nis - biology

NIS - BIOLOGY

Lecture 81 – Lecture 82 – Lecture 83Modern Classification

Ozgur Unal

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Determining Species

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Do these two organisms belong to the same species?

How about these?

Determining Species

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It isn’t always easy to define species.There are different definitions of species.As knowledge increases, definitions change.Organisms that are different species by one definition might be the same species by another definition.Typological species conceptBiological species conceptPhylogenetic species concept

Determining Species

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Typological species concept:Aristotle and Linnaeus thought of each species as a distinct different group of organisms based on physical similarities typological species conceptIt is based on the idea that species are unchanging, distinct and natural types.The type specimen was an individual of the species that best displayed the characteristics of that species.When another specimen was found that varied significantly from the type specimen, it was classified as a different species.Check out Figure 17.6!!

Determining Species

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Biological species concept:Two evolutionary biologists, Theodosius Dobzhansky and Ernst Mayr, redefined the term species in the 1930s and 1940s.Species a groups of organisms that is able to interbreed and produce fertile offsprings in a natural setting.Examples??There are limitations to this definition.Example: Wolves and dogsThis definition does NOT account for extinct species or species that reproduce asexually.However, this definition still works in most everyday experiences of classification.

Determining Species

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Phylogenetic species concept:In the 1940s, evolutionary species concept was proposed as a companion to the biological species concept.The evolutionary species concept defines species in terms of populations and ancestry.According to this concept, two or more groups that evolve independently from an ancestral population are classified as different species.More recently, this concept has developed into the phylogenetic species concept.Phylogeny is the evolutionary history of species.

Determining Species

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Phylogenetic species concept:The phylogenetic species concept defines a species as a cluster of organisms that is distinct from other clusters and shows evidence of a pattern of ancestry and descent.When a phylogenic species branches, it becomes two different phylogenetic species.Example: Recall allopatric speciationThis definition can account for the extinct species and species that reproduce asexually.Check out Table 17.2!!

Characters

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How can you distinguish the following veggies?

These veggies belong to the same species called Brassica oleracea.

Characters

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To classify a species, scientists often construct patterns of descent, or phylogenies, by using characters.Characters can be morphological or biochemical.Shared morphological characters suggest that species are related closely and evolved from a recent common ancestor.

Morphological Characters

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Remember that analogous characters do not indicate a close evolutionary relationship.Homologous characters might perform different functions, but show an anatomical similarity inherited from a common ancestor.Example: birds and dinosaursSimilar features: large hollow spacesSome fossil dinosaur bones had feathers.The evidence provided by these morphological characters indicates that modern birds are related more closely to theropod dinosaurs than they are related to other reptiles.

Biochemical Characters

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Scientists use biochemical characters, such as aminoacids and nucleotides, to help them determine evolutionary relationships among species Figure 15.9!!Chromosome structure and number is also powerful clue Mustard family (Cruciferae) members have almost identical chromosome structure common ancestorCheck out Figure 17.8!!


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DNA and RNA analyses are also powerful tools for reconstructing phylogenies.The greater the number of shared DNA sequences between species, the greater the number of shared genes the greater the evidence that the species share a recent common ancestor


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Compare the elephants shown in Figure 17.9!!Asiatic elephant Elephas maximusAfrican elephant (savannah) Loxodonta africanaAfican elephant (forest) Loxodonta cyclotisRecent DNA studies show that African elephants diverged from a common ancestor about 2.5 million years ago.2.5 million years ago... How do we know that?


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Molecular clocks:Mutations occur randomly in DNA.Mutations can be harmful, beneficial or neutral.As time passes mutations accumulate in the chromosomes.Systematists can use these mutations to help them determine the degree of relationship among species.A molecular clock is a model that is used to compare DNA sequences from two different species to estimate how long the species have been evolving since they diverged from a common ancestor.Check out Figure 17.10!!


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Molecular clocks:The differences between DNA sequences in the genes of different species indicate the presence of mutations.The more mutations the more time has passed since divergenceHow about the rate of mutations?The speed by which mutations occur is not always the same in a single gene difficult to read the molecular clock!Many factors affect the rate of mutation..Researchers try to compare genes that accumulate mutations at a relatively constant rate such as the gene for cytochrome c

Phylogenetic Resconstruction

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Remember pedigrees?


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The most common systems of classification today are based on a method of analysis called cladistics.Cladistics is a method that classifies organisms according to the order that they diverged from a common ancestor.Character types:Scientists consider two main types of characters when doing cladistics analysis: Ancestral trait and Derived traitExample: Birds and mammalsAncestral trait: backboneDerived trait: feathers (for birds) and hair (for mammals)


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Cladograms:Systematists use shared derived characters to make a cladogram.A cladogram is a branching diagram that represents the proposed phylogeny or evolutionary history of a species or group.A cladogram is a model similar to pedigrees. How?The groups used in cladograms are called clades.A clade is one branch of the cladogram.


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The outgroup is the species or group of species on a cladogram that has more ancestral characters with respect to the other organisms being compared.The cladogram is constructed by sequencing the order in which derived characters evolved with repsect to the outgroup.The closeness of clades in the clagodram indicate the number of characters shared.The nodes where the branches originate represent a common ancestor.The greater the number of derived characters shared by groups, the more recently the groups share a common ancestor.A cladogram is also called a phylogenetic tree.Check out Figure 17.12!!


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