27.3 Animal Phylogeny
3 min read•june 14, 2024
Animal phylogeny unveils the intricate relationships between diverse animal groups. From sponges to humans, this field explores how different species are connected through evolutionary history, shedding light on the development of complex body plans and adaptations.
Recent advances in genetic and molecular techniques have revolutionized our understanding of animal relationships. These new insights have led to significant changes in classification systems, reshaping our view of how different animal groups are related and how they evolved over time.
Animal Phylogeny
Branches of metazoan phylogeny
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- (animals) split into two main branches
- includes sponges () which lack true tissues and organs
- encompasses all other animal phyla with true tissues and organs
- consists of jellyfish, corals, sea anemones () and comb jellies () with radial symmetry
- includes animals with bilateral symmetry
- develops mouth first during embryogenesis (annelids, arthropods, mollusks)
- undergoes molting (shedding exoskeleton) and includes nematodes (roundworms) and arthropods (insects, crustaceans, arachnids)
- named for spiral cleavage pattern in early development and includes annelids (segmented worms) and mollusks (snails, clams, octopuses)
- develops anus first during embryogenesis
- includes sea stars and sea urchins with pentaradial symmetry as adults
- includes tunicates, lancelets, and vertebrates with a and dorsal nerve cord
- develops mouth first during embryogenesis (annelids, arthropods, mollusks)
Methods for constructing animal phylogeny
- provides evidence from comparative anatomy
- Examines body structures and organs across different animal groups
- Tracks embryological development and patterns (, )
- Notes presence or absence of key features like segmentation and body cavities
- Genetic data compares DNA sequences and genome organization
- Analyzes similarities and differences in DNA sequences across taxa
- Examines genome structure and organization (, gene order)
- Identifies presence or absence of specific genes and gene families
- Molecular data looks at amino acid sequences and gene expression
- Compares amino acid sequences of homologous proteins across species
- Analyzes gene expression patterns during development and in different tissues
- Uses molecular markers like and to infer evolutionary relationships
- Phylogenetic analysis methods reconstruct evolutionary trees
- aims to minimize the total number of character changes required
- estimates the most probable tree based on an evolutionary model
- incorporates prior probabilities to estimate posterior probability of trees
- focuses on shared derived characteristics (synapomorphies) to determine evolutionary relationships
Recent changes in animal classification
- Reclassification of some phyla based on molecular evidence
- removed from Porifera and placed in its own phylum
- , formerly considered protists, now placed within Cnidaria
- Restructuring of Protostomia into Ecdysozoa and Spiralia
- Molecular data supports these two major protostome clades
- Annelids now grouped with mollusks in Spiralia rather than with arthropods
- Recognition of new phyla that were previously overlooked or misclassified
- and now considered separate phyla instead of flatworms
- discovered as a new phylum closely related to rotifers
- Revisions within Deuterostomia based on molecular phylogenies
- Xenoturbellida and Acoelomorpha now included in Deuterostomia
- now a separate phylum from Chordata rather than a subphylum
- Improved resolution of deep evolutionary relationships among animals
- Bilateria confirmed as a including Protostomia and Deuterostomia
- Strong support for Deuterostomia and Protostomia as the two major bilaterian clades
Evolutionary concepts in animal phylogeny
- refers to similarities due to common ancestry
- results in similar traits evolving independently in different lineages
- techniques estimate divergence times based on genetic differences
- Monophyletic groups include all descendants of a common ancestor
Key Terms to Review (41)
Acoelomorpha: Acoelomorpha is a phylum of simple, worm-like organisms that are characterized by their lack of a true coelom, or body cavity. These animals are primarily marine and exhibit bilateral symmetry, which connects them to other bilaterally symmetrical organisms. Acoelomorpha represents a critical evolutionary stage between simpler organisms like cnidarians and more complex bilaterians, helping scientists understand the evolution of body plans in the animal kingdom.
