Bio II Ch 24- The Origin of Species

Speciation

-The process by which one species splits into two or more species, is at the focal point of evolutionary theory

-Forms a conceptual bridge between microevolution and macroevolution

Microevolution

Consists of changes in allele frequency in a population over time

Macroevolution

Refers to broad patterns of evolutionary change above the species level

The Biological Species Concept

States that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with members of other such groups.

Reproductive isolation

The existence of biological factors (barriers) that impede two species from producing viable, fertile offspring.

Hybrids

The offspring that result from mating between different species.

EX: A mule; sterile because it is hybrid, isn’t a species

Gametic Isolation

Sperm of one species may not be able to fertilize eggs of another species

Postzygotic barriers

Prevent the hybrid zygote from developing into a viable, fertile adult by:

1. Reduced hybrid viability

2. Reduced hybrid fertility

3. Hybrid breakdown

Reduced hybrid viability

Genes of different parent species may interact in ways that impair the hybrid’s development or survival in its environment

Reduced hybrid fertility

Meiosis may fail to produce normal gametes, resulting in sterility, if the parent species have chromosomes or different number or structure.

Hybrid breakdown

First-generation hybrids are viable and fertile, but offspring in the next generation are feeble or sterile

Morphological species concept

Defines a species by structural features. Applies to sexual and asexual species but relies on subjective criteria.

Ecological species concept

Defines a species in terms of its ecological niche (EX: occupation/role). Applies to sexual and asexual species and emphasizes the role of disruptive selection.

Two ways Speciation can occur

1. Allopatric

2. Sympatric

Allopatric speciation

groups become reproductively isolated and diverge due to a geographical barrier.

groups become reproductively isolated and diverge due to a geographical barrier. EX: Galapagos Islands

Sympatric speciation

-Speciation occurs in populations that live in the same geographic area.

-Can occur if gene flow is reduced by factors including:

EX: Polyploidy (extra sets of chromosomes), sexual selection, habitual differentiation.

POLYPLOIDY

Presence of extra sets of chromosomes due to accidents during cell division

-Much more common in plants than animals

-Can produce new biological species in sympatry within a single generation

Autopolyploid

An individual with more than two chromosome sets derived from a single species

The offspring resulting from mating between polyploids and diploids have reduced fertility

EX: Cotton, bananas, potatoes

Allopolyploid

Species with multiple sets of chromosomes derived from a different species

Can successfully mate with each other, but cannot interbreed with either parent species

EX: Mules, rapeseed

Hybrid zone**** (STUDY CH 24 34-40)

A region in which members of different species mate and produce hybrids

EX: Two species of toad interbreed in a long narrow hybrid zone

Three possible outcomes for hybrid zones over time:

1. Reinforcement

2. Fusion

3. Stability

Reinforcement (hybrid zones)****

-When hybrids are less fit than parent species, this action of reproductive barriers may occur through strong selection for prezygotic barriers.

-Over time, the rate of hybridization decreases.

-Where this action occurs, reproductive barriers should be stronger for sympatric than allopatric species

EX: Female flycatchers recognize and select males of their own species when choosing males from sympatric populations. They make mistakes when selecting males from the more similar allopatric populations.

Fusion (hybrid zones) *****

If hybrids are as fit as parents, there can be substantial gene flow between species.

If gene flow is great enough, reproductive barriers weaken and the parent species can fuse into a single species. EX: Pollution in Lake Victoria has reduced the ability of female fish to distinguish males of a different species from males of their own.

Stability (hybrid zone) ****

Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone

EX: Parent species of Bombina routinely migrate into the narrow hybrid zone, resulting in ongoing hybridization.

The Time Course of Speciation ***

The rate of speciation can be studied using the fossil record, morphological data, or molecular data!!

Punctuated equilibria

- The fossil record includes examples of species that appear suddenly, persist unchanged, then disappear.

• Describes the periods of apparent stasis punctuated by sudden change.