Introduction to Physical Anthropology and Evolution
Physical Anthropology:
- Two main aspects: Human evolution and human variation.
- Human Evolution: Evolution of Homo sapiens from ancestors.
- Human Variation: Differences among individual populations.
Evolution:
- Definition: Origin of all life through common descent from a last universal ancestor (3.5–3.8 billion years ago).
- Processes:
- Speciation: Formation of new species.
- Anagenesis: Change within species.
- Extinction: Loss of species.
- Reconstruction: Phylogenetics uses shared morphological and biochemical traits, including DNA sequences, to reconstruct the "tree of life".
Biodiversity:
- Extinction Rate: Over 99% of all species are extinct.
- Current Species: Approximately 10–14 million species on Earth.
Darwinian Theory of Evolution by Natural Selection:
- Adaptation: Organisms adapt to their environments by accumulating beneficial mutations.
- Mechanism of Inheritance: Initially unknown, leading Darwin to partially accept Lamarckian inheritance of acquired traits.
- Acceptance: Darwin's theory accepted across biology.
Human Evolution and Genetics:
- Primates: Humans are closely related to great apes (Huxley, Broca, Duckworth).
- Mendelian Genetics: Rediscovered in the 20th century, explaining inheritance mechanisms.
- Modern Genetic Theory:
- Evolution: Changes in gene (allele) frequencies.
- Key Processes: Natural selection, mutation, genetic drift, migration.
- Morphological Changes: Evolution also viewed through morphological changes over time.
Synthetic Theory of Evolution:
- Functional Adaptation: Continuous production of variation and natural selection lead to diverse life forms.
Evolution: Definition and Basic Concepts
Definition of Evolution
- General Definition: Evolutionary processes lead to diversity at every biological level (species, organisms, molecular).
- Historical Context:
- First used by Herbert Spencer (1852).
- Denotes historical development of life.
- Types of Evolution:
- Micro-evolution: Changes within an organism over time.
- Macro-evolution: Transformation from one being to another.
- Various Contexts:
- Geological evolution.
- Evolution of solar systems, technology (automobiles, radios, etc.).
- Cultural evolution.
- Organic evolution (changes in living things, plants, and animals).
- Definitions by Key Scientists:
- Charles Darwin (1859): “Descent with modification” - closely related species resemble due to inheritance but differ due to hereditary differences.
- Dodson and Dodson (1976): Evolution as the divergence of related populations, giving rise to new species.
- Dobzhansky (1951): Evolution as the development of dissimilarities between ancestral and descendant populations.
Basic Concepts of Evolution
- Pre-Darwinian Beliefs:
- Life's diversity attributed to divine creation, considered perfect and unchanging.
- Fossil discoveries and geological evidence suggested that life and Earth were much older and had changed over time.
- Evolutionary Thought Development:
- Pre-Darwin biologists proposed mechanisms for evolution.
- Darwin and Wallace (1858, 1859) formalized the mechanism of evolution through natural selection.
- Evolutionary Principles:
- Speciation: Formation of new species.
- Irreversibility: Evolutionary changes are generally not reversible.
- Parallelism: Similar traits evolve independently in related species.
- Convergence: Unrelated species evolve similar traits.
- Adaptive Radiation: Rapid evolution of diversely adapted species from a common ancestor.
- Extinction: Complete loss of species.
Speciation: The Evolutionary Process
Speciation is the process by which new biological species arise, playing a crucial role in evolution. The term was first coined by Orator F. Cook in 1906. According to Mayr (1970), speciation is the creation of new species from one original species, a fundamental aspect of evolution.
What is a Species?
- A species is a basic unit of biological classification.
- Defined as a group of organisms capable of interbreeding and producing fertile offspring.
Mechanisms of Speciation
- Instantaneous Speciation: Occurs through genetic or cytological changes in individuals.
- Gradual Speciation: Occurs through populations, either geographically or sympatrically.
Geographic Modes of Speciation
Allopatric Speciation
- Also known as geographic speciation.
- Occurs when populations are geographically isolated by barriers (e.g., mountains, rivers).
- Isolated populations experience different selective pressures leading to genotypic and phenotypic changes.
Parapatric Speciation
- A small population enters a new habitat with partial separation between zones.
- Some interbreeding occurs, but reduced hybrid suitability leads to selection for mechanisms that prevent interbreeding.
Sympatric Speciation
- Occurs within the same geographical area.
- Descendant species form from a single ancestor.
