Toward the end of the carboniferous period, major changes in the climate occurred. The current day European and North American continents slammed together, forming the Appalachian mountains (which were taller, at that time, than the present-day Himalayas). Fossil and geologic records show a tendency toward a drier climate, with evidence of glaciation and lowered sea levels. Inland seas were increasingly diverted into distinct river channels as woody debris channelled the movement of waterways. In short, the terrestrial surface began to dry out and there was much more of it. The ancestors of birds, reptiles and mammals were adapting eggs that could survive outside of the water -- plants were working toward a similar strategy. Around this time, a group of animals likely took flight for the first time -- the insects! The presence of flying insects allowed for another option for distributing pollen, as well as a large source of potential herbivory.
The plants that would become the gymnosperms evolved xerophytic leaves (see lab 7) to prevent desiccation in the dry air. Some would have the ability to grow wider (and thus taller) via the production of a new layer of secondary xylem (wood) each year. These plants could also produce exterior layers of dead cells, unlike the living epidermis, called bark. Together, the production of bark and wood are part of a process called secondary growth. To increase the chances of fertilization in the absence of water, gametes began to be dispersed aerially via pollen. Perhaps most importantly, the zygote and female gametophyte were surrounded in a protective coating and dispersed as seeds. Both seeds and pollen develop within structures called cones.
The fossil record shows gymnosperms diversifying in a dry period called the Permian that followed the swampy Carboniferous period. Extant groups of gymnosperms include conifers, cycads (somewhat similar in appearance to palms), gnetophytes, and a single species from the ginkgophytes, Ginkgo biloba. Of the approximately 1,000 species of gymnosperms alive today, about 600 of these are conifers, 58 of which can be found in California. In fact, some of the oldest (bristlecone pine), tallest (coast redwood), and most massive (giant sequoia) organisms on the planet are conifers and all are native to California.
See this open-access paper for recent genetic work on the evolutionary relationships between gymnosperms: http://dx.doi.org/10.1098/rspb.2018.1012