SPECIES AND SPECIATION

VARIATION

How much variation in living organisms and fossils is a result of ontogeny, sexual dimorphism, ecophenotypic variation, deformation of fossils, etc.?  And how much variation can be the result of individuals belonging to a different species?  These are very important questions that biologists and paleontologists must grapple with when identifying, classifying, and determining the evolutionary relationships of organisms.  Biologists and paleontologists must be able to define what a species represents and must understand how speciation occurs.

IMPORTANCE OF SPECIES

The species is the fundamental taxonomic unit of biological entities, it is real.  All other taxonomic categories are arbitrary and constructed by man, but the species has a reality in nature, not just in the minds of scientists.  Organisms of the same species recognize each other as such.  The formation of new species is the basic mechanism of evolutionary change.  However, how we define species is another matter.

• Typological Species Concept: Pre-Darwinian view that God created the ideal type for each species.  Any deviation from the ideal type was viewed as an imperfection from God's blueprint.  Species were fixed and unchanging.
• With Darwin's publication of "The Origin of Species by Means of Natural Selection ........", it became apparent that species were not static and did not fit a "blueprint" type.  Because of the natural variation in populations, changes in species would occur with time because of the continued struggle for existence and the continued selection for the most fit.  Thus variation is the basis for evolution and natural selection works on population variation to generate new species.
• Biological Species Concept: "a species is an array of populations which are actually or potentially interbreeding, and which are reproductively isolated from other such arrays under natural conditions." (Ernst Mayr, 1963).  Or, in other words, a species is a group of naturally interbreeding or potentially interbreeding populations with a common gene pool and reproductively isolated from other species..
• Morphological Species Concept: "a species is a diagnosable cluster of individuals within which there is a pattern of ancestry and descent, and beyond which there is not." (Eldredge and Cracraft, 1980).  Of course this definition implies morphologic similarity, but also evolutionary relationships.  Perhaps the above definition, because of the evolutionary implications, is really that for a biological species.  Morphological species are defined solely by morphological criteria and are also referred to as morphospecies.  The morphospecies concept is most often used by paleontologist to define species, however, paleontologists are really using the morphospecies concept as a way to recognize ancient species, they are not (or perhaps should not be) really defining a species with this concept.  Even biologists, in practice, use the concept of the morphospecies (we can't always actual observe living populations interbreeding).  However, most biologists (and paleontologists should) recognize that the morphospecies concept is not a definition of species, but rather a useful concept in the recognition of different species (both in living organisms and for fossils).
•  Paleontological Species Concept: Alan Shaw (1964) describes a paleontological species in the following manner: ".....objects of organic origin that are of sufficiently distinctive and consistent morphology so that a competent paleontologist could define them so that another competent paleontologist could recognize them."  In practice, of course, this is typically how paleontologists recognize species.  However, perhaps paleontologists should use the biological definition of what a species is and is not.  Certainly, biologists have information about modern species that is not available to the paleontologists, however, paleontologists have information that may help in the recognition of species in the biological sense; such as paleoecologic relationships, biostratigraphic data, paleobiogeographical distributions, and sedimentological information which may help in recognizing fossils as valid biological species.
• The following definition of the paleontological species is offered by Maddocks, 1999 (professor at the University of Houston), "A paleontological species is a group of fossil populations showing similarity and range of variability within, and differing from other such populations, such that the best explanation of these relationships is that in life they were members of a species."  Of course, here she means members of a biological species as biological species has been defined above.
• Some other species concept terms:
• Ecological Species Concept:  no two biological species occupy the same niche, so this concept basically has the same meaning as the biological species concept.
• Asexual Species: difficult situation.  Certainly, the biological species definition doesn't apply well here.  Organisms that reproduce asexually surely are very similar genetically (which would isolate them from other species).  For fossils, often the best thing we can do here is use morphologic traits to recognize different asexual species.
• Sibling Species: Some modern organisms that are very closely related and have very similar morphologic characteristics may not be part of a naturally interbreeding (or potentially interbreeding) population (i.e. they are reproductively isolated), yet it is morphologically difficult to tell them apart.  However, in modern sibling species we can often observe their behavior and reproductive habits and determine that they are separate species.  This is much more difficult for extinct organisms.  Perhaps, in most cases, sibling species cannot be differentiated in the fossil record.
• Evolutionary Species Concept: a "lineage evolving separately from others and with its own unitary evolutionary role and tendencies." Simpson, 1961.

