Biology II, Chapter 24

Sympatric Speciation

"Same Country." Speciation takes place in geographically overlapping populations. Can occur if gene flow is reduced by factors including polyploidy, sexual selection, habitat differentiation.

Species in Latin

"kind" or "appearance"

Where reinforcement occurs...

...Reproductive barriers should be stronger for sympatric than allopatric species. For example, in populations of flycatchers, males are more similar in allopatric populations than sympatric populations.

Evidence of Allopatric Speciation: Reproductive barriers can develop...

...When laboratory populations are experimentally isolated and subjected to different environmental conditions. Can also occur in nature. Isthmus of Panama.

Speciation can occur rapidly or slowly...

...and can result from changes in few or many genes. Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species.

Evidence of Allopatric Speciation: Reproductive isolation between populations generally increases...

...as the distance between them increases. Reproductive barriers are intrinsic to the organisms themselves; physical separation alone is not a biological barrier

The biological species concept emphasizes...

...reproductive isolation

Regions with many geographic barriers...

...typically have more species than do regions with fewer barriers.

Speciation can take place with or without geographic separation:

Allopatric speciation and sympatric speciation

Autopolyploidy

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

The time course of speciation

Broad patterns in speciation can be studied using fossil record, morphological data, or molecular data.

Process of Allopatric speciation: reproductive isolation

Can arise as a by-product of genetic divergence. For example, isolated populations of mosquitofish have evolved reproductive isolation as a result of selection under different levels of predation.

Sexual selection

Can drive sympatric speciation. ___ ___ for mates of different colors has likely contributed to speciation of cichlid fish in Lake Victoria.

Patterns within Hybrid zones

Can occur in a single band where adjacent species meet (two species of toad) or can be more complex if parent species are found in patches within the same region.

Microevolution

Consists of changes in allele frequency in a population over time

Stability

Continued formation of hybrid individuals. Extensive gene flow from outside the hybrid zone can overwhelm selection for reproductive isolation inside the hybrid zone. For example, parent species of Bombina routinely migrate into the narrow hybrid zone resulting in ongoing hybridization.

Prezygotic barrier: Behavioral isolation

Courtship rituals and other behaviors unique to a species are effective barriers to mating

Phylogenetic Species Concept

Defines a species as the smallest group of individuals on a phylogenetic tree. It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species.

Morphological Species Concept

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

Genetics of speciation: how many genes change when a new species forms?

Depending on the species in question, speciation might require change in a single gene or many genes. For example in Japanese snails, the direction of shell spiral affects mating and is controlled by a single gene. Flower color influences pollinator preference. Pollination that is dominated by either hummingbirds or bees can lead to reproductive isolation. Speciation can be influence by larger numbers of gene and gene interactions.

Process of Allopatric speciation: definition of a barrier

Depends on the ability of a population to disperse. For example, a canyon may create a barrier for small rodents, but not birds, coyotes or pollen

Postzygotic barrier: Reduced hybrid fertility

Even if hybrids are vigorous, they may be sterile

At least five new plant species have originated by polyploidy speciation

For example, in the genus Tragopogon, two allopolyploid species have evolved from three diploid parent species.

Postzygotic barrier: Reduced hybrid viability

Genes of the different parent species may interact and impair the hybrid's development or survival in its environment

Summary of Allopatric Speciation

Geographic isolation restricts gene flow between populations. Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations. Even if contact is restored between populations, interbreeding is prevented

Time course: patterns in fossil records

Includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear. Punctuated equilibria describes periods of apparent stasis punctuated by sudden change

From speciation to macroevolution

Macroevolution is the cumulative effect of many speciation and extinction events.

Prezygotic barrier: Mechanical isolation

Morphological differences can prevent successful completion of mating

Biologists compare _____ when grouping organisms

Morphology, physiology, biochemistry, and DNA sequences

Evolutionary theory

Must explain how new species originate and how populations evolve

Hybrids

Offspring of crosses between different species

Other definitions of species

Other concepts emphasize the unity within a species rather than the separateness of a different species

Oats, Cotton, Potatoes, Tobacco, Wheat

Polyploid species

Polyploidy

Presence of extra sets of chromosomes due to accidents during cell division. More common in plants than animals. Can produce new biological species in sympatry with a single generation

Niles Eldredge and Stephen Jay Gould

Punctuated equilibria. Model contrasts with a model of gradual change in a species over time.

Interval between speciation events

Range from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years.

Hybrids often have (increased or reduced) fitness compared to parent species.

Reduced

Macroevolution

Refers to broad patterns of evolutionary change above the species level

Hybrid zones

Region in which members of different species mate and produce hybrids. Hybrids are the result of mating between species with incomplete reproductive barriers. ___ ___ reveal factors that cause reproductive isolation.

Summary of Sympatric Speciation

Reproductive barrier isolates ta subset of a population without geographic separation from the parent species. Can result from polyploidy, natural selection, or sexual selection.

Postzygotic barrier: Hybrid breakdown

Some first generation hybrids are fetile, but when they mate with each other or with either parent species, offspring of the next generation are feeble or sterile.

Prezygotic barrier: Temporal isolation

Species that breed at different times of the day, different seasons, or different years cannot mix their gametes.

Allopolyploidy

Species with multiple sets of chromosomes derived from different species

Prezygotic barrier: Gametic isolation

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

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 populations. Gene flow between populations holds a species together genetically.

Reinforcement

Strengthening reproductive barriers. When hybrids are less fit than parent species, ____ of reproductive barriers may occur through strong selection for prezygotic barriers. Rate of hybridization decreases.

Habitat Differentiation

Sympatric speciation can also result from the appearance of new ecological niches. For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees

Limitations of the Biological Species Concept

The biological species concept cannot be applied to fossils or asexual organisms (including ALL prokaryotes). Emphasizes absence of gene flow. However, gene flow can occur between distinct species, for example: grizzly bear + polar bear = grolar bears

Reproductive isolation

The existence of biological factors (barriers) that impede two species from producing viable, fertile offspring. Can be classified by whether factors act before or after fertilization.

Speciation

The origin of new species. Focal point of evolutionary theory

Time course: Speciation rates

The punctuated patter in the fossil record and evidence from lab studies suggest that speciation can be rapid. For example, sunflower originated from the hybridization of two other sunflower species.

Process of Allopatric speciation: separate populations may evolve independently

Through mutation, natural selection, and genetic drift

Prezygotic barrier: Habitat isolation

Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers.

Ecological Species Concept

Views a species in terms of its ecological niche. It applies to sexual and asexual species and emphasizes the role of disruptive selection.

Fusion

Weakening reproductive barriers. 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. For example, pollution has reduced ability of female cichlids to distinguish males of another species.

Hybrid Zones over time

When closely related species meet in a hybrid zone, there are three possible outcomes: reinforcement, fusion, or stability

Prezygotic barriers

block fertilization from occurring by impeding different species from attempting to mate, preventing the successful completion of mating, hindering fertilization if mating is successful.

Allopatric Speciation

"Other Country." Gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations. For example, flightless cormorant of the Galapagos likely originated from a flying species on the mainland

Postzygotic barriers

prevent the hybrid zygote from developing into a viable, fertile adult; reduced hybrid viability, reduced hybrid fertility, hybrid breakdown