Archaeocyatha: Archaeocyatha are an extinct group of primitive marine animals that existed during the Cambrian period, around 505 to 510 million years ago. They are significant as they are some of the earliest known reef builders, contributing to the development of marine ecosystems and influencing the evolution of other marine life forms.
Bayesian inference: Bayesian inference is a statistical method that applies Bayes' theorem to update the probability estimate for a hypothesis as more evidence or information becomes available. It allows researchers to incorporate prior knowledge alongside new data to make more informed conclusions about evolutionary relationships and phylogenetic trees, especially in the context of biological research.
Bilateria: Bilateria refers to a major clade of animals characterized by bilateral symmetry, which means their bodies can be divided into mirror-image halves along a single plane. This body plan includes a distinct head and tail end, allowing for more complex movement and organization of body systems. Bilaterians are crucial to understanding the evolutionary relationships among various animal groups, as they encompass a vast diversity of organisms with varying degrees of complexity.
Cephalochordata: Cephalochordata is a subphylum of small, fish-like marine animals known as lancelets. They are characterized by their elongated bodies and retention of the notochord throughout life.
Chordata: Chordata is a phylum that includes animals characterized by having a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some stage of their development. This diverse group encompasses various organisms ranging from fish to mammals, showcasing a wide variety of adaptations and evolutionary significance within the animal kingdom.
Cladistics: Cladistics is a method of classifying organisms based on common ancestry and evolutionary relationships. It uses shared derived traits to construct a cladogram that illustrates these relationships.
Cladistics: Cladistics is a method of classifying organisms based on their evolutionary relationships, specifically by analyzing shared characteristics and common ancestry. This approach focuses on grouping species into clades, which are branches on a phylogenetic tree that share a common ancestor, providing insights into the evolutionary history and relationships among different organisms.
Cnidaria: Cnidaria is a phylum of simple, aquatic animals known for their unique stinging cells called cnidocytes. These organisms, which include jellyfish, corals, and sea anemones, exhibit a range of body forms and life cycles, showcasing both polyp and medusa stages. Cnidarians are key players in marine ecosystems, contributing to biodiversity and forming essential habitats.
Convergent evolution: Convergent evolution is the process where unrelated species develop similar traits independently, often due to similar environmental pressures or ecological functions. It results in analogous structures that serve similar purposes but do not share a common ancestry.
Convergent Evolution: Convergent evolution refers to the process where organisms from different evolutionary backgrounds develop similar traits or adaptations as a response to similar environmental challenges. This phenomenon highlights how different species can evolve similar solutions despite not sharing a recent common ancestor, revealing the power of natural selection in shaping life forms in analogous ways.
Ctenophora: Ctenophora, commonly known as comb jellies, is a phylum of marine invertebrates characterized by their gelatinous bodies and unique locomotion using cilia. These organisms are known for their distinct features such as bioluminescence and their role in marine ecosystems as both predators and prey. The ctenophores possess a simple body structure, but they display a variety of forms and sizes, making them an intriguing subject in the study of animal phylogeny.
Deuterostomia: Deuterostomia is a major lineage of animals characterized by a specific developmental pattern where the blastopore, the initial opening formed during embryonic development, becomes the anus, while the mouth develops from a secondary opening. This group includes important animal phyla such as echinoderms and chordates, linking them through shared developmental traits and evolutionary relationships.
Ecdysozoa: Ecdysozoa is a major clade of animals characterized by their ability to undergo ecdysis, or molting, which allows them to shed their exoskeleton or cuticle as they grow. This group includes diverse organisms such as arthropods and nematodes, highlighting the evolutionary significance of molting in adaptation and survival across various habitats.
Echinodermata: Echinodermata is a phylum of marine animals characterized by their unique radial symmetry, spiny skin, and a water vascular system. This group includes well-known organisms like starfish, sea urchins, and sea cucumbers, which play important roles in marine ecosystems. Echinoderms are known for their ability to regenerate lost body parts, their distinctive tube feet for movement and feeding, and their fascinating reproductive strategies.
