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2.29: Evolution and Adaptation To Extremes

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    Evolution and Adaptation To Extremes

    Adaptation is the driving force of evolution on many levels (microscopic to massive organisms; individual species to diverse communities). Environmental changes over time force species and communities (ecosystems) to adapt to special niches. Figure 2.75 shows the evolution and diversification of plants through geologic time. Some species able to spread across large regions by adapting to variable climate conditions that match their reproductive and feeding cycles. Ancient lineages that have survived extinction are often better adapted to living in harsh environments (such as lichens, mosses, and club mosses living in barren, rocky settings, Figure 2.76). Species like the Giant Sequoias that live in isolated communities in California's Sierra Nevada Range are remnant populations was once a much more widespread forest community that existed during the last ice age (Figure 2.77).

    Plant evolution through geologic time. Primitive plants (lichens, mosses, club mosses) growing on or around a boulder in the San Diego desert. A forest of giant sequoias in Yosemite National Park, California.
    Figure 2.75. Evolution involving competition and adaptations have led to a diversification of plants through geologic time. Figure 2.76. Ancient lineages of early plants (such lichens, mosses, and club mosses) have adapted to harsh environments on rocky settings. Figure 2.77. Giant Sequoias (the world's largest trees) in Yosemite National Park, CA are adapted to local climate conditions.

    Organisms that have adapted to living in vernal pools illustrate adaptation to extreme environmental conditions. A vernal pool is a small pool or pond that forms temporarily, such as after a summer thunderstorm, seasonal precipitation (Figure 2.79). During a short period when water is present, a variety of species have adapted to completing their entire life cycle in a matter of days to weeks before the water dries up or becomes too salty. Amazingly, species like tadpole shrimp, fairy shrimp, and other desert species have adapted to these extreme environmental conditions. Tadpole shrimp have fossil ancestry dating back to marine environments in middle Paleozoic time. Tadpole shrimp have basically survived longer than any known species by being able to adapt to a variety of extreme environment conditions (Figure 2.80).

    A staghorm coral reef with a variety of fish. A vernal pool that formed after summer thunderstorms on bedrock on the Navajo Reservation, northern Arizona. Tadpole shrimp in a vernal pool.
    Figure 2.78. Healthy coral reef communities are adapted to a stable yet limited range of environmental conditions: clear, shallow, warm seawater with good circulation. Today, reef communities worldwide are threatened by rapidly changing environmental conditions largely influenced by human activities (pollution, heat from global warming, and resource exploitation). Figure 2.79. Vernal pools like this one form in after a desert summer thunderstorm. Within days, species such as tadpole shrimp hatch, feed on limited food supply, grow to adult size, reproduce (producing cysts and eggs, both sexually and asexually) before dying off when the water dries up, sometimes for many years between periods of precipitation. Figure 2.80. Tadpole shrimp are brachiopod crustaceans that appeared in the marine fossil record about 400 million years ago, but are only found today in vernal pool habitats. Their body plan has remained more or less consistent over the course of the past 250 million years. These species have adapted to survive some of the harshest climate extremes on Earth.

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