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

Fig. 9.1

  1. Geographic variation
    • Broad range of variation within species

Fig. 12.19

    • Gradient of variation between species

Fig. 15.1

  1. 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. 9.23

·        Hybridization may not be uniform across the species range

Fig. 15.3

·        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

 

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

  1. 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)

  1. Meet but don’t mate (behavioral isolation)

·        Behavioral differences prevent mating

Fig. 15.5

·        Pollen transferred by different pollinators

Fig. 15.7

 

2.  Postmating prezygotic barriers

Three kinds

  1. Lack of fit (mechanical isolation)
  1. Lack of stimulation (behavioral isolation)

Fig. 15.8

  1.  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

  1. Extrinsic (external)

·        Lower fitness for ecological reasons

Example

Hybrids don’t effectively attract either pollinator

Fig. 15.7

·        Lower fitness for behavioral reasons

Fig. 15.5

 

  1.  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. 15.9

·        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 1-2%) 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.