Lecture 4 Flashcards
the biological species concept
can be considered equal to a group of organisms capable of producing fertile offspring
A more practical definition of species
a group of organisms which can and usually do interbreed to produce fertile offspring
Ring species
two end populations are reproductively isolated from one another but indirectly connected by a continuous chain of reproducing populations
its debatable how to classify such populations
the morphological species concept
defines a species by morphological characteristics, applies to both sexual and asexual organisms
mainly the species concept for extinct species where other info isn’t available
subjective criteria
cryptic species
species that are reproductively isolates so are separate biological species but morphologically similar
it may not have been initially recognised that they were separate species in term of the biological species concept
speciation
the formation of a new species
the source of biodiversity
a focal point of evolutionary theory
above the microevolutionary level of allele frequencies in populations
Macroevolution
evolutionary change above the species level
Allopatric speciation
gene flow is interrupted or reduced when a population is divided by an environmental barrier
two populations become isolated from each other over time and differences form
Sympatric speciation
speciation takes place in geographically overlapping populations
e.g due to reproductive barriers
Reproductive isolation
reproductive isolation is the existence of reproductive barriers that stop two species from interbreeding to produce viable and fertile offspring
Hybrids
are the offspring of crosses between different species
they occasionally occur and can be infertile or occasionally fertile
two main biological mechanisms of reproductive isolation
pre zygotic barriers- before fertilisation
postzygotic barriers- after fertilisation
Three prezygotic barriers
impeding different species from attempting to mate
preventing the successful completion of mating
hindering fertilisation if mating is successful
impeding different species from attempting to mate
habitat isolation in different habitats
temporal isolation in time
behavioural isolation by different behaviours
preventing successful completion of mating
mechanical isolation that prevents mating
hindering fertilisation if mating is successful
gametic isolation
Habitat isolation
two species encounter each other rarely or not at all as they occupy different habitats even though not isolated by physical barriers
temporal isolation
species that breed at different times of the day, different seasons or different years cannot interbreed
behavioural isolation
courtship rituals and other behaviours are unique to a species
mechanical isolation
morphological differences can prevent successful mating
gametic isolation
sperm of one species may not be able to fertilise egg of another
Postzygotic barriers
block hybrid survival and reproduction
three types of post zygotic barriers
reducing hybrid survival reduced- hybrid viability
reducing hybrid mating success
-reduced hybrid fertility in 1st generation
reducing ongoing hybrid mating success - hybrid breakdown 2nd generation
reduce hybrid viability
genes of the different parent species may combine to impair the hybrids development or health
reduced hybrid fertility
even if hybrids are healthy they may be sterile
hybrid breakdown
some first generation hybrids are fertile but when they breed with other species or with either parent species, offspring of the second generation are feeble or sterile
Hybrid zones
hybrids can occur when the reproductive barriers between species are incomplete
a hybrid zone is a region in which members of different species mate and produce hybrids
hybrid zones can tell us about mechanisms of reproductive isolation
a barrier to gene flow forms and the OG pop splits into two populations partially reproductively isolated but retain a hybrid zone
there are three possible long term outcomes of hybrid zones
reinforcement
fusion
stability
Reinforcement
differences increase until hybrids are no longer formed
-hybrids become less viable or fertile
over time the rates of hybridisation decreases
-finally the 2 species do not produce hybrids
-pre-zygotic mechanisms appear
fusion
barriers weaken until there’s one species
-fusion of populations can occur if there is continues substantial gene flow in a hybrid zone
-reproductive barriers weaken and 2 species fuse into one
stability
hybrids continue to be produced
-a hybrid zone may be stable over a long period of time, if some gene flow continues to occur
-usually involves only a small overlap between populations preventing fusion or reinforcement
