Biol 4802 Evolution
Lecture 23, Chapter 17
Topics for today:
1. What is a species?
2. What factors prevent gene flow?
3. How are species diagnosed?
Speciation bridges microevolution and macroevolution
Microevolution
 Short-term change
 Genetic divergence among populations within a species
Macroevolution
 Long-term change
 Speciation and higher taxonomic levels
Cladogenesis
Anagenesis
Morphological definitions are often not adequate
1. Variation with populations:
 Segregating polymorphism
 Environmentally induced variation
Fig. 13.20 old and new
Fig. 9.1 old and new
2. Geographic variation
 Broad range of variation within species
Fig. 12.19 old and new
 Gradient of variation between species
Fig. 17.1 new 15.1 old
3. Sibling species (cryptic)
 Species that are not morphologically distinct but differ in other traits
Parasitoid species specialize on different ant species
How should species be defined?
Define in a way that: Coyne and Orr 2004
 Allows systematic classification of organisms
 Corresponds to discrete groups of similar organisms
 Reflects how discrete groups arise in nature
 Represents products of evolutionary history
 Applies to the largest possible variety of organisms
No single definition meets all of these criteria
“No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely
what he means when he speaks of a species (Darwin 1859).”
Slew of species definitions
Biological species concept
 Emphasizes the process by which species arise
 Minimal gene exchange even when species co-occur
Limitations
 Only works for sexual outcrossing organisms
 Many species can hybridize
Fig. 9.25 new 9.23 old
 Hybridization may not be uniform across the species range
Fig. 17.4 new 15.3 old
 Difficult to evaluate if populations do not co-occur
Separated for 50 million yrs
Still forms fully fertile hybrids
Phylogenetic species concept
 Emphasizes the outcome or products of evolution
 A species can be defined by the fixation of even a single base pair
Limitations
 “Good” species according to the Biological Species Concept not necessarily detected
 Diverged populations may also merit species status
Genus Greya
Fig. 15.4 old only
Species have reduced gene flow
What factors prevent gene flow?
 Physical barriers by themselves are not adequate but may play a role
Three main categories
1. Premating barriers
2. Postmating prezygotic barriers
3. Postzygotic barriers
1. Premating Barriers
Two possibilities
A. Don’t meet (ecological isolation)
 Breed at different times (temporal)
Example
Henosephilachna nipponica
 Breeds early.
 Overwinters as nymph.
Henosephilachna yasutomii
 Breeds late.
 Overwinters as egg.
Form fertile hybrids
 Breed in different habitats (spatial)
B. Meet but don’t mate (behavioral isolation)
 Behavioral differences prevent mating
Fig. 17.5 new 15.5 old
 Pollen transferred by different pollinators
Fig. 17.7 new 15.7 old
2. Postmating prezygotic barriers
Three kinds
A. Lack of fit (mechanical isolation)
B. Lack of stimulation (behavioral isolation)
Fig. 17.8 new 15.8 old
C. Gametes don’t join (gametic isolation)
 Intrinsic incompatibility
 Competition between conspecifics
Examples
 Sperm tails labeled with florescent tag
Birkhead, T. 1999. Nature 400:406
 Pollen tubes growing toward specific protein emitted by ovary
Lord, E. 2003 PNAS
3. Postzygotic barriers
Two kinds
A. Extrinsic (external)
 Lower fitness for ecological reasons
Example
Hybrids don’t effectively attract either pollinator
Fig. 17.7 new 15.7 old
 Lower fitness for behavioral reasons
Fig. 17.5 new 15.5 old
B. Intrinsic (internal - genetic incompatibility)
 Developmental problems
Example
Campanulastrum americana, American Bellflower
o Hybrids have more deformities
Etterson, Keller, and Galloway. 2007. Evolution 61:2671-2683
o Lowest survival in F2
 F2 breakdown
o Loss of positive epistasis
 Hybrid sterility – Haldane’s Rule
o Heterogametic sex shows greatest hybrid sterility
How are species are diagnosed?
 Seldom done through direct testing
 Phenotypic characters often used
Fig. 17.9 new 15.9 old
 Barcoding
Barcode of life
What is CBOL? http://www.barcoding.si.edu/
The Consortium for the Barcode of Life (CBOL) is an international initiative devoted to
developing DNA barcoding as a global standard for the identification of biological
species. DNA barcoding is a new technique that uses a short DNA sequence from a
standardized and agreed-upon position in the genome as a molecular diagnostic for
species-level identification. DNA barcode sequences are very short relative to the entire
genome and they can be obtained reasonably quickly and cheaply. The "Folmer region"
at the 5' end of the cytochrome c oxidase subunit 1 mitochondrial region (COI) is
emerging as the standard barcode region for almost all groups of higher animals. This
region is 648 nucleotide base pairs long in most groups and is flanked by regions of
conserved sequences, making it relatively easy to isolate and analyze. A growing number
of studies have shown that COI sequence variability is very low (generally less than 12%) and that the COI sequences of even closely related species differ by several percent,
making it possible to identify species with high confidence. For those groups in which
COI is unable to resolve species-level differences, CBOL recommends the use of an
additional gene region. In some groups, COI is not an effective barcode region and a
different standard region must be identified. In all cases, DNA barcoding is based on the
use of a short, standard region that enables cost-effective species identification.