Speciation: How Species Form Modes of Speciation A species is a population of organisms that can interbreed and produce a viable offspring. Speciation is the formation of a new species (______________). In order for a new species to form, individuals from the original species must evolve to become reproductively _________ from the remainder of the population. The creation of a new species is continuous (_____________) and it is often difficult to pinpoint exactly when a new species forms. Reproductive Isolation biological barriers that keep species from reproducing Pre-zygotic isolating mechanisms either ____________ between species or prevent __________ of the eggs. These include: Behavioral isolation – different species use different _______ and other __________ clues to find and attract a mate. Examples: songs of birds and species specific pheromones Ecological/Habitat isolation - two species may live in the same region but in different habitats and may never ___________ each other. Example: elevation within a mountain range Temporal isolation - many species are kept separate by ________________ (mate at different times of the day, different seasons, or even different years) Mechanical isolation - may attempt to mate but are ___________________________ (lock and key genitals of some insects) Gametic isolation - if gametes of different species do meet, male gametes may not be able to __________ and fertilize an egg of a different species. Example: chemical markers on the surface of fish eggs. Post-zygotic barriers are when sometimes a sperm from one species is able to fertilize the egg of another species and a zygote is produced, but post-zygotic barriers prevent these hybrid zygotes from developing into ___________, _____________ individuals. These include: Zygotic Mortality – mating and fertilization is possible, but genetic differences result in a zygote that is unable to develop past an __________________. Example: goat and a sheep Hybrid Inviability – the hybrid individual develops and either ______ before birth or if born alive cannot survive to _______________. Example: tiger and leopard Hybrid Infertility - offspring produced is __________. Example: donkey and horse = mule Types of Speciation Sympatric Speciation – populations within the _______________________ and become reproductively isolated. Example: a mutation called polyploidy, where an organism has three or more sets of chromosomes; common in plants that self-pollinate - potatoes Example: two different species breed to produce a sterile offspring that then reproduces asexually - wheat. Allopatric Speciation – formation of a new species as a result of evolutionary changes ___________________ _________________. i.e. mountains, rivers or roadways. Eventually the split populations become so distinct that they are unable to breed even if brought back together. Example: Darwin's Finches Parapatric Speciation – the formation of a new species in an area where an organisms range does not significantly ______. Individuals at the periphery of a population tend to have a slightly different __________ than that of the parent population and are subjected to the founder effect and genetic drift. The Speed of Evolution Current models of evolution suggest that main types of change are at work: Gradualism – a model of evolution that views evolutionary change as _________________________. Big changes occur by the accumulation of many small changes. Punctuated Equilibrium – a model of evolution that views history as long periods of ___________ (equilibrium) interrupted by periods of __________. It seems that a lot of species undergo changes when they first diverge from the parent species. After that, the change is relative slow. Human Impact on Speciation and Mass Extinctions Human activities can affect the genetic diversity of a population in various ways: In addition to the affect humans have on species, there have been ____ notable ____________ events identified throughout history where ____________ in the environment caused by a major shift in the ___________ on the planet. The most recent of these was the extinction of the dinosaurs thought to have been caused by the ice age triggered by the impact of a large asteroid. This event was the catalyst for the adaptive radiation of _________.