Tinn Oive, Oakley Todd H
Ecology Evolution and Marine Biology, University of California-Santa Barbara, Santa Barbara, CA 93106, USA.
Mol Phylogenet Evol. 2008 Jul;48(1):157-67. doi: 10.1016/j.ympev.2008.03.001. Epub 2008 Mar 13.
Dating evolutionary origins of taxa is essential for understanding rates and timing of evolutionary events, often inciting intense debate when molecular estimates differ from first fossil appearances. For numerous reasons, ostracods present a challenging case study of rates of evolution and congruence of fossil and molecular divergence time estimates. On the one hand, ostracods have one of the densest fossil records of any metazoan group. However, taxonomy of fossil ostracods is controversial, owing at least in part to homoplasy of carapaces, the most commonly fossilized part. In addition, rates of evolution are variable in ostracods. Here, we report evidence of extreme variation in the rate of molecular evolution in different ostracod groups. This rate is significantly elevated in Halocyprid ostracods, a widespread planktonic group, consistent with previous observations that planktonic groups show elevated rates of molecular evolution. At the same time, the rate of molecular evolution is slow in the lineage leading to Manawa staceyi, a relict species that we estimate diverged approximately 500 million years ago from its closest known living relative. We also report multiple cases of significant incongruence between fossil and molecular estimates of divergence times in Ostracoda. Although relaxed clock methods improve the congruence of fossil and molecular divergence estimates over strict clock models, incongruence is present regardless of method. We hypothesize that this observed incongruence is driven largely by problems with taxonomy of fossil Ostracoda. Our results illustrate the difficulty in consistently estimating lineage divergence times, even in the presence of a voluminous fossil record.
确定生物分类单元的进化起源对于理解进化事件的速率和时间至关重要，当分子估计与首次化石出现的时间不同时，常常引发激烈的争论。由于多种原因，介形虫是进化速率以及化石和分子分歧时间估计一致性方面具有挑战性的案例研究对象。一方面，介形虫拥有后生动物群体中最密集的化石记录之一。然而，化石介形虫的分类存在争议，至少部分原因是甲壳（最常形成化石的部分）存在同形现象。此外，介形虫的进化速率是可变的。在这里，我们报告了不同介形虫群体分子进化速率存在极端差异的证据。在广泛分布的浮游类群——海萤介形虫中，这种速率显著提高，这与之前观察到的浮游类群分子进化速率升高一致。与此同时，在导致史氏玛那瓦介形虫（一种残遗物种，我们估计它在大约5亿年前与其已知最近的现存亲缘物种分化）的谱系中，分子进化速率较慢。我们还报告了介形虫化石和分子分歧时间估计之间存在多个显著不一致的案例。尽管宽松时钟方法比严格时钟模型提高了化石和分子分歧估计的一致性，但无论采用何种方法，不一致性仍然存在。我们推测观察到的这种不一致主要是由化石介形虫分类问题导致的。我们的结果说明了即使存在大量化石记录，持续估计谱系分歧时间也存在困难。
