Sunday, August 12, 2007

Trilobite variation declined after the Cambrian Explosion

From an evolutionary perspective, the more variable a species is, the more raw material natural selection has to operate on. So a highly variable species will evolve more rapidly than others.

Is that statement true?

Paleontologists for decades have suspected that highly variable species evolved more rapidly than others, and several studies have approached questions pertaining to it--but this is the first to convincingly document it in any group.

Most studies have focused on the variability that occurs between species rather than within them, but one recent study analyzed 982 species of trilobites, ancient relatives of spiders and horseshoe crabs.

When did Trilobites live?
Trilobites have been extinct for over 250 million years. They were once the most common creatures in the world's oceans. They ranged in size from nearly microscopic to more than a foot long, though most of the 17,000 known species measured from one to four inches. They were very diverse.

Trilobites were among the creatures that emerged 500 million years ago, during what paleontologists call "the Cambrian explosion," or "the Cambrian radiation." Before this time, life on Earth was limited mostly to bacteria, algae, single-celled organisms and only the simplest animal groups. But during the Cambrian Period, more complex creatures with skeletons, eyes and limbs emerged with amazing suddenness.

What does the research show?

So the question is what fueled the Cambrian radiation, and why was that event so singular? The answer: It appears that organisms displayed "rampant" within-species variation in the 'warm afterglow' of the Cambrian explosion, but not later.

A study focused on actively evolving characteristics during the Cambrian time. The trilobite head alone displayed many different characteristics. There were differences in ornamentation, number and placement of spines, and the shape of head segments. Overall, approximately 35 percent of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving. But more than 70 percent of early and middle Cambrian species exhibited variation, while only 13 percent of later trilobite species did so.

Conclusion: There's hardly any variation in the post-Cambrian. Even the presence or absence or the kind of ornamentation on the head shield varies within these Cambrian trilobites and doesn't vary in the post-Cambrian trilobites.

Why does variation withing a species decline through time?
Paleontologists have proposed two ideas to account for why variation within species declined through time.

1)Ecological. In the very early Cambrian seas, fewer organisms existed than today, which meant that they faced less competition for food. You didn't really have to be tightly specialized to make a living in the Cambrian. But as evolution gave rise to more varieties of organisms, ecological communities became more diverse. You had to be very fine-tuned to your particular niche to make a living and to beat out competitors for a limited resource. More organizms in the ocean meant that there must be more genetic variation in order to survive.

2) The genomic hypothesis offers a second explanation for the decline of within-species variation over time. According to this idea, internal processes in the organism were the key factors. Various developmental processes interact with one another to control the growth and formation of body parts as any organism progresses from egg to adult.

It's been suggested that early on in evolutionary history, in the Cambrian Period, the degree to which these different developmental processes interacted with each other within the organism was a lot less. As a result, the constraints on what the final organism looked like were relatively low.

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