Macroevolution

macroevolution

1. Factors of large evolutionary changes(Macroevolution)

2. Macroevolution Macroevolutionary  studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes (typically described as changes in allele frequencies) within a species or population.

3. Adaptive radiations The term adaptive radiation was coined by H.F. Osborn (1902 ). It is the evolution of a number of descendants with a great variety of adaptations to different niches from a single ancestor.

4. The phenomenon of adaptive radiation is the diversification of a dominant evolutionary group into a large number of subsidiary types adapted to more restrictive modes of life (different adaptive zones) within the range of the larger group

5. According to George Gaylord Simpson (1940, 1953), adaptive radiation is the rapid proliferation of new taxa (species ) from a single ancestral group . Certain authors of evolution biology such as Savage ( 1969),Stanley (1979) and Volpe (1985) have used an entirely new term macroevolution for the Osborn’s law of adaptive radiation .

6. Examples of Adaptive Radiation Adaptive radiation in Darwin’s finches . A classical example of speciation involving the interplay of complex forces (such as isolation, competition, adaptation, etc.) leading to adaptive radiation at the species level is provided by Darwin’s finches.

7. These birds belong to largest family of birds , the Fringillidae and live in Galapagos Islands in the Pacific ocean. The study of these small dark birds in their native habitat gave Darwin his first insight into evolutionary processes .

8. The present-day assemblages of Darwin’s finches descended from small sparrow-like birds that once inhabited the mainland of South America. The ancestors of Darwin’s finches were early migrants to the Galapagos Islands and probably the first land birds to reach the islands. These early colonists have given rise to 14 distinct species, each well adapted to a specific niche. Thirteen of these species occur in the Galapagos, one is found in the small isolated Cocos Island, northeast of Galapagos.

9. Not all 13 species are found on each island. These 14 species belong to 4 genera : 1. Geospiza 2. Camarhynchus 3 . Certhidea and 4 . Pinaroloxias .

10. The most conspicuous difference among the species are in the sizes and shapes of the beak, which are correlated with marked differences in feeding habits .

11. The ancestral finch was a ground-dwelling, seed-eating finch. After the burst of speciation in the Galapagos, a total of 14 species exist: ground-dwelling seed-eaters; some living on cactuses and eating seeds; some living in trees and eating seeds; and species of tree-dwelling insect-eaters.

12. Orthogenesis Orthogenesis is the assumption that evolutionary changes or variations are produced in straight lines. The observed tendency of a part or organ to change progressively in size is called orthogenesis or evolution in a straight line. The theory of orthogenesis was proposed by Eimer in 1897.

13. Evidences of orthogenesis Analogous or parallel variations are of frequent observation. They are the modification of similar characters which appear in different branches of the same large group or in unrelated group. For example the total reduction of side toes among artiodactyl in several unrelated genera such as giraffe,camel and prong-buck( Antilocarpa )

14. Another example existed between the pseudohorses of South America and true horses of nothern hemisphere,both lines showing a three-toed and finally a one-toed modification. There are several structures which evolved beyond the point of usefulness,such as tusks of Jefferson mammoth and antlers of Irish deer.

15. Allometry Allometry  is an important method for describing morphological evolution. It is the relation between the size of an organism and the size of any of its parts .

16. Within a taxon , most animals share a common body plan that comprises a certain number of body parts arranged in a particular way. For example, all insects share a head, thorax, abdomen, one pair of antennae, two pairs of wings and three pairs of legs. Yet out of this simple body plan has arisen the enormous diversity of insect forms that we see around us. Most of this diversity is a result of changes in the relative size of the different body parts.