28-1-2014                                            CLASS WORK -5

Class X-D&H                                        BIOLOGY

SPECIATION – Origin of new species from the existing one due to reproductive isolation of a part of its population is called speciation.

The factors that could lead to rise of a new species are:

Gene Flow – It means exchange of genetic material by interbreeding between populations of the same species or between individuals within a population. It increases the variation in the genetic composition of a population.

Genetic Drift – The elimination of the genes of certain traits from the small population when a section of the species population dies of natural calamity or migrate to other region is called genetic drift. It alters the gene frequency of the remaining population Genetic drift can lead to accumulation of changes in the generations. 

Natural Selection – According to Darwin natural selection also plays an important role in bringing about evolution of new species of plants and animals. According to him variations existed between the individuals of a population and some natural phenomena eliminated those individuals which were less adapted. The surviving population would pass the hereditary advantageous features to their offsprings. With time this process would give rise to organisms different from the original population and new species are formed

Reproductive Isolation – Genetic drift and natural selection make the geographically isolated sub-populations of species to become more and more different from each other. Ultimately reproductive isolation occurs between individuals of these two groups. As a result they are unable to reproduce even if they happen to meet somehow. These two groups then transform into new species. This can happen in a number of ways

1.     DNA changes are severe enough such as a change in the number of chromosomes eventually the germ cells of the two groups cannot fuse with each other.

2.     It prevents interbreeding of two groups due to differences in mating behavior. (A new variation emerges in which green females will not mate with red males but only with green males. This allows very strong natural selection for greenness. Now if such a green female beetle meets a red male from the other group her behavior will ensure that there is no reproduction between them. Effectively new species of beetles are being generated)                 Recapitulation Exercise

Q.1 What factors could lead to the rise of a new species?

Q.2 Will geographical isolation be a major factor in the speciation of a self pollinating plant species? Why or why not?

Q.3 Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?                                       SEEMA MISRA