School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA
School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA.
Philos Trans R Soc Lond B Biol Sci. 2017 Dec 5;372(1735). doi: 10.1098/rstb.2016.0420.
Evolutionary transitions in individuality (ETIs) occur when formerly autonomous organisms evolve to become parts of a new, 'higher-level' organism. One of the first major hurdles that must be overcome during an ETI is the emergence of Darwinian evolvability in the higher-level entity (e.g. a multicellular group), and the loss of Darwinian autonomy in the lower-level units (e.g. individual cells). Here, we examine how simple higher-level life cycles are a key innovation during an ETI, allowing this transfer of fitness to occur 'for free'. Specifically, we show how novel life cycles can arise and lead to the origin of higher-level individuals by (i) mitigating conflicts between levels of selection, (ii) engendering the expression of heritable higher-level traits and (iii) allowing selection to efficiently act on these emergent higher-level traits. Further, we compute how canonical early life cycles vary in their ability to fix beneficial mutations via mathematical modelling. Life cycles that lack a persistent lower-level stage and develop clonally are far more likely to fix 'ratcheting' mutations that limit evolutionary reversion to the pre-ETI state. By stabilizing the fragile first steps of an evolutionary transition in individuality, nascent higher-level life cycles may play a crucial role in the origin of complex life.This article is part of the themed issue 'Process and pattern in innovations from cells to societies'.
个体进化转变（ETI）发生在原本自主的生物体进化成为新的、“更高层次”生物体的一部分时。在 ETI 过程中，必须克服的第一个主要障碍之一是在更高层次实体（例如多细胞群体）中出现达尔文可进化性，以及在较低层次单元（例如单个细胞）中失去达尔文自主性。在这里，我们研究了简单的高级生命周期如何成为 ETI 期间的关键创新，允许这种适应性的转移“免费”发生。具体来说，我们展示了新的生命周期如何通过（i）减轻选择层次之间的冲突，（ii）产生可遗传的高级特征的表达，以及（iii）允许选择有效地作用于这些新兴的高级特征，从而产生并导致更高层次个体的起源。此外，我们通过数学建模计算了典型的早期生命周期在通过固定有益突变来固定有益突变的能力上的差异。缺乏持久的低级阶段并且以克隆方式发育的生命周期更有可能固定限制向 ETI 前状态进化逆转的“棘轮”突变。通过稳定个体进化转变脆弱的第一步，新生的高级生命周期可能在复杂生命的起源中发挥关键作用。本文是“从细胞到社会的创新中的过程和模式”主题问题的一部分。
