Evolution and Animal Diversity: Study Notes for General Biology

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Evolution: Patterns, Processes, and Evidence

Definition and Overview

Evolution is the process by which species accumulate differences from their ancestors as they adapt to different environments over time. This results in changes in the genetic composition of populations from generation to generation.

Evolutionary change can be observed as both a pattern (the diversity and unity of life) and a process (the mechanisms that drive change).
Darwin described evolution as "descent with modification."

Key Observations About Life

Unity of life: All living organisms share certain characteristics.
Diversity of life: There is a vast variety of living forms.
Adaptation: Organisms are well-suited to their environments.

Taxonomy and Classification

Carolus Linnaeus developed a hierarchical system to classify organisms, grouping similar species into increasingly general categories.

Taxonomic hierarchy: Domain > Kingdom > Phylum > Class > Order > Family > Genus > Species
Binomial nomenclature: Each species is given a two-part scientific name (Genus species).

Theories of Evolution

Catastrophism (Cuvier): Many species were created originally, but periodic catastrophes caused extinctions, with new species appearing in the fossil record.
Lamarck's Hypothesis: Species evolve through use and disuse of body parts and inheritance of acquired characteristics. (This mechanism is not supported by modern genetics.)
Darwin's Theory: Natural selection acts on heritable variation, leading to adaptation and speciation.

Mechanisms of Evolution

Mutation: Random changes in DNA that create genetic variation.
Natural Selection: Individuals with favorable inherited traits are more likely to survive and reproduce.
Artificial Selection: Humans selectively breed organisms for desired traits.
Genetic Drift: Random fluctuations in allele frequencies, especially in small populations (e.g., founder effect, bottleneck effect).
Gene Flow: Movement of alleles between populations, reducing genetic differences.

Darwin's Observations

Populations show variation due to random mutation.
All species can produce more offspring than the environment can support; many offspring fail to survive and reproduce.

Evidence for Evolution

Direct Observation: Natural selection in response to environmental changes (e.g., drug-resistant bacteria).
Homology: Similarities due to shared ancestry (anatomical, molecular, embryological).
Analogy: Similar features due to convergent evolution, not common ancestry.
Fossil Record: Documents patterns of evolution, origins of new groups, and transitional forms.
Endemic Species: Species found only in one specific location, often due to evolutionary isolation.

Phylogeny and Systematics

Phylogeny

Phylogeny is the evolutionary history of a species or group of related species. Systematics is the discipline that classifies organisms and determines their evolutionary relationships.

Closely related organisms share more genes.
Phylogenetic trees represent evolutionary relationships; each branch point (node) indicates divergence from a common ancestor.
Sister taxa: Groups sharing an immediate common ancestor.
Basal taxon: A lineage that diverged early from the rest of the group.

Microevolution: The Smallest Unit of Evolution

Genetic Variation

Genetic variation is the raw material for evolution and is reflected in phenotypic differences among individuals.

Arises from new alleles and genes produced by mutation, gene duplication, or other processes.
Point mutations: Single nucleotide changes in DNA.
Mutation rates are generally low in animals and plants, higher in prokaryotes.

Mechanisms Affecting Allele Frequencies

Natural Selection: Increases frequency of advantageous alleles.
Genetic Drift: Random changes, especially significant in small populations.
Founder Effect: Small group starts a new population with different allele frequencies.
Bottleneck Effect: Sudden reduction in population size alters allele frequencies.
Gene Flow: Movement of alleles between populations, reducing differences.

Animal Evolution and Diversity

Origins and Major Groups

Animals likely evolved from single-celled eukaryotes similar to modern choanoflagellates. Over 1.3 million animal species have been named.

Sponges and Cnidarians: Early-diverging animal groups; sponges lack true tissues.
Bilaterians: Animals with bilateral symmetry and a complete digestive tract.

Animal Body Plans

Diploblastic: Two germ layers (ectoderm and endoderm).
Triploblastic: Three germ layers (ectoderm, mesoderm, endoderm).
Body cavity (coelom): Present or absent; provides space for organs.

Vertebrate Evolution

Vertebrates: Animals with a backbone and skull; first appeared ~500 million years ago.
Jawless vertebrates: First vertebrate lineage.
Gnathostomes: Jawed vertebrates.
Dipnoi (lungfish): Have both gills and lungs; bridge between aquatic and terrestrial vertebrates.

Life on Land: Challenges and Adaptations

Higher oxygen levels, new food sources, and fewer competitors on land.
Challenges: Scarce water, temperature fluctuations, lack of buoyancy/support.
Arthropods: First animals to colonize land (~400 million years ago).

Terrestrial Vertebrates and Tetrapods

Tetrapods: Vertebrates with limbs and digits; key adaptation for land colonization.
Neck allows head movement independent of body.
Tiktaalik: Transitional fossil bridging aquatic and terrestrial vertebrates.
First tetrapods appeared ~365 million years ago.

Amphibians

Three groups: Salamanders (tailed), Frogs (tailless), Caecilians (legless).
Most have a dual life: aquatic larvae, terrestrial adults (semi-terrestrial).

Reptiles and Birds

Reptiles appeared ~310 million years ago.
Most are ectothermic (rely on external heat); birds are endothermic (regulate body temperature metabolically).
Evolution of flight in birds (~160 million years ago) aided in finding food and escaping predators.

Mammals

Mammals have hair/fur and mammary glands (produce milk).
Evolved from synapsids; appeared ~140 million years ago.
Three main groups:
- Monotremes: Egg-laying mammals (e.g., platypus).
- Marsupials: Young born early, develop in pouch (e.g., kangaroos, opossums).
- Eutherians (placental mammals): Young develop inside the mother; includes most mammals.

Primates and Human Evolution

Primates: Grasping hands/feet, opposable thumbs, large brains, forward-facing eyes.
Humans (Homo sapiens): Upright posture, bipedal locomotion, large brains, tool use.
Homo habilis: Early human ancestor (2.4–1.6 million years ago).
Neanderthals (Homo neanderthalensis): Lived ~350,000–30,000 years ago.

Key Terms and Concepts Table

Term	Definition	Example/Application
Natural Selection	Process by which individuals with advantageous traits survive and reproduce more successfully	Antibiotic resistance in bacteria
Genetic Drift	Random changes in allele frequencies in a population	Bottleneck effect after a natural disaster
Gene Flow	Movement of alleles between populations	Migration of individuals between populations
Homology	Similarity due to shared ancestry	Forelimbs of humans, cats, whales, and bats
Analogy	Similarity due to convergent evolution	Wings of birds and insects
Endemic Species	Species found only in one specific location	Galápagos finches

Important Equations

Hardy-Weinberg Equation:

Where p and q are the frequencies of two alleles in a population.

Summary

Evolution explains the diversity and unity of life through mechanisms such as natural selection, genetic drift, and gene flow. The fossil record, comparative anatomy, embryology, and molecular evidence all support the theory of evolution. Animal diversity reflects evolutionary history, with major adaptations enabling the colonization of new environments, including land. Understanding these concepts is fundamental to the study of biology.