Eumetazoa: Eumetazoa is a clade of animals that encompasses all multicellular organisms with true tissues, representing a significant branch in the animal kingdom. This group is defined by the presence of organized tissues and a body plan that is more complex than that of simpler organisms, such as sponges. Eumetazoans are distinguished from other animals by their developmental patterns and the presence of specialized cells and structures.
Gastrulation: Gastrulation is a critical phase in embryonic development where the single-layered blastula reorganizes into a multi-layered structure called the gastrula. This process establishes the three primary germ layers: ectoderm, mesoderm, and endoderm, which are essential for forming various tissues and organs in the developing organism.
Germ layers: Germ layers are the primary layers of cells that form during embryonic development. These layers differentiate into various tissues and organs in animals.
Germ layers: Germ layers are the primary layers of cells that form during embryonic development in animals, specifically the ectoderm, mesoderm, and endoderm. These layers give rise to all the organs and tissues in an animal's body, playing a crucial role in determining the overall structure and function of the organism. The presence and arrangement of germ layers are key features that differentiate various animal groups and reflect their evolutionary relationships.
Hemichordata: Hemichordata is a phylum of marine organisms that exhibit characteristics of both echinoderms and chordates, including a unique body plan that includes a proboscis, a collar, and a trunk. These organisms play a significant role in understanding the evolutionary relationships among animal phyla, particularly in the context of deuterostome development.
Homology: Homology refers to the similarity in structure, function, or genetic makeup between different species that is derived from a common ancestor. This concept is crucial in understanding evolutionary relationships and how organisms have diverged from shared lineages over time. Homologous structures provide insight into how diverse life forms are interconnected through evolution, emphasizing the unity of life on Earth.
Maximum likelihood: Maximum likelihood is a statistical method used to estimate the parameters of a model by maximizing the likelihood function, which measures how well the model explains the observed data. This approach is crucial in inferring evolutionary relationships and constructing phylogenetic trees, as it allows researchers to evaluate different models of evolution and identify the one that best fits the genetic or morphological data collected from various organisms.
Metazoa: Metazoa refers to a major clade of multicellular animals that are characterized by their complex tissue structures and diverse body plans. This group encompasses all animals except for the unicellular organisms, establishing a fundamental distinction in the evolutionary history of life forms. Metazoa includes a wide variety of organisms, from simple sponges to complex mammals, highlighting the evolutionary significance of multicellularity and specialization in animal life.
Micrognathozoa: Micrognathozoa is a phylum of tiny, aquatic animals that are known for their unique jaw structure and specialized morphology. These organisms are primarily found in freshwater environments and are characterized by their complex jaws that enable them to feed on microorganisms. They play a crucial role in the ecosystem, serving as a part of the food web and contributing to the cycling of nutrients in aquatic habitats.
Microsatellites: Microsatellites are short, repeating sequences of DNA, typically 2 to 6 base pairs in length, that are found throughout the genomes of many organisms. These sequences are highly variable among individuals and are often used as genetic markers in studies of population genetics and animal phylogeny due to their high mutation rates and ability to provide insight into evolutionary relationships.
Molecular clock: A molecular clock is a technique that uses the mutation rates of biomolecules to estimate the time of evolutionary events. This concept connects molecular biology with evolutionary biology by allowing scientists to infer the timing of species divergence based on genetic differences. By analyzing specific genes or proteins, researchers can estimate when two species diverged from a common ancestor, providing insights into evolutionary history and relationships.
Monophyletic group: A monophyletic group, also known as a clade, is a group of organisms that includes an ancestor and all of its descendants. This concept is important in understanding evolutionary relationships and constructing phylogenetic trees, which visually represent the evolutionary history of species. Monophyletic groups are distinct from paraphyletic and polyphyletic groups, as they accurately reflect the shared evolutionary history of a lineage.
Morphological data: Morphological data refers to the structural features and forms of organisms, including their shape, size, and arrangement of body parts. This type of data is crucial in understanding the evolutionary relationships among animals and plays a significant role in constructing phylogenetic trees, which depict the evolutionary pathways and connections among different species.
