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Evolutionary Species Concept
- Species are populations with a shared evolutionary history and fate.
- But does not help explaining how to seperate species.
- Developed by George Simpson and E.O. Wiley
Phenetic Species Concept
- Delimits species based on shared traits.
- But gives all traits equal weight.
Biological Species Concept
- Delimits species based on populations capable of interbreeding.
- "groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups"
- By Ernst Mayr
- Difficult to use with fossils and disregards asexual organisms.
Phylogenetic Species Concept
- Delimits species by resolving the smallest monophyletic clade with members uniquely sharing the derived traits
- "The smallest monophyletic group distinguished by a shared derived character."
- Speciation takes place in populations that are geographically isolated from one another.
- Genetic drift, accumulation of mutations and natural selection cause populations to diverge as different alleles are fixed within them.
- Dumbbell model-a population is divided and isolated into smaller population which diverge.
- Peripheral isolate model- small amount of the population leaves and is isolated, which causes divergence.
- "Two adjacent populations diverge into separate species without a geographic barrier to dispersal."
- A gradient or cline causes adjacent populations to experience different selective conditions -but the populations can still mate, generating hybrids.
- "An area in which diverging populations encounter each other, mate and produce hybrid offspring."
- Essential to parapatric speciation.
- Speciation in the absence of barriers.
- Niche partitioning and adaptive radiation
Diversification of a species or single ancestral type into several forms that are each adaptively specialized to a specific environmental niche.
Reproductive Isolation Mechanisms
- Deters hybridization between two populations, once in place may lead to completion of the speciation process.
- Prezygotic isolating mechanisms
- Postzygotic isolating mechanisms
Prezygotic isolating mechanisms
- Prevents mating or successful fertilization between different species.
- behavioral, temporal, habitat
Postzygotic isolating mechanisms
Affects zygote/embryo survival.
Zygote doesn’t survive, sterile hybrids, backcrosses or F2 hybrids are sterile.
Matings between species with distinct chromosome arrangements can result in dysfunctional gametes (extra or no copies of important genes are passed on) -hybrid likely has reduced fitness and is cleared by selection.
Haldane's Dominance Rule
- If hybrids have reduced fitness, then hybrids will be under strong negative selection in heterogametic sexes.
- “When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic] sex.”