation of phylogenetic trees.

05:01

🔍 Common Misconceptions About Evolutionary Trees

The second paragraph focuses on debunking common misconceptions related to evolutionary trees. It clarifies that the proximity of tips does not indicate relatedness, and that the order of branching (topology) is what matters, not the physical distance on the tree. It also corrects the misunderstanding that the root represents the most primitive species and the tips the most advanced, emphasizing that evolution is not a ladder of progress but a process of natural selection. The paragraph further explains that outgroups are not static but part of the broader evolutionary picture, and refutes the idea that new species are hybrids of their ancestors, instead highlighting the role of natural selection in species formation. The paragraph ends by encouraging viewers to explore more resources for a deeper understanding of evolutionary trees.

Keywords

💡Phylogeny

Phylogeny refers to the evolutionary relationships among various species, represented as a tree-like diagram that shows the paths of evolution and relatedness. In the video, phylogeny is central to understanding the theme as it allows viewers to visualize how different species are connected through common ancestors, as exemplified by the root of the tree representing the common ancestor of all organisms.

💡Common Ancestor

A common ancestor is a species from which multiple species have evolved. The script explains that the root of a phylogenetic tree is the common ancestor of all species depicted, and internal nodes represent other common ancestors, illustrating how species diverge and evolve from shared evolutionary origins.

💡Diversification

Diversification is the process by which a single lineage splits into two or more distinct lineages. The script mentions that nodes in a phylogenetic tree indicate diversification, showing the branching out of species from a common ancestor into different evolutionary paths.

💡Terminal Nodes

Terminal nodes, or tips, represent the current or extant species in a phylogenetic tree. The script clarifies that these nodes, although they may not appear related, demonstrate the relationships between species that may not have been previously considered.

💡Branch

A branch in an evolutionary tree signifies a lineage or a path of evolution from a common ancestor to a descendant species. The script uses the term to describe the connection between sister taxa, which share the same direct common ancestor.

💡Sister Taxa

Sister taxa are two species or groups that have a more recent common ancestor than they do with any other species. The script explains that these groups are part of the same branch, indicating a close evolutionary relationship.

💡Ingroup

The ingroup in a phylogenetic tree consists of the species that share common characteristics relevant to the specific tree being analyzed. The script mentions that the ingroup is part of the tree that is being specifically classified, showing the shared traits among the species of interest.

💡Outgroup

An outgroup is a species or group included in a phylogenetic analysis to provide a point of reference for understanding the relationships among the ingroup. The script notes that the outgroup is less closely related to the ingroup but is crucial for showing the broader picture of evolution.

💡Cladistics

Cladistics is a method of classifying organisms based on shared characteristics and evolutionary relatedness. The script discusses cladistics as a way to understand the order of branching in a phylogenetic tree, emphasizing the importance of topology over branch lengths.

💡Monophyletic Group

A monophyletic group, also known as a clade, includes all descendants of a common ancestor. The script provides a helpful tip for identifying monophyletic groups by counting the number of nodes, which corresponds to the number of common ancestors.

💡Paraphyletic Group

A paraphyletic group includes some descendants of a common ancestor but not all. The script explains that in a paraphyletic group, the recent common ancestor is shown, but not all descendants are included, indicating an incomplete representation of evolutionary relationships.

💡Polyphyletic Group

A polyphyletic group consists of species that are not each other's closest relatives but are grouped together due to shared characteristics. The script mentions that a polyphyletic group does not include a common ancestor but includes distant relatives, which is not a true reflection of evolutionary relationships.

💡Soft Polytomy

Soft polytomy in a phylogenetic tree indicates that the pattern of branching is unknown or that the relationships among certain species are not fully understood. The script uses soft polytomy as an example of an evolutionary relationship that requires further study.

💡Misconceptions

The script addresses several misconceptions about phylogenetic trees, such as the incorrect assumption that tips beside each other are more related or that the outgroup undergoes no evolutionary change. These misconceptions are important to clarify to ensure a correct understanding of evolutionary relationships.

Highlights

Understanding evolutionary trees, also known as phylogenies, helps to comprehend the paths of evolution and relatedness of species.

The root of a phylogeny represents the common ancestor of all organisms in the tree.

Internal nodes in a phylogeny signify common ancestors and diversification of species.

Terminal nodes or tips represent current organisms and their relationships.

Branches in a phylogenetic tree are part of sister taxa, sharing a direct common ancestor.

The ingroup in a phylogeny shows species with common characteristics relevant to the tree.

The outgroup in a phylogeny is a species included for broader evolutionary context but not fitting the specific classification.

Cladistics is a method of classifying organisms based on shared characteristics and evolutionary relatedness.

In a cladogram, branch lengths are not significant; only the order of branching matters.

A phylogram shows branch lengths that are significant, indicating time or quantity of divergence.

Monotypic groups in phylogenies show a common ancestor in all of their descendants.

