Lecture 3

14 January 2003

Species and Speciation – What are species and how do they arise?

Chapter 22 in Gill

Key terms

Biological Species Concept

Phylogenetic Species Concept

Sympatry vs. Allopatry

Vicariance

“diagnosable”

leks

polytypy/polytypic

assortative mating

heritability

sister species

1.      Species definitions

A.     Traditionally- the BSC, as defined by Mayr in 1970 – “groups of interbreeding natural populations reproductively isolated from other such groups.”

                        1. Guesswork involved

2.      Challenged by the Phylogenetic Species Concept, which recognizes any isolated, diagnosable population as a species.

A.     9,600 species goes to 20,000+

B.     recognizes separate evolutionary tracks of isolated populations

C.     important for conservation?

3.      The process

A.     Underlying paradigm is natural selection – species accumulate heritable adaptations to their environment.

B.     Populations are separated

                                                   i.      Island colonization

                                                 ii.      Vicariant event

1.      mountains

2.      rivers

3.      glaciers

C.     Separate populations will be under different selective regimes

D.     Genetic differences accumulate

E.      Subsequent sympatry tests limits of reproductive isolation.

                                                   i.      Ecological

                                                 ii.      Social/behavioral

F.      Hybridization zones – stable or not?

4.      Geographical variation

A.     33% N.A. species have recognized subspecies

                                                   i.      complicated by

1.      clines - balance between selection and gene flow

2.      environmental influences on phenotype – transplant experiments by Fran James

                                                 ii.      can be measured with new biochemical techniques

1.      allozymes

2.      mtDNA

5.      Population structure

A.     Gene flow determined by movement of young from nest to eventual breeding territory

                                                   i.      Philopatric species stick close to home

                                                 ii.      Some species females return to natal territories, in others it’s the males that move

                                                iii.      High dispersal rates mean large effective population sizes (demes)

1.      low effective population size increases chances of random events fixing genes.

a.       Colonial species along coastlines

b.      Nonmonogamous breeding systems (few males dominate contribution to next generation – leks)

2.      small populations on islands have founder effects –

a.       sampling

b.      inbreeding

c.       evolve faster than larger continental pops.

6.      Geographical Isolation

A.     Vicariant events likely to lead to slower speciation than

B.     Island colonization – genetic founder effects, etc.

7.      Secondary contact and hybridization

A.     Genetic introgression

                                                   i.      Audubon vs. Myrtle warblers

1.      secondary contact 7500 ybp at end of Wisconsin glaciation

2.      contact in narrow mt. passes so exchange of genes slow, but now 150 km wide

3.      lead ornithologists to lump the two forms into “yellow-rumped” warbler

                                                 ii.      11 of 14 pairs of species that meet in the great plains hybridize

1.      where stable hybridization zones exist, the BSC is applied and the species lumped

2.      where limited hybridization occurs with most individuals distinct (e.g. Lazuli and Indigo Buntings) species are recognized as distinct.

B.     Stable hybrid zones

                                                   i.      Sinks constantly replenished by immigrants from parent populations, or

                                                 ii.      “bounded superiority” where ecological conditions are intermediate so intermediate individuals formed by interbreeding are superior to either parental form.

C.     Problems with hybrids

                                                   i.      Infertility – often the heterogametic sex (females in birds)

                                                 ii.      Blended characters lead to “diluted sex appeal”

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