在漫长的进化过程中，哺乳动物的基因组不断缩小，并且这种趋势今日仍未停止。
最新一项研究中科学家分析了7种哺乳动物、8种非哺乳类动物、3种植物的基因组，重点对长末端重复序列（LTR）予以分析。长末端重复序列是反转录病毒DNA基因组两端各有一个的长末端重复序列(5'—LTR和3'—LTR)，不编码蛋白质，但含有启动子，增强子等调控元件，是一种常见的转座子。
转座子插入DNA序列后通常会造成基因的失活，在人类基因组中转座子序列占到总DNA的45%，LTRs可占人体总DNA的8%。LTRs在基因组中一直处于非常活跃状态，在进化过程中，一种LTRs在不同物种基因组中的含量随时间不断变化。通过研究发现，某一种LTRs在不同物种中某一进化时间中存在高峰，然后这种LTRs会随基因组进化不断丢失。这个丢失现象有些是随机的，有些是由于自然选择淘汰掉多余的基因。老的LTR虽然在物种进化过程中不断减少，但是有新的LTR会出现在物种基因组中，因此基因组中的LTR处于一个不断更新状态，随着基因组进化总的LTRs是一个下降趋势。
但是研究人员发现在人类、狗、牛、老鼠等哺乳动物基因组中，新的和老的LTRs都呈现一个明显的下降趋势，科学家解释说造成这个现象的原因可能是种群数量巨大加快了自然选择作用。（生物谷Bioon.com）
生物谷推荐原始出处：
Genome Biology and Evolution (2009) Vol. 2009:2; doi:10.1093/gbe/evp007 published on May 22,2009
Independent Mammalian Genome Contractions Following the KT Boundary
Mina Rho*, Mo Zhou*, Xiang Gao, Sun Kim*, Haixu Tang* and Michael Lynch
Although it is generally accepted that major changes in the earth's history are significant drivers of phylogenetic diversification and extinction, such episodes may also have long-lasting effects on genomic architecture. Here we show that widespread reductions in genome size have occurred in multiple lineages of mammals subsequent to the Cretaceous–Tertiary (KT) boundary, whereas there is no evidence for such changes in other vertebrate, invertebrate, or land plant lineages. Although the mechanisms remain unclear, such shifts in mammalian genome evolution may be a consequence of an increase in the efficiency of selection against excess DNA resulting from post-KT population size expansions. Independent historical changes in genome architecture in diverse lineages raise a significant challenge to the idea that genome size is finely tuned to achieve adaptive phenotypic modifications and suggest that attempts to use phylogenetic analysis to infer ancestral genome sizes may be problematical.
