EVOLUTION 1B

1 EJC H2 Biology T2W1 Evolution 1B Evolution 1B 1. Introduction What are species, exactly? Defining and recognising a species has been a controversial issue for a long time. There are many definitions for the concept of species. There is no universal species concept – one that applies to all organisms. Instead, scientists have been proposing dif ferent species concepts for years, and each is based on slightly different biological reasoning. Species concepts do not only define what a species is, but by defining what a species is, they also clarify what speciation is. 2. Learning Outcomes (h) Explain t he various concepts of the species (biological, ecological, morphological, genetic and phylogenetic concepts) (j) Explain how new species are formed with respect to geographical isolation (allopatric speciation) and behavioural or physiological isolation withi n the same geographical location (sympatric speciation) 3. References Campbell, N.A. and Reece, J.B. (2008). Biology, 9th edition. Pearson. Contents 1. Introduction ............................................................................................................... 1 2. Learning Outcomes.................................................................................................... 1 3. References ................................................................................................................. 1 4. Concepts of Species ................................................................................................... 2 5. Speciation .................................................................................................................. 5

2 EJC H2 Biology T2W1 Evolution 1B 4. Concepts of Species Biological species concept A species is a group of organisms capable of interbreeding and producing fertile, viable offspring . Organisms of the same species are reproductively isolated from those of a different species. (Note: For most purposes, the biological species concept is used.) Advantage: To determine if organisms are of the same species, they can be studied to see if they can interbreed and produce fertile, viable offspring. Limitation: 1) This definition of species cannot be applied to asexually reproducing organisms and extinct species whose breeding behaviour cannot be observed in nature. 2) In plants, two different species can mate and give rise to viable hybrids offspring that are potentially fertile because t hey are capable of polyploidy. Polyploidy is rare in animals. Genetic species concept A species is a group of genetically compatible interbreeding organisms in a natural population that is genetically isolated from other such groups. Organisms in a species have sufficient similarity in their DNA sequences and share the same number of chromosomes. The focus on genetic isolation rather than reproductive isolation distinguishes the Genetic Species Concept from the Biological Species Concept. Genetic isolation results as 2 genomes diverge to the point that they are genetically distinct and e volve independently of each other. e.g. genetic changes that lead to behavioural changes or changes in pheromones and odors associated with species recognition. Genetic isolation maintains the integrity of gene pools in the natural population, but if the 2 sister species are mated deliberately or incidentally, fertile viable offspring may be possible. There are instances where a species has achieved genetic isolat ion but not reproductive isolation. e.g. two related species, mule deer and white -tailed deer have distinctive morphology, mating behaviour and habitat preference. While they keep mostly to themselves, occasional interspecific mating and captive breeding r esulted in fertile hybrids. Genetic isolation is achieved as the 2 species have genetically determined mechanisms in place to prevent or minimize breeding but reproductive isolation has not been achieved because if a fertilization event occurs, fertile off spring is still possible. Two reproductively isolated species, however, will always be genetically isolated. Advantage: The genetic species concept uses genetic data from mitochondrial and nuclear DNA to identify species and species boundaries and helps to deduce evolutionary relationships. Notes to self

3 EJC H2 Biology T2W1 Evolution 1B Limitation: Technology required to study the DNA sequences is currently quite expensive and not accessible to everyone. However , the cost is reducing rapidly and its use has become more widespread recently. Ecological species concept A species is a group of organisms sharing the same ecological niche. Niche refers to both the place where an organism lives and its interactions with the environment, i.e. the roles that an organism plays in its habitat (predator, prey, primary producer or consumer, decomposer etc). Advantage: Every organism has a niche. Limitation: 1) This definition cannot be applied to unrelated species that occupy similar niche. e.g. the striped possum, a marsupial mammal from Australia and the aye-aye a placental mammal from Madagascar. 2) Furthermore, many animals are elusive and determining their niches can be time-consuming and difficult. Morphological species concept A species is a group of organisms sharing similar body shape and other structural features. Advantage: Every organism has morphological features that can be easily studied as long as you have a specimen. Limitation: 1) This definition makes it difficult to determine the degree of difference that is required to indicate separate species as well as what structural features should be used to distinguish the differences. 2) Some organisms may be superficially similar but have different evolutionary origins e.g. the marsupial mole from Australia and the placental mole from Africa. Fig. 1: marsupial mole versus placental mole. Marsupial “mole” Placental mole Notes to self

4 EJC H2 Biology T2W1 Evolution 1B This node represents the most recent common ancestor of species B and C. Phylogenetic species concept A species is the smallest group of organisms that share a most recent common ancestor and can be distinguished from other such groups. The phylogenetic history of a species can be obtained by comparing homologous morphological structures and/or homologous molecular sequences , with those of other organisms. Fig. 2: Phylogenetic tree. A diagram that depicts the lines of evolutionary descent of different species from a common ancestor. Advantage: 1) Using this definition to classify species will avoid mistakenly classifying organisms based on superficial morphological similarities as the characteristics that are compared are based on common ancestry/homology. 2) To determine the phylogeny, genetic, DNA sequences, amino acid sequences, morphological, fossil evidence, hybridization(mating) studies are conducted, so phylogenetic concept of the species potentially presents the most accurate representation of a species. 3) It has the added advantage of providing accurate historical information about the speciation event that the biological concept of a species cannot. Limitation: It is difficult to come up with an accurate phylogenetic tree as multiple evidences need to be gathered and processed . This means that the accuracy of the phylogenetic tree is dependent on the availability, diversity and accuracy of the source data. If for instance, DNA evidence was not used, the phylogenetic tree may not be that reliable. Notes to self

5 EJC H2 Biology T2W1 Evolution 1B 5. Speciation Speciation is a process by which one or more new species arise from a previously existing species. Disruption of gene flow is required for speciation to occur. Recall: G