12.0.3 12.6: With the evolution of the seed, gymnosperms became the dominant plants on earth.

Gymnosperms include four major groups: the conifers, cycads, gnetophytes, and ginkgo (just one species) (FIGURE 12-14). All of the 900 or more species of gymnosperms are seed-bearing plants that produce ovules on the edge of a cone-like structure. During the middle of the Mesozoic era, 160 million years ago, the forests that dinosaurs walked through consisted entirely of gymnosperms—pine trees, redwoods, cycads, and their relatives. Flowering plants (angiosperms) did not appear for another 35 million years. Indeed, when it comes to population size and habitat range, pine trees and their relatives are among the most evolutionarily successful groups of plants on earth, growing on every continent except Antarctica and extending from sea level to the tree line on mountains (FIGURE 12-15).

Pines, spruces, firs, redwoods, and their relatives are familiar to residents of the temperate regions: many of these conifers have needle-like leaves—but that is not true of all gymnosperms. The ginkgo has distinctive fan-shaped leaves, and cycads have palm-like fronds with many small leaflets (see Figure 12-15). Both are nearly identical to fossils from the Mesozoic era.

The reproductive structures of gymnosperms—the cones—are male or female. The pine cones you are probably familiar with are the female cones, which produce the ovules and, eventually, the seeds (FIGURE 12-16). The male cones are smaller and release pollen that is blown by the wind, and some of it reaches the ovules, which lie beneath the protruding scales of the female cones. The quantity of pollen released by conifers is beyond imagination— pollen-heavy air in a pine forest becomes hazy, and every surface is coated with yellow. Wind dispersal is clearly an inefficient method of pollination (getting the pollen to the vicinity of the ovule): billions of pollen grains are wasted for each grain that lands on a female cone and produces sperm to fertilize an egg in an ovule. Nonetheless, this “brute force” method of ensuring fertilization works: for more than 200 million years, pines have been successfully pollinated by wind.

When the pollen arrives at the female cone, a pollen tube forms and transports the haploid sperm to the ovule, where fertilization occurs and a diploid embryo begins to grow. The embryo develops slowly within the female cone over the course of many months, until the scales of the cone open and release the seed, ready to sprout into a new plant (FIGURE 12-17). With the evolution of seeds, gymnosperms developed a life cycle with no free-living haploid stage such as we see in non-vascular plants.

This evolutionary change in life history probably reflects the different degrees of evolutionary fitness of haploid versus diploid organisms. A haploid organism has just one copy of each chromosome, which means it has just one copy of each allele. If the haploid organism carries a defective allele for a critically important gene, the organism is doomed, because it has only that one defective version of the gene. In contrast, diploid organisms have two sets of chromosomes and thus two copies of each allele. If one copy is defective, the second copy can function as a backup, enabling the plant to produce the gene product. As a result, mutations are much less likely to be lethal for diploid organisms than for haploid organisms.