lec3paleontology

GEOLOGY 397 - SPECIAL TOPICS: INTRODUCTION TO PALEONTOLOGY
Geology Department - West Virginia University at Parkersburg

LECTURE 3: SPECIES AND SPECIATION

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. However, how we define species is another matter.

THE SPECIES CONCEPT

Typolocial 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 "On 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.
Biological Species Concept: "a species is an array of populations which are acturally or potentially interbreeding, and which are reproductively isolated from other such arays under natural conditions." (Ernst Mayr, 1963, also check out this link: What is a Species, and What is Not? ). 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. (See this link, Species Concepts ).
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 biolgical species. Morphological species are defined soley 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.
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 (see this link, GEOL 3330: THE SPECIES ): "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 morpholoigcally 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 organsims. Perhaps, in most cases, sibling species cannot be differentiated from fossils.
Evolutionary Species Concept: a "lineage evolving separately from others and with its own unitary evolutionary role and tendencies." Simpson, 1961.

HOW DOES BIOLOGIC SPECIATION OCCUR?

Reproductive Isolation: most speciation occurrs 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).. 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 Prinicple:
Clines and the subspecies concept:
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.
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.

Some Problems in Recognizing Species in Paleontology

Anagenesis: phyletic gradualism in the geologic record and the problem of pseudoextinction. 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: Eldredge and 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 , Evolutionary Genetics , Punctuated Equilibrium at Twenty )