Phyletic gradualism and punctuated equilibrium in the late Neogene planktonic foraminiferal clade Globoconella
Phyletic gradualism and punctuated equilibrium in the late Neogene planktonic foraminiferal clade Globoconella
复制标题
新近纪晚期浮游有孔虫进化枝 Globoconella 的系统进化论和间断平衡
作者：
K. Wei;J. Kennett
古生物学研究中的大量地理覆盖对于测试系统渐进主义和间断平衡的进化模型至关重要。我们利用来自西南太平洋纬度线的四个深海钻探项目地点（DSDP 284、207A、208 和 588）的标本，对新近纪晚期浮游有孔虫分支 Globoconella 进行了多变量形态测量研究。在晚中新世（7Ma至5Ma）期间，祖先物种Globorotalia（Globoconella）conomiozea的种群形成了一个地理斜线，显示出从温带地点（DSDP 284和207A）到温暖的亚热带地点（DSDP 208和588）的连续形态变化。与北方种群相比，居住在南方的种群在最后的螺旋中具有更高的圆锥角和更少的室。然而，随着时间的推移，整个谱系的种群表现出一致的、方向性的趋势，即具有更大的锥角和更少的腔室。在中新世/上新世边界，塔斯曼锋（副热带辐散）的强化可能将暖亚热带的外围种群与温带水团的中心种群隔离开来。进化趋势变得脱钩：中心种群逐渐失去龙骨并转变为G.(G.)sphericomiozea，而暖亚热带地区的外围种群保留了龙骨并进化为扁平种G.(G.)pliozea。 G.(G.)conomiozeaterminalis（一种保留龙骨的形态）向G.(G.)sphericomiozea（一种缺乏龙骨的形态）的逐渐转变发生在大约0.2my的间隔内，所有测量的形态变量都显示出连续稳定的变化。中心种群的进化遵循系统渐进主义模型。在外围种群中，来自祖先 G. (G.) conomiozeaterminalis 的后代种 G. (G.) pliozea 的起源在不到 0.01 m.y 的间隔内非常迅速地发生。 G. (G.) pliozea 的种群规模在约 5.05 Ma 的初期阶段很小，但在接下来的 0.2 m.y 中迅速增加并成为优势。当祖先物种 G. (G.) conomiozeaterminalis 局部灭绝时。物种形成后，G. (G.) pliozea 表现出约 0.6 m.y. 的形态停滞，直到中心种群 G. (G.) punctulata 迁移回温暖的亚热带；在接下来的 0.5 米内由于它们的同源性，这两个姐妹物种之间没有杂交的迹象。 G. (G.) pliozea 的进化遵循间断平衡模型。 Globoconella 进化枝的进化表现出系统渐进性和间断平衡。这两种“另类”进化模型是相辅相成的，而不是相互排斥的。这两种模型对于提供 Globoconella 进化的完整图像都是不可或缺的。
Substantial geographic coverage in paleontological study is essential in testing evolutionary models of phyletic gradualism and punctuated equilibrium. We present a multivariate morphometric study of the late Neogene planktonic foraminiferal clade Globoconella using specimens from four Deep Sea Drilling Project sites (DSDP 284, 207A, 208, and 588) along a latitudinal traverse in the southwest Pacific. During the Late Miocene (7 Ma to 5 Ma), populations of the ancestral species Globorotalia (Globoconella) conomiozea formed a geographic cline showing continuous morphological variation from the temperate sites (DSDP 284 and 207A) to the warm subtropical sites (DSDP 208 and 588). Populations living to the south had higher conical angle and fewer chambers in the final whorl compared to the northern populations. Nevertheless, populations across the entire cline exhibited a coherent, directional trend towards having larger conical angle and fewer chambers through time. At the Miocene/Pliocene boundary, the intensification of the Tasman Front (Subtropical Divergence) possibly isolated the peripheral populations in the warm subtropics from the central stocks of the temperate water masses. The evolutionary trends became decoupled: the central populations gradually lost their keel and transformed into G. (G.) sphericomiozea, while the peripheral populations in the warm subtropical areas retained their keel and evolved into a flattened species, G. (G.) pliozea. The gradual transformation of G. (G.) conomiozea terminalis (a form retaining a keel) into G. (G.) sphericomiozea (a form lacking a keel) occurred during an interval of about 0.2 m.y., with all measured morphologic variables showing continuous and steady changes. The evolution of the central populations follows the model of phyletic gradualism. In peripheral populations, the origin of the descendant species G. (G.) pliozea from the ancestor G. (G.) conomiozea terminalis occurred very rapidly within an interval of less than 0.01 m.y. The population size of G. (G.) pliozea was small at the incipient stage at about 5.05 Ma, but increased rapidly to become dominant during the next 0.2 m.y. when the ancestral species G. (G.) conomiozea terminalis became locally extinct. Following speciation, G. (G.) pliozea exhibited morphological stasis for about 0.6 m.y., until the central stock form G. (G.) puncticulata migrated back to the warm subtropics; during the next 0.5 m.y. of their sympatry, there is no sign of hybridization between these two sister species. The evolution of G. (G.) pliozea follows the model of punctuated equilibrium. The evolution of the Globoconella clade shows both phyletic gradualism and punctuated equilibrium. These two “alternative” evolutionary models complement each other rather than being mutually exclusive. Both models are indispensable towards providing a complete picture of the evolution of Globoconella.