The Process of Speciation• Every individual alive today, the highest as well as

the lowest, is derived in an unbroken line from the first and lowest forms

August Frederick Lopold Weismann

(1834-1914)

• From the remotest past which Science can fathom, up to the novelties of yesterday, that in which Progress essentially consists, is the transformation of the homogeneous to the heterogeneous

Hebert Spencer (1820-1903)

The Process of SpeciationWe wish to ask:• What is biological evolution?• How are the theories of microevolution and

macroevolution related?• What is a species, and what are the different ways is

can be defined?• How is reproductive isolation important to speciation,

and what forms can it take?• Why should natural selection reinforce reproductive

isolation?• Can species be formed in ways other than geographic

isolation?

Biological Evolution Defined

• Changes in the genetic composition of a population with the passage of each generation – Micro-evolution– Small, gradual changes

• The gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity• Macro-evolution and taxonomic hierarchy• May not be so gradual

Species formation

• To ask, “how are species formed”, it helps first to ask, “What is a species?”

• Two main definitions– Morphological species– Biological species

Morphological Species Concept

• “members of a species are individuals that look similar to one another”

-based on appearance and professional judgment-good enough for Linneaus

• Criticisms-arbitrary-may fail to discriminate (e.g. mimicry complexes)

Biological Species Concept• “a species is a group of actually or potentially

interbreeding individuals who are reproductively isolated from other such groups”

-critical test of ‘species-hood’ –ability to produce successful, fertile offspring

-species is evolutionary unit

• Criticisms

-useless with fossils, museum specimens

-can not use with spatially disjunct populations

- hybridization

How does Reproductive Isolation Arise?

• First, what are reproductive barriers ?• Pre-zygotic isolating mechanisms

- ecological - temporal- behavioral- mechanical

• Post-zygotic isolating mechanisms- hybrid invariability- hybrid sterility- hybrid less fit

Courtship displays are a behavioral, pre-zygotic isolating mechanism

By adapting to breed at different times (temporal isolation), these four frog species achieve at least partial pre-zygotic isolation.

Post-zygotic reproductive isolation

A sheep-goat chimera produced by injecting a goat inner cell mass into a sheep blastocyst. The chimera was carried to term in a sheep recipient female. Dept Animal Science, UC-Davis

Speciation by Geographic Isolation

• Intact population becomes separated by geographic barrier (note link to earth processes)

• Different geographic regions have different environments, competitors, predators

• Separated populations are genetically independent (different mutations)

• Given enough time, the two populations may diverge enough to become distinct species

• If reproductively isolated, two species

How geographic isolation may lead to the splitting of one species into two, following the allopatric model of speciation

How might geographic isolation occur?

Dispersal

Continental drift

Climate fluctuations, including glaciation

Drifting Continents and Biogeography

                                                                                                                                  

Freshwater and land dinosaurs became isolated on different continents as plates drifted apart.

100-million- year old Rugops primus

Five species of toucans occur in the rain forest of South America. How did they arise?

Contemporary rainfall. Areas receiving > 2500 mm annually are suggested Pleistocene forest refugia

Savannahs occur where rainfall is < 1500 mm annually

Proposed pleistocene refugia for forest-dwelling plans and animals

Adaptive radiation on island chains

• Adaptive radiation is the evolution of many diversely adapted species from a common ancestor

• Example is Darwin’s finches in the Galapagos

• Multiple events of colonisation, adaptation, speciation and recolonisation

Conditions favouring allopatric speciation

– A small, isolated population is more likely to change substantially enough to become a new species than a large one

– The geographic isolation of a small population usually occurs at the fringe of the parent population’s range

– Peripheral isolates are good candidates for speciation, although most peripheral isolates do not survive long enough to speciate

Sympatric speciation

• Sympatric speciation is the formation of a new species within the range of the parent population– Reproductive isolation without geographical isolation

• Many plant species have evolved through polyploidy:– Autopolyploids have chromosomes derived from a single

species

– Allopolyploids arise from two different species:• More common that autopolyploidy

Polyploidy in plants

Species Formation and the Hierarchy of Life

• Speciation results in the splitting of one ancestral species into two (or more) descendent species

• Extended over great expanses of time, this same process gave rise to the entire tree of life

• The link from micro-evolution to macro-evolution is time – lots and lots of time

Species Formation and the Tree of Life

• Time + speciation => diversity

• Outcome depends on:

- strength of selection pressure

- length of time

• Explains why species is not an unambiguous entity

- continuum of differentiation

Species formation (Revisited)

• Cladogenesis – the splitting of one lineage into two

• Geographic (allopatric) isolation is the prerequisite condition for reproductive isolation to evolve (animals)

• Except in plants, where hybridization and/or chromosome doubling can instantly produce reproductively isolated, new species (see notes)

• How can we re-construct phylogenetic trees?

A B C D E F G Living speciesFossil species

Tim

e

Ancestral (common) species

A phylogenetic tree

Clocks in Molecules"One of the most startling things that has come to light is that molecular evolution seems to happen at a surprisingly constant rate. If you plot molecular changes in a particular lineage of organisms against time they seem to accumulate these changes at a remarkably constant rate, and that has come to be known as the molecular clock.“ Linda Partridge

This suggests that molecular evolution is constant enough to provide a molecular clock of evolution.

This means that the amount of molecular change between two species measures how long ago they shared a common ancestor.

Figure: the rate of evolution of hemoglobin. Each point on the graph is for a pair of species, or groups of species. From Kimura (1983).

A phylogenetic tree constructed from similarities between cytochrome c molecules in various organisms.

Summary

• Biological evolution - genetic change (microevolution)

- change from one form into another over time (macroevolution)

• Species definitions- morphological, biological

• Reproductive isolation- central to biological species concept

• Allopatric speciation- main mechanism in animals

- long-term geographic isolation is prerequisite

• Diversity and the tree of life - repeated splittings over great expanse of time