Lecture 21 Flashcards
- Natural selection, genetic drift, mutation, migration
Sources of MICROEVOLUTION
– Can these processes explain MACROEVOLUTION
(taxonomic diversification)?
– Link between micro and macro a source of doubts for
early researchers studying evolution
What IS a species?
The species problem”
– The problem of how best to define a species
* There are many species concepts:
* Biological
* Recognition
* Cohesion
* Darwinian
* Evolutionary
* Taxonomic
* Phenetic
* Genetic
* Ecological
* Phylogenetic
What IS a species?
Traditionally defined by phenotypic similarity
* Fairly easy to identify within a region (sympatric)
* But problems arise from gradual differences
across regions (allopatric)
* Genetic similarity also used to identify and define
species (phylogenetic species concept)
Two Main Species Concepts
Taxonomic (or morphological)
* Based primarily on distinct measurable differences
Biological
* Based on inter-fertility among individuals
Concepts vary among groups of organisms:
No universal species concept
Darwin and the Origin of Species
Species defined as groups of organisms
that are sufficiently similar in phenotype
The Biological Species Concept (BSC)
a group of interbreeding
natural populations that are
reproductively isolated
from other such groups
-The BSC helps frame “the species problem”
as a tractable research question
Biological Species Concept:
Some Points to Highlight
Focuses on the PROCESS
* Geographic isolation alone is NOT sufficient
* Isolation does NOT have to be absolute
* Question of what cutoff?
* Must be possibly interbreeding IN THE WILD
* Does not apply well for bacteria, asexuals, highly
self-fertilizing species …or fossils
Allopatric speciation
Allopatric speciation
much more common than sympatric
& easier to evolve,
due to evolution with
minimal gene flow
Pre-zygotic
RI barriers
Fertilization, Finding a compatible mate & mating
Post-zygotic
RI barriers
Development & growth of zygote (F1), Adult survival & reproduction (F1), Growth, survival, reproduction of offspring (F2)
Pre-zygotic barriers
prevent mating or
fertilization so no zygote gets formed
* Geographical, ecological
* Temporal, behavioral (mate recognition)
* Mechanical (genital structure compatibility)
* Cellular (sperm-egg compatibility)
Pre-zygotic Isolation in Apple Maggot Flies:
Habitat and Temporal Isolation
Host shift from hawthorns after arrival of domesticated apples in 1800s
* Differences in timing of host plant fruiting (apple vs hawthorn)
* Different timing of fly mating on preferred host plant
* Reduces fly gene flow by 94% in sympatry (same region)
Pre-zygotic Isolation in Abalone
Binding of sperm lysin protein to egg
vitelline envelope receptor (VERL) required
for fertilization
– Molecular “lock and key”
* Lysin/VERL interaction has coevolved
– Different evolutionary changes in
different species
– Causes reproductive isolation due to
fertilization incompatibility
Post-zygotic barriers
prevent proper
functioning of zygotes once they are formed
* Caused by combinations of genes with low fitness in the
hybrid
* Intrinsic:
– Inviability, sterility, or abnormal development of hybrids
* Extrinsic:
– Ecological mismatch of hybrid phenotype to environment
Cannot be directly favored by natural selection:
* Arise as an indirect byproduct of evolution acting
separately in different populations
Relation Between Genetic Distance*
and Post-zygotic Isolation in Fruit Flies
The more that fly pairs are
genetically differentiated,
the more likely they are to be
reproductively isolated
* D is a proxy for time
* The plot can be viewed as a
time course for the evolution of
reproductive isolation
Genetic distance (D) is a measure of the degree
of genetic differentiation between samples
Extrinsic Post-zygotic Isolation
Hybrids have aberrant colour patterns
* Higher predation risk
* Lower mating success
Is Adaptive Evolution Required
for Speciation?
Local adaptation by different populations can
lead to reproductive isolation and speciation
– Distinct evolutionary responses to different
selective pressures
– LA not absolutely necessary, but accelerates
population divergence and evolution of RI
* Termed ecological speciation
* Much current research aims to determine:
– The biotic & abiotic agents of selection
– The underlying‘ speciation genes’
Eda: A Key Gene for Understanding
Adaptation and Speciation During Colonization
of Freshwater Lakes from Marine Environments
In marine environment:
Bony armor protects
against large fish predation
In freshwater:
Loss of plates increases growth rate
– Greater winter survival
– Earlier breeding
How and why do organisms diverge and diversify?
2) How can we explain patterns of variation across species?
3) Can the evolutionary processes of mutation, selection, drift
explain how we got millions of diverse species on Earth?
Allows speciation to occur
* Populations continue evolving and adapting independently,
driving macroevolutionary diversification
-Populations diverge genetically as a result of evolutionary
forces (mutation, natural selection, genetic drift)
* Become reproductively isolated, often as an incidental
byproduct of evolutionary change within populations
Adaptive Radiation
The evolution of ecological and phenotypic diversity
within a rapidly multiplying lineage as a result of
speciation
– Originates from a single common ancestor
– The process results in an array of many species
– The species differ in traits allowing exploitation of a
range of habitats and resources
* Four features commonly identify an adaptive radiation
1) Recent common ancestry from a single species
2) Phenotype-environment correlation
3) Trait utility
4) Rapid speciation
What Causes Adaptive Radiations?
1) Ecological opportunity
* Abundant resources
* Few competitors
* Often encountered on oceanic islands or their aquatic
counterparts e.g. African rift lakes
2) Origin of a key innovation
* e.g. toepad in Anoles; floral nectar spur in Columbines
3) High rates of speciation characterize the clade
* Test by comparing island to mainland clade
* e.g. Darwin’s finches & Hawaiian honeycreepers also
radiated on mainland (although not as much), whereas
Galápagos mockingbirds have not radiated on islands or
continents
Hybridization
The exchange of genes between species
as a result of occasional inter-species mating
– Sometimes can reverse speciation process to merge
two groups into one
* Varies across Tree of Life
– Common in plants and fish, rare in mammals
* Can result in complex patterns of variation
– Can be evolutionarily significant for speciation,
especially by polyploidy
Polyploidy
An organism, tissue, or cell with more than two
complete sets of homologous chromosomes
* Allopolyploidy (e.g. AA x AA AA AA)
– Arises from duplicated karyotype following
hybridization between species
– Commonest type of polyploidy
* Autopolyploidy (e.g. AA AA AA)
– Arises from duplicated karyotype within a
species (e.g. non-disjunction)
Evolutionary Significance of Polyploidy
Polyploids are reproductively isolated from
their diploid parents
– Hence a form of sympatric speciation
* Polyploids exhibit novel phenotypes
– Allows exploitation of new habitats
* Polyploids often show hybrid vigor due to
heterozygosity, particularly in allopolyploids
* Polyploid origin for approx 50% of flowering plants
– Many crops plants & invasive species
