Microevolution
Biodiversity
• Number and kinds of species on the earth
Traditional
Classification
Prior to 1859, Aristotelian essentialism dominated
– Classification and taxonomy treated as single activity
– Species were eternal, immutable, and discrete
– Morphological characters (‘essential traits’) used to
describe a species
– Observed variation among individuals treated as
aberrations around a type (idealized) form for the species
– Combined with Linnaean hierarchy (1700’s), this approach
generated species classifications
Hierarchical structure of life
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Molecule
Cell
Tissue
Organ
Organ system
Organism
Population
Community
Ecosystem
Fossils
• Fossils of extinct animals
related to those in the same
area.
• Succession of fossil types
Pakistan's Siwalik Hills
Geographic relationships of similar
species (‘Sarawak Law’)
There are similar species in different geographic
places with similar climate.
Hydromedusa
maximiliani
Emydura
macquarii
Disharmony of island biotas
• the relative proportions
of different taxa are not
the same on islands as
they are on the
mainland.
• represents a "grab-bag"
rather than a broad
range of all possible
taxa -->
those groups capable of
long-distance dispersal
Shared embryonic features
• Comparative embryology shows how
different adult structures of many
animals have the same embryonic
precursors.
Vestigial organs in many species
• Organs or
structures
remaining or
surviving in a
degenerate,
atrophied, or
imperfect
condition or
form.
Common ‘design blueprints’ for
structures with very different
functions
Shift in biological thought
Radical shift in thinking during 19th century
away from Aristotelian essentialism
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Hutton & Lyell: uniformitarianism (old = current
processes)
Lamarck: inheritance of acquired traits
Species were not eternal (many fossils of extinct
forms discovered)
Species were not immutable (e.g., domestication
by humans through breeding designs)
Variation in nature was not aberrant, but
common and important for survival of individuals
Elements of microevolution
• Evolution: change in populations over time
• Individuals develop but do not evolve
• Darwin’s theory of evolution by natural selection:
(descent with modification)
– Individuals within a population vary for some trait
– Some of these variations are heritable
– Often, more offspring are produced than can survive (limited
resources)
– Survival and reproduction is not random: individuals with the
most favorable variations are "naturally selected".
Darwinian Evolution by
Natural Selection
Theory not accepted until after 70 yrs
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1860’s: world’s leading physicist Lord Kelvin = earth’s
not old enough for gradual Darwinian evolution to
account for the observed diversity
– Based on wrong assumptions about the sources of
energy heating the earth
– Not known at that time that the sun is fission
powered
– Decay of radioactive elements unknown until 1900’s
– Gross underestimate of Earth’s age
Darwinian Evolution by
Natural Selection
Theory not accepted until after 70 yrs
• Because Darwin had no idea how variability was
generated in populations
– Mutation was unknown
– Critics: the amount of genetic variability was strictly
limited and natural selection would halt when variability
ran out.
– 1900s: Thomas Morgan experimental work with fruit flies
– Mutation generated new variation in every generation and
in every trait.
Darwinian Evolution by
Natural Selection
Darwin had no idea how variation was passed on from parent to offspring
– Lack of a satisfactory theory of inheritance
– Mendel’s theory of the particulate inheritance of genetic
“factors” (i.e., genes) from 1865-1866 was unknown until 1900.
– Mendel’s laws of segregation and independent assortment
– 1953: Watson and Crick discovered the nature of DNA as the hereditary
genetic material
– Darwin adopted the theory of blending inheritance
– Heritable variation will be lost each generation = natural selection could not
operate
– How discrete units of heredity (genes) produce continuous variation?
– How discrete genes could account for speciation?
Fig. 2-10, Ridley 2004, Evolution
Anti-Darwinian ideas
• Neo-Lamarckism
– Inheritance of acquired traits
– Refuted by experiments of Weismann
• Orthogenesis
– Straight-line inheritance: Spp evolve towards a fate. No
proposed mechanism
• Mutationist
– Discrete new phenotypes derive from mutations = speciation
– De Vries, Morgan, Goldschmidt
Evolutionary (Modern) Synthesis
• 1930’s-1940’s
• Neo-Darwinism
• Reconciled Darwin’s theory with genetics
– Fisher, Haldane, Wright: Mutation + Nat. Sel. = Adaptive evolution
– Chetverikov, Dobzhansky: genetic var & change in natural pops.
• Other contributors
– Mayr, Rensch, Stebbins, Simpson
– Mutation + Recombination + Nat. Sel. within spp (µevol) = speciation
& macroevolution
Evolutionary (Modern) Synthesis
• Foundations of modern evolutionary biology
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Phenotype ≠ genotype. Genotype + environment = phenotype
Acquired characteristics are not inherited
Hereditary variation based on non-blending genes
Polygenic inheritance = Continuous phenotypes
Alleles: isostable forms of a gene w/ small to large phenotypic effect
Mutation + recombination = genetic variation of natural populations
Evolutionary change is a population process
Evolutionary (Modern) Synthesis
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Genetic drift & Natural selection = genotypic changes in populations
Nat. Sec. can produce slight or large changes (adaptations)
Nat. pops are genetically variable and can evol fast if env. changes
Genetic differentiation of populations in ≠ geographic regions
(adaptive = from Nat. Sel.): often polygenic and quantitative traits =
evolution is gradual
Populations have phenotypically different genotypes
Species are distinct non-interbreeding “gene pools”
Speciation = origin of >1 species from a single common ancestor
(often by genetic differentiation of geographically isolated pops)
Higher taxa arise by accumulation of small differences (gradual)
Fossil record has gaps (incomplete) but shows gradual evolution
Modern Evolutionary Biology
– > 1950’s: Molecular biology
– Molecular evolution, mutation, genetic variation, species differences,
development, phylogenetics
– Neutral theory of mol. evol. (Kimura)
– > 1960’s: Evolutionary ecology
– Ecology, behavior, reproductive biol, life history.
– EvoDevo
– Evolutionary Genomics
Neutralist-Selectionist Debate
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1960’s to present
There are four mechanisms for evolutionary change with debatable
relative importance (mainly 1 vs 3)
1. Natural selection
2. Mutation (more a source of variation and not a mechanism for
evolution per se)
3. Random genetic drift
4. Migration (gene flow)
Organic Evolution
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Evolution: changes in a population’s gene pool
(genetic makeup) over time.
Natural selection: differential survival and reproduction of individuals
that differ from one another in [heritable] characteristics (phenotypes).
Survival of the fittest, not the strongest (really, of the good enough).
Fitness: survival + reproductive output (via female and male functions).