Species
an evolutionary independent population or group of populations
- genetically separate enough from other taxa that evolutionary mechanisms that affect one species are not shared by another
4 criteria to identify a species:
1) Biological species concept
2) Morphospecies concept
3) ecological species concept
4) phylogenetic species concept
What is the biological species concept?
1/4 criteria to identify a species
- species = member of a population(s) that can interbreed and (potentially) produce fertile offspring
- species are reproductively isolated due to reproductive barriers
the reproductive barriers of BSC
1) prezygotic isolation: prevents individuals from successfully mating or sperm from fertilizing the egg.
5 common barriers…
a) Temporal: Populations are isolated because they breed at different times.
- ie: Bishop pines and Monterey pines release their pollen at different times of the year.
b) Habitat: Populations are isolated because they breed in different habitats (in the same area).
- ie: hawthrorn & apple maggot fly
c) Gametic: Matings fail because eggs and sperm are incompatible.
- ie: Differences in the shape of bindin protein determine whether sea urchin sperm will penetrate eggs
d) Behavioural: Populations do not interbreed because they have different mating rituals.
- ie: fireflies have a sspecies-specific sequene of flashes that attract mates
e) Mechanical: male and female reproductive structures are incompatible.
- ie: spiked penis of a beetle
2) postsygotic isolation: individualds of adifferent species CAN mste but thir offspring dont survive or are infertile
2 common barriers….
a) Hybrid viability: Hybrid offspring do not develop normally and die as embryos.
b) hybrid sterility: Hybrid offspring mature but are infertile as adults.
- ie: A female horse can mate with a male donkey to produce a sturdy and long-lived mule. However, the mule is sterile.
BSC disadvantages
- if nearby populations don’t overlap its hard to tell if they are reproductively isolated or not
- doesn’t apply to asexual populations
- cant be tested on fossils
Morphospecies concept
1/4 criteria to identify a species
- same species if they have the same/similar morphology/anatomy (look the same).
Benefits and disadvantages to the Morphospecies concept
Good: widely applicable (can be used with sexual, asexual species and fossils)
Bad:
1) cannot identify cryptic species (species that can’t be distinguished from similar species by an anatomical trait)
2) naming +1 species when there’s really one polymorphic species (a species where individuals look different at different life stages)
ecological species concept
1/4 criteria to identify a species
- organism that have the same ecological niche (share the same habitat, food, predators, have the same range of environmental tolerances, et )
emphasizes natural selection
Benefits and disadvantages of the ecological species concept
Good: useful when identifying asexual species (ie bacteria)
Bad:
1) some species change their niche overtime
- ie: caddisfly -> larva(aquatic), adult (terrestrial)
2) some species share the same niche
- ie: purple shore crab and the yellow shore crab
the phylogenetic species concept
1/4 criteria to identify a species
- a species is the smallest non-divisible monophyletic group
- *if you snip a branch off the tree, everything that falls under it is a monophyletic group.
monophyletic group
evolutionary unit that includes an ancestral population and its descents only (no other descendants!)
- aka: clade, lineage
synapomorphy
a trait unique to a monophyletic group
Benefits and disadvantages of the phylogenetic species concept
Good:
- can be applied to any population (fossil, sexual, asexual)
Bad:
- carefully constructed phylogenies are only available for a few species
- cost lots of $$, time and analysis to construct new phylogenies
speciation
a lineage splitting event that produces 2 or more separate species
3 steps to speciation
1) population must be isolated, STOP GENE FLOW
- so mutation, natural selection and/or drift can act on a population independently
2) Populations genetically diverge due to mutation, genetic drift, natural selection)
3) prezygotic/postzygotic barriers to reproduction evolve (in case of recontact – to satisfy the BSC)
two types of speciation
1) allopatric speciation: geographic isolation
due to…
a) dispersal - some individuals disperse to a new location (founder effect)
b) vicariance - physical barriers separate a population (ie.canyons, coastal mountains)
2) sympatric speciation: reproductive isolation (live in the same geographic area such that breeding is possible)
a) Disruptive selection: individuals in a population become reproductively isolated by adapting to different ecological niches (habitat preference, food, etc)
what happens when isolated population some into contact?
- distinctions may be reinforces if hybrids have low fitness
- hybrid zones may be established if hybrids have intermediate fitness
- speciation by hybridization may occur if hybrids have high fitness
- *FINISH READ THIS SECTION (24.4)
paraphyletic group
- not all descendants of a common ancestor are included
INCLDES ancestor, not all descendants
polyphyletic group
a group composed of a collection of the organisms in which the most recent common ancestor of all the included organisms is NOT included
synapomorphies
shared derived characteristics (ie, hair in mammals)
- more closely relates species should share more synapomorphies
Two categories off homolgous traits
1) Ancestral traits: traits that were inherited from a distant ancestor
2) Derived traits: those that appear (via mutation) in the most recent ancestor
**depends which taxa you are referring to.
homologous trait
- aka homologs
- traits shared by two or more species due to a shared ancestry
therefore used to build a phylogenetic tree
analogous trait (aka homoplasies)
- two structures that look the same and have the same function BUT weren’t inherited from a common ancestor
- arise when different species live in similar ways and/or a similar environment, and so face the same environmental factors => similar problems can lead to similar solutions.
convergent evolution
when organisms independently evolve similar traits
- arise when different species live in similar ways and/or a similar environment, and so face the same environmental factors => similar problems can lead to similar solutions.
