Notes on species
Darwin's thesis was called "Origin of species". First step in macroevolutionary divergence.But two central questions remain.
What is a species?
How are species formed?
There is universal recognition that the species is a fundamental natural unit (in other words, a construct of the human mind, the way we naturally think about or fundamentally group natural units).
Often primitive tribes and formal taxonomic distinctions as to local "species" agree. Kalam of New Guinea recognize 174 species, all but 4 agree with those established by taxonomists. But there are over 30 definitions used in the natural sciences.
In introductory courses, Biological species definition is stressed, but there are actually about 30 species definitions used in biology.
A. Biological species: populations that are reproductively isolated. http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_41
This concept unites population genetics with taxonomy because it equates the species with a gene pool. Assume genes in a pool have been selected to interact well.There are several practical problems with applying this definition (fossils, asexuals, etc.).
Unfortunately, the species concept has political overtones
(16) The term "species " includes any subspecies of fish or wildlife or plants, and any distinct population segment of any species or vertebrate fish or wildlife which interbreeds when mature" Sec. 4 : Endangered species act of 1973
Example: Red wolves are not not reproductively isolated from coyotes. Before 1930s there are some data that red wolves were a distinct type. After 1930s, their morphology becomes more more like coyotes (wolves could not find mates?). Mitochondrial data shows no difference between red wolves and coyotes.
a. The species as an ecological unit.
Can assume that the morphology and behavior of organisms are adapted to the resources they use and habitats they live in.
Leads to concept of niche or an organisms "job."
Implies that if species are distinct, microhabitats must be distinct or there must be discrete adaptive zones perhaps even with gaps between zones.
Simplest example is two parasites that occupy two different hosts. These hosts may be active at different times of the day or reproduce at different times in the season. Gut morphology may vary somewhat or the morphology exploited by the parasite. If these two species interbreed, the offspring may be "adapted" with a mixture of traits inherited from both parents for an nonexistent species of host.
b. Morphological (Phenetic and for practical purposes phyletic) species :A species is defined by means of easily identifiable morphological characters. This is the way most biological species are identified in the field. This is the way fossils are divided into species.
One species may change into a different species, based on morphological data. This was part of Darwin's theory. Over time, descendants diverge from their ancestors. When the difference is arbitrarily large enough they are recognized as new species according to some morphological criteria.
Gradual change in one species is not the only type of change that occurs over time. At times species are expected to split and give rise to two or more species, a process called speciation,our next topic.
Take home message: The reason we are interested in speciation is that potential for two populations who have diverged into two species to "unlock" evolutionary futures. The reason we are interested in the process of speciation is that we do recognize species as distinct categories however we struggle to define them. Mutation and other agents are also cause differentiation within the genetic pool of the two populations on which selection acts.
Example: A case in point would be cats and dogs, which shared a common ancestor in the distance past; yet we do not expect a dog and cat to mate and produce offspring, or for that matter, to even live together in harmony.
Although sometimes reversible, we look upon speciation as that first visible sign of divergence that can lead to the isolation of two populations so that they in the future continue to pursue two different evolutionary pathways.
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Speciation: Process of a single species becoming two or more species.
Most common method is known as Allopatric speciation. Visit this website for background on allopatric speciation.
http://evolution.berkeley.edu/evolibrary/article/side_0_0/speciationmodes_01
Allopatric speciation requires a period of geographic isolation.
Recently, biologists have postulated another way speciation can occur.
Sympatric speciation: Speciation occurs without populations becoming geographically isolated.
Example: apple maggot fly Rhagolettis pomenella.
Records exist of apple infestations from the 1860's. Formally this species was only found on Hawthorn trees
Selection must be strong to maintain isolation in the face of gene flow. So the differences in microhabitat must be such that they dictate strong selective pressure. Situations in which this occurs are difficult to visualize.
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Postulated by Eldredge and Gould
Presented in introductory texts as simply a difference of opinion regarding rates of evolution.
However, the significance for how species are formed is more important.
http://evolution.berkeley.edu/evolibrary/article/side_0_0/punctuated_01
Punctuated Equilibrium is characterized by rapid change. The small, isolated population experiences rapid change because of a novel environment and small population size: The environment in the newly formed habitat exerts new selection pressures on the isolated population. Unfortunately, the Berkeley website uses strong selection to indicate that evolution is rapid, actually selection is relaxed in that many new phenotypes that probably would be out competed in the parental population can make it in the new founder population.
Also, their small population size means that genetic drift influences their evolution.
The isolated population undergoes rapid evolutionary change. This is based on the model of allopatric (peripatric) speciation and so requires geographical isolation. It is also essentially a group (species) selection explantation.
