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- Evolution is a theory to explain the origin and
diversity of organisms. It posits that all living and
extinct organisms were produced by two processes: descent
with modification and diversification from a common
ancestor.
- Life is believed to have evolved several billion
years ago as a simple single-celled organism with the
capacities to acquire energy from its environment and
reproduce itself.
- Over time, populations of this one organism became
modified relative to its ancestral form and also
subdivided into new populations which subsequently
became modified and subdivided to form new organisms,
and so on, and so on.
- The modification of life forms over time is driven
by two processes: genetic mutation and natural
selection. [Genetic mutations are random changes
in DNA sequence that occur from one generation to the
next. Natural selection is the process by which
organisms with traits that are favored by the current
environment survive and reproduce better than
organisms with less favored traits. The result is that
the genes for favored traits are passed on to the next
generation at higher frequency than the genes for less
favored traits.]
- Genetic changes that accumulative within
populations and genetic differences that accumulate
between recently separated populations both lead to
speciation.
- Speciation is the formation of new species.
- Species are generally recognized as the
fundamental unit of evolution or the taxonomic unit
(taxon) that undergoes evolutionary change. Species
are usually grouped into higher-order taxa (e.g.,
genus, family, order, etc.) that may or may not
reflect evolutionary history.
- There are different ways of defining species and
different models for explaining how speciation occurs.
Some definitions and models are specific to the type
of organism being considered.
- Speciation, which always involves genetic change,
may also be accompanied by biochemical, morphological
and behavioral changes.
- Phylogenetics is the branch of biology that deals
with reconstructing the evolutionary history and
evolutionary relationships of a group of taxa.
[Evolutionary history refers to the sequence and
pattern of branching points in the ancestral lineages of
a group of taxa. Evolutionary relationships refer to the
nested grouping of taxa that emerges from a particular
sequence and pattern of branching points.]
- The task of reconstructing evolutionary history or
relationships involves several key concepts: homology,
character, cladistics, clade, monophyly,
synapomorphy.
- Homology describes the relationship between two or
more physical features or characters that were derived
from the same character in an ancestral species.
Homology usually refers to characters in different
species that evolved from a single character in a
common ancestor of those species.
- Cladistics is a method of phylogenetic
reconstruction that is based on the distribution of
homologous characters among the group of taxa being
considered.
- Characters used for cladistic analysis may be
morphological, behavioral, biochemical, and molecular.
Molecular characters include the base pair sequences
of specific pieces of DNA and RNA, the amino acid
sequences of specific proteins, the maps of
restriction enzyme sites in DNA, and chromosomal
characteristics.
- A clade is a branch of an evolutionary tree,
meaning an ancestral lineage and all of its descendent
lineages.
- Monophyly means that all members of a group of
taxa are derived from a single common ancestor and no
other taxa are derived from that ancestor.
- A synapomorphy, meaning a shared, derived
character, is a character that is common and specific
to all members of a monophyletic group. It is presumed
to have evolved in the most recent common ancestor of
that group.
- There are several lines of evidence for a single tree
of life.
- Miller-Urey simulated the conditions that are
believed to have existed on the primitive Earth. These
conditions in the laboratory produced complex organic
molecules such as amino acids (the building blocks of
life).
- The major taxa of organisms (and their major
subdivisions) are viruses, prions, eubacteria,
archaeabacteria, and eukaryotes, the latter of which
includes protista, animalia (diploblasts and
triploblasts: protostomes and deuterostomes), plantae
(nonvascular and vascular: seedless and seeded:
gymnosperm and angiosperm), and fungi (zygomycetes,
ascomycetes, deuteromycetes, and basidiomycetes). These
groups all share fundamental similarities in their basic
biochemistry and molecular design, which suggest that
they all evolved from the same ancestral organism.
Further, these groups can all be distinguished from each
other by more specific differences in molecular, cellular
and/or developmental characters. Because of their
fundamental roles in organism design, development and
function, these characters are generally considered to
have evolved their different states only once. The
distribution of these states among organisms thus serves
to distinguish the major taxa and subtaxa as monophyletic
groups (or clades) and supports one particular branching
pattern that unites these clades. This pattern is
generally recognized as the tree of life.
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