Descent with Modification, Phylogeny, and Diversity of Life: Study Guide Notes

Descent with Modification

Darwinian Evolution: Major Concepts

Darwinian evolution is based on two major concepts: descent with modification and natural selection.

• Descent with modification: All organisms are related through descent from a common ancestor, but have accumulated diverse modifications over time.

• Natural selection: The process by which individuals with advantageous traits survive and reproduce more successfully, leading to adaptation.

Evidence for Descent from a Common Ancestor

There are five major lines of evidence supporting common ancestry:

• Fossil record: Fossils are preserved remains or traces of organisms, typically found in sedimentary rocks. These rocks form in layers called strata, which represent different periods in geologic time. Fossils provide direct evidence of past life and evolutionary transitions.

• Homology: Similarities in structure or genetics due to shared ancestry. Homologies can be anatomical (e.g., forelimb bones in vertebrates), molecular (e.g., genetic code), or developmental.

• Biogeography: The geographic distribution of species. Organisms from the same region tend to be more closely related, supporting descent from a common ancestor.

• Direct observation: Evolutionary changes observed in real time, such as antibiotic resistance in bacteria.

• Comparative embryology: Similarities in embryonic development among different species indicate common ancestry.

Homology is central, as it underlies molecular, anatomical, and developmental similarities. The shared genetic code of all life is a molecular homology, indicating a universal ancestor.

Key Terms and Concepts

• Fossil: Any preserved remains, impression, or trace of a once-living organism.

• Strata: Layers of sedimentary rock, each representing a specific interval of geologic time.

• Origination: The process by which new species arise.

• Extinction: The end of an organism or group of organisms.

• Hierarchical organization: Life is organized in nested levels (e.g., species, genus, family, etc.). If life were not hierarchical, we would not see such nested patterns of similarity.

• Vestigial structures: Remnants of features that served important functions in ancestors but are now reduced or unused (e.g., human appendix).

Natural Selection

Fitness, Adaptation, and Mutation

• Fitness: The ability of an organism to survive and reproduce in its environment. Fitness is influenced by genes, traits, phenotypes, survival, and reproduction.

• Adaptation: A heritable trait that increases fitness in a specific environment.

• Mutation: The ultimate source of new genetic variation and traits.

Conditions for Natural Selection

• Variation in traits

• Heritability of traits

• Differential survival and reproduction

Acclimation vs. Adaptation

• Acclimation: Short-term physiological adjustment to environmental change; not heritable.

• Adaptation: Genetic change in a population over generations; heritable.

Species and Speciation

• Biological species concept: Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.

• Allopatric speciation: Formation of new species due to geographic isolation.

Reproductive Isolation Mechanisms

Type	Mechanism	Pre-zygotic/Postzygotic
Temporal isolation	Breed at different times	Pre-zygotic
Habitat isolation	Live in different habitats	Pre-zygotic
Behavioral isolation	Different mating behaviors	Pre-zygotic
Mechanical isolation	Incompatible reproductive structures	Pre-zygotic
Gametic isolation	Gametes cannot fuse	Pre-zygotic
Reduced hybrid viability	Hybrids do not develop properly	Postzygotic
Reduced hybrid fertility	Hybrids are sterile	Postzygotic
Hybrid breakdown	Hybrid offspring are weak or sterile	Postzygotic

Adaptive Radiation and Sexual Selection

• Adaptive radiation: Rapid evolution of many diverse species from a common ancestor, often following colonization of new environments (e.g., Darwin's finches).

• Sexual selection: Evolution of traits that increase mating success, sometimes at the expense of survival (e.g., peacock's tail).

• Evolutionary trade-offs: Traits that are beneficial in one context may be costly in another (e.g., bright coloration attracts mates but also predators).

Phylogeny and Tree Thinking

Phylogenetic Trees and Evolutionary Relationships

A phylogenetic tree is a diagram representing a hypothesis about evolutionary relationships among species.

• Both phylogenetic trees and the Linnean hierarchy group species by relatedness, but not all Linnean groups are monophyletic (i.e., containing all descendants of a common ancestor).

