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