CH 24: Origin of Species and Macroevolution

BIO 1134

Species

• are the real and objective items of nature’s morphology

• are many ways to define a species in evolutionary biology

• the majority of species concepts describe species in terms of reproductive isolation, molecular features, ecological factors, and evolutionary relationships

Stephen Jay Gould

(1992)

• emphasizes that the species is the only objective, real category of the Linnaean hierarchy (Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species)

• paleontologist and evolutionary biologist

• Harvard professor

Species concept

is a way of defining the concept of a species and/or of providing an approach to distinguish one species from another

Type of species concepts

• Biological species concepts

• Evolutionary lineage concepts

• Ecological species concepts

• General lineage concept

Biological species concept

individuals of the same species can interbreed and produce viable, fertile offspring, but cannot successfully interbreed with members of other species

Evolutionary lineage concept

series of species in a line of descent where each new species evolved from the immediate, previous ancestor

Ecological species concept

each species occupies an ecological niche

General lineage concept

each species is a population of an independently evolving lineage

Speciation

can occur when some event separates a population by time, space, or genetics. population then diverge along their own evolutionary path (either gradually or rapidly)

• when one ancestral species evolves into one or more new species

Time effect on speciation

• In the gradual model a species acquires small adaptions to its environment over millions of years

  • Ex: horses

• in the punctuated equilibrium model, there are periods of stasis interrupted by bursts of speciation

  • Ex: during the cambrian explosion period the rate of evolution was an order higher

Space effect on speciation

• allopatric speciation arises by geographic isolation of a few members of the original population or species. genetic drift is more pronounced, and natural selection takes the isolate in a different direction.

• Ex: a group of squirrels travel together and then get separated on opposite ends of a canyon leading to isolation between members

Genetic effect on speciation

• Sympatric speciation most often arises by instantaneous speciation through polyploidy

  • drastic changes in chromosome number

  • chromosomal rearrangements

  • disruptive selection

  • Ex: new species arises in midst of old species with interbreeding of different wheats to make a hybrid and turn to modern day wheat

Hybrid zone

are areas where separated populations interbreed

• Ex: different bird populations overlap on a map

How do we declare a population a new, or different species? (Species in the language of the Biological Species Concepts)

Biological Species Concepts

• proposed by Ernst Mayr in 1942

• a biological species is a group of interbreeding natural populations that are reproductively isolated from other such groups

Reproductive barriers

maintain species

• prezygotic isolating mechanisms

• postzygotic isolating mechanisms

Prezygotic isolating mechanisms (preventing)

Maintain biological species by preventing mating attempts

• Behavioral isolation

  • members of a species often identify each other through specific rituals (Ex: peacocks)

• Temporal isolation

  • many species reproduce only at specific times (Ex: frogs)

• Habitat isolation

  • if species live in slightly different habitats they may never meet (Ex: desert)

Prezygotic isolating mechanisms (attempting)

Maintain biological species after mating attempt before fertilization (mating attempted)

• Mechanical isolation

  • members of different species often cannot mate because their anatomies are incompatible (Ex: wasp and bee)

• Gametic isolation

  • the gametes (sperm and egg) of different species usually cannot fertilize each other

Postzygotic isolating mechanisms (fertilization)

Maintain biological species after fertilization

• Hybrid inviability

  • the fertilized egg fails to develop

• Hybrid sterility

  • an interspecies hybrid survives, but it is sterile

• Hybrid breakdown

  • the F1 interspecies hybrid is viable and fertile but succeeding generations (F2 and so on) becoming increasing inviable

What does the Biological Species Concept apply to?

• sexually reproducing species

• living (not extinct) species

• species with gene flow (immigration) between populations

• Look at slide show for ex

What does the Biological Species Concept not apply to?

• species that reproduce asexually

• extinct organisms (fossils only)

• species with limited gene flow between populations