Protistiary - Homology

<%@LANGUAGE="JAVASCRIPT" CODEPAGE="1252"%> Protistiary - Homology

INVENTORY OF CELL COMPONENTS - HOMOLOGIES
Homologous characters are defined as those characters that come from a single evolutionary source. They share a common evolutionary history. Other types of characters are those that have converged to seem similar. These are referred to as homoplasious characters. We use homologies to establish relationships, so it is important that we can distinguish homologous similarities from homoplasious similarities. As we progress towards a better understanding of evolutionary history, we refine our understanding of which characters are homologous and remove the noise created by mis-interpreting homoplasious characters as homologous.	In this group of protists, parasitism has evolved twice, and is homoplasious. A group called 'parasitic prodiscea' would not define a monophyletic lineage. On the other hand, the symbiotic acquisition of a green chloroplast within the euglenids defines a subset of them, and that is a monophyletic group. In this group, plastids may be secondarily lost - as is the case with Astasia. The 'loss of' is a a character state in its own right. The continued retention of a feature is NOT a requirement for membership of the group defined by the evolutionary innovation. Image by D J Patterson.
Each character is an evolutionary innovation. It appears at a point in an evolutionary history. The character is an apomorphic feature - an evolutionary innovation. The character can be used to define a group that contains all of the descendents from that event. When used to define a subset of life, the character is referred to as a synapomorphy - shared derived character (shared between the common descendents of that point). Ideally, we can identify members of a monophyletic group because they all share the characters. This is rarely simple, because characters may subsequently change or be lost. Synapomorphies are important in reconstructing evolutionary history because they define single lines of evolution. Homoplasious features (wings) define polyphyletic groups (birds, bats and insects). Because evolution is progressive, apomorphies appear in succession. Synapomorphies can be predicted to form nested sets; those characters that appear earlier define more extensive groups, and those that appear later define smaller subsets. The structure is the same as the more conventional classification schemes.	The stars represent evolutionary events, and these events leave a legacy that is inherited by all descendents - although they may continue to be modified later in evolutionary history. Image by D J Patterson.
We cannot be sure if things are homologous, so homologies are best regarded working hypotheses. As homologies and monophyletic groupings are different sides of the same coin, the stability of groups is a good measure of the homology of the character or characters that were offered as synapomorphies. How do we decide if we think things are homologous: They will have a similar composition (microtubular roots are not homologous with fibrous roots). They will have a similar architectural or positional relationship - just as the wings of birds and the flippers of seals derive from the shoulder region, so certain microtubular roots may derive from a particular part of a particular basal body. There is likely to be a similar functional role. Homologous organelles will co-distribute with other homologous organelles. Because of the nested or successive character of evolution, the consequences of later evolutionary events should co-occur in the same organisms as the consequences of earlier evolutionary events. If there is inconsistency in the distribution of characters, it is a good sign that we have misunderstood a homology. A persistent example of this lies with the 'chromists,' a group which is based on the common presence of a particular type of chloroplast that contains chlorophylls a and c. The chromists contain three major subgroups: stramenopiles (also referred to as heterokonts or chrysophytes), haptophytes (prymnesiophytes) and cryptophytes. Sequences and cytology are fairly different among these three groups - therefore violating the expectation of co-distribution of features. Convergence in plastids seems to explain this grouping, and this argument is made credible by examples of plastids moving laterally from one lineage to another (from the green algae to the euglenids; or by kleptoplasty).
There is a simple logical relationship between evolutionary history, the sequence in which synapomorphies appear, and classification schemes. If we get the game right, then all of these elements become interchangeable. A measure of the success with which we do this is the robustness or volatility of the taxonomic groups that are created from our thinking about evolutionary history.	Taxonomic structure overlain on inferred evolutionary tree. Image by D J Patterson.
protistiary home page top of electron microscopy exercise

INVENTORY OF CELL COMPONENTS - HOMOLOGIES

Homologous characters are defined as those characters that come from a single evolutionary source. They share a common evolutionary history.

Other types of characters are those that have converged to seem similar. These are referred to as homoplasious characters.

We use homologies to establish relationships, so it is important that we can distinguish homologous similarities from homoplasious similarities. As we progress towards a better understanding of evolutionary history, we refine our understanding of which characters are homologous and remove the noise created by mis-interpreting homoplasious characters as homologous.

In this group of protists, parasitism has evolved twice, and is homoplasious. A group called 'parasitic prodiscea' would not define a monophyletic lineage. On the other hand, the symbiotic acquisition of a green chloroplast within the euglenids defines a subset of them, and that is a monophyletic group. In this group, plastids may be secondarily lost - as is the case with Astasia. The 'loss of' is a a character state in its own right. The continued retention of a feature is NOT a requirement for membership of the group defined by the evolutionary innovation. Image by D J Patterson.

Each character is an evolutionary innovation. It appears at a point in an evolutionary history. The character is an apomorphic feature - an evolutionary innovation. The character can be used to define a group that contains all of the descendents from that event. When used to define a subset of life, the character is referred to as a synapomorphy - shared derived character (shared between the common descendents of that point). Ideally, we can identify members of a monophyletic group because they all share the characters. This is rarely simple, because characters may subsequently change or be lost.

Synapomorphies are important in reconstructing evolutionary history because they define single lines of evolution. Homoplasious features (wings) define polyphyletic groups (birds, bats and insects).

Because evolution is progressive, apomorphies appear in succession. Synapomorphies can be predicted to form nested sets; those characters that appear earlier define more extensive groups, and those that appear later define smaller subsets. The structure is the same as the more conventional classification schemes.

The stars represent evolutionary events, and these events leave a legacy that is inherited by all descendents - although they may continue to be modified later in evolutionary history. Image by D J Patterson.

We cannot be sure if things are homologous, so homologies are best regarded working hypotheses. As homologies and monophyletic groupings are different sides of the same coin, the stability of groups is a good measure of the homology of the character or characters that were offered as synapomorphies.

How do we decide if we think things are homologous:

They will have a similar composition (microtubular roots are not homologous with fibrous roots).
They will have a similar architectural or positional relationship - just as the wings of birds and the flippers of seals derive from the shoulder region, so certain microtubular roots may derive from a particular part of a particular basal body.
There is likely to be a similar functional role.
Homologous organelles will co-distribute with other homologous organelles. Because of the nested or successive character of evolution, the consequences of later evolutionary events should co-occur in the same organisms as the consequences of earlier evolutionary events. If there is inconsistency in the distribution of characters, it is a good sign that we have misunderstood a homology. A persistent example of this lies with the 'chromists,' a group which is based on the common presence of a particular type of chloroplast that contains chlorophylls a and c. The chromists contain three major subgroups: stramenopiles (also referred to as heterokonts or chrysophytes), haptophytes (prymnesiophytes) and cryptophytes. Sequences and cytology are fairly different among these three groups - therefore violating the expectation of co-distribution of features. Convergence in plastids seems to explain this grouping, and this argument is made credible by examples of plastids moving laterally from one lineage to another (from the green algae to the euglenids; or by kleptoplasty).

There is a simple logical relationship between evolutionary history, the sequence in which synapomorphies appear, and classification schemes. If we get the game right, then all of these elements become interchangeable.

A measure of the success with which we do this is the robustness or volatility of the taxonomic groups that are created from our thinking about evolutionary history.

Taxonomic structure overlain on inferred evolutionary tree. Image by D J Patterson.

protistiary home page top of electron microscopy exercise