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Evolution—The Theory and Its Supporting Evidence
How to explain fossil record?
• tremendous variation and diversity in the rock record of life.
Organisms in the same species share a common
DNA pool•DNA defines the species•there is variation within species
•gene pool: •total sum of genetic information present in all members of a species
•a species’ gene pool changes slightly with each organism’s birth and death
How gene pools can vary
•DNA can be changed by:•mutation•sex (recombination)
•changes can be:• neutral, advantageous, or deleterious
•natural selection: •advantageous mutations aid reproduction and are eventually amplified within the gene pool.
Organic Evolution
• All life on Earth is descended from other, earlier life
• All life is interrelated
• Natural selection is the mechanism which drives changes in species.
Charles Darwin
• H.M.S. Beagle cruise 1831-1836
www.rit.edu/~rhrsbi/GalapagosPages/Darwin.html
Precursor ideas to Darwin
• Organisms present in fossil record are different from those alive today;
• Geologic time might be long (uniformitarianism);
• Organisms can be classified by shared characteristics;
• Inheritance of acquired characteristics;• Continuous, spontaneous generation of
life.
Charles Darwin
• H.M.S. Beagle cruise 1831-1836
• Natural selection – through time, populations become better fitted to their environments as poorly-adapted members fail to reproduce offspring as successfully (mechanism)
• The best adapted are most likely to survive
Galapagos Finches
• Helped form Darwin’s theory of natural selection
• Variety between populations of different islands
• Adapted to exploit different food sources
Galápagos Finches
– Beak shape varies depending on dietBerr
y eater
Insect eaters
Cactus eaters
Seed eaters
More on Darwin
• On the Origin of Species by Means of Natural Selection published in 1859
• 4 main arguments for evolution– Branching organization of life– Homology– Vestigial structures– Embryonic history
Branching organization of life
• Nature organized into hierarchy of groups (Linnaeus);
• Kingdom – Phylum – Class – Order – Family – Genus – Species
• Confirmed by modern genetics
What does species mean?
• Most specific classification
• Organisms capable of interbreeding
• Individuals that in nature interbreed and produce fertile offspring
• Goats and sheep do not interbreed in nature, so they are separate species
• Yet in captivity they can produce fertile offspring
Species
Homology
• Similarity in structure between parts of different organisms due to evolutionary differentiation from the same part
• Similarity attributable to common origin
• Legs, hands, wings, flippers
Homology
• Homologous – similar elements derived from common ancestor (eg. wings of bats and our finger bones)
• Analogous – similar elements without common ancestor (eg. wings of bats and wings of insects)
• Forelimbs of humans, whales, dogs, and birds are made up of the same bones
Homologous Structures
– Also have similar arrangement of muscles, nerves and blood vessels
• Wings of insects and bats serve the same function but differ considerably in structure and embryological development
Analogous Structures
Vestigial Structures
• Remnants of features no longer used
Human tail bones
Whale hip and leg bones
Horse splint bones (ancient side toes)
• Vestigial structures are nonfunctional remnants of structures in organisms that were functional in their ancestors
Vestigial Structures
• Why do dogs have tiny, functionless toes on their feet (dewclaws)?
• Ancestral dogs had five toes on each foot
• As they evolved they became toe-walkers with only four toes on the ground
• Big toes and thumbs were lost or reduced to their present state
• Normally a horse’s back foot has only one functional toe, the third
Remnants of Toes in Horses
• Splints are small remnants of toes 2 and 4 that remain as vestiges
Embryonic History (Ontogeny)
• All vertebrate embryos start out very similar– Gill slits– Long tail
• One idea: organisms evolve by adding stages to their embryonic development
Embryonic History (Ontogeny)
Embryonic History (Ontogeny)• YES: We can learn about
evolutionary history by looking at how embryos develop
• NO: organisms do not evolve by adding stages to their embryonic development
Example of the Peppered Moths
Phyletic Gradualism vs. Punctuated Equilibrium
• Gradual transformations from one species to another
• Rare and sudden speciation events
• Evidence of both
• Horse example – first thought to be example of gradualism, but many species living at same time
• Still debated
Patterns of Evolution
• Divergence – new species develop traits that differentiate them from their ancestors
• Adaptive radiation – mammals filled ecological niches vacated by dinosaurs
• Convergence – unrelated animals develop similar body forms to fill same niche
• Divergent evolution of a variety of placental mammals from a common ancestor
• Divergence accounts for descendants that differ from their ancestors and from one another
Divergent Evolution
• Convergent evolution takes place when distantly related organisms give rise to species that
Convergent Evolution
resemble one another because they adapt in comparable
ways
Recent Ideas
• Neutralism – most genetic changes are adaptively neutral
• Inheritance of acquired characteristics (e.g., immunity passed on to offspring)?
• Can natural selection account for macroevolution (major evolutionary changes, complex structures)?
• Perhaps as many as 99% of all species that ever existed are now extinct
• The continual extinction of species is referred to as background extinction
• Different from mass extinction during which accelerated extinction rates sharply reduce Earth’s biotic diversity
Background and Mass Extinction
• The mass extinction of dinosaurs is well known
• Greatest mass extinction occurred millions of years before
• More than 90% of all species died out– we will discuss these extinctions and
their possible causes later in the term
Mass Extinction