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Species and speciation Updated: |
Topics for February 18
Species and Speciation
Phylogeny and classification
What is a species?
Individuals which belong to the same species are "similar" (but what about sexual dimorphism? conspicuous phenotypic differences?, ...)
A biological species is defined as a population or group of populations whose members have the potential to interbreed and produce fertile offspring.
A species is...
A group of individuals which interbreed and therefore represent a common gene pool.
If there are reproductive barriers that prevent (permanently) two populations from interbreeding, they belong to separate species.
As a practical matter, other criteria are often used. Behavior, appearance, location, etc.
An aside about spelling
The singular of species is....
Species
The plural of species is...
Species
Similar species are grouped together as a genus (singular). The plural is genera: two or more genera.
Speciation: the division of a species into two or more species.
A variety of mechanisms have been discovered which can cause speciation--the division of one species (ancestral) into two or more species (descendant). (see fig 15.3)
The key is reproductive isolation. Mechanisms introduce barriers to reproduction. The barriers may be increased by selection, or erased by interbreeding. Time will tell which.
Significance of reproductive barriers
The significance of reproductive barriers is that they maintain genetic isolation between two populations. If such barriers are complete, the populations represent distinct species.
Barriers may arise by a variety of different means. Example: geographic isolation followed by drift, mutation, or selection until reproductive isolation is complete.
The process of speciation
Many different mechanisms have been studied.
Two examples:
*Allopatric speciation--speciation based on geographic separation,
*Polyploidy--speciation based on a chromosome mechanism.
Allopatric speciation
Geographic isolation is one of the mechanisms which can bring about reproductive isolation.
Allopatric speciation means: speciation which follows (over time) after geographic isolation. The initial barrier to reproduction is physical separation. Given sufficient time (many generations) sufficient differences may accumulate to make separation permanent.
Example of allopatric speciation
Blue-headed wrasse (Caribbean) and the rainbow wrasse (Pacific) are closely similar. Their ancestral common population was split by the growth of the Isthmus of Panama about 5 million years ago. (In this case, the separation is known from geological data.)
Since this allopatric separation occurred, the two species have changed independently.
Logical consequences of geographic separation.
After separation, the isolated groups go their separate evolutionary pathways.
Recently separated species share most traits.
New traits arise independently by drift or by selection.
After a very long time, separated groups become less similar, but may retain some features in common.
Polyploidy: chromosomal changes that cause speciation.
On rare occasions, plants produce a hybrid. Typically, such hybrids are sterile, and are not capable of sexual reproduction.
Occasionally, such hybrids nevertheless produce flowers with pollen, etc.
The gametes of such plants commonly have a chromosome number that is the sum of the chromosomes in the original plants that hybridized in the first place.
Example of polyploidy: wheat.
Modern wheat is the descendent of two hybridizations that occurred in the past.
The first hybridization, between Einkorn wheat and a wild wheat produced Emmer wheat.
The second hybridization, between Emmer wheat and a second wild wheat produced modern bread wheat.
The study of wheat and its history.
The karyotype of modern bread wheat reveals its ancestry.
Similar hybrids have been reproduced experimentally to test this hypothesis.
In contrast to allopatric speciation, which occurs gradually, speciation by polyploidy is abrupt. Although it occurs rarely, many modern species of plants are polyploids.
Speciation: a dynamic process
Speciation is a dynamic process--it is taking place in many places in many populations, but it is being reversed in many places by interbreeding.
We should expect to see: populations with the potential to diverge (e.g. Snail p238), populations which have diverged horses and donkeys), populations which might be in the process (deermice).
Reproductive barriers--many types. (see p241).
Barriers to reproduction may prevent any mating: behavioral (courtship, etc.); habitat (populations live in different habitats, as with horses and zebras), etc. Such barriers are prezygotic barriers. No fertilization.
Barriers to reproduction may prevent subsequent reproductive success: sterility (hybrids die or are infertile), etc. Such barriers are postzygotic barriers.
Phylogeny: The evolutionary history of a group of species.
The classification of living species are based on evidence of common ancestry. The evidence is shared inherited traits.
Important caveat: similarities based on common ancestry (homologies) must be separated from similarities based on evolutionary convergence (analogies).
Similarities due to evolutionary convergence: analogy
Similar traits which have a separate evolutionary origin are called analogies.
Example: streamline form of shark, penguin, and porpoise (p255).
Homologies: traits with a common origin.
Homologies exist because of derivation from a common ancestor.
Example (p254): The array of bones in the forelimb of quadrapeds: humerus, radius, ulna, carpels, metacarpels, phalanges. (Notice that in this example the homologous structures no longer serve precisely the same function.)
How can we distinguish analogy from homology?
It’s not always easy, and some classifications have been undone by more complete information.
Clues: Development: homologies follow a common developmental pathway. Relationship among traits: the relationship among the bones of the forelimb remain the same. Neutral features: non-adaptive features are more reliable clues!
The basic rules for constructing phylogenies: shared traits.
Individual taxa (species, genera, families, etc.) are assigned to:
The same group if they share a homologous trait,
To separate groups if the traits are not shared.
Groups: large and small
Members of a large group may share an ancestral trait: e.g. mammals, reptiles, fish, birds share a conspicuous feature (vertebral column).
A smaller group is identified by a derived trait not shared by the large group. e.g. mammals are separated from other vertebrates based on milk for their young.
Ancestral traits and derived traits.
Ancestral traits are shared throughout the larger group.
Derived traits are present only in a smaller group. The smaller group is defined and identified by having the derived trait. The derived trait is a feature which was present in the ancestor of the members of the smaller group.
Construction of phylogenetic trees.
A phylogenetic tree is constructed based on the patterns of ancestral and derived traits.
The various branches are based on having or not having a particular trait or group of traits. (Derived traits are most useful!)
Derived traits are evidence of a shared evolutionary heritage.
The logic of using ancestral or derived traits for classification.
Ancestral traits already existed in the ancestral group. Such traits indicate affinity with a larger taxonomic unit, but don’t identify a species as part of a smaller group. Example: mammals are all vertebrates, along with many other species.
Derived traits are unique to a group, and identify a species as belonging to the smaller taxonomic unit. Only mammals nurse their young.
Newer data and newer methods reinforce many past decisions.
Molecular biology has introduced many new techniques.
Classification based on prior information (fossils, morphology, behavior, etc.) can be re-examined with molecular data.
Example: Cytochrome C data on page 256.
Molecular methods can help resolve old controversies.
Example: Are Pandas bears? Or Raccoons?
Yes. (see p257)
Molecular data indicates that Red Pandas are more closely related to raccoons.
Molecular data indicates that Giant Pandas are more closely related to other bears.
The similarities (between the two pandas) are analogies, due to natural selection.
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