Agood scientific theory is often

many phylogenetic trees have a single lineage at the base representing a common ancestor. a lineage that evolved early from the root and remains unbranched is called basal taxon. when two lineages stem from the same branch point, they are called sister taxa.

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explanation:

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.