Alana Wehbe

EVIDENCE OF EVOLUTION

PalaeontologyFossils• If there have been no geological upheavals, older

sedimentary rocks contain earlier fossils. These simple, water living fossils can be compared to the fossils of younger organisms found in the upper parts of sedimentary rock, which are both simple and complex, and both water and land living.

• Fossils can also be compared with the skeletal structure of modern organisms to find similarities and differences, showing how they have changed over time.

Transitional Forms

Transition fossils are fossils which appear to have the characteristics of two different groups of organisms. They show the intermediate states between an ancestral form and that of its descendants. Transition fossils are evidence of change over time.

For example, Archaeopteryx (pictured) is similar to an extinct group of dinosaurs (theropods), except for its feathers and wishbone. Its skeleton is not unlike that of a reptile, but its feathers are singularly birdlike.

Biogeography• Darwin and Wallace noticed that species living in the same area were more similar to each other than to species living in similar habitats far apart. This shows that species evolved due to the selective pressure in their own habitats.• As continents collide, their species can mingle, and when the continents separate, they take their new species with them. For example, Australia, Africa, South America and New Zealand were all once joined. The continents split off one by one, first Africa, then New Zealand, and then finally Australia and South America. The evolutionary tree of some groups of species — such as tiny insects known as midges — show the same pattern. South American and Australian midges, for example, are more closely related to one another than they are to New Zealand species, and the midges of all three land masses are more closely related to one another than they are to African species.

Comparative Embryology• Homologous structures not present in the adult organism often appear in some stage of embryonic development.• Species that have little resemblance in their adult form may have similar embryonic stages.• For example, the human embryo passes through a stage in which it has gill structures like those of a fish. For a large portion of its development it also possesses a tail, much like those of our close primate relatives

Homologous StructuresHomologous structures are structures found in plants or animals that have the same origin, but not necessarily exactly the same form or the same function.

The development of homologous structures is an example of divergent evolution, which occurs when a number of species develop from one common ancestor, becoming less alike over time due to their different habitats.

The pentadactyl limbs of vertebrate animals,

which have the same basic structural plan but

have altered to different environments and

lifestyles, suggests that these groups of animals

originated from a common ancestor.

Comparative DNA• DNA and the proteins it produces can provide evidence about evolution.• If two new species evolve from a common ancestor, their DNA and protein molecules slowly change and become different. The number of differences is proportional to the time since they separated. This allows the construction of evolutionary trees.• Genes are slightly different in each species, but their striking similarities reveal their common ancestry. • DNA and RNA possess a simple four-base code that provides the recipe for all living things. This code is a homology that links all life on earth to a common ancestor.