Classification • Organisms are classified into a hierarchical classification that groups closely related organisms and progressively includes more and more organisms.
Species • The species is the basic biological unit around which classifications are based. • However, what constitutes a species can be difficult to define and there are multiple definitions of species in use today.
What is a species? • The species is a basic biological unit and humans seem to intuitively recognize species. • However, why do species exist? • Why don’t we see a smooth continuous blending of one species into another?
Why do we see discrete species? • Because intermediate forms between closely related organisms are usually selected against. • If they were not selected against, then the two forms would merge into one as their gene pools mixed.
Why do we see discrete species? • Organisms are very well adapted to their environments having evolved over millions of years. • Each organism has specialized characteristics such as camouflage, feeding structures, behavior, and genitalia that equip it to survive well in its environment.
Why do we see discrete species? • An offspring that results from a cross between members of two different species or between members of different populations that have been evolving in isolation from each other, will probably have traits intermediate between its parents. • As a result, it likely will be less well adapted to its environment than either parental form and be selected against. • Thus, we see distinctively different species.
What is a species? • John Ray (1627-1705) gave first general definition of a species. • A species consists of all individuals that can breed together and produce fertile offspring.
A female donkey mated to a male horse produces what?
A mule (which is sterile) Hence, donkeys and horses are separate species.
Biological Species Concept • Ray’s idea was updated into the Biological Species Concept. Two definitions of the BSC are given below: • “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” Ernst Mayr. • “A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature.” Ernst Mayr.
Biological Species Concept • The biological species concept emphasizes that a species is an interbreeding population of individuals sharing common descent and that members of that community because they share a niche constitute an ecological entity in nature. • Members of a species we expect to be similar to each other but different from other organisms,
Criticisms of the Biological Species Concept • The BSC has been criticized for several reasons: • 1. It applies only to sexually reproducing species. • 2. Distinguishing between species on the basis of reproductive separation is problematic because it can be difficult to determine how much reproductive separation is needed to distinguish between species. • 3. The definition refers only to current populations and ignores the species status of ancestral populations.
Evolutionary Species Concept • George Gaylord Simpson proposed the Evolutionary Species Concept in the 1940’s to add an evolutionary time dimension to the Biological Species Concept.
Evolutionary Species Concept • Evolutionary species concept “A single lineage of ancestor-descendant populations that maintains its identity from other such lineages and that has its own evolutionary tendencies and historical fate.”
Evolutionary Species Concept • Definition applies to both sexually and asexually reproducing species and emphasizes common descent. As long as diagnostic features are maintained a lineage will be recognized as a single species.
Phylogenetic species concept • A third species concept is the phylogenetic species concept. • “an irreducible (basal) grouping of organisms diagnosably distinct from other such groupings and within which there is a parental pattern of ancestry and descent.”
Phylogenetic species concept • The phylogenetic species concept also emphasizes common descent and covers both sexually and asexually reproducing organisms. • Under the PSC any population that has become separated and has undergone character evolution will be recognized as a species.
Phylogenetic species concept • Criterion of irrreducibility requires that no more than one diagnosibly distinct population can be included in a single species. Thus, the emphasis is placed on monophyly: lineages that contain all the descendents of a single common ancestor. • Main difference in practice between ESC and PSC is that PSC recognizes as species the smallest groupings of organisms that have undergone independent evolutionary change.
Phylogenetic species concept • The ESC would group into one species a series of geographically disjunct populations that show some genetic divergence, but the PSC would treat them as discrete species. • Thus, subspecies under the ESC would be species under the PSC and in general more species would be recognized under the PSC than either the BSC or ESC.
Typological Species concept • For historical interest this is the pre-Darwinian idea that species are defined by fixed and unchanging features and do not change over time (i.e., evolve). • Biologists discarded the idea after Darwin’s theory of evolution by natural selection became established. • Creationist’s still cling to the typological species concept and you’ll often see “types” referred to in creationist writings.
Applications of species concepts • Diversification in marine copepods. • Copepods are small abundant crustaceans. Numerous populations of Eurytemoraaffinis have been described from estuaries in the northern hemisphere and traditionally grouped into one species on the basis of similarity of appearance.
Diversification in marine copepods • A study by Lee (2000) in which she compared gene sequences of populations and also carried out breeding trials showed that at least 8 phylogenetic species exist, which are reproductively isolated. • Clearly, assuming species identity on the basis of morphology alone will underestimate species diversity.
How many species of African elephants are there? • Traditionally one species of elephant Loxodonta africana has been recognized in Africa (a second species Elephasmaximus occurs in Asia). • However, recent morphological studies have pointed out that forest dwelling elephants in West Africa appear to differ from elephants found in Savannah habitats elsewhere on the continent.
How many species of African elephants are there? • A comparison of DNA from 21 populations suggests that two phylogenetic species exist and it has been suggested by Roca et al. (2001) that forest elephants be named Loxodonta cyclotis. • Whether the two populations are capable of interbreeding is unclear, but the clear genetic differences between populations suggest that conservation biologists should be attempting to conserve members of both populations.
How species form • Classically, speciation has been viewed as a three stage process: • Isolation of populations. • Divergence in traits of separated populations (e.g. mating system or habitat use). • Reproductive isolation of populations that maintains isolation when populations come into contact again (secondary contact).
How species form • Recent research shows that steps one and two may take place simultaneously in the same place and often the third step does not occur.
