AP Bio Chapter 17: Speciation

speciation
divergence of biological lineages and emergences of reproductive isolation between lineages
-Different species concepts proposed by different biologists are different ways to approach question “what are species”
-Requires interruption of gene flow within a species whose members formly exchanged genes; single lineage can change over time without creating a new species
-Species: groups of organisms that mate with each other
reproductive isolation
when two groups of organisms can no longer exchange genes
-its evolution is most important factor in long-term isolation of sexually reproducing lineages from one another; necessary for lineages to remain distinct over time
-If a group of individuals only reproduce with one another, they are a distinct species: their genes recombinate and they are an independent evolutionary lineage: a separate branch on the tree of life
biological species concept
species are groups of actually or potentially interbreeding populations which are productively isolated from other such groups (proposed by Mayr)
-Actually: individuals live in same area and interbreed
-Potentially: individuals do not live in the same area nor interbreed, but they would interbreed if they did live in the same area
morphological species concept
-Linneaus described species based on their appearance; members of a species look alike because they share many alleles, made binomial system of nomenclature (classified on appearance alone)
-Limitations: members of same species don’t always look alike (e.g., male and female wood ducks) (sexual dimorphism)
-Cryptic species—two or more species that are morphologically indistinguishable but do not interbreed; species change little in appearance over large geographical distances
Lineage Species Concept
species are branches on the tree of life
-One species splits into two daughter species; evolve as distinct lineages; allows biologists to consider species over evolutionary time (long-term approach); splitting can take forever or can be quick
-Lineage: ancestor-descendant series of populations followed over time
-accommodates species that reproduce asexually
Three Species Concepts emphasize different aspects of species/speciation
1) Morphological species concepts: practical aspects of recognizing species; sometimes over or underestimates actual number of species
2) Mayr’s concept: reproductive isolation allows sexual species to evolve independently; most important for understanding origin of species because it is responsible for morphological distinctiveness in species (mutations that cause morphological change cannot spread between reproductively isolated species)
3) Lineage species concepts: sexual species are maintained by reproductive isolation, but species are a lineage over evolutionary time
Incompatibilities between genes
-can produce reproductive isolation
-If a new allele that causes reproductive incompatibility arises, it will not spread because the individual cannot reproduce
-Dobzkhansky-Muller Model; gene flow is interrupted and genetic drift can occur independently
Dobzkhansky-Muller Model
1) Ancestral population separates into two separate lineages that evolve independently
2) A new allele becomes fixed in each lineage, but at two different genes (locus)
3) Two descendant lineages become reproductively incompatible over time (two alleles are incompatible; hybrids are not possible)
Eg) in bats of genus Rhogeessa
-Works for pairs of individual genes and for chromosomal rearrangement
Dobzkhansky-Muller Model: chromosomal rearrangement
-Polymorphism in centric fusion causes few problems in meiosis because chromosomes can still align and assort normally
-If a different centric fusion becomes fixed in a second linage, they cannot have hybrids
Reproductive isolation increases as genetic divergence increases
-Reproductive incompatibility develops slowly due to slow pace of incompatible genes accumulating in each lineage
-Partial reproduction evolved in strains of Phlox, isolated by humans; in 1835, Drummond did experiments with plants and their reproductive compatibility reduced 14-50% over two centuries
how can speciation occur?
1) allopatric speciation
2) sympatric speciation
allopatric speciation
when populations are separated by physical or geographical barriers
-Dominant mode of speciation
-Barriers form when continents drift, sea levels rise/fall, and climates change
-Populations separated are usually large
-New lineages evolve differences from genetic drift and adaptations to new environment
-Can also result when some members cross existing barrier and establish isolated population; eg. Darwin’s finches in the Galapagos (islands have different environmental conditions)
sympatric speciation
occurs without physical barriers/isolation
-By disruptive selection: individuals with certain genotypes prefer distinct microhabitats where mating takes place; (eg. two groups of apple maggot flies; some lay eggs on harthorn, but some began to lay on apples and now prefer that if they lay their eggs early; two groups are becoming distinct species)
-Usually caused by polyploidy: duplication of sets of chromosomes within individuals (autopolyploidy & allopolyploidy)
sympatric speciation: autopolyploidy
chromosome duplication in a single species
-Can result in complete reproductive isolation in two generations
-Eg) Two unreduced diploid gametes combine to form a tetraploid; tetraploid and diploid individuals are reproductively isolated because their hybrid offspring are triploid; even if offspring survive they cannot reproduce. tetraploid + tetraploid can mate normally; can result in complete reproductive isolation in two generations
sympatric speciation: allopolyploidy
combining chromosomes of two different species
-When individuals of two different species interbreed; hybridization disrupts normal meiosis (can result in chromosomal doubling)
-Often fertile because each chromosome has almost identical partner to pair with during meiosis
reinforcement of reproductive isolation: If two incipient species come into contact, what keeps them from merging back into a single species?