- Gene flow tends to erase genetic differences unless specific mechanisms prevent it.
- Common in invertebrates like insects dependent on different host plants in the same area.
Quantum Speciation
- Defined by Grant (1971) as the rapid budding off of a new and very different daughter species from a semi-isolated peripheral population.
- Involves adaptive radiation and genetic variability in ecological islands.
- Quick process, may occur in just a few generations, often involving genetic drift.
Speciation
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Instantaneous Gradual
Speciation Speciation
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Geographic Sympatric Speciation
(Allopatric) |
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Geographical | |
Barriers Parapatric Quantum
Speciation Speciation
Irreversibility in Primate Evolution
Irreversibility is a significant principle proposed by Louis Dollo in 1893, known as Dollo's Law of Irreversibility. It asserts that once an organism evolves a particular structure or adaptation, it cannot revert back to a previous form seen in its ancestors. This concept is crucial in understanding evolutionary trajectories and adaptations over time.
Dollo's Law of Irreversibility
Definition: An organism, once it has evolved a specific trait or structure, cannot revert to a previous state already present in its evolutionary lineage.
Example: Flying reptiles (Pterosaurs) provide a clear illustration:
- After the extinction of flying reptiles, two distinct lineages evolved adaptations for flight: birds and bats.
- Birds and bats developed wings independently from each other and from the flying reptiles.
- This demonstrates that once flight was lost (due to extinction), it evolved anew in unrelated lineages, following different evolutionary paths.
Implications of Irreversibility
- Evolutionary Trajectories: Structures and adaptations evolve in response to specific environmental pressures and ecological niches.
- Divergence: Once a lineage loses a particular trait or adaptation, subsequent adaptations or structures that fulfill similar functions evolve independently and cannot revert to the original form.
Parallelism and Convergence in Evolution
Parallelism and Convergence are fundamental concepts in evolutionary biology that explain the development of similar traits or behaviors in different species. These phenomena highlight evolutionary opportunities and the adaptive responses of organisms to their environments.
Parallelism
Definition: Parallelism refers to the independent development of similar adaptive features in closely related species or groups. It suggests that these similarities are due to a shared genetic heritage within a particular taxonomic lineage.
Example:
- Rodents: Various rodent species in different geographic regions evolved similar adaptations in response to similar environmental pressures, such as burrowing habits or tooth structure.
- Monkeys: Old World and New World monkeys independently evolved similar features due to their shared ancestry from prosimian ancestors.
Convergence
Definition: Convergence involves the development of similar adaptive traits or behaviors in species or groups that are not closely related phylogenetically. It implies that these similarities arise due to similar selective pressures from the environment.
Example:
- Flight in Animals: Flying has evolved independently in insects, birds, and bats. Each group developed wings and flight capabilities through different genetic pathways, demonstrating convergent evolution.
- Eyesight: The complex eye structure in octopuses and vertebrates (like humans) evolved independently to optimize vision in their respective environments.
Differentiating Parallelism and Convergence
Rod and Cone Cells in Vertebrate Eyes:
- Rods: Found in nocturnal vertebrates and those in dim light; evolved independently in various species, demonstrating convergent evolution.
- Cones: Sensitive to higher light intensities and evolved independently in species with different ecological needs; examples include diurnal animals like primates and birds.
Homologous vs. Analogous Structures:
- Homologous: Structures that share a common ancestry, such as the forelimb of a bat and the wing of a bird, both derived from the same ancestral limb structure.
- Analogous: Structures that serve similar functions but do not share a common ancestry, such as the wings of insects and birds, evolved independently for flight.
Adaptive Radiation
Definition: Adaptive radiation refers to the rapid diversification of species from a single ancestral lineage into a variety of ecological niches, driven by environmental changes and the availability of new resources.
Key Features of Adaptive Radiation
Common Ancestry:
- Adaptive radiation originates from a single ancestral species that diversifies rapidly into multiple descendant species.
- It does not necessarily require all descendant species to be monophyletic but shares recent ancestry.
Phenotype-Environment Correlation:
- There is a significant correlation between the environment and the morphological or physiological traits of organisms that enable them to exploit new ecological niches.
Trait Utility:
- Traits that confer fitness advantages in specific environments undergo rapid evolution and diversification.
- This allows species to effectively utilize available resources and adapt to new ecological challenges.
Rapid Speciation:
- Adaptive radiation is characterized by bursts of speciation events coinciding with ecological and phenotypic divergence.
