Allopatric speciation Flashcards
(19 cards)
What are the three definitions of species?
The biological species concept, the morphospecies concept, the phylogenetic species concept
Describe the biological species concept
Proposed by Ernst Mayer. Still the flagship definition that is used in legal contexts
‘If populations of organisms do not hybridise, or if they fail to produce fertile offspring when they do, then they are reproductively isolated and can be considered as good species’
What are the shortcommings of the biological species conceot?
Many populations (e.g. extinct taxa) cannot be tested for reproductive isolation. Irrelevant to sexual taxa. Cannot be applied to many plants where hydbridisation occurs routinely.
What is the phylogenetic species concept?
Based on the principle of monophyly. Monophyletic groups contain individuals that are all descendants of a single ancestor. Species are defined as the smallest diagnosable monophyletic group. Any population that forms an independent branch on a phylogeny is a species. Assmes that these units must have been isolated for sufficiently long enough that each possesses diagnostic traits
Shortcommings of the phylogenetic species concept?
Phylogenies are only available for a limited number of speces
Measures of different characters often produce different phylogenies for the same set of taxa
Different species may be diagnosed if they contain small genetic differences, yet these differences may be irrelevant in terms of interbreeding
Describe the morphospecies concept
Species are defined on the basis of consistent morphological differences. Universal. Often used by paleontologists who cant compare interbreeding
Shortcommings of the morphospecies concept?#
Some species show polymorphic morphology.
Could be subjective. Difficuly to apply consistently.
Difficult to apply to cryptid species i.e. populations that are reproductively isolated byt which differ in attributes other than morphology
Give an example about defining a species
The red wolf. Similar to grey wolf and coyotes.
- Underwent massive population decline. 14 captured for breeding and reintroduction.
- Skulls before 1939 suggesting morphospecies due to distinct characteristics
- After 1930, resembled coyotes, resembling hybridisation.
So biologiacal species concept not applicable today, but maybe 70 years ago?
Describe reproductive isolation
Ernst Mayer
- Gene flow tends to homogenise allele frequencies and prevent populations differentiating. For speciation to occur, there must be reproductive isolation.
- What about intermediates?
Main mechanism of speciation is allopatric speciation. Populations that are prevented from exchanging genes by a geographic isolation
- Vicariant and peripatric
Describe vicariant speciation
Populations are divided by emergence of an extrinsic barrier. Generally divides widespread and large populations
e.g. snapping shrimp. Isthmus of Panama is a land bridge dividing the oceans. Seven morphospecies identified, with similar morphology, on different sides of the bridge
What is peripatric speciation?
A colony diverges from a widespread parent population and acquires reproductive isolation. If the isolated population is small, genetic drift may be important in the evolution of a new species (founder effects)
e.g. marine lakes of Palau jellyfish. Lakes formed in chronological series as sea level rose. Diversity of forms that are morphologically, behaviourally and genetically distinct
What is divergence?
Seen in examples of allopatric speciation. Once reproductively isolated, populations diverge. Is it natural selection or genetic drift? Theoretically, in peripatrix speciation, drift is considered important but has been difficult to prove.
Natural selection and divergence
Good evidence for importance of selection from ‘parallel speciation’ in three-spined sticklebacks. Freshwater populations assume to have a large marine ancestor.
Small pares of open water and benthic forms evolved in several lakes from “double invasion” (allopatric) in which repeated invasions of marine forms have subsequently differentiated into benthic and limnetic forms.
Because similar forms have evolved in several diffreent lakes, evolution of body form is adaptive (related to haitat use/ecology)
Can sexual selection and assortative mating produce reproductive isolation and promote speciation?
Sexual selection is a mode of natural selection where typically members of one biological sex choose mates of the other sex with whom to mate.
Assortative mating is a mating pattern and form of sexual selection in which individuals with similar genotypes/phenotypes mate with one another more frequenly than would be expected under a random mating pattern.
Can sexual selection and assortative mating produce reproductive isolation and promote speciation? Evidence
Female choice of elaborate traits may promote divergence among isolated populations
Species diversity is highest in clades of bird species where the sexes are dimorphic, rather than in related monomorphic clades - sexual selection increases speciation rate
Secondary contact
In allopatric speciation, what happens if geographic isolation breaks down – populations come back into secodnary contact
Hybridisation may be possible, or barriers to gene flow may have evolved:
- prezygotic: behavior, mechanical, gametic incompatibility
- postzygotic: includes zygotic mortality, hybrid inviability and hybrid sterility
Describe barriers to gene flow in secondary contact
If populations have diverged in terms of their ecology or mating system, hybrids may be less fit. Selection against hybridisation
Any increase in prezygotic isolation in response to selection against hybridisation is called reinforcement. Closing stage of speciation between taxa that can, feasbily, reproduce.
Describe reproductive character displacement
Differences in traits involved with mate choice or mating compatibility among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimised or lost where the species distributions do not overlap.
Give an example of secondary contact
Reproductive character displacement in stag beetles
In sympatry, they are strikingly different in colour patterning. In allopatry, very similar in colour
Vary in body number and genetical length in sympatry, not allopatry. Genital morphology is important for prezygotic isolation in these beetles
