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Evolutionary Biology
Speciation 1
| Question | Answer |
|---|---|
| What is speciation? | Evolution of reproductive isolation within an ancestral species, resulting in two or more descendant species |
| Cladogenesis | The formation of a new group of organisms or higher taxon by evolutionary divergence from an ancestral form |
| Anagenesis | The gradual evolution of a species that continues to exist as an interbreeding population |
| Why is it important? | 1. Speciation generates biodiversity But note that much diversity exists within species as well, and may itself contribute to speciation |
| Why is it important? | 2. Speciation events in very distant evolutionary past generate higher-level branches (e.g. phyla) |
| Why is it important? | 3. Study of speciation illuminates role of genetic processes and natural selection in evolution |
| Three methods have been used to demonstrate that species exist | 1) common sense 2) folk 3) statistical methods |
| Common sense? | Species are real because everyone recognizes they are real Nobody claims there is a continuum between crows and eagles |
| Do species exist? | Common sense argument does not provide hard evidence for existence of species Humans separate the continuous spectrum of light into 7 distinct colours - units? We can compare diverse human communities ideas of species to see if there is concordance |
| ‘Folk’ vs Linnaean | Survey a regions indigenous peoples and compare lists of animal and plant types with Linnaean species |
| 1) One to one correspondence | - evidence for reality of species |
| 2) Folk species are under-differentiated – 2 or more Linnean species per folk species | - evidence for reality of species if untrained observers cannot tell cryptic species apart |
| 3) Folk species are over-differentiated | - evidence against the reality of species |
| Is the folk argument convincing? | All humans have same visual system Do animals recognise the same species as we do? e.g. Mates or pollinators |
| Statistically identifiable clusters | If we use statistical tools do we see sympatric organisms falling into distinct clusters? Could use morphology, behaviour, molecular characteristics. Statistical methods will not identify clusters if they do not exist! |
| Goals of species concepts: | Helps with systematic classification Corresponds to discrete entities we see in nature Helps understanding of how species arise |
| Goals of species concepts: | Represents evolutionary history of organisms Applies to largest possible number of organisms |
| Biological species concept | Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups |
| Morphological species concept | A lineage judged to be evolutionarily independent on the basis of size, shape or other morphological features |
| Phylogenetic species concept | e.g. A species is the smallest monophyletic group of common ancestry |
| Genotypic cluster species concept | Species are groups that remain recognizable in sympatry because of the morphological and genotypic gaps between them |
| Example of cryptic species | Pipistrelle Bat (Pipistrellus pipistrellus) - Two forms with differences in frequency of calls identified by differences in mitochondrial DNA (11% divergence) - Now recognised as two species |
| Incomplete reproductive isolation - Greenish Warbler | Northern forms (viridanus and plumbeitarsus) do not interbreed (and have distinct songs and plumages and are distinct genetically), southern forms do interbreed (and have less distinct songs and plumages and are less distinct genetically) |
| Geographically variable degree of hybridisation in non-ring species | Spotted Towhee (Pipilo maculatus) and Collared Towhee (P. ocai) Hybridise in some areas where they co-occur but not in others |
| Example of application of phylogenetic species concept - elephants | Phylogeny based on 4 nuclear genes Forest v. Savannah Elephants show 58% of difference shown between African and Asian Elephants in characteristics |
| Species created by hybridisation | Heliconius cydno and Heliconius melpomene crossed in laboratory to re-create Heliconius heurippa H. heurippa: Occurs in nature (in Colombia) Does not breed with putative parent species Hybrid genome |
| Wolves and coyotes | Under MSC, Red Wolf is a species Under BSC, it is not a species as it hybridises with the Coyote Phylogenetic SC - suggests red wolves result from hybrids between the Gray Wolf and the Coyote! |
| Species concepts: summary | There is no consensus on what is the best species concept One should not necessarily expect a universally-applicable or totally watertight species concept – on evolutionary grounds |
| Species concepts: summary | Most species concepts agree on evolutionary independence as a key criterion Biological species concept remains most widely-followed, but not perfect |
| Speciation in the lab | Fruitfly drosophila Result = positive assortative mating: individuals from each group prefer to mate with opposite sex from same group, even across cages - Reproductive isolation as a by-product of adaptation to media (not separation per se) |
| Summary | Speciation – the formation of new species – is important because it is a major process in the generation of biodiversity Speciation must occur in nature and important elements of speciation have also been observed to occur in the laboratory |
| Summary | There is no universally agreed best species concept, and may never be, but the most widely-adopted is the biological species concept Speciation can occur by fusion of lineages as well as by fission |
Created by:
reub8n
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