Speciation and Macroevolution

Chapter 17

Microevolution

Changes in allele frequencies within a population's gene pool.

Macroevolution

Evolution on a large scale that results in the formation of one or more new species.

Speciation

The creation of one species from a previous species.

Species concept

A definition or method used to determine what makes organisms members of the same species.

Morphologic Species Concept

Defines species based on physical characteristics or diagnostic features that distinguish one species from another.

Advantage of the Morphologic Species Concept

Can be useful for organisms that can be easily observed and compared by their physical characteristics.

Disadvantage of the Morphologic Species Concept

Not as useful for microbial organisms or extinct organisms.

Cryptic species

Species that may look very similar but are actually different species.

Evolutionary Species Concept

Defines a species based on diagnostic features of organisms that share an evolutionary lineage.

Advantage of the Evolutionary Species Concept

Can help identify possible speciation in extinct lineages.

Disadvantage of the Evolutionary Species Concept

Fossil evidence is often incomplete, especially because soft tissues are usually not preserved, so conclusions can be tentative.

Phylogenetic Species Concept

Defines species based on their relationship to a shared common ancestor.

Evidence used by the Phylogenetic Species Concept

Morphological, behavioral, and genetic evidence.

Biologic Species Concept

Defines species based on the ability of organisms to interbreed and produce fertile offspring in nature.

Reproductive isolation

When organisms do not reproduce because of a physiological, behavioral, or genetic process.

Advantage of the Biologic Species Concept

Helps determine whether sexually reproducing populations are reproductively isolated.

Disadvantage of the Biologic Species Concept

Only applies to organisms that reproduce sexually and is not useful for fossils.

Prezygotic isolating mechanism

A mechanism that prevents mating attempts or prevents successful fertilization before a zygote forms.

Postzygotic isolating mechanism

A mechanism that prevents hybrid offspring from developing normally or reproducing.

Habitat isolation

A prezygotic barrier in which species live in different habitats and therefore do not normally meet to mate.

Temporal isolation

A prezygotic barrier in which species reproduce at different times.

Behavioral isolation

A prezygotic barrier in which species have different courtship behaviors or patterns.

Mechanical isolation

A prezygotic barrier in which reproductive structures are incompatible.

Gamete isolation

A prezygotic barrier in which gametes meet but cannot fuse to form a zygote.

Hybrid inviability

A postzygotic barrier in which a hybrid zygote is not viable and may die.

Hybrid sterility

A postzygotic barrier in which a hybrid develops into an adult but cannot reproduce.

Example of hybrid sterility

A mule is a horse-donkey hybrid that is sterile.

Five prezygotic barriers

Habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gamete isolation.

Two postzygotic barriers

Hybrid inviability and hybrid sterility.

Allopatric speciation

The formation of new species caused by geographic isolation.

Geographic isolation

Physical separation of populations by a geographic barrier.

Example of allopatric speciation

Ensatina salamanders in California are separated by the Central Valley.

Sympatric speciation

The formation of new species without geographic isolation.

Polyploidy

A condition in which an organism has more than two sets of chromosomes.

Autoploidy

A form of sympatric speciation in which a new polyploid species comes from one diploid species.

Alloploidy

A form of sympatric speciation in which a new polyploid species comes from two different species.

Nondisjunction

The failure of chromosomes to separate properly, which can produce gametes with abnormal chromosome numbers.

Allopatric vs. sympatric speciation

Allopatric speciation occurs because populations are geographically separated; sympatric speciation occurs without geographic separation.

Convergent evolution

When organisms experience similar environmental pressures and independently evolve similar forms or traits.

Example of convergent evolution

Cichlid fish in different African lakes experience similar pressures and evolve into similar forms.

Coevolution

When two organisms influence each other's evolution and adapt in response to each other.

Example of coevolution

Flowering plants and their pollinators can adapt over time in ways that benefit both organisms.

Convergent evolution vs. coevolution

Convergent evolution results from similar environmental pressures; coevolution results from interactions between organisms.

Gradualistic equilibrium model

The model in which reproductively isolated groups slowly adapt in their own ways until they become different species.

Gradualistic model

Species change gradually over a long period of time, producing many transitional forms.

Punctuated equilibrium model

The model in which environmental changes can cause rapid evolutionary change and fast speciation with few transitional forms.

Gradualistic vs. punctuated equilibrium

Gradualistic evolution involves slow, continuous change; punctuated equilibrium involves relatively rapid changes separated by periods of little change.

Macroevolution and fossils

Macroevolution is best observed in the fossil record because it involves large-scale evolutionary changes and the formation of new species.

Is macroevolution goal-oriented?

No. Macroevolution is not goal-oriented; evolution is reactive to environmental pressures.