BIS 2B Lecture Notes - Lecture 17: Anagenesis, Phylogenetic Tree, Cladogenesis

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Lecture 17Lecture 17
Creation of species does not always include isolation and bifurcation. Time can result in
accumulated changes such that individuals in the present would not interbreed with their
direction progenitors, numerous generations back.
Phylogenetic tree: Map of
relatedness among different
species/history of relationships
among a group of organisms.
Time moving from far in the past
to the present- if things change
thru time, you would expect there
to be a # of different character
differences that would increase as
you go backwards in time. The
process of changing along one of
these branches is called anagenesis.
Bifurcation- two
separate populations
and a barrier to
reproduction b/w
those populations
such that they are
now evolving
independently from
one another.
Butterfly is
undergoing changes
because of natural
selection in color,
labeled as agenesis
because it’s a
population that’s
changing through
time.
Process of speciation requires both the breaking apart/barriers (cladogenesis) and also
transformation (anagenesis).
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3 kinds of evolutionary changes influence the shape of phylogenetic trees:
1. The origin of new lineages by speciation (CLADOGENESIS)
2. Within-lineage change (ANAGENESIS) (can also lead to new species formation after
sufficient time and differentiation, this is a different process)
Natural selection + random processes (mutation, genetic drift)
3. The disappearance of lineages thru EXTINCTION
A, B, C, D would represent 4 distinct
Species
A & B sister species (two taxa that
are each other’s closest relative)
C & D NOT sister species
The central problem of speciation: so how does cladogenesis
lead to speciation
?
How can barriers to gene flow arise in the first place?
Can a mutation arise that makes one individual different from the others in a population, and less  
likely to mate with them? (is that likely to create new species?)
NotNot typicallytypically,, if such a mutation arose, it would generally not spread, since it would give the
mutant lower reproductive success (i.e. fitness) (think of challard duck).
EXCEPT in some self-compatible organisms (adaptive radiations… think of flowers
example- simple differences in genome because failure of meiosis to appropriately segregate
chromosomes into gametes that lead to nearly instantaneous speciation because they are
self-fertile… so a population of mutants can expand)
1. Imagine 2 populations, one with A1A1 individuals, the other with A2A2 individuals
2. A1A2 individuals (hybrids) have low reproductive success or are inviable
3. These are thus “good” biological species
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Document Summary

Creation of species does not always include isolation and bifurcation. Time can result in accumulated changes such that individuals in the present would not interbreed with their direction progenitors, numerous generations back. of. Phylogenetic tree: map of among relatedness different species/history relationships among a group of organisms. Time moving from far in the past to the present- if things change thru time, you would expect there to be a # of different character differences that would increase as you go backwards in time. The process of changing along one of these branches is called anagenesis . Bifurcation- two separate populations and a barrier to reproduction b/w those populations such that they are now evolving independently from one another. Butterfly is undergoing changes because of natural selection in color, labeled as agenesis because it"s a population that"s changing through time. Process of speciation requires both the breaking apart/barriers (cladogenesis) and also transformation (anagenesis).

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