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Speciation | OAT Species • Species - a group of individuals who can interbreed and yield fertile offspring • Through a process called speciation, individuals who were once able to interbreed become sexually isolated o They become unable to yield fertile offspring • After speciation occurs, the individuals are classified as different species Plant Cell with 3 Chromosomes • Allopatric (Geographic) Speciation • Allopatric speciation occurs when a physical barrier separates members of a species • Once separated, the individuals are exposed to different environments o And therefore, different selective forces • Through a process termed adaptive radiation, the nowseparated populations will evolve in response to their unique environments • After many generations of evolution, the populations may become so evolutionarily distanced that they can no longer interbreed o And thus, speciation has occurred If this cell undergoes meiosis I, we’d expect each tetrad to be pulled to opposite ends of the cell o Such that 2 new cells are yielded, each with 3 chromosomes Plant Cell Undergoing Meiosis I The tetrads are separated and pulled to opposite ends of the cell. • But suppose that nondisjunction occurs, such that one of the centrosomes fails to reel in its 3 chromosomes Nondisjunction In an event known as nondisjunction, chromosomes are not evenly distributed during meiosis I. One cell will end up with 6 chromosomes. Sympatric Speciation • Sympatric speciation is speciation that occurs in the absence of a physical barrier • Suppose we have a plant cell that contains 3 chromosomes • • • Mutant 6-chromosomed cells would be created Meiosis II would then form mutant 6-chromosomed gametes The 6-chromosomed gametes may be capable of growing into full-fledged plants 1 © 2017 J Co Review, Inc., Accessed by Guest on 05-10-2017 Speciation | OAT • o However, they will need to be fertilized by other 6chromosomed gametes 6-Chromosomed Gamete The original plant would ordinarily yield 3chromosomed gametes. But nondisjunction induced the generation of mutant 6chromosomed gametes. These gametes must be fertilized by other 6-chromosomed gametes. • • The 6-chromosomed mutant will no longer be able to breed with the 3-chromosomed plant o By virtue of this polyploidy, speciation has occurred o Plants of the two genotypes can breed, but cannot interbreed Thus, they are different species • Reproductive Isolation • Both allopatric and sympatric speciation yield the same resultreproductive isolation o Two species are left unable to breed with one another • There are a two categories of reproductive isolation o Pre-zygotic isolation o Post-zygotic isolation • Pre-zygotic isolation occurs when the speciation is such that the members of the 2 species don’t have the opportunity to reproduce o A zygote never even forms Examples of pre-zyogtic isolation: o Geographic isolation – the species are separated by a physical barrier o Temporal isolation – the species mate during different times of the year o Mechanical isolation – the sizes/shapes of the species’ reproductive organs are not compatible o Behavioral isolation – mating rituals differ between the populations Post-zygotic isolation occurs when speciation is such that a zygote forms, but that zygote is sterile or not viable for survival Examples of post-zygotic isolation o A horse and a donkey can breed to yield a mule But the mule is sterile o Because the horse and donkey could not yield fertile offspring, they are reproductively isolated o But since the isolation occurred after the zygote (the mule) formed, this is post-zygotic isolation Post-Zygotic Isolation A horse and donkey can mate, and successfully yield a child (a mule). However, that mule is infertile- it cannot reproduce- and so the horse and donkey are still considered to be reproductively isolated. 2 © 2017 J Co Review, Inc., Accessed by Guest on 05-10-2017 Speciation | OAT • Other post-zygotic barriers to reproduction include: o Zygote abnormality – the zygote forms, but dies during development o Low viability – the zygote develops, but dies prior to reaching adulthood Convergent Evolution The wings of birds and bats both serve to facilitate flight. But birds and bats do not derive their wings from a common ancestor. Trends in Evolution • Divergent evolution – multiple species retain a similar structure that they acquired by virtue of their common ancestor o i.e. humans and chimpanzees both have arms/legs by virtue of their common ancestor: 4-legged tetrapods • • There’s a tendency to think that evolution is a diversifying force, always making species more diverse o This is not true Consider a population of genetically-diverse horses A Population of Genetically Diverse Horses • Divergent Evolution Humans and chimpanzees both have arms/legs by virtue of their common ancestor: 4-legged tetrapods. • Convergent evolution – multiple species evolve similar structures, but did not acquire those structures by virtue of a common ancestor o i.e. birds and bats both have wings to facilitate flight, but did not acquire their wings from a common ancestor Suppose that the environment that these horses occupy becomes exceedingly hot o So hot that only a few members are able to withstand the heat Rising Temperature of the Enivironment With the increase in temperature, only a few members of the once-diverse population are able to survive. 3 © 2017 J Co Review, Inc., Accessed by Guest on 05-10-2017 Speciation | OAT • • Much of the population’s diversity has thus been lost This phenomenon is called evolutionary bottleneck and it causes a population to converge on a less-diverse population Polymorphisms • Certain physical differences exist among members of all species o i.e. subtle differences in height • These subtle differences are not evolutionary differences • Rather, they are slight genetic differences called polymorphisms Polymorphisms Certain genetic differences (i.e. subtle differences in height) are not evolutionary differences. They are the products of slight genetic differences called polymorphisms. 4 © 2017 J Co Review, Inc., Accessed by Guest on 05-10-2017