Paraphyletic groups include a recent common ancestor but not all descendants.

Polyphyletic groups include distant relatives but do not include a common ancestor.

Soft polytomies in phylogenies indicate unknown patterns of branching and relationships.

Common misconceptions about phylogenies include misunderstanding the relatedness indicated by the order of tips.

In phylogenetic trees, the root represents the common ancestor, and the tips represent evolved species.

Evolutionary trees do not indicate levels of advancement or primitiveness; they show common ancestry and divergence.

Outgroups in phylogenies undergo evolutionary changes, though they may not be depicted in the specific tree.

New species evolve through natural selection, not as hybrids of their ancestors.

Transcripts

welcome to a video tutorial on how to

understand evolutionary

trees an evolutionary tree is called a

philogyny phogy show Paths of evolution

and relatedness of species which makes

them helpful because they allow us to

understand Evolution on a larger

scale this is the anatomy of a

philogyny the root is at the very bottom

and this is where the rest of the trees

stems from the root is the common

ancestor of all organisms present in the

tree the internal nodes represent other

common ancestors present in the tree

nodes also indicate diversification of

species in this tree the heart and the

circle both share two common ancestors

in addition to the

root the terminal nodes or the tips

represent current organisms all although

they may not seem related they show

relationships between species that you

may not have even thought

of a branch is also part of an

evolutionary

tree these groups are sister taxa they

come from the same direct common

ancestor the ingroup shows the species

with common characteristics relevant to

this specific

tree the outgroup is a species that is

included in the philogyny but does not

fit in well with the tree specific

classification however they are all

related note that the outgroup does not

have any branching occurring this does

not mean that the outgroup doesn't

undergo any evolutionary change

branching is just not shown in the

outgroup because the out group itself

does not fit in with the common

characteristics of the tree as a whole

in this way the out group acts as a

reminder to biologists of the grand

scheme of evolution and that species

that may not look similar can still be

related

it's important to note that trees can be

represented in many ways however they

still tell us the same information

remember the direction of the branch is

irrelevant but rather the topology or

the order of the branches urrent is what

matters nodes can be rotated so the

order of the tips can be changed do not

read across the tips

moving on to cladistics cladistics is a

term used for classifying organisms

based on evolutionary relatedness or

shared characteristics in a cladogram

the branch lengths are not significant

only the topology the order of branching

matters in a phram the branch lengths

are significant they show the time or

quantity of

Divergence within an evolutionary tree

there are different groups this is a

monoptic group it can also be called a

CLA monoptic groups and clayes show the

common ancestor in all of their

descendants as a helpful tip to find the

number of clayes count the number of

nodes remember each node and the root

are common ancestor so the number of

nodes is equal to the number of clayes

this is a paraphyletic

group the recent common ancestor is

shown but not all descendants are

included this is a polyphilic

group The Reason common ancestor is not

included but distant relatives are

included a tree that looks like this is

called a soft polytomy soft polytomy

indicates that the pattern of branching

is unknown so this is an evolutionary

relationship that is not fully

understood congratulations you just got

through tree thinking 101 we're going to

end things off by addressing some common

misconceptions misconception number one

tips beside each other are more related

than tips further

apart

wrong the order of the tips do not show

relatedness topology does it is

important to remember to not read across

the tips

as an example let's look at this tree

the blue circle is more closely related

to the red heart than it is to the green

triangle even though the circle and the

triangle are right next to each other

this is because the circle and the Heart

share a more recent common

ancestor again do not read across the

tips misconception number two looking at

this

phram the yellow star is the ancestor

and the green triangle is the most

recently evolved

species wrong

in the PHR tree the root is the ancestor

and the tips are the evolved species

read from root to tip to determine the

time of Divergence in a

phog let's try another example looking

at this tree the swirl is more evolved

and advance than the smiley

face

wrong nodes simply indicate the number

of common ancestor they do not indicate

levels of advancement or show

primitiveness remember Evolution occurs

by means of natural selection so the

odds of survival are higher for

organisms that have traits that work for

them in that specific environment at

that specific time a trait that led to

survival previously may not do so now

and a trait that leads to survival now

may not later advancement and

primitiveness are Concepts that are

irrelevant to tree thinking

misconception number three looking at

this tree the outgroup in this case s

under goes no evolutionary change

wrong Oak groups are less closely

related to other taxa in the tree out

groups are meant to show the bigger

picture of evolution their evolutionary

changes exist but aren't shown in the

specific tree because they do not fit in

with the characteristics of that tree

one more to go misconception number four

species that arise from no are hybrids

of their ancestors so for example in

this tree the circle is a hybrid of the

flower the X and the

diamond

wrong again species evolve based on

natural selection which is non-random

differences in survival and or

reproduction among individuals on the

basis of hereditable traits new species

are not accommodation of

ancestors congratulations you now know

the basics of evolutionary trees and the

common misconceptions associated with

them good job looking for more check out

some of our resources used for this

video for more in-depth material