Myxozoa: Myxozoa are a group of microscopic, parasitic organisms that belong to the phylum Cnidaria, specifically characterized by their complex life cycles and association with aquatic hosts, primarily fish. These tiny parasites have a unique morphology and life history, showcasing how certain adaptations can lead to specialized ecological niches in animal phylogeny.
Notochord: The notochord is a flexible, rod-like structure found in the embryos of all chordates, serving as a primary support structure that defines the body's axis. It plays a crucial role in the development and organization of the vertebrate body plan, influencing the formation of the spine and other skeletal structures.
Parazoa: Parazoa refers to a group of simple multicellular animals that lack true tissues and organs, primarily represented by sponges. These organisms are characterized by their porous bodies and ability to filter feed, connecting them to the broader evolutionary tree of animals and providing insights into the early stages of animal evolution.
Parsimony: Parsimony is a principle often used in evolutionary biology, which states that the simplest explanation or pathway with the least number of assumptions is preferred when constructing phylogenetic trees. This concept emphasizes the idea that the best hypothesis or tree is one that requires the fewest changes, aligning with Occam's Razor, which advocates for simplicity in explanations.
Phylogenetic tree: A phylogenetic tree is a diagram that represents the evolutionary relationships among various biological species based on similarities and differences in their physical or genetic characteristics. This tree helps to visualize how different organisms are related through common ancestry, showcasing both the divergence and convergence of species over time.
Porifera: Porifera, commonly known as sponges, is a phylum of simple, multicellular animals characterized by a porous body structure and a lack of true tissues and organs. They are primarily aquatic organisms that play a crucial role in marine and freshwater ecosystems, acting as filter feeders and contributing to nutrient cycling.
Protostomia: Protostomia is a major lineage in the animal kingdom characterized by the developmental pattern where the mouth forms before the anus during embryonic development. This group includes a diverse range of animals such as arthropods, mollusks, and annelids, showcasing varied body plans and ecological roles. The unique developmental sequence in protostomes sets them apart from deuterostomes, which develop the anus first, leading to significant differences in anatomy and evolutionary adaptations.
Radiata: Radiata refers to a group of animals characterized by their radial symmetry, where their body parts are arranged around a central axis. This term primarily includes two major phyla: Cnidaria, which encompasses jellyfish, corals, and sea anemones, and Ctenophora, known as comb jellies. Radiata are primarily aquatic organisms and have evolved various adaptations for life in marine environments.
SNPs: Single Nucleotide Polymorphisms (SNPs) are the most common type of genetic variation among individuals, where a single nucleotide in the DNA sequence is altered. These variations can have significant implications for gene function, disease susceptibility, and individual responses to drugs. SNPs serve as valuable markers in genomics for understanding the genetic basis of complex traits and evolutionary relationships among species.
Spiralia: Spiralia refers to a major clade of animals that are characterized by their spiral cleavage during embryonic development. This group includes various organisms, such as mollusks, annelids, and certain other invertebrates, which share developmental and morphological traits that distinguish them from other animal lineages.
Synapomorphy: A synapomorphy is a shared derived characteristic that is used to define a group of organisms in phylogenetic analysis. This concept is crucial for understanding evolutionary relationships as it helps to identify common ancestors and distinguish between different lineages based on unique traits that have evolved in a specific lineage but are not present in distant relatives. Synapomorphies are fundamental for constructing accurate phylogenetic trees and clarifying evolutionary history.
Synteny: Synteny refers to the conservation of blocks of order within two sets of chromosomes that are inherited from a common ancestor. It highlights the genetic similarity between species by showing that certain genes remain in the same relative positions on their respective chromosomes, which can be useful for understanding evolutionary relationships and mapping genomes.
Xenoturbellida: Xenoturbellida is a small group of marine invertebrates that belong to the phylum Xenacoelomorpha, characterized by their flattened, worm-like bodies and simple body structure. These organisms are significant in understanding animal phylogeny due to their unique evolutionary position and the insights they provide into the early development of bilateral animals, linking them to both deuterostomes and protostomes.