Download Reproductive Isolating Mechanisms: Prezygotic Reproductive

Reproductive Isolating Mechanisms:
Prezygotic
• Prezygotic - before fertilization,
mechanisms which lower the
probability that hybrid zygotes will
be formed.
• Differences in mating or courtship
behavior in animals; sexually
selected traits often appear to
evolve rapidly and show marked
differences between otherwise
similar species
• Differences in phenology
(flowering periods in plants)
• Ecological differences which
minimize contact (e.g., use of
different microhabitats)
Reproductive Isolating Mechanisms:
Postzygotic
Postzygotic - after fertilization,
processes which act to reduce the
viability or fertility of hybrids
• Incompatibility of nuclear
genomes, maternal/paternal
genome conflict (e.g., genomic
imprinting)
• Incompatibility of nuclear and
cytoplasmic (e.g., mitochondrial)
genomes
• Problems in crossing over during
meiosis causes sterility (e.g.,
Mules)
• Low ecologically-based fitness of
hybrids
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Haldane’s Rule
• In hybrids produced by
interpopulation or
interspecies crosses, the
heterogametic sex is more
likely to be inviable and/or
infertile
– Mammals and most
insects: males are the
heterogametic sex (XY)
– Birds and butterflies:
females are
heterogametic sex (ZW)
– http://discover.npr.org/fea
tures/feature.jhtml?wfId=
1303260
New Topic: Phylogeny
• Systematics - scientific study of biological
diversity
• Taxonomy - theory and practice of classifying
organisms
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A.J. Cain cerca 1960
• “Taxonomists are trained like performing
monkeys -- almost solely by imitation”
• Cain was lamenting the lack of a conceptual
basis for Systematics
Systematics Now
• With the advent of Cladistics (see below) and the
development of molecular genetic methods, systematics has
become a rigorous scientific discipline over the past two
decades.
• Why is Sytematics important? - Because evolution is about
history. Testing hypotheses about evolutionary mechanisms
requires rigorous data on phylogenetic relationships
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Phylogeny
A biological classification which reflects
evolutionary relationships, i.e., ancestordescendent relationships; A branching pattern
showing ancestor-descendent relationships
Three Schools of Systematics
• Phenetic Systematics: classification based strictly
on overall similarity using multivariate statistical
methods
• Cladistics or Phylogenetic Systematics:
classification based on recency of common ancestry
• Evolutionary Systematics: classification based on a
combination of recency of common ancestry and
morphological similarity
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Cladistics is the proper approach
• Critical unit in cladistics is the monophyletic unit,
i.e., an ancestor and ALL its descendent species
• Paraphyletic unit - an ancestor and some but not all
of its descendants, e.g., reptiles
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How Do We Reconstruct
Phylogeny?
• Fossils
• Development
• Distribution of character states in living
species
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Fossils
• In principle, fossils could provide direct evidence of evolutionary transitions.
• In practice, for most taxa, the data are too fragmentary.
• Also, dead fossils may not have left descendants whereas living organisms
definitely had ancestors.
• Fossils are useful for designating outgroups and estimating divergence time
of taxa
Traits Used in Reconstructing
Phylogenies
• Structures in early
developmental stages
sometimes show
evolutionary
relationships not evident
in adults.
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VON BAER’s LAW
The features
common to a
higher-level
taxonomic
group often
appear in
development
before specific
characters of
lower-level taxa
Development: Haeckel’s “Law”
• Ontogeny recapitulates phylogeny (sort of, some
times)
• To repeat stages from the evolution of the species
during the embryonic period of an organism’s life
• In the course of its development, an individual
successively passes through the adult forms of its
ancestors
• Ontogeny = development of the individual organism
• Phylogeny = evolutionary history of species
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Haeckel’s “Law” cont...
• Terminal additions to
development lead to
recapitulation (i.e.,
phylogenetically new
features are added to the
ancestral ontogeny)
• This appears to be
sufficiently common that
recapitulation was noticed
• Law is violated most
strongly by paedomorphic
species (species in which the
juvenile morphology of the
ancestor is retained
throughout life)
Distribution of Character States in
Modern Organisms
• The distribution of character states in a set
of organisms provides information on
evolutionary relationships
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Characters & Phylogeny
Reconstruction
• Classification should be based on shared, derived
characters
• Shared, primitive characters - provide no information
on evolutionary relationships
• Outgroup comparison - used to determine which
character states are primitive and which are derived,
which is called polarity of character states.
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Cladistic Analysis: an example
How do we determine the best
evolutionary tree?
• Principle of
Parsimony: find the
evolutionary tree
which minimizes the
number of character
state changes
summed over all
characters
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Types of Characters
• Phenotypic Characters:
– Morphological Characters
– Physiological Characters
– Behavioral Characters
• Molecular Characters
– Proteins
– Nucleic Acids: DNA, RNA
Advantages of Molecular Characters
Different regions of the genome
evolve at different rates:
• Slowly evolving regions are
useful for broad-scale
phylogenetic questions, i.e.,
higher relationships such as
kingdoms, phyla, classes, etc.
– Ribosomal RNA genes:
sequence divergence evolves
very slowly and can be used
to examine relationships of
five kingdoms; phyla, etc.
– DNA polymerase gene
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Advantages of Molecular Characters cont...
• Rapidly-evolving
regions are useful
for fine-scale
phylogenetic
resolution:
populations, species
and genera.
– Mitochondrial
genes: have base
substitution rates
10X higher than
nuclear genes
Major Breakthrough in Molecular
Phylogenetics: PCR
UNIVERSAL PRIMERS - Regions of conserved
(invariant) DNA sequence across taxa which allows
for amplification of DNA from geneticallyuncharacterized species
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Universal Primers - named after prominent
conservative politicians (conserved primers)
C1-J-1751 (alias Ron)
G T (Degenerate Ron works better)
5’GGATCACCTGATATAGCATTCCC 3 BEE
...G.T.....C........... FLY
...G.T..A........T..T.. LOCUST
..TG.T..C...........T.. COW
...G.C..A...........T.. TOAD
..TG.G.G..A...G..C..... SEA URCHIN
...G...........AG...T.. ROUNDWORM
C1-N-2191 (alias Nancy)
5 CCCGGTAAAATTAAAATATAAACTTC
..A.......................
..T.......................
.....C..............T.....
..A.................G.....
3 FLY
BEE
LOCUST
SPRINGTAIL
COW
Lesson - Nancy is more conservative than Ron
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Molecular Characters
Advantages:
• No environmental effects
• Can evolve in clock-like fashion (rate of substitution constant)
• DNA is ubiquitous to life and homologous sequences exist
among all taxa
• Different regions of genome evolve at different rates and rate
variation can be exploited for addressing phylogenetic questions
at different levels
• Protein-coding genes provide a model of evolutionary change:
– Genetic code is degenerate in the third position, e.g., CUU,
CUC, CUA, CUG all code for the amino acid leucine;
Changes in third position are synonymous or silent
mutations; Because most mutations are deleterious, we
expect a higher rate of base substitution at synonymous sites
Molecular Characters
Disadvantages:
• Costly in terms of time and time and money
• Sequences can be subject to convergence
– Only 4 states per character for sequence data
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