Lecture #20—Species Formation
Previous lectures described how mutation, sexual reproduction and genetic recombination produce variation in species, and how various abiotic and biotic forces act to select out some individuals and not others. This causes a shift in the population characteristics over time—i.e. evolution. But how does this process lead to new species? We examine this topic in detail in this lecture.
WHAT IS A SPECIES?
Typological species—This is the concept that group of organisms whose members share certain characteristics that distinguish them from other species. These are usually differences in anatomy. When taxonomists classify organisms they are usually applying this definition. When biologists first applied this definition they thought that species could never change—so that they thought that if they put one example of the species this would be sufficient (type specimen). But when evolution was accepted, many specimens needed to be collected to show the variation in the species.
Biological species-- Reproductively isolated group of actually interbreeding natural populations that produce fertile offspring. This is the modern definition of a species but it is hard to apply because
1. Most organisms don’t live in the same area and they never have a chance to interbreed.
Organisms that live in the same area are said to be sympatric.
Organisms that live in different areas are said to be allopatric.
2. Polymorphic species—individuals of these species interbreed but have great physical differences among them.
3. Sibling species—these are different species (don’t interbreed) but they look virtually identical.
4. Ring species—these are species that stretch over a large area and show gradual shifts in anatomy; all of the individuals can interbreed but the ends of the range are so different that they cannot.
5. Asexual species—these are organisms which do not reproduce by sex; so we cannot apply the definition. People who study such organisms like bacteria or protozoa like amoebae classify them by the typological method. If they look different enough, they put groups into different species.
6. Fossil species—fossils can’t breed so we cannot apply the biological species definition. But paleontologists still designate species on the typological method; if two organisms look sufficiently different we should put them into different species categories.
REPRODUCTIVE ISOLATING MECHANISMS: WHAT KEEPS SPECIES FROM INTERBREEDING?
Naturally, if populations live in different parts of the world they can’t interbreed (they are geographically isolated), but this is not what is being considered here. Reproductive isolating mechanisms deal with the behavioral or physical reasons that stop two populations from interbreeding even if they were living together. There are two major categories: mechanisms that come into play before a zygote is formed (prezygotic) and ones that can occur after a zygote is formed (postzygotic).
Prezygotic Mechanisms
1. Ecological (Habitat) Isolation—populations live in same area but choose different parts of the habitat.
2. Behavioral Isolation—populations have different behavioral patterns. E.g. mating signals (frog calls; firefly flashing patterns).
3. Mechanical Isolation—sexes from different populations cannot successfully copulate. This is a common in insects and spiders.
4. Gametic Isolation—Egg and sperm don’t fuse because the cell receptors are incompatible. Common in sea urchins and in flowers.
Postzygotic Mechanisms
1. Hybrid inviability—fertilization occurs but the zygote or embryo dies. E.g. sheep and goats hybridize but the embryo dies.
2. Hybrid sterility—fertilization and development succeed but the hybrid individuals are sterile. E.g. horses and donkeys interbreed to produce mules but they are sterile.
STEPS IN SPECIES FORMATION:
Allopatric Speciation: this is the most common method of species formation; two or more populations of a species get separated and then specialize for different environments. They become so different that they can no longer interbreed even if they got back together. Two steps are involved: first separation and then development of reproductive isolating mechanisms.
1. Geographic separation—populations become separated. This may occur because a population disperses to different environments (e.g. islands) or because barriers develop (glaciers, mountains, rivers, tectonic plates move) separating groups of the species.
2. Reproductive isolation develops—this occurs over time since the populations are in different environments and subject to different selection pressures or because of random genetic drift.
e.g. squirrels on opposite sides of the grand canyon.
Parapatric Speciation is when two adjacent populations differentiate into two or more species. This separation occurs even with limited interbreeding because the environments are significantly different with different selective pressures acting.
Sympatric Speciation: this is the process where a new species evolve from a single ancestral species while living in the same geographic region.
Gradual sympatric speciation—two populations of the same species gradually become different even though they live in the same habitat.
This can occur if there is disruptive selection. That is, two populations specialize for different parts of the habitat say by eating different things, or preferring different places to live in the habitat. This has been observed among Galapagos finches specializing on different sized seeds; this leads to birds with different beaks sizes.
Instantaneous speciation—new species form in one generation.
a. By mutation—not probable because any mutation that produces individuals that are so different they can’t breed with their parent species, is likely very drastic and the mutant’s survival is probably compromised. Also, who is the mutate going to breed with?
But this can occur by polyploidy—If a mutation occurs which doubles the chromosome number and the individuals survive, the resulting organism will have double the number of chromosomes as the parent. Common in plants because they can self-fertilize. Varieties of tulips, crocuses, irises and primroses have all been created this way by treating flowers with the chemical colchicine.
b. Hybrid polyploidy—two species interbreed and their chromosome number is the sum of the two. Again this is common in plants
About half of all Angiosperms appear to be have evolved as a result of some sort of polyploid speciation
Terms/Concepts to define
Taxonomy
Binomial System of Nomenclature
Carl Linnaeus
Typological species
Biological species definition
Sympatric species
Allopatric species
Polymorphic species
Sibling species
Ring species
Asexual species
Fossil species
Reproductive Isolating Mechanisms
Prezygotic Isolating Mechanisms
Postzygotic Isolating Mechanisms
Allopatric speciation
Sympatric speciation
Disruptive selection
Instantaneous speciation
Polyploid speciation
Can you answer these questions?
1. The biological species definition is the accepted modern way to define a species, but it really is seldom used in practice. Explain why this is.
2. It is said that all of the higher taxonomic categories are arbitrary while the category of species has a real biological basis. Explain the reasoning behind this comment.
3. Most of the animals in the world don’t attempt to breed together. What kind of reproductive isolating mechanism is at work?
4. The Galapagos islands are populated by many species that are unique to specific islands. What kind of speciation process do you expect to have occurred? Describe what must have happened in some detail assuming the islands once did not have any animals present. Remember they are volcanic and were at one time devoid of all life.
5. Explain why disruptive selection provides a ready explanation of how sympatric speciation could develop.