Darwin’s finches Their common ancestor arrived on the Galapagos about million years ago. During T r e e f i n c h es G r o u n d f i n c h e s G r a s p i n g b i l l s P r o b i n g b i l l s C r u s h i n g b i l l s Warbler finch ( Certhidea olivacea ) Woodpecker finch ( Cactospiza pallida ) Cactus ground finch ( Geospiza scandens ) Sharp-beaked ground finch ( Geospiza difficilis ) Small insectivorous tree finch ( Camarhynchus parvulus ) Large insectivorous tree finch ( Camarhynchus psittacula ) Vegetarian tree finch ( Platyspiza crassirostris ) Warbler finch Cactus eater Insect eaters Bud eater Parrot-like bill Small ground finch ( Geospiza fuliginosa ) Medium ground finch ( Geospiza fortis ) Seed eaters Large ground finch ( Geospiza magnirostris ) Fig . Darwin’s finches XII Std Biology-Zoology Chapter- XII Std Biology-Zoology Chapter- Evolution with average phenotypes survive whereas the extreme individuals from both the ends are eliminated. There is no speciation but the phenotypic stability is maintained within the population over generation.
For example, measurements of sparrows that survived the storm clustered around the mean, and the sparrows that failed to survive the storm clustered around the extremes of the variation showing stabilizing selection. ii. Directional Selection: The environment which undergoes gradual change is subjected to directional selection (Fig. .7b) .
This type of selection removes the individuals from one end towards the other end of phenotypic distribution. For example, size differences between male and female sparrows. Both male and female look alike externally but differ in body weight. Females show directional selection in relation to body weight.
iii. Disruptive Selection (centrifugal selection): When homogenous environment changes into heterogenous environment this type of selection is operational (Fig. .7c). The organisms of both the extreme phenotypes are selected whereas individuals with average phenotype are eliminated.
This results in splitting of the population into sub population/species. This is a rare form of selection but leads to formation of two