Chapter 8 evidence for evolution

Evolution on large & small scales

• There are two different types of evolution based on a scale
• Macroevolution- refers to large scale changes that occur over extended time periods, such as the formation of new species and groups
• Microevolution- refers to small scale changes that affect just one or a few genes and happen in populations over shorter timescales
• These are not different processes, they just occur on different timescales

Anatomy & Embryology

• Darwin's idea of descent with modification means all species can be traced back to a common ancestor
• Closely related groups of species have more recent common ancestors
• This is used to figure out how organisms are related based on their share features, which are seen through anatomy and embryology

Homologous Features

• If two or more species share a unique physical feature, such as a complex bone structure or body plan, they may have inherited this feature from a common ancestor.
• Physical features that are shared due to evolutionary history are homologous features.
• - For example, whales, humans, dogs, and birds all have a 4-limbed arm structure that is different for different environments. It is probable that there was a common ancestor with this basic layout.
• Homologous features can be seen in embryos
• - Most embryos have gill slits and tails due to a common ancestor
• Vestigial structures- homologous structures that have lost their major ancestral function; usually reduced
• - For example, tailbones in humans, hind legs in whales, and underdeveloped legs in some snakes
• Divergent evolution- a common ancestor evolves into two different organisms; can be shown with homologous and vestigial structures

Analogous Features

• Not all similar features are a result of common ancestry
• Analogous features- features that evolved independently in organisms that live in similar environments
• Convergent evolution- two unrelated organisms evolve to have the same traits because they live in similar environments and face similar selective pressures

Molecular Biology

• All living organisms share things in common:
• The same genetic material(DNA/RNA)
• The same, or highly similar, genetic codes
• The same basic process of gene expression(transcription and translation)
• The same molecular building blocks, such as amino acids
• Such evidence suggests that all living things are descended from a common ancestor with DNA
• Homologous genes- related genes found within different species
• - Homologous genes are used to figure out how certain species are evolutionarily related; two species will have the "same" gene if they both inherited it from a common ancestor
• - The more DNA differences in homologous genes between two species, the more distantly related the two species are

Biogeography

• Why do different organisms live in different places? This is answered with evolution and the shifting of tectonic plates over geological time.
• Pangea- ancient supercontinent(all continents combined)
• Broad groupings of animals that evolved before Pangea split up are distributed worldwide
• Broad groupings of animals that evolved after Pangea split up appear uniquely in small regions of the world
• - Marsupials in Australia carry a unique pouch to develop their young. Marsupials are very diverse because Australia is surrounded by water. There were no placental mammals(mammals that develop their young in a placenta)to compete with.

Fossil Record

• Fossils- preserved remains of previously living organisms or their traces, dating back to the distant past
• The fossil record is not complete. Most organisms never fossilized and fossils are very hard to find.
• The age of fossils is determined by the rocks they are found in.
• Strata- layers of rock that determine the age of fossils; the top layer is the youngest and the bottom layer is the oldest
• Fossils can be dated through radiometric dating- a process that measures the radioactive decay of certain elements
• Fossils show the existence of now-extinct species. Different organisms lived on the earth during different periods of time
• - Ex: Horse lineage fossils show adaptations that lead to modern-day horses.
• Transitional Species- species that occurred in between an ancestor and a present-day species