More Evidence of Evolution More Evidence of Evolution

BiogeographyBiogeography• Biogeography is the study of the past and

present geographical distribution of organisms.

• There are patterns in how species occur geographically, based on such factors as where they evolved, how far they dispersed, and how changes in the Earth affected their distribution.

Example from the Fossil Record:

The discovery of Mesosaurus fossils in South America and Africa can be explained by the fact that the continents once formed a single land mass. Mesosaurus, an aquatic reptile adapted to fresh water, lived in the Permian period, more than 250 mya. South America, Africa, and the other continents were still joined at that time.

Modern Examples of BiogeographyModern Examples of Biogeography• In geographically isolated locations,

existing species are similar to those on the nearest mainland, but are different enough to be considered separate species.

• This is best explained through common ancestry (species evolved from a common ancestor in a particular geographic region).

Modern ExamplesModern Examples of Biogeographyof Biogeography• More Examples:

– Hawaiian Islands - 1 mammal species (bat)

– Galapagos Islands - finches, tortoises, and marine iguanas (all similar to mainland species)

– Australia-unique mammalian species

Homologous StructuresHomologous Structures•…similar structure found in more than one species that share a common ancestor. •Homologous structures are revealed by comparing anatomy, cellular similarities and differences, and studying embryological development.•Homologous structures are best explained as having all derived from the same body part in a common ancestor.

Mammal ForelimbsMammal Forelimbs

Plant LeavesPlant Leaves

Vestigial StructuresVestigial Structures• …a type of homologous structure that is no

longer used as it is in most other organisms that have it.

• “Vestigial” does not necessarily mean useless or nonfunctional. It literally means “not fully developed.”

Examples of Vestigial StructuresExamples of Vestigial Structures•Hipbones and ear muscles of whales•Hipbones and rudimentary legs of some snakes•Muscles in the human tailbone and human ear, human wisdom teeth, and “goose bumps”•Fingernails on manatee flippers

Another Example• Human appendix = small,

finger-sized structure found near junction of small and large intestine.

• Thought to have descended from an organ (cecum) in a distant herbivorous ancestor.

• Modern herbivores maintain a cecum; a sac that houses bacteria essential to the digestion of cellulose in plants.

• Human appendix, although homologous, does not contain a significant number of these bacteria (can’t digest cellulose).

More Examples of Vestigial StructuresMore Examples of Vestigial StructuresBlind cave fish and salamanders have rudimentary eyes.

The ostrich has wings which are useless as wings.

Dandelions reproduce asexually, yet still produce flowers and pollen.

Flightless beetles havefunctionless wings under fused wing-

covers that never open.

AtavismsAtavisms• …traits from a distant evolutionary

ancestor that have reappeared in modern-day organisms

Hindlimb bones from a humpback whale

Hind flippers on a dolphinTails in humans

Hindlimbs with toeson a snake

Other AtavismsOther Atavisms• Additional nipples occurring in

mammals, including humans• Dew claws in many dog breeds• Extra toes in horses

Similarities During DevelopmentSimilarities During Development• Related organisms have similar

developmental stages despite adult differences.

• We can observe certain structures appearing early in development, which disappear later as the organism matures.

• This evidence further supports the hypothesis that all animals are related and descended, with modification, from a common ancestor.

Examples of Developmental Similarities• Snake embryos and whale

embryos develop hind limb buds.

• Horse embryos develop three toes (adults have one toe).

• Human embryos have pharyngeal pouches, a tail, and six aortic arches (all features found in embryonic fish).

• Human embryos develop a coat of hair called lanugo that is shed before birth.

Molecular BiologyMolecular Biology•We can compare DNA and protein sequences: high similarity in sequences demonstrates a high degree of relatedness.•For example, fossil evidence and anatomical similarities suggest that humans are closely related to chimpanzees and gorillas.•If this hypothesis is supported, we would expect to see a high degree of similarity in the DNA and protein sequences.•If this hypothesis isn’t supported, we would expect to see a low degree of similarity (more distantly related).

Molecular Biology ExampleMolecular Biology Example•In fact, data supports the hypothesis that humans are more closely related to primates than to other vertebrates.•Hemoglobin protein sequence is highly similar in humans and primates.

•Also, there is approximately only 5% difference in the total DNA between humans and chimpanzees.•Keep in mind that the genetic code is almost universal: thus, organisms are all related to one another, but with varying degrees of relatedness.

Embryo similarity: baleen whale embryos develop teeth, which vanish before birth

Homologous structure: whaleflippers have the same bones as other mammal forelimbsVestigial structure: whales have

pelvic bones that do not function in locomotion

Atavism: some whales have beenobserved with hindlimbs

Molecular data: whale DNA is most similar to hippos

Conclusion•When independent evidence supports the same inference, the inference is strengthened.• All available data sets lead to the same conclusion: living organisms have evolved from common ancestors.

Fossil data: fossil whales havecharacteristics found today onlyin hoofed mammals

Whales: Vivid Examples of Evolution