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

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 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 dependin g on diet Berry eater Insect eaters Cactus eaters Seed eaters

More on Darwin On the Origin of Species by Means of Natural Selection published in 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)

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