Chapter 25PHYLOGENY AND SYSTEMATICS
Phylogeny is the evolutionary history of a species or group of species.
- It includes the description and explanation of the sequence in time of the morphological, ecological and biogeographical changes of the taxon.
- And the theoretical explanations of the origin of the taxon.
THE FOSSIL RECORD AND GEOLOGIC TIME
Fossils are the preserved remnant of organisms. Fossils include impressions and casts.
Fossils are found embedded in layers, or strata, of sedimentary rock.
Sedimentary: hardened eroded material due to pressure or chemical means; about 8% of the crust, e. g. limestone, sandstone, and shale.
Sedimentary rocks form from layers of minerals that settle out of water, or dust, sand, silt and other eroded material that are carried to ponds and lakes and the particles settle to the bottom.
Dead organisms also settle at the bottom and are covered with these particles.
Hard parts like shells and bones preserve well; sometimes dissolved minerals penetrate the tissues and replace the organic material petrifying the organism.
Tracks and impressions are preserved when the soft sediment hardens.
Sometimes the entire organism including the soft parts are preserved when the body falls in a place where bacteria and fungi cannot decompose the body, e. g. insects trapped in resin, mammoths frozen in the Siberian permafrost.
Paleontology is the study of fossils.
Paleontologists use several methods to date fossils.
Trapping the organism in sediments freezes the fossil in time. The fossils in a stratum are a sample of the organisms that lived and died at the time of the sedimentation.
The geologic time scale is the chronology of the Earth's history.
The earth's history is divided into eons, eras, periods and epochs.
These divisions correspond to transitions in the composition of the fossil record.
Climatic changes are also deduced from the composition of the rock layers.
There are two eons:
Precambrian, 4500 m.y.a. to 570 m.y.a.
Phanerozoic, 570 m.y.a. to the present.
There are four eras: Precambrian (beginning - 543 mya), Paleozoic (543-245 mya), Mesozoic (245-65 mya) and Cenozoic (65 mya-present).
The absolute ages of fossils in years can be determined by radiometric dating. Radiometric dating measures the fixed rate of decay of the radioactive isotope.
Half-life is the time it takes for 50% of the original sample to decay.
- Carbon-14 has a half-life of 5730 years; uranium-238 has a half-life of 4.5 million years.
The fossil record is an incomplete chronicle of the evolutionary history of organisms. It favors organisms with parts easily fossilized.
THE THEORY OF CONTINENTAL DRIFT
Continental drift is the process by which tectonic plates on the Earth's surface move closer or apart relative to each other.
Continental drift has had a great impact in the distribution of species over the earth (biogeography) and evolution.
The movement of continental plates is called plate tectonics.
Continental drift is an on-going process that has changed the configuration of the continents since the formation of the landmasses.
This hypothesis first proposed in 1912 by Alfred Wegener, a German Meteorologist, was not widely accepted until after 1960.
Wegener's evidence for continental drift includes:
(1) Continental fit.
(2) Glacial deposits indicating high latitudes on portions of the continents that are now in warm latitudes,
(3) Alignment of geologic features such as mountain belts when continents are reassembled, and
(4) Occurrence of identical fossils, of the same age, on landmasses that are no longer connected.
Wegener proposed that landmasses were joined at one time into a huge supercontinent called Pangaea.
Pangaea broke up about 180 million years ago with the plates carrying the different continents with the species they had.
Continental drift subjected landmasses to …
- New climatic conditions.
- New geographical connections.
This caused the separation of groups that were related and brought into competition groups that were previously separated and had evolved their unique adaptations.
The results were increase complexity and extinction.
There have been several mass extinctions.
Evolutionary history has been characterized by long, relatively stable periods interrupted by intervals of extensive species turnover.
Mass extinctions have been followed by periods of extensive adaptive radiation.
SYSTEMATICS
Taxonomy is the branch of biology that identifies, name and describes organisms.
Systematics is the study of the diversity and evolutionary relationship of organisms.
Nomenclature is the system of use to name different taxa.
Classification refers to the establishing, defining and ranking of taxa in a hierarchical series of groups.
BINOMIAL SYSTEM
Carolus Linnaeus established the binomial system of nomenclature in the mid-18th. Century.
- Each species has a two-part name: the genus name and the species epithet.
The species is the basic unit of classification.
Populations of the same species show distinct characteristics that distinguish them form other populations.
These geographical variants may be classified as subspecies.
Plant subspecies may be referred as varieties, and bacteria subspecies may be referred as strains.
The hierarchical system is divided into kingdom, phylum, class, order, family, genus and species.
- Plural of species is species and of genus is genera.
A taxon is formal grouping of organisms at any level including all the subordinate groups.
The hierarchical system is based on groups of groups, e.g. several species forma genus; several genera form a family; several families form an order, etc.
Originally there were two kingdoms: Animalia and Plantae.
Over the years other kingdoms were recognized: Monera (or Prokaryotae), Fungi, Protista (or Protoctista).
Most biologists recognize six kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae and Animalia.
Domain is a new taxon above the kingdom based on fundamental molecular differences, e.g. gene sequencing.
There are three domains recognized by many biologists: Archaea, Eubacteria and Eukarya.
Phylogenetic trees reflect the hierarchical classification of the taxonomic groups.
Cladistics is one method of analyzing the species evolutionary relationships.
Cladistics begins with the premise that al taxa are monophyletic.
- Each taxon or clade consists of a common ancestor and all its descendants.
- It is based only on shared derived characters.
- Results are presented in a diagram called cladogram.
It is a tree constructed from a series of dichotomies.
The branch point represents the divergence of two species.
The branching is inferred by analyzing homologies and identifying shared characters.
Monophyletic groups include all the descendants of the most recent common ancestor.
Organisms in a polyphyletic group evolved from different ancestors.
Paraphyletic group include some but not all the organisms descended from a common ancestor.
Systematists consider structural, physiological, behavioral and molecular traits in the evaluation of similarities between two species.
Molecular data
Species-to-species comparisons of the amino acid sequences in proteins and the base sequences in nucleic acids reveal phylogenetic relationships.
The principle of parsimony:
- Among phylogenetic hypotheses, the most parsimonious tree is the one that requires the fewest evolutionary changes.
The best hypothesis is the one that incorporates the greatest amount of data, e. g. anatomical, molecular, etc.
Phylogenetic trees are hypothetical.
The base sequence of some regions of DNA change at a rate consistent enough to serve as clocks to date episodes in past evolution.
Molecular clocks.
Molecular clock is a new method of timing the development of traits and species, provided by molecular biology.
Molecular clocks may keep tack of evolutionary time.
- Homologous DNA sequences or their protein products are compared for taxa that are known to have diverged from common ancestors during certain periods in the past.
- The number of nucleotides and amino acid substitutions is proportional to the time that has elapsed since the lineages branched.
- See example of tuna and sharks when compared to bats and dolphins, page 503.
The base sequence of some regions of DNA change at a rate consistent enough to serve as clocks to date episodes in past evolution.
No gene marks time accurately.
Some sections of the genome evolve in fits and starts that are not at all clockwise.
Genes that make good molecular clocks are accurate only in a statistical sense of a fairly smooth average rate of change.
Many biologists are skeptical about the use of molecular to record evolutionary events in time.