Forum > Bio-Diversity > Can evolution run backwards?

Posted by Susan Sharma on July 06, 2006


Can two species that have evolved from one species collapse into one once again? In other words, can evolution run backwards?

Two fledgling species can become different enough genetically so that they can no longer hybridize effectively. But, if the barriers to gene flow come down too soon the two may hybridize and merge again. A recent issue of New Scientist ( 20 May 2006) describes two studies that point to such a possibility. One study relates to two finch ( a small seed-eating songbird) species on the Santa Cruz island ( off the coast of California )-one with large bills and one with small bills-but rarely medium sized ones. This feature reflects two populations specializing in eating two different sizes of seeds. This was in 1960. Four decades later the researchers found that only birds living in sparsely settled parts of the island still showed two different bill sizes. Near the island's only town, birds with middle-sized bills had become more common! The earlier two distinct groups had collapsed into one! What could be the reason for the change? The researchers attribute this to the fact that people are providing bird feeders filled with rice and hence it is no longer a disadvantage to have an intermediate beak! Apparently everybody can eat this rice! Is the impact of human beings on environment forcing evolution into reverse?

Another study relates to Homo Sapiens-human beings. It is really asking who we are and where we came from! True, humans did not evolve from modern apes, but humans and modern apes shared a common ancestor, a species that no longer exists. In other words, we are cousins. Evolution is not a ladder. it is a branching bush. because we shared a recent common ancestorwith chimpanzees and gorillas, we have many anatomical, genetic, biochemical, and even behaviuoral similarities with the African great apes. We are less similar to the Asian apes -orangutans and gibbons- and even less similar to monkeys, because we shared common ancestors with these groups in the more distant past. In this study, genomes of humans, chimps and gorillas were compared using a "molecular clock" to estimate how long ago the three groups diverged. The further back two species diverged, the more differences would have accumulated between their genome sequences. The study suggests that the two lineages split over 6.3 million years ago. But later both the species re-hybridized in a "reverse speciation" event! Complete speciation between humans nad chimpanzees took place less than 6.3 million years. Natural selection then favoured those hybrid individuals whose chromosomes carried fewest of the genes that lower fertility! Evolution just selected what worked! May be, hybridization between the two fledgling species might have provided traits that saved our ancestors from extinction! The growing genomic information should bring us closer to the understanding of the key steps in evolution-the origin of species. Surely every bit of bio-diversity is invaluable. We never know which one would trigger the next innovation. 

Excerpts from article by Dr.V.B.Kamble at


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