T6 - speciation Flashcards
What is speciation?
Process by which one species splits into two or more species
- eventually reaches an irreversible endpoint when 2 taxa cannot form fertile hybrids (they become entirely separate gene pools that alleles can move between)
What is a species?
Species = latin for ‘appearance’ or ‘kind’
What are biological species? (BSC)
Groups that do (or potentially can) interbreed and produce fertile offspring in nature and that do not form fertile offspring with members of other groups
- this ability means that gene flow is possible, allowing the exchange of alleles to slow or prevent accumulation of genetic differences
What is reproductive isolation?
The opposite of compatibility - the result of repreoductive barriers between the genetic species
= Biological features of organisms that reduce/prevent gene exchange with members of other groups
What are advantages and disadvantages of the BSC?
A;
- biological relevance = lack of gene exchange between populations and lack of gene flow means they are evolving independently
- provides a straightforward approach to studying speciation (how reprod barriers evolve)
D;
- cant be applied to asexual taxa such as prokaryotes (they dont mate) or fossils (cannot test is gene flow is possible)
- can be hard to assay reproductive barriers (sometimes results are behaviours that are hard to measure)
- many ‘good species’ exchange genes but NS maintains divergence despite ongoing gene flow
What is morphological vs ecological concepts ?
Morphological = defines species based on morphological similarity
- can be applied to fossils and asexual taxa but is subjective and misses cryptic species (those that look the same but house different habits/behaviours/genetics - i.e. superficially the same)
- groups that are morphologically distinct from one another are classified as different species
Ecological = defines species based on their ecological niche (physical and environmental conditions it requires and the interactions it has with other species
- can be applied to asexual taxa but not fossils; is subjective because niches are hard to quantify
What are the 2 types of reproductive barriers? how are they distinguished from each other?
Prezygotic = block fertilization by reducing the likelihood of mating, by preventing an attempted mating from being successful, or by impeding fertilization after a successful sperm transfer
Postzygotic = reducing the survival/reproductive success of hybrids (reducing fitness of the hybrid zygote) if the hybrid dies, the hybrid grows up but is infertile or the hybrid survives and can reproduce but has a lowered fitness
What is ecologically dependent postzygotic isolation?
Reduction in hybrid fitness due to a mismatch between the phenotype and the environment (hybrids fall between niches)
- can only be produced by divergent NS
What is allopatric vs sympatric?
Allopatric = occuring in separate, non overlapping geographic areas
- easy for speciation, all evolutionary processes can occur with no repercussions of gene flow (constantly homogenizes population)
Sympatric = occuring in the same, overlapping geographic area
- harder for divergence
- harder to separate the 2 without gene flow
Parapatric = something in between (partial overlap)
What happens in allpatric speciation?
- gene flow is interrupted by some extrinsic barrier
- populations evolve independently eventually accumluating reproductive barrier
- if there is secondary contact, populations are reproductively isolated
What happens in sympatric speciation?
- speciation occurs in the absence of any extrinsic or geographic barrier
- requires gene flow between 2 groups to be reduced to some factor (polyploidization, strongly divergent natural/sexual selection between habitats) -> ‘like mates with like’
- generally thought to involve the evolution of prezygotic barriers that prevent mating (behavioural/temporal/habitat isolation)
- requires some disruptive selection
What is biogeographic evidence or allopatric speciation?
- many closely related species pairs have geographic ranges that abut one another
- isolated habitats often harbour endemic species
- reproductive isolation among populations increases with geographic distance between them
What would NOT increase the likelihood of allopatric speciation?
Ongoing gene flow
Allopatric -> you dont want gene flow
What is ecological speciation?
Reproductive isolation that arises as a by-product of adaptation to different environments or niches
- NS differs in each population due to the unique biotic/abiotic environments they inhabit
- Divergent NS
- Phenotypic differences that evolve casue reproductive isolation as a side-effect
- populations may live in the same environments but utilize the habitat in different ways (using the habitat to their advantage differently)
What is polyploidization?
Occurs in sympatry = an increase in the number of whole sets of chromosomes
- postzygotic can be instantaneous (production of gamates with unbalanced numbers of chromosomes)
- common in ferns and flowering plants because many plants self-fertilize meaning they are self compatible to mate
