lecture 18- speciation Flashcards
speciation
single ancestral species becomes two or more descendant species
what is a species
most resolved level of organization for how we classify the organisms in the world around us
evidence about species: species identification does not differ between cultures/peoples
evidence about species: animals can recognize different species
evidence about species: identification of statistical clusters
how many species exist on earth?
1.2 million now
biological species concept
reproductively isolated
reproductive barriers: prezygotic isolation
before the formation of the zygote
act to prevent or limit mating from even occurring (before a hybrid is produced)
reproductive barriers
features of organisms (or their ecology) that prevent or limit their ability to produce viable and fertile offspring with another species
reproductive barriers: postzygotic isolation
after the formation of the zygote
reduce the likelihood that a hybrid offspring will survive or itself reproduce after mating has occurred
5 examples of the reproductive barriers in prezygotic isolation
ecological: separated in space
temporal: separated in time
sexual/behavioral: separated by preference
mechanical: mating cannot physically occur (snails left vs right curl)
gametic incompatibility: eggs and sperm cannot combine (marine animals)
5 examples of prezygotic reproductive barriers: behavioral isolation
usually involves variation in male mating rituals or signals, and female recognition and preference for species-specific displays
caused by differences in behavioral
preferences, and most often these include mating rituals or signals.
These barriers may be substantial – as in the case of humans and
chimpanzees, our closest relatives – or subtle – as in the case of song
differences between closely related crickets
5 examples of prezygotic reproductive barriers: gametic incompatibility
corals and urchins
occurs when the egg and sperm themselves
are not compatible. This is common in broadcast spawners, that is
organisms that eject gametes into the water column, with the hope
that they will encounter each other (but do not fertilize if they meet a
gamete from another species)
postzygotic isolation: hybrid inviability
the offspring does not survive or is impaired in some way
postzygotic isolation: hybrid sterility
the offspring may be perfectly viable (horse plus donkey equal mule) but is infertile. often this is the case when there are chromosomal differences between the parents (so meiosis cant work properly)
speciation
the process where a lineage splits during evolution: a single ancestral species becomes 2 or more descendant species (darwin finch)
allopatric speciation
speciation in the absence of gene flow (geographically isolated populations)
allopatric speciation: vicariance
population split by extrinsic event
allopatric speciation: dispersal
some individuals disperse to a new isolated location
sympatry
populations in the same place
sympatric speciation
the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region
sympatric speciation: chromosomal rearrangement
plant evolution; new polyploid can sometimes self-fertilize
sympatric speciation: gradual divergence
for non chromosomal sympatric speciation to occur, there must be strong disruptive selection
disruptive selection
selection that favors different parts of the spectrum of variation in the population (counteracts the homogenizing influence of gene flow)
dobzhansky-muller model: speciation through epistasis
The Dobzhansky–Muller model describes the negative epistatic interactions that occur between different alleles (versions) of different genes with a different evolutionary history. These genetic incompatibilities can occur when populations are hybridising (decreased fitness of hybrids)
epistasis
interaction between genes
mapping the genetic incompatibilities between closely related species
limitations of bsc: fossils (reproductive isolation makes a species)
Fossils. In general (but not always), species are morphologically
distinct from each other, so we can categorize individuals according
to their morphological affinities. This provides us with a way, for
example, of classifying different fossils, or classifying extinct versus
living organisms. If they are morphologically similar enough (i.e., they
cluster in morphospace), we assign them to the same species; if
they’re not, we classify them as different species.
limitations of bsc: asexual species (reproductive isolation makes a species)
Most microbes reproduce asexually, therefore,
generally we define microbial species based on genetic similarity
(i.e., by comparing their genome sequences). However, even
normally asexual species occasionally undergo a form of genetic
exchange, corresponding roughly to sexual exchange of genetic
material. For example, some microbes conjugate – exchange
genetic material – via sex pili. Thus, many microbial species show the
same clustering in morphospace as we see in sexual ones
limitations of bsc: hybridization (reproductive isolation makes a species)
Some species – e.g., the tiger and the lion – that do
not normally encounter each other in nature can still produce viable
and fertile offspring (i.e., tigons and ligers) in captivity. Yet, I don’t
think any of us would question that tigers and lions are distinct
species. While hybridization can occasionally occur between closely
related species, if it’s not frequent enough to compromise the
identity of the species (i.e., cause them to merge into one), we are
comfortable considering both species to be “good biological
species”
5 examples of prezygotic reproductive barriers: mechanical isolation
occurs when the male and female reproductive
equipment is incompatible, thus mating cannot occur effectively for
physical reasons
5 examples of prezygotic reproductive barriers: temporal isolation
occurs when two species are separated in time. For
example, two species of plants may flower at different times and
therefore are unable to cross pollinate.
5 examples of prezygotic reproductive barriers: ecological isolation
occurs when two species are separated in space.
For example, while lions and tigers can mate and produce hybrid
offspring (e.g., the liger), they never meet in nature
genetic divergence
once two populations have diverged beyond a certain point, individuals in the two populations will no longer be reproductively compatible. In some cases, incompatibility may arise relatively rapidly. In other cases, the process can be gradual. There is no set rule for the rate at which speciation occurs, though it appears in general to occur slowly and gradually
Sympatric speciation: chromosomal rearrangement
Chromosomal
rearrangements are the result of large-scale changes in one or more
chromosomes, or in the number of chromosomes, that in effect
instantaneously isolate individuals from other members of the population.
Specifically, individuals (or populations) with different chromosome content
(or arrangements of chromosomes) have a hard time pairing properly,
resulting in offspring with non-viable combinations of chromosomes. This
seems to have been an important mode of speciation in plants.
Sympatric speciation: disruptive selection
disruptive selection is the result of selection favoring different extremes of a trait distribution and acting against intermediate phenotypes (i.e., hybrids)
disruptive selection can counteract the effect of gene flow, preventing the
diverging populations from becoming genetically homogeneous because
the hybrid offspring are at a disadvantage. While it is uncertain how
common this mechanism is in natural populations, what is clear is that very powerful disruptive selection is required to counteract the homogenizing effect of gene flow
