Evolution doesn't favour sexual reproduction
STUDIES reveal embarrassing exceptions to the common view that sexual reporduction scores over asexual reproduction ('mechanical' division into two, the way bacteria or amoeba does) in many ways. It is believed sexual reproduction enables individuals to generate a progeny with genetic variety that helps it to cope with future contingencies. Sexual reproduction helps concentrate 'good genes' within individuals and
gets rid of 'bad genes '. These presumptions have strengthened the belief that sexual reproduction is, more or less, necessary for a successful evolution of
a species, especially in the case of
multi-cellular higher organisms (Science, Vol 288, No 5469).
However, Tiny roundworm-like freshwater creatures known as rotifers stand up as exceptions. One class of rotifers, known as the bdelloids, do not have male counterparts. Reproduction occurs solely between females by a process known as parthenogenesis. Bdelloids are ancient creatures, dating back to at least 35 million years. Molecular biologists David Welch and Mathew Meselson performed tests based on the expectation that sexual reproduction entails a pairing of chromosomes and shuffling of genes between them during a process known as meiosis. Any species lineage that has a history of sexual reproduction should possess similar genetic sequences in different pairs of chromosomes. On the contrary, a long history of asexual reproduction will permit different
chromosomes to evolve independently; without sharing DNA sequences.
Four different species of bdelloids sampled by Welch and Meselson had dissimilar pairs of DNA. Scientists have been left wondering how these creatures have managed to flout what seems to be such an important rule in evolution -- no sex, no multicellularity, no genetic complexity.
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