BIO SCI 94 Midterm: Bio94 Midterm 2 Key Ideas
Course CodeBIO SCI 94
ProfessorNancy Aguilar- Roca
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Chapter 28: Phylogenies and the History of Life
●Phylogeny is the evolutionary history of a group of organisms
●A phylogenetic tree is a graphical summary of this history, showing the
ancestor-descendant relationships among populations, species…
●Tree of life is the most universal of a phylogenetic tree, depicting all the
relationships among all living organisms on earth
●Branches tell time; the left are more ancient than the most recent ones.
●Tips represent a living or extinct group of genes, species, families, phyla or
●Nodes shows common ancestors and split into branches.
●Homology is similarities in organisms due to common ancestry,
●Synapomorphy is a trait found in two or more taxa that is present in their most
recent common ancestor but is missing in more distant ancestors. This helps
recognize monophyletic groups. The trait for the monophyletic group.
●Monophyletic group is an evolutionary unit that includes an ancestral
population and all of its descendants, but no others.
●Homoplasy is similarities in organisms due to reasons other than common
ancestry. A common cause of homoplasy is convergent evolution which occurs
when natural selection favors similar solutions to the problems posed by a
similar way of making a living in different species. Convergent evolution is the
independent evolution of similar traits in distantly related organisms due to
adaptations to similar environments and way of life.
●Polyphyletic group is an unnatural group that does not include the most recent
●Paraphyletic group includes an ancestral population and some of its
descendants but not all.
●SINEs (short interspersed nuclear elements) are parasitic DNA sequences,
which insert themselves into genomes of mammals. This supports the close
relationship between whales and hippos.
●Fossils are physical evidence from an organism that lived in the past. Fossil
records provide direct evidence about what organisms looked like in the past,
where they lived and when they existed. Fossils form when organisms are
buried in ash, sand, mud or other sediment.
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●Precambrian: 1) Life was exclusively unicellular for most of history 2)oxygen
was absent from the oceans and atmosphere for almost 2 billion years after
the origin of life (photosynthesis bacteria were responsible for the creation of
●Paleozoic Era (542 billion years ago-251) includes the origin and initial
diversification of animals, land plants, fungi and land animals.
●Mesozoic Era includes the most dominant plants, gymnosperms and the most
dominant vertebrates, dinosaurs. Ended with the extinction of dinosaurs.
●Cenozoic Era (recent life) on land, angiosperms were the most dominant plants
and mammals were the largest vertebrates.
●Adaptive radiation is when a single lineage rapidly produces many descendant
species with a wide range of adaptive forms.
●Two mechanisms trigger Adaptive Radiation 1) Ecological opportunity (new
resources) 2) Morphological innovation (one that allowed descendants to live in
new areas, make full use of new sources of food or move in new ways).
●Cambrian Explosion is the adaptive radiation of animals. An increase in size and
morphological complexity occurred and diversification in how they made a
●Triggers of Cambrian Explosion: 1)Higher oxygen levels 2)evolution of predation
(when predators evolved, they exerted selection pressures for shells, hard
exoskeletons, rapid movement and other adaptations for prey to defend
themselves) 3)new niches bring rise to more new niches (driving speciation and
ecological diversification) 4)new genes, new bodies (Hox genes organized the
development of the body)
●Mass extinction refers to the rapid extinction of a large number of lineages
scattered throughout the tree of life. More specifically, it occurs when at least
60% of the species are wiped within a million years. Mass extinctions result
from extraordinary, sudden and temporary changes in the environment.
Species do not die because individuals are poorly adapted to normal or
gradually changing environments. They die because of exposure to
exceptionally harsh or short term conditions such as huge volcanic eruptions
or catastrophic sea-level changes.
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