BIOSC 0160 Lecture Notes - Lecture 15: Biofilm, Heterosis, Staphylococcus Epidermidis
22.1 Species are Reproductively Isolated Lineages on the Tree of Life:
Species: groups of organisms that mate with each other.
Species are the result of the process of speciation: the divergence of biological lineages and the
emergence of the reproductive isolate between lineages.
Three major classes of species concepts:
● Recognize by appearance: Carolus Linnaeus developed binomial nomenclature. Since
didn’t know about their genetics, classified them by appearance. Used morphological
species concept. A construct that assumes that a species comprises individuals that look
alike and that individuals that do not look alike are in a different species. This works out
because individuals that look alike often do share several similar genes that code for
those features. Limitations: males, females, young in same species don’t always look
alike. Also there are species that are visually identical but genetically/behaviorally
different=cryptic species.
Appearance, genetics, behavior?
Reproductive isolation is key:
Reproductive isolation: a state in which two groups of organisms can no longer exchange
genes. Ex: if group A mate and reproduce only with each other, then group A = distinct species
within which genes recombine=indep evolutionary lineage-a separate branch on the tree of life.
Ernst Mayr: biological species concept: species=groups of actually or potentially interbreeding
natural populations which are reproductively isolated from other such groups. This definition
limited to one point in time and does not include asexual organisms.
Lineage species concept: species as branches on the tree of life. Allows biologists to consider
species over evolutionary time.
Lineage: ancestor-descendant series of populations followed over time. Each species starts with
speciation event by which one lineage splits into two on the tree and ends at extinction or at
another speciation event in which it produces two daughter species. Limitation: sometimes hard
to tell whether a species will split at some point in the future or rejoin with another.
Dif species are not mutually exclusive. Rarely 100% isolated and distinct so some overlap.
22.2 Speciation is a Natural consequence of Population Subdivision:
A lineage can change over time without giving rise to a new species. Speciation requires the
interruption of gene flow within a species whose members previously exchanged genes.
Incompatibilities between genes can produce reproductive isolation:
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Dobzhansky-Muller model is quite simple: Assume a species is split by a new mountain range.
Each develops a new allele at a certain locus. These may still be compatible if came back
together an interbreed, but the offspring may or may not be inferior. Overtime build up several of
these allele differences and down the line become incompatible.
Reproductive isolation develops with increasing genetic divergence:
The amount of time it takes and how isolation occurs can vary.
22.3 Speciation May Occur through Geographic Isolation or in Sympatry:
Physical barriers give rise to allopatric speciation: Dominant mode of speciation in most groups
of organisms. Change may include new mountain range, body of water, dry land, etc. Any
habitat inhospitable to the species. As result: sister species form on either side of barriere bc
common ancestor recently but adapt to new envt, some genetic mutations, or genetic shift.
Allopatric speciation can also occur when members of a population cross an existing barrier and
establish a new isolated population.
Sympatric speciation occurs without physical barriers:
● Disruptive selection: when individuals with a certain genotype have a preference for
distinct microhabitats where mating takes place. Ex: a species thats initially all the same,
develops preferences for specific things within their shared habitat, then those with
shared preferences tend to mate amongst themselves.
● Polyploidy: duplication of a set of chromosomes within individuals.
○ Autopolyploidy: when polyploidy arises from chromosome duplication in a single
species. In this case the tetraploids and the original diploids are isolated from each
other because their hybrid offspring are triploids which don’t survive or are sterile
b/c their chromosomes do not segregate evenly during meiosis. But a tetraploid
can mate with another tetraploid.
○ Or allopolyploidy arises from combining of the chromosomes of two different
species. This hybridization usually disrupts normal meiosis
New species arise by polyploids more easily in plants than animals b/c plants of many species
can reproduce by self-fertilization. If polyploidy arises in several offspring of single parent plant
then siblings can fertilize one another.
22.4 Reproductive Isolation is Reinforced When Diverging Species Come into Contact:
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Reinforcement: mechanisms that prevent hybridization: Ex: if two diverging species can still
mate and reproduce but their offspring are not as successful, then selection favors more
homogeneous parents.
Prezygotic isolating mechanisms: mechanisms that prevent hybridization from occuring. These
come into play before fertilization.
● Mechanical isolation: mismatch shape or size of reproductive organs that doesn’t allow
reproduction. Ex: one orchid produces look and smell of a female wasp, when a male
comes and tries to mate with it, his mating behavior gets pollen on hair and then
complementary shapes stigmata are designed to get that pollen from the wasp. Insects
that do not try to mate with the flower don’t pick up the pollen.
● Temporal Isolation: many organisms have distinct mating seasons, if two closely related
species breed at separate times, they’ll never breed with each other.
● Behavioral Isolation: mating calls of similar types of frogs diverge quickly so females
won’t even recognize sister species as males to mate with b/c the call is dif. Also
preferences for dif colors or patterns, etc. Sometimes mating success depends on behavior
of individuals of another species: Ex: flowers of different shapes or patterns may or many
not attract their pollinators. If not, they’re not likely to reproduce.
● Habitat Isolation: when two closely related species evolve preferences for living or
mating different habitats won’t come into contact to breed with each other
● Gametic isolation: sperm of ones species won’t attach to eggs of another b/c the egg
doesn’t release the appropriate attractice chemicals or sperm can’t penetrate the egg. Ex:
this important for species that spawn by releasing sperm into the water, needs to meet the
right type of egg and not with others.
Postzygotic isolating mechanisms: mechanisms that reduce the fitness of hybrid offspring.
Results in selection against hybridization which in turns leads to reinforcement of prezygotic
isolating mechanisms.
● Low hybrid zygote viability: hybrid zygotes may fail to mature normally, either dying
during dvlpt or developing phenotypic abnormalities that prevent them from becoming
reproductively capable adults.
● Low hybrid adult viability: hybrid offspring less likely to survive than non-hybrids
● Hybrid infertility: hybrids may mature into infertile adults. Ex: mules are sterile.
Hybrid zones may form if reproductive isolation is incomplete:
Hybrid zone: areas where two closely related species overlap and hybridize. The first gen of
offspring usually crosses between the two different types, then subsequent genes include a
variety of individual with varying proportions of their genres derived from original two species.
Hybrids zones often have recombinant individuals from many gens of hybridization.
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Document Summary
22. 1 species are reproductively isolated lineages on the tree of life: Species: groups of organisms that mate with each other. Species are the result of the process of speciation: the divergence of biological lineages and the emergence of the reproductive isolate between lineages. Recognize by appearance: carolus linnaeus developed binomial nomenclature. Since didn"t know about their genetics, classified them by appearance. A construct that assumes that a species comprises individuals that look alike and that individuals that do not look alike are in a different species. This works out because individuals that look alike often do share several similar genes that code for those features. Limitations: males, females, young in same species don"t always look alike. Also there are species that are visually identical but genetically/behaviorally different=cryptic species. Reproductive isolation: a state in which two groups of organisms can no longer exchange genes.
