Mechanisms of Speciation

Chapter 12

I) What is a species

A) Intuitive classification

B) Biological reality

C) Specie Concepts

1) Biological Species Concept – Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.

a) more mechanistically based

b) can't deal with clonal or asexual organisms

c) hard to test in every situation

d) cannot be used with fossil

2) Evolutionary Species Concept – A lineage (ancestral to descendant sequence) evolving separately from others and with it own unitary evolutionary role and tendencies.

a) difficult to define lineage

b) can be used with fossils and clonal/asexual organisms

c) difficult to define evolutionary role or tendency.

3) Phylogenetic Species Concept: – an irreducible cluster of organisms that is diagnosably distinct form other such clusters, and within which there is a parental pattern of ancestry and descent

a) overall similarity/differences between groups to set limits to species.

b) monophyletic unit is basis of designation

c) testable

d) arbitrary in the exact magnitude of differences necessary to reach species status.

e) not consistent between taxonomic groups

f) can be used with fossils

g) may be trivial amount of difference

4) Morphospecies Concept – defined by morphological differences between groups

a) widely applicable (everything has morphology)

b) works well with fossils

c) arbitrary in the exact magnitude of differences necessary to reach species status.

d) may not represent biological reality (ecotypic variation)

5) Recognition Species Concept – Species are set of organism that share a common mate recognition system.

a) mechanistic

b) unclear how to treat clonal organisms

c) difficult to define "mate recognition system" in every case

d) cannot be used with fossil

6) Ecological Species Concept – Sets of organisms occupying a single ecological niche.

a) competition for resources keep species apart

b) difficult to define what is a niche

c) difficult to use with fossils

II) Species Concepts – Biological Species Concept

A Reproductive Isolating Barriers

1) Prezygotic (premating)

a) Potential mates do not meet

i) Temporal isolation

ii) Spatial isolation

b) Potential mates meet but do not mate

i) Behavioral – do not recognize each other as mates

c) Copulation occurs but no transfer of male gametes takes place

i) Mechanical isolation – lock and key

d) Gamete transfer occurs, but egg is not fertilized

i) Incompatibility of gametes

2) Postzygotic

a) Zygote dies

b) Hybrid inviability – F₁ hybrid has reduced viability

c) Hybrid sterility – F₁ hybrid is viable but has reduced fertility

d) Hybrid breakdown – F₁ hybrid is viable and fertile, but later generations have reduced viability or fertility

II) Isolation and Genetic Differentiation

A) Genetic Similarity and Geographic Distance – assuming limited gene flow between populations.

B) Genetic Similarity and Time – assuming that two populations have been isolated sometime in the past.

C) A phylogeny perspective

1) Panmixia – All individuals are equally likely to mate with all other individuals. One continuous population.

2) Polyphyle – After population isolation (e.g., speciation) some individuals in each population (species) are more closely related to individuals of the other population (species) than they are to individuals of their own population.

3) Paraphyle – individuals in one population are more closely related to individuals in the other population than they are to individuals in their own population. Individuals in the other populations are all each others most closets relative.

4) Monophyle (reciprocal) – all individuals in both populations are more closely related to other individuals in their own population than they are to any of the individuals in the other population.

III) Models of Speciation

A) Patterns of Species Distribution

1) Sympatric – species are found in completely or partially overlapping areas.

2) Parapatric – Species are found in connected but not overlapping areas.

3) Allopatric – species occupy geographically separated areas

B) Mechanisms of Speciation – Three Types

1) Allopatric Speciation – differentiation and speciation (i.e., production of reproductive isolating barrier) occur due to an extrinsic or physically isolating barrier. Two Types

a) Vicariance – geological or physical barrier causes isolation

b) Founder Event – rare colonization of new area and highly limited connection to parental population

2) Parapatric – evolution of reproductive isolating barriers between geographic subsections of a continuously distributed species.

3) Sympatric – evolution of reproductive isolating barriers within a randomly mating population (i.e., no physical, geological, or geographic isolation).

C) Allopatric Speciation – most strongly supported type.

1) Vicariant speciation

Widespread, continuously distributed species

Geographic Barrier is Established and reduced gene flow between subsections

Isolated Populations Diverge Over Time and either prezygotic or postzygotic isolating mechanisms develop.

Species Maintained After Removal of Barrier

2) Founder Event (Allopatric)

One of a few number of individuals leave parental range and start a new population.

Individuals experience reduced intraspecific competition and population size increases rapidly. May be different selection pressures. Divergence in small populations can be very fast.

D) Mechanisms of Divergence – when in isolation, subsections can diverge in several ways.

1) Adaptive Divergence – Natural selection pressure is different in one population relative to the other.

2) Genetic Drift – Even if environments and therefore natural selection pressure is the same, due to finite population size, each populations will drift and most likely not in the same way.

3) Peak Shift – most (if not all) quantative traits have more than one fitness optimum (i.e., peak). Small population size increases the strength of drift relative to selection and can result in the population shifting from one peak to another.

IV) Alternatives to Allopatric Speciation

A) Parapatric Speciation – A continuously distributed species is likely to cross ecologically different geographically localized areas. Natural Selection pressures may be different in each area. Speciation can occur as individuals adapt to localized areas.

B) Sympatric Speciation – new species arise (i.e., reproductive isolating barriers) within the range of a randomly reproducing species (i.e., not extrinsic isolating barrier).