快猫短视频

WHICH came first鈥攖he butterfly or the caterpillar? Well both, says a
British zoologist. Creatures such as butterflies did not gradually evolve until
they ended up with distinct larval stages, but are actually the result of a
genetic merger between a separate larva-like animal and a butterfly-like adult,
he says. Most biologists are extremely sceptical, but they鈥檒l need to rewrite
all their classification books if the theory is right.

Many invertebrates have separate larval and adult stages for good reason.
Free-swimming marine larvae, such as barnacle larvae, can disperse, unlike their
rock-bound parents. Larvae and grubs can also set up shop in ecological niches
adults can鈥檛 reach. But since larvae often look nothing like adults, how did
such complex life histories evolve?

Most biologists think that the same animal evolved both larval and adult
forms. They suggest that different environmental pressures on the juveniles
could have made them evolve into radically different forms.

But Donald Williamson of Liverpool University disagrees: 鈥淚 don鈥檛 think that
is an adequate explanation.鈥� He is convinced that the adult forms evolved first,
and then merged or 鈥渉ybridised鈥� with other adult animals to acquire the genetic
blueprint for larvae. This, he says, would explain why caterpillars and their
butterflies look so different.

Different species of animal can鈥檛 normally interbreed, but it does happen
occasionally, says Williamson. 鈥淚t only has to work once, and it鈥檚 done,鈥� he
says. Williamson claims to have created hybrids using sea squirt eggs and sea
urchin sperm, though none lived to reproduce
(快猫短视频, 22 May 1999, p 58).

鈥淲hat he is advocating is what no one else in his field would ever consider,鈥�
says Michael Hart, an evolutionary biologist at Dalhousie University in Nova
Scotia. Hart has tried sequencing the DNA from Williamson鈥檚 hybrids, but has
only found evidence of sea urchin genes.

Williamson argues that his hybrids amount to proof in principle. 鈥淵ou can鈥檛
repeat evolution,鈥� he says, speculating that the genetic barriers to
hybridisation were probably lower millions of years ago. However, Hart says that
differences in genes and chromosome structure between widely different species
would make them incompatible.

But Williamson says that hybridisation would also explain why widely
different animals have larvae that look alike. Sawflies, scorpion-flies and
butterflies and moths all have similar caterpillars, he says. He suggests that
the genome of a caterpillar-like adult animal, similar to the modern-day velvet
worm Peripatus, crossed into the ancestors of these three insect taxa
independently after they had diverged. Other researchers say these are examples
of convergent evolution or common ancestry, says Hart.

To convince the sceptics, Williamson will have to provide solid examples of
modern-day hybrids. 鈥淚 really like heretical ideas,鈥� says Hart, 鈥渂ut
extraordinary claims require extraordinary evidence.鈥