-If reproductive isolation is incomplete, hybridization occurs
-Hybrids are less fit; selection favors non-hybridizing parents
-Selection reinforces (strengthens) isolating mechanisms (prezygotic and prozygotic)
reinforcement: prezygotic isolating mechanisms
prevent hybridization between species
1) Mechanical isolation: differences in sizes and shapes of reproductive organs prevent union of gametes from different species
2) Temporal Isolation: species breed at different times of year or times of day
3) Behavioral Isolation: individuals don’t recognize mating behaviors of other species
4) Habitat isolation: two closely related species evolve preferences for living or mating in different habitats
5) Gametic isolation: sperm and eggs of different species will not fuse; do not release appropriate attractive chemicals or sperm cannot penetrate the egg
examples of prezygotic isolating mechanisms
1) Mechanical isolation: flower color and shape influences which pollinators are attracted, or alters where pollen is deposited, two sympatric species of columbines (Aquilegia) have diverged in flower color, structure, and orientation; hummingbirds pollinate one, the other by hawkmoths
2) Temporal Isolation: closely related leopard frog species
3) Behavioral Isolation: mating calls of male frogs and coloration of male cichlid fish species
4) Habitat isolation: Rhagoletis flies
5) Gametic isolation: aquatic animals release gametes into water
postzygotic isolating mechanisms
result in selection against hybridization
-Reduce fitness of hybrid offspring; gametes fused but offspring are not fit and sometimes never survive to maturity
-Individuals that avoid interbreeding will have advantageous offspring
-To detect prezygotic isolating mechanisms: compare sympatric and allopatric populations of potentially hybridizing species; sympatric populations are expected to evolve more effective prezygotic reproductive barriers than do allopatric populations
types of postzygotic isolating mechanisms
1) Low hybrid zygote viability: fails to mature normally and dies during development or has phenotype abnormalities that prevent it from reproducing
2) Low hybrid adult viability: have lower survivorship than non-hybrid
3) Hybrid infertility: may mature into infertile adults (Eg. mules are healthy but they cannot reproduce)
hybrid zones
if reproductive isolation is incomplete, they may form where population ranges overlap
-may persist for long periods
-remains narrow because of strong selection against hybrids
-Eg) ranges of European Bombina toad species; individuals of both species move short distances into it
polyploidy is important to plants
-70% of flowering and 95% of ferns are from polypoidization
-new species can arise by polyploidy easily among plants because they can reproduce by self-fertilization
taxonomy/systematics
branch of science about classification/study and classification of biodiversity
cladistics
an approach to biological classification in which organisms are grouped together based on whether or not they have one or more shared unique characteristics that come from the group’s common ancestor
Three Domains
1) Eukarya: multicellular/eukaryotic, have membranes bound by nucleus, fungi, plants, animals
2) Archaea: prokaryotic, extreme living environments
3) Bacteria: prokaryotic, microorganism, in most habitats, can reproduce through spores
6 Kingdoms
1) plants: autotrophs (make own food), eukaroytic
2) animals: eukaryotic, largest kingdom, heterotrophs
3) archaebacteria: prokaryotic/unicellular, extreme environment
4) eubacteria: prokaryotic, most bacteria, complex
5) fungi: multicellular, not autotrophs, mushroom/mold/mildew
6) protists: most are unicellular, but complex cells (none of members are similar: all microscopic organisms that are not bacteria/plants/fungi), slime molds & algae
Lamarck’s idea
suggested that species change over time but no mechanism for how; also though individuals could change over their lifetime
natural selection occurs through interaction of
genetic variation in a population and the environment
natural selection
reduces genetic variability; removes individuals with deleterious mutations
phenotype variations resemble bel curve
contain a large range of variability; bell shows continuous variation of phenotypes