• Nodes represent most recent common ancestors; branching points correspond to speciation events.

Homology vs. Analogy

• Homology: Similarity due to shared ancestry.

• Analogy (convergent evolution): Similarity due to independent evolution in similar environments (e.g., wings in bats and birds).

• Homoplasy: Similar traits not due to common ancestry (includes analogy).

Building and Interpreting Phylogenetic Trees

• Homologies are used to construct trees; the principle of parsimony favors the tree with the fewest evolutionary changes.

• Important to use many characters to avoid errors due to convergence/homoplasy and to increase confidence in relationships.

• Molecular clock: Uses mutation rates and fossil ages to estimate divergence times. Assumptions include constant mutation rates; violations can mislead interpretations of tree topology and branch lengths.

Tree Interpretation Skills

• Identify most recent common ancestors, outgroups, monophyletic/paraphyletic/polyphyletic groups, and polytomies.

• Map evolutionary events (e.g., trait changes) and identify convergent evolution.

• Determine most parsimonious trees using character data matrices.

Bacteria and Bacterial Diversity

Domains and Relationships

• Domain Bacteria and Domain Archaea are both prokaryotic; Archaea is more closely related to Domain Eukarya. Bacteria is the outgroup.

• The most ancient organisms likely used RNA as genetic material.

Prokaryotic Metabolism

• Prokaryotes use diverse energy and carbon sources: phototrophs, chemotrophs, autotrophs, heterotrophs.

• Photosynthesis evolved in bacteria about 2.5–3 billion years ago, leading to increased atmospheric oxygen.

Genetic Exchange in Prokaryotes

Process	Description	Importance
Conjugation	Direct transfer of DNA between cells via pilus	Increases genetic diversity
Transformation	Uptake of free DNA from environment	Allows acquisition of new traits
Transduction	Transfer of DNA by bacteriophages (viruses)	Facilitates gene transfer between bacteria

Protists and Protist Diversity

• Domain Eukarya includes protists, which are eukaryotic organisms.

• Endosymbiosis played a key role in the origin of eukaryotes (e.g., mitochondria and chloroplasts).

• Plants, animals, and fungi are not sister taxa to protists; they are derived from within the protist lineage.

• Complex multicellularity arose several times independently among eukaryotes.

Fungus and Fungal Diversity

• Kingdom Fungi includes divisions Ascomycota and Basidiomycota.

• Fungi are more closely related to animals than to plants.

• Fungi are heterotrophic, absorbing nutrients from their environment.

• Hyphae are thread-like structures that form the mycelium and are essential for nutrient absorption.

• Fungi can reproduce both sexually and asexually.

• Plasmogamy: Fusion of cytoplasm from two parent mycelia. Karyogamy: Fusion of nuclei from the fused cells.

• Fungi are important decomposers and symbionts in ecosystems.

• Lichens are symbiotic associations between fungi and photosynthetic organisms (algae or cyanobacteria). Mycorrhizae are mutualistic associations between fungi and plant roots.

Animal Diversity

Major Groups and Characteristics

• Kingdom Animalia includes Metazoa, Eumetazoa, Bilaterians, Deuterostomes, Chordates, Vertebrates, Amniotes, Mammals, Monotremes, Marsupials, Eutherians/Placentals, and Primates.

• Bony fishes, lobe-finned fishes, and amphibians are key vertebrate groups.

Key Evolutionary Innovations

Characteristic	Where it Appeared
Notochord	Chordates
Vertebrae	Vertebrates
Jaws	Gnathostomes
Lobed fins	Lobe-finned fishes
Limbs with digits	Tetrapods
Amniotic egg	Amniotes

Chordate and Vertebrate Features

• Four shared characteristics of chordates: notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail.

• Amphibians share moist skin, aquatic larvae, and metamorphosis.

• Amniotes (reptiles, birds, mammals) have the amniotic egg, allowing reproduction away from water.

• Birds' closest relatives are reptiles, specifically crocodilians.

• Present-day hominid diversity is low compared to the past 3 million years, when multiple hominid species coexisted. Modern humans are now the only extant hominids.