Class XD&H                                        Class Work-6

9-2-2014                                                Biology

Evolution by stages –

1

Evolution is a continuous and gradual process; complicated organs did not evolve by a single DNA change but were created bit by bit over generations e.g. eye is a complex organ of sight in animals. Flat worms have very simple eyes,even these rudimentary structures in flat worm provide advantage to the animal to ensure its survival. From this basic design more complex eyes later evolved in different organisms like eyes in insects, octopus other invertebrates and all vertebrates in cluding human beings have different structures and also have separate evolutionary origin. The complex structure of eye in these animals has been created in stages over many generations.

2

Some organ developed for one particular function but later became useful for quite a different function e.g. Feathers developed to provide warmth in cold weather to the animal but later became useful for flight

3

Some dissimilar looking structures also evolved from common ancestors. The

        current example of such a process is wild cabbage plant from which different

        vegetables are generated by artificial selection rather than natural selection

a)

Selection for short distance between the leaves has led to formation of cabbage that we eat.

b)

Selection for arrested flower development has led to broccoli

c)

Selection for sterile flowers has made cauliflower

d)

Selection for swollen stem has formed kohlrabi

e)

Selection for large leaves has formed a leafy vegetable kale

f)

Selection for coloured leaves formed red cabbage

Evolution verses progress - Evolution cannot be called progress from lower forms to higher forms. It is basically forming more complex designs while the simpler once also keep growing. Evolution is generation of diversity with the help of environmental selection. Bacteria which were formed first have the capacity to live in diverse conditions and are still flourishing on the other hand human beings which are highly evolved species cannot be called the pinnacle of evolution but yet another species in the evolving life forms.

Human Evolution - Human evolution has been studied with the help of excavation, time dating and fossil study. All human beings belong to single species i.e Homo sapiens. Human species have come from Africa. Some of our ancestors left Africa while others stayed on. These migrants slowly spread across the planet i.e. West Asia, Central Asia, Eurasia, South Asia and East Asia. They traveled to Indonesia, Philippines, Australia and America. They traveled forward and backward sometimes separating and sometimes coming back to mix with each other. They had come into beings as an accident of evolution.

                                                                                                                Seema Misra