Protists and Evolutionary Groups (PDF)

# **The Plastid-Bearing Lineage of Protists** The plastid-bearing lineage of protists evolved into red algae and green algae. - On several occasions during eukaryotic evolution, red and green algae underwent **secondary endosymbiosis**. In which they were ingested by a **heterotrophic eukaryote**....

# **The Plastid-Bearing Lineage of Protists** The plastid-bearing lineage of protists evolved into red algae and green algae. - On several occasions during eukaryotic evolution, red and green algae underwent **secondary endosymbiosis**. In which they were ingested by a **heterotrophic eukaryote**. ## **Diagram:** A diagram depicts this process, showing the following: * **Left side:** A cyanobacterium, a red alga, and a green alga. Arrows indicate the **primary endosymbiosis** of a cyanobacterium by a heterotrophic eukaryote, which eventually lost its membrane. Subsequent secondary endosymbiosis events are indicated for the red alga and the green alga. * **Right side:** Various protist groups: Dinoflagellates, Apicomplexans, Stramenopiles, Euglenids, and Chlorarachniophytes. Arrows point from a plastid to these groups. ## **The Five Supergroups of Eukaryotes** A tree diagram shows the relationships between the five supergroups of eukaryotes: * **Excavata:** Contains Diplomonads, Parabasalids, and Euglenozoans * **Chromalveolata:** Contains Dinoflagellates, Apicomplexans, Ciliates, Diatoms, Golden Algae, Brown Algae, and Oomycetes. * **Rhizarla:** Contains Chlorarachniophytes, Forams, and Radiolarians. * **Archaeplastida:** Contains Red Algae, Chlorophytes, Charophyceans, and Land plants. * **Unikonta:** Contains Slime molds, Gymnamoebas, Entamoebas, Nucleariids, Fungi, Choanoflagellates, and Animals. ## **Endosymbiosis in Eukaryotic Evolution** - There is significant evidence that much protist diversity has its origins in **endosymbiosis**. - **Mitochondria evolved by endosymbiosis of an aerobic prokaryote.** - **Plastids evolved by endosymbiosis of a photosynthetic cyanobacterium.** ## **Protists - Nutritionally Diverse** Protists are the most nutritionally diverse of all eukaryotes. They consist of 3 main types: - **Photoautotrophs:** Contain chloroplasts. - **Heterotrophs:** Absorb organic molecules or ingest larger food particles (phagocytosis and/or absorption). - **Mixotrophs:** Combine photosynthesis and heterotrophic nutrition. ## **Most Eukaryotes are Single-Celled Organisms** - Protists are eukaryotes and thus have organelles making them more complex than prokaryotes. - While many protists are unicellular, some are colonial or **multicellular**. ## **Structural and Functional Diversity in Protists** Protists exhibit more structural and functional diversity than any other group of eukaryotes. ## **Kingdom Protista** - The kingdom Protista is a **highly diverse collection of organisms**. - **Defined by exclusion** - meaning it includes organisms which do not belong to any other kingdom. - The classification of the kingdom Protista is being reconsidered, and it may be as many as **20 kingdoms**. ## **Overview: Living Small** - *Protist* is the informal name of the kingdom of **unicellular and multicellular eukaryotes**. - Advances in eukaryotic **systematics** have caused the classification of protists to change significantly. - Protists constitute a **paraphyletic group,** and **Protista is no longer valid as a kingdom.** ## **Evolutionary Groups** A diagram represents the evolutionary relationships between various groups, including: - prokaryotes - single-celled eukaryotes - multicellular eukaryotes - archaea - animals - fungi - plants - red algae - green algae - land plants - Choanoflagellates - amphibians - reptiles - mammals ## **Paraphyletic Group** - A paraphyletic group includes a common ancestor and some of its descendants, but not all. ## **Polyphyletic Group** - A polyphyletic group includes multiple descendants, but not the common ancestor. ## **Monophyletic Group** - A monophyletic group includes a common ancestor and all of its descendants, sometimes referred to as a clade. <start_of_image> Diagrams illustrate these concepts using examples of animals, including: - Giraffe - Bat - Turtle - Crocodile - Stegosaurus - Tyrannosaurus - Velociraptor - Hawk ## **Modern Evolutionary Biologists Prefer to Recognize Only Monophyletic Clades as Higher Taxa:** - An example is given for the **Reptilia group**. Traditionally, the Reptilia group was considered to include snakes and crocodiles but not birds. However, this classification is paraphyletic, as it does not include all the descendants from the most recent common ancestor of snakes and crocodiles. ## **Similarity Versus Relationship** - A diagram shows the following examples: - Chimpanzees and monkeys are more closely related to each other than to rodents and are also more similar. - Dolphins are closely related to hippopotamuses and other mammals, but they superficially look more like sharks. - Crocodiles and birds share a more recent common ancestor than either does with lizards, but birds look very different because they have undergone more evolutionary changes than crocodiles. ## **Evolutionary Novelties** - Most novel biological structures evolve in many stages from previously existing structures. - Complex eyes have evolved from simple photosensitive cells independently many times. - **Exaptations** are structures that evolve in one context but become co-opted for a different function. For example, feathers were originally involved in thermal regulation before providing insulation and flight. ## **Hox Genes and Vertebrate Evolution** - A diagram shows the hypothetical evolutionary history of Hox gene duplication in vertebrates. - A single Hox cluster in an invertebrate ancestor. - Two Hox clusters in early, jawless vertebrates. - Four Hox clusters in vertebrates with jaws. - The evolution of vertebrates from invertebrates was associated with alterations in Hox genes. - Multiple duplications of Hox genes occurred in the vertebrate lineage. - These duplications may have been important in the evolution of new vertebrate characteristics. - Evolutionary ancestors had fewer Hox genes. Modern organisms have the genetic potential to have a relatively complex body. ## **Homeotic Genes** - Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower's parts are arranged. - One class of homeotic genes, the **Hox genes**, provide positional information in animals. ## **Neoteny (Paedomorphosis, Progensis)** - Possessing in the adult stage of features typical of the juvenile stage of the organism's ancestor. - **Neoteny** is illustrated by comparing human skull growth with that of a chimpanzee. -Humans exhibit a delayed growth rate compared to chimpanzees and retain more juvenile features.