- These speciation events enable the exploitation of diverse ecological niches.
Examples and Illustrations
Mammalian Evolution:
- Following the extinction of dinosaurs at the end of the Mesozoic era, mammals underwent extensive adaptive radiation during the Tertiary period (Cenozoic era).
- Rodents adapted for gnawing, carnivores specialized in hunting, hoofed animals evolved for grazing, primates and sloths adapted to arboreal lifestyles, whales, seals, and sea cows adapted to marine life, and bats evolved flight capabilities.
- Each lineage diversified further into subgroups adapted to various habitats, demonstrating parallel evolution and convergence.
Darwin’s Finches:
- Location: Galapagos Islands, characterized by patchy and diverse ecological landscapes.
- Adaptive Features: Different species of finches evolved distinct beak shapes and sizes to exploit various food resources (seeds, insects, probing into cacti).
- Consequence: Reduced competition among species sharing the same environment due to specialized adaptations, facilitating coexistence.
Mechanisms and Evolutionary Significance
- Geographic Isolation: Disjointed landscapes and islands promote adaptive radiation by creating isolated ecological niches.
- Morphological Evidence: Fossil morphology and comparative studies reveal evolutionary relationships and adaptations across different lineages.
- Diversification: Enables species to occupy and thrive in diverse habitats by evolving specialized traits and behaviors.
Adaptive Radiation
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Rapid diversification Common ancestry
into new forms (from single lineage)
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Phenotype-environment Trait utility
correlation (fitness advantages)
│ │
Speciation bursts Ecological niches
(phenotypic divergence) availability
Extinction
Definition: Extinction occurs when all individuals of a species cease to exist. It is a natural process in evolution, where species disappear due to inability to adapt to changing environments or competition with other species.
Key Points
- Definition: The complete disappearance of a species from the evolutionary record when the last individual dies.
- Natural Process: Extinction is part of the evolutionary process, balancing speciation by removing less adapted species.
- Causes: Extinction can result from environmental changes, competition, or evolutionary transformations.
- Examples:
- Mass Extinctions: Events like the Cretaceous–Tertiary extinction wiped out non-avian dinosaurs.
- Current Extinction: Human activities are causing an ongoing mass extinction known as the Holocene extinction.
- Functionally Extinct: When very few individuals of a species remain, unable to reproduce effectively, they are functionally extinct.
- Ecological Impact: Extinctions affect ecosystems by altering food webs and ecological dynamics.
- Evolutionary Implications: Mass extinctions can lead to bursts of rapid evolution and speciation among surviving species.
Types of Extinction
Background Extinction:
- Continuous low-level extinction due to competition between species for resources.
Mass Extinction:
- Sudden and drastic loss of species across many taxonomic groups, altering biodiversity significantly.
Types of Extinction
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Background Extinction Mass Extinction
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Continuous, low-level Sudden and drastic loss
extinction events of species across many
│ taxonomic groups
Due to competition Alters biodiversity and
between species ecological dynamics
Evolutionary Significance
- Survival of the Fittest: Extinction can be seen as a form of natural selection, where species unable to cope with changes in their environment perish.
- Bursts of Evolution: Mass extinctions create opportunities for rapid evolutionary change and diversification among surviving species.
- Human Impact: Current extinction rates are accelerated due to human activities like habitat destruction, pollution, and climate change.
Pseudoextinction
Definition: Pseudoextinction refers to the situation where a parent species becomes extinct, but its descendant species or subspecies continue to exist.
Key Points
- Definition: Extinction of a parent species while its daughter species or subspecies remain alive.
- Example: Prehistoric species like Eohippus (an ancient horse-like animal) are pseudoextinct because although Eohippus itself is extinct, its descendants such as horses, zebras, and donkeys still exist.
- Evolutionary Perspective: Pseudoextinction highlights the evolutionary process where species evolve into new forms, but the original ancestral species ceases to exist.
- Continued Lineage: Despite the extinction of the parent species, its evolutionary lineage continues through its descendant species.
- Significance: Shows the continuity and branching of evolutionary lineages over time.
Example: Eohippus and Descendant Species
- Eohippus: An extinct genus of small horses known from the early Eocene epoch.
- Descendants: Modern-day horses, zebras, and donkeys evolved from Eohippus through adaptive changes over millions of years.
- Pseudoextinction: Eohippus is pseudoextinct because it no longer exists as a species, but its evolutionary legacy is carried on by its descendant species.
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