Speciation Flashcards
Species
There are various way we can define a species:
Morphologically → Organisms are classified as the same species if they appear identical by morphological (Anatomical Criteria)
Biologically → A group of similar individuals, capable of interbreeding, that produce fertile offspring
Evolutionarily → Members of interbreeding groups or populations that are reproductively isolated from other groups and evolve independently
→ There is no clear definition and there is always exceptions.
Speciation
The process by which new biological species arise
→ For a lineage to spilt once and for all, the two species must have genetic differences that are expressed in some way that causes matings between them to either not happen or to be unsuccessful.
↪ These don’t have to be huge differences. A small change in the timing, location or rituals of matings is enough.
→ The change might evolve by natural selection or genetic drift or any form of evolution
→ We can assume that speciation is in the process when two population are reproductively isolated from each other.
Reproductive Isolation
The inability of species to breed successfully with related species due to geographical, behavioral, physiological, or genetic barriers/differences.
(Different species is used in the definition as when one species forms two groups that are reproductively isolated, those two groups will lead to the formation of two new species)
Reproductive Isolating Mechanisms
Any behavioral, structural or biochemical trait that prevents individuals from reproducing successfully together.
There are two types: Prezygotic Mechanisms and Postzygotic Mechanisms
Allopatric Speciation
Occurs when populations of a species become geographically isolated.
↪ Something extrinsic to the organisms control that causes reproductive isolation, creating two new species (causing a lineage to speciate)
↪ Often a physical barrier (like a river forming)
Sympatric Speciation
The evolution of populations within the same geographic area into separate species.
→ Gene flow is reduced between populations
↪ Does not require a lot of geographic distance to reduce the gene flow amongst the populations.
→ Often organisms exploit new environmental niche which reduces gene flow (when herbivores insects try a new host plant)
Adaptive Radiation
When a single species evolves into a number of distinct/closely related species in one speciation event.
→ Is the possible result of Allopatric or sympatric speciation (Mainly Allopatric)
→ Each new species fills a different ecological niche (Darwin’s Finches: Look at slides for examples)
→ Usually occurs when a variety of new resources become available
→ Increases biodiversity
Prezygotic Mechanisms
Prevent interspecies mating and fertilization. (Effects before the zygote of a male and female gamete has formed)
~~The following are Prezygotic barriers~~
Mechanisms that prevent mating include…
→ Temporal
→ Ecological
→ Behavioral
Mechanisms that prevent fertilization include…
→ Mechanical
→ Gametic
Postzygotic Mechanisms
Prevent maturation and reproduction in offspring from interspecies reproduction. (Effects after the zygote of a male and female gamete is formed)
~~There are three types of postzygotic barriers~~
→ Zygotic Mortality
→ Hybrid In-viability
→ Hybrid Infertility
Temporal Isolation: Prezygotic Barrier
→ Two species breed at different times (During different times of day, seasons or even years)
Example: Red and black sea urchins live in the same location, but release their gametes at different times of the year.
Ecological Isolation: Prezygotic Barrier
→ Two species breed during the same season and in the same general area but not in the exact same place
Example: Bufo americanus (breed in quiet streams) and Bufo woodhousei (breeds in rain pools)
Behavioral Isolation: Prezygotic Barrier
→ There is little to no sexual interaction between the females and males of different species due to distinct behaviors. (This can be due to things like mating rituals. Mating Rituals prevent members of other species from recognizing or selecting a mate)
Ex: Two different species of gray tree frog (Hyla versicolor and Hyla chrysoscelis) are identical in appearance and inhabit the same regions of the USA. However, they have very distinctly different mating calls.
Mechanical Isolation: Prezygotic Barriers
→ Occurs when female and male sex organs are not compatible.
→ For plants this occurs when flower structures prevent certain animals from pollinating a plant.
↪ Important to plants that depend on animal pollinators.
Ex: If a bird beak fits in one species of flower, it likely will not be long enough to fit into a longer flower of a different species, therefore, it will not able to fertilize it/help the flower reproduce.
Gametic Isolation: Prezygotic Barriers
→ Male and female may mate, but the gametes do not unite to form a zygote (Can be due to different chromosome counts)
→ Many mammal sperm cannot survive in the female of different species
Ex: Sea urchins release eggs and sperm into the sea, but fertilization occurs only if species-specific - molecules on the surface of the egg and sperm attach to each other
Hybrid Inviability: Postzygotic Barrier
→ Fertilization can occur but the genes of the two parents are not compatible and the hybrids do not survive
