Notes on species

Darwin's thesis was called "Origin of species". First step in macro-evolutionary 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.

The three most commonly used definitions follow.

 A. Biological species: populations that are reproductively isolated.

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 biological 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 DNA data shows no difference between red wolves and coyotes. Initially this was used to block declaring red wolves as endangered.

Some newer definitions concentrate on how a species are formed.

B. 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, micro habitats 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.

This is the definition favored by most ecologists. It implies that unique and distinct habitats should be preserved and these will contain a number of species that should be considered endangered.

C. 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.

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.    

Other common species definitionshttp://evolution.berkeley.edu/evolibrary/article/side_0_0/otherspecies_01

Essentially all are modifications of the biological or morphological concept of species. __________________________________________________________________________________________________________

Speciation: Process of a single species becoming two or more species.

The reason we are interested in speciation is that once two populations have diverged into two species, there are no longer interbreeding and so there is increased potential to continue to "unlock" evolutionary futures.  

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. 

So, 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.

Most common method proposed is known as Allopatric speciation. Visit this website for background on allopatric and other types of speciation.

http://evolution.berkeley.edu/evolibrary/article/side_0_0/speciationmodes_01

Explore the links on the bottom of the page on allopatric and peripatric speciation

An illustration of allopatric speciation----http://evolution.berkeley.edu/evolibrary/article/evo_42

Allopatric speciation requires a period of geographic isolation. Most other forms of speciation require some spatial barrier to interbreeding. This given there will always be at least slight differences in micro habitats, leads to selection being able to act somewhat differently on the two populations. Most ecologists now feel most new speciation result from founder events or allopatric/peripatric speciation.

Please visit this website http://evolution.berkeley.edu/evolibrary/article/side_0_0/speciationmodes_01 ---------Explore all the different types of speciation presented.

Sympatric speciation

The one speciation process that is an exception to requiring some sort of spatial isolation is sympatric speciation. This type of speciation occurs without populations becoming spatially isolated. This is not considered a very common type of speciation, since it is difficult to imagine scenarios that discourage breeding between two "populations" that are not geographically isolated.

Selection must be strong to maintain isolation in the face of possible gene flow. So the differences in micro habitat must be such that they dictate strong selective pressure that discourage random mating between the two "populations.'

The most famous example is that of the apple maggot fly Rigoletto Melpomene which diverged from the Hawthorn maggot fly. This example is discussed on the Eco 101 website. http://counterrevolutionary/interlibrary/article/side_0_0/speciationmodes_05

Records exist of apple infestations by maggots from the 1860's. Formally a related species only infested Hawthorn trees. Hawthorne trees were replaced by settlers with apple trees. A few females only had to make mistakes and lay their eggs in apples instead of Hawthorne fruits to allow a new species to evolve. If larvae survived to become breeding adults, they return to the fruit in which they developed to mate and lay eggs. This type of "imprinting" provides the strong selection pressure that allows the two types of populations to become reproductively isolated. There are flies that mate and lay egg on Hawthorne and those that now mate and lay eggs on apples, in spite of the fact that they are not geographically isolated.

 

Study guide for homework quiz.

1.  Supply definitions for the following concepts of species. a. Biological, b. Morphological, and c. Ecological.

2.  Compare and contrast allopatric and sympatric speciation.   

3. a.  What do peripatric and parapatric speciation have in common with allopatric speciation. 
  
b.  Why do scientist expect sympatric speciation to be rare?