SPECIATION

• SPECIES - a group of organisms which can successfully breed with each other, but have limited success breeding with other groups.
• RANGE - the area in which a species is found.
• ENDEMIC - native to one area or continent only.

Speciation is the evolution of new species. It occurs when a population of interbreeding individuals is split up into separate populations. These separate populations then continue to evolve independently of each other. Over time, they may become separate species and be unable to breed with the populations from whom they were initially separated. There are three main types of speciation: range splitting speciation, island-colonisation speciation and non-geographic speciation.

1. Range splitting speciation occurs when geographic divisions, such as changes in the environment or landscape, split a species' range. This type of speciation happens over a long period of time. For example, when ocean temperatures around southern Australia cooled during the ice ages, fish species were forced to separate into two groups, as they retreated into warmer waters off the east and west coasts.These two groups then evolved into two different but related species. There are at least 18 examples of such closely related fish species in the east and west of Australia.
2. Island colonisation speciation occurs when an area, such as an island, is colonised only once, for instance by a single pregnant female. This type of speciation is more rapid and can be seen in the Drosophila fruit flies in Hawaii. About 750 species of Drosophila are native to the archipelago of Hawaii, and 98% of these are endemic to one island only. Most species groups are derived from a species on another island, meaning that a pregnant female has left one island and colonised another.
3. Non-geographic speciation occurs when changes in mating time, behaviour or flowering season prevent individuals of what had been the same species from interbreeding.

MODES OF SPECIATION

1. GEOGRAPHIC (ALLOPATRIC) SPECIATION: A geographic barrier of some sort divides a population, gene flow between the now separate populations stops, each population evolves independently of each other, and thus diverge. Result: new species, since after genetic divergence the two populations can no longer interbreed even if the barrier removed. (Figure 1).

A) How big must the barrier be? Depends on the type of organism. For large mammals, oceans, high mountain ranges, or glaciers can suffice. For terrestrial animals in general, deep river valleys, wide rivers, or deserts serve as barriers. Some studies have shown divergence between populations of snails separated by a large parking lot! (They aren't new species yet---it takes a lot longer to do that)

B) What are the divergence events? Ultimately they are genetic in nature; changes in chromosome structure, number, and alleles. They are maintained because they might have a selective advantage in that population. Different sides of the barrier could have different environments, thus leading to different traits being selective on either side

C) What mechanisms reproductively separate the species? These are generally called reproductive isolating mechanisms, and can be pre-zygotic (gamete transfer is prevented) or post-zygotic (prevent fertile and/or viable hybrids), as follows:

• Temporal isolation: potential mates do not meet because they mate or are active at different times of year or day.
• Habitat/Ecological isolation: potential mates do not meet because of different habitat use. Ex: Ambystoma texanum salamanders breed in ponds, but Ambystoma barbouri breeds in streams
• Ethological isolation: potential mates meet but do not mate because they don't recognize each other behaviorally (Figure 3). Fireflies (Lampyridae) are classic examples: their flash patterns differ from species to species
• Mechanical isolation: mating attempted but no sperm transfer because genitalia don't fit or get stuck. Many insects require a precise "lock and key" fit for sperm transfer (Figure 4).
• Gamete mortality: sperm transfer takes place but egg not fertilized; common in plants where pollen is incompatible with the female plant
• Zygotic mortality: egg fertilized but zygote dies
• Hybrid inviability: zygote produces a hybrid of reduced viability, so often dies post-birth
• Hybrid sterility: hybrid is fully viable but sterile. Mules, for example (Figure 5).
• Hybrid breakdown: hybrid is viable and fertile but its progeny are neither

2. SYMPATRIC SPECIATION: speciation in the absence of any obvious geographical barriers; in these cases, gene flow reduced or negated by ethological, ecological, or other non-geographic modes (Figure 6).

DOES THIS REALLY HAPPEN? Yes, it does. The scientific literature is filled with many examples of this having happenned, especially in plants but also more rarely in animals. In such cases, it often happens by one of the following mechanisms:

• Crossing between species of plants produces hybrids with one copy of chromosomes from each parent. Normally this produces a sterile plant, but if the chromosomes double (through abnormal mitosis, which does happen), a POLYPLOID is the result---in this case, a TETRAPLOID, with four sets of chromosomes, two from each parent. Did you know that wheat is a HEXAPLOID? Its chromosomes come from three distinct species! Organisms that are polyploid and whose chromosomes come from separate species are called ALLOPOLYPLOIDS, and they are REPRODUCTIVELY ISOLATED from the parent species! Still not convinced? Let's diagram it on the board...
• Alternate use of host plants is thought to be a common mode of speciation, especially for insects. Individuals from a population may spread out to other host plants. If the preference for host plant is heritable, then the populations become ecologically and behaviorally isolated. This has been observed in many insects, such as apple maggot flies and leafhoppers.

3. PARAPATRIC SPECIATION: Exploitation of previously unexploited habitats, usually preceded by some genetic change, followed by reduced gene flow between the populations. This differs from sympatric modes because there is usually a spreading out of the population into a new range without the presence of a geographical barrier. As an example, consider morabine grasshoppers in Australia. There are 240 species, each similar in gross morphology but none with sympatric ranges, and each species HAS A DIFFERENT SET OF CHROMOSOME SHAPES AND SIZES that nevertheless can be seen to have been derived in some way from each other.