• Reproductive Isolation: most speciation occurs as a result of reproductive isolation. How does this reproductive isolation come about?
• Peripherally isolated populations: If a portion of the population becomes isolated to the fringe of the main population, then the gene pool is smaller and any unusual gene frequencies (say from original variability in the population or from new alleles introduced by mutation) have a higher probability of becoming dominant in the peripheral population (i.e. there is no longer gene flow with the main population).  A novel beneficial mutation in a large gene pool may be hybridized out (or at least may not spread rapidly).  But in a small peripherally isolated population (with a small gene pool), a novel beneficial mutation may soon become dominant in the population by natural selection.  Eventually, the gene pool of the peripheral population may become so different that they are reproductively isolated from the main population, even if other isolating barriers are removed.
• Founder Principle:  A few individuals that become isolated from the main population.  Oceanic Islands - small isolated population with a small gene pool.  These small populations evolve rapidly compared the the larger gene pool on the mainland.  Example: Galapagos Islands compared to mainland South America.
• Clines and the subspecies concept: individuals of the main population that may migrate to the periphery of the main population may not mix with the entire gene pool and may develop into subspecies.  This may eventually result in a gradient in features from one subspecies to another as the population spreads out (migrates) from a region.  These gradients in features are referred to as clines.  Ring species may also develop (see gull example, fig. 3.2, in Prothero).
• Allopatric Speciation Model: Geographically isolated portions of a population.  Over time, these isolated variants become reproductively isolated from the parent population and become new species.  Most speciation is thought to result from allopatric speciation.  This type of speciation is not always due to geographic barriers (body of water, mountains, desert, etc.).  Climatic differences can also result in geographically isolated populations.  For example, in cold water a certain species may grow larger, thus increasing volume at a faster rate than surface area (higher SA/V ratio) to retain more heat (for vertebrates this might mean having a larger body, with shorter, stubbier ears and limbs), whereas in hotter climates the species may evolve toward being smaller to have a smaller SA/V ratio (for vertebrates this might mean having a smaller body, with longer, slimmer ears and limbs for dissipating heat more quickly).  Eventually such climatic difference of a once continuous population may result in reproductive isolation.
• Sympatric Speciation: Any factor which causes reproductive isolation results in speciation.  Speciation is not always a result of sharp geographic isolation.  Note: sympatric means living together in the same area, allopatric means living in different areas.

Paleontological species are morphological species (morphospecies).  Biologist have living organisms to work with, but they lack the time factor.  Paleontologists can see how populations have changed over time.

• Anagenesis: phyletic gradualism in the geologic record and the problem of pseudoextinction.  If we find fossil populations that gradually change from one to the other (anagenesis or phyletic gradualism), how do we decide where to divide them into two distinct species (or should we?).  In practice, this is usually not a problem because of the incompleteness of the fossil record (gaps in the rock record).  But some continuous uninterrupted transformations exist and it is rather arbitrary where we split species in such a case.
• Biostratigraphers like to split often - more zones they can use for correlation.
• But when we arbitrarily split a continuum to create species we are arbitrarily causing extinction of one species (the parent species becomes extinct).  This is pseudoextinction, which is not the same as true extinction of a lineage.
• Some argue that anagenetic lineages should not be subdivided into species (evolutionary species concept of Simpson, 1961).  Simpson and others would say that species should only be recognized by branching events.  Each branch is a different species.
• Problems with this too though, end members of anagenetic sequences may be very different and most paleontologists would call them different species.
• Should paleontologists use the Evolutionary Species Concept of Simpson (1961) to avoid problems with anagenesis.  Does each branch of a family tree represent a new species?  Problems here too!!!
• Punctuated Equilibria: Niles Eldredge and Stephen J. Gould (1972) first formally proposed this idea.  This concept basically says that species do not change much (stasis, i.e. in stable equilibrium) over long periods of time, but that speciation occurs rapidly (i.e. stasis is punctuated by the sudden introduction of new species, either due to migration of peripherally isolated variants back into a particular area or perhaps by environmental stress that results in rapid change in the gene pool, and thus speciation).  The Punctuated Equilibria Model appears to be the best explanation of what we observe in the fossil record, however, there are examples of phyletic gradualism.  (See these links:  Punctuated Equilibria http://www.talkorigins.org/faqs/punc-eq.html#pe-vs-pg, Evolutionary Genetics http://www.zoology.ubc.ca/~bio336/Bio336/Lectures/Lecture23/Overheads.html)
• Should we expect to see gradual evolutionary change?  Most modern allopatric speciation occurs rapidly (geological speaking) in peripherally isolated populations.
• Based on what we see in living organisms, shouldn't we see fossil populations that are not changing for long periods of time, punctuated by sudden appearance of new species in the rock record.
• Thus Punctuated Equilibrium.
• The Eldredge and Gould (1972) paper generated much controversy concerning the nature of speciation and the controversy has not subsided much today.
• But most paleontologists see the punctuated pattern in the rock record.
• So most fossil species can "be defined by branching speciation events."(Prothero, 1998).