L10. Diversification: Origin of Species Flashcards
Species
two things in biology:
- taxonomic rank: the most specific unit of taxonomic classification
- biological classification: a way we try to classify individuals based on a set of characters
Speciation
- mechanism of change
- creation of new species
- dependant on limited or absent gene flow between populations (if gene flows continues then they will remain genetically similar and won’t become separate species)
- only arise when there is minimal or no gene flow between two populations
Mechanisms of speciation
- change in gene flow
- mutation
- genetic drift
- natural selection
Act separately on each isolated population
Factors that may cause a population to stop exchanging genes
- geographic or physical barriers
- ex. mountains, rivers, deserts
- physically isolated populations rarely interbreed and therefore characterized by the lack of gene flow
Where did the variation that natural selection acts on come from?
- either already existed when they separated or newly arose from chance mutations
Mutation
random change that happens when DNA is replicated. Most are not adaptive. But some create new characters
What is genetic drift
Based on probability, the frequency of different genes in a population can change over time, even when not under selection pressure
- likely to occur in small populations, by chance
- whole traits may even be lost
How do we define a species (species concepts)
3 ways:
- morphological: phenotypic characteristics most important to this way of thought, useful application is looking at fossils. Limitations: doesn’t take modern genetics into account, not accurate, coarse grained way of differentiating groups
- Biological: defines a species as members of populations that actually or potentially interbreed (mate with each other) in nature and produce offspring that are fertile. Limitations: assumes sexual reproduction, very hard to test
- phylogenetic: species are groups of individuals that share a unique common ancestor, determined by showing that individuals share traits unique to that species. Causes them to cluster distinctly from other groups
Why do we care about how many different species there are?
conservation
How/why is gene flow halted between populations
- Allopatry: species in geographic isolation
- Parapatry: species with ranges directly adjacent to each other, ranges overlap slightly
- Sympatry: species existing in the same geographic location
Allopatric Speciation
Speciation caused by the geographic isolation of two different populations
- believed to explain the greatest number of species
- barrier depends on the type of organism (ex. marine vs. terrestrial)
- very important example is the break up of Pangea
- Speciation increased after super continent division
- believed to be the primary speciation which takes place on islands
Parapatric Speciation
speciation when the ranges of two speciating populations are actually overlapping, but gene flow is reduced
- could be due to the development of new niches
- could be due to huge ranges, when the populations at the far edges become very different. Eventually, individuals in the middle either disappear or become one of the two populations
- overlap zone in middle is hard to explain and is why the theory is contentious
Sympatric Speciation
A species arises from within a range overlapping with its origin species
- reproductive isolation: reproductive methods are incompatible, restricting gene flow, even in overlapping populations
Hybrid Speciation
- organisms that we consider to be different species can interbreed
- offspring is often sterile but not always
- rare in animals
- we don’t see often in the wild because ranges may not overlap or they may have different courtship rituals
- plants readily hybridize
Despeciation
- when two species become one
- when distinct lineages to collapse into a single lineage