Genetic Isolation: physical isolation • Physical separation reduces or stops gene flow between populations and as a result there may be a balance between gene flow and natural selection (recall the Lake Erie water snake example from chapter 6). • On the islands selection favors elimination of alleles for banding, but migration constantly introduces them. If the islands were to be completely separated so no snakes migrated natural selection would result in the island populations becoming different from the mainland ones.
Allopatric speciation • This is the essence of Ernst Mayr’s allopatric model of speciation. • A physical barrier isolates a population or populations from the rest of the species and selection favors genetic divergence of that population.
Allopatric speciation • Separation of populations can occur by two major means: • Dispersal of some individuals across a barrier. • Development of a new barrier that separates populations [Vicariance] (the vicariance event could be e.g. change in flow of a river, lava flow, development of a mountain range, habitat destruction)
Geographic isolation through dispersal • We have already encountered example sof speciation after individuals crossed a barrier. • The ancestors of Darwin’s finches colonized the Galapagos Islands after dispersing from South America and speciated into the current range of species. • Similarly, the Hawaiian Islands were colonized by ancestral Drosophila fruit flies that appear to have speciated to produce more than 500 endemic species of Drosophila on the islands.
Evidence for founder hypothesis of speciation in Hawaiian Islands • The main hypothesis for how the Hawaiian Islands became populated with a diverse variety of endemic species most of which occur on only a single island is the founder hypothesis. • According to the founder hypothesis new species are formed when a small population of individuals disperses to a new island and after being separated diverges from the ancestral form.
Evidence for founder hypothesis of speciation in Hawaiian Islands • The Hawaiian Islands were formed by a stationary geological “hot spot” over which the continental plate drifts northwest. • Periodically, the hot spot produces magma flows, which form islands that are then carried away on the plate and ultimately erode away. Thus the newest islands are close to the hot spot and the oldest further northwest.
Evidence for founder hypothesis of speciation in Hawaiian Islands • Based on the geological information te founder hypothesis makes two predictions about the pattern of speciation that should be observed. • Closely related species should be found on adjacent islands and • Some speciation sequences should match the sequence in which islands formed.
Evidence for founder hypothesis of speciation in Hawaiian Islands • A study of mitochondrial DNA of four species of closely related Drosophila by DeSalle and Giddings (1986) found the predicted patterns. • The most recent species occur on the youngest islands and several of the branching events match the order of island formation.
Geographic isolation through vicariance events • There are many ways in which a species distribution may be split into two by a physical event. Some such as mountain formation are slow, others such as a lava flow are rapid. • The Isthmus of Panama closed about 3 million years ago separating marine populations on either side. Did these populations speciate?
Geographic isolation through vicariance events • A DNA sequence study by Knowlton et al. 1993 of snapping shrimp populations from both sides of the isthmus suggests they did. • Seven pairs of morphologically closely related species pairs occur, one of each pair on each side of the isthmus and the DNA sequence results confirm that these are each others closest relatives, which is consistent with the vicariance hypothesis.
Phylogenetic tree of numbered species of snapping shrimp. P and C refer to Pacific and Caribbean species respectively.
Geographic isolation through vicariance events • Mating experiments with the snapping shrimp fund that males and females with the greatest genetic divergence were least interested in each other and almost none the pairs produced clutches that yielded fertile young.
Polyploidization as a mechanism of speciation • Polyploidy (production of multiple sets of chromosomes) appears to have played a major role in the speciation patterns of plants. • An estimated 70% of flowering plants appear to have had polyploid events in their evolutionary history as have 95% of fern species.
Mechansims of divergence • Dispersal, vicariance and polyploidization create opportunity for speciation to take place. • For speciation to occur populations must diverge genetically from each other.
Genetic drift • Genetic drift is a sampling phenomenon in which only some alleles occur in a population as a result of its small size because of founder effect and bottlenecking. • If a population remains small for a period of time many alleles may be lost from the gene pool.
Genetic drift • The length of time the population is bottlenecked has a strong influence on how great allele frequency changes will be. Theoretical studies show that if populations remain very small for only a short time then only rare alleles are likely to be lost and little effect on speciation is likely. • Thus, scientists are increasingly focusing on natural selection as a more important force driving speciation than drift.
Natural selection and speciation in apple and hawthorn maggot flies • The apple maggot fly (Rhagolestis pomonella) is a major pest of apples that occurs throughout the northeastern U.S. It also parasitizes hawthorn trees a close relative of apples. • Maggot flies recognize trees on the basis of visual, tactile and olfactory cues and mate on or near the fruit.
Natural selection and speciation in apple and hawthorn maggot flies • Eggs are laid on fruits and larvae develop in them. When the fruit falls the larvae burrow into the ground and pupate emerging as adults the next year. • Apple trees are a novel food source for these native flies, which exploited apples after they were introduced about 300 years ago.
Natural selection and speciation in apple and hawthorn maggot flies • The question is does the new food source represent an island and are the populations that breed on apples genetically distinct form those that breed on hawthorn trees? • Do apple and hawthorn populations interbreed or not and are they diverging?
Natural selection and speciation in apple and hawthorn maggot flies • Hawthorn and apple trees are often in very close proximity so it would seem hard for the populations to diverge. • However, a protein electrophoresis study by Feder et al. (1988,1990) showed that the populations are genetically distinct.
Natural selection and speciation in apple and hawthorn maggot flies • Each population shows a strong preference for its own fruit type, which because mating takes place on fruit results in strong nonrandom mating. • There is gene flow between populations because about 6% of matings are cross-population matings, but despite this gene flow natural selection appears to driving the populations apart.