Historically, insects have been the most significant herbivores, especially the larvae of insects. The evolution of land plants is closely associated with the evolution of insects. While most plant defences are directed against insects, other defences have evolved that are aimed at vertebrate herbivores, such as birds and mammals.
The study of plant defences against herbivory is important, not only from an evolutionary viewpoint, but also because these defences can be used in agriculture, including human and livestock food sources.
Plant chemicals have evolved to affect insects. Some of these biochemical pathways exist in vertebrates, including humans. Many pharmaceuticals are got from the chemicals plants use to protect themselves from herbivores. Examples are opium, aspirin, cocaine, and atropine.
Pyrrolizidine alkaloids are produced by plants as a defence mechanism against insect herbivores. More than 660 PAs and PA N-oxides have been identified in over 6,000 plants. About half of them exhibit hepatotoxicity, that is, they attack liver function.
The earliest land plants evolved from aquatic plants around 450 million years ago (mya) in the Ordovician period. These early land plants had no vascular tissue and needed free water for their reproduction. Vascular plants appeared later and their diversification began in the Devonian era (about 400 mya). They had adaptations such as protective coatings to reduce evaporation from their tissues.
Reproduction and dispersal of vascular plants in these dry conditions was achieved through the evolution of specialized seed structures. The diversification of flowering plants (angiosperms) during the Cretaceous period is associated with the sudden burst of speciation in insects. This diversification of insects represented a major selective force in plant evolution, and led to selection of plants that had defensive adaptations. Early insect herbivores bit or chewed vegetation; but the evolution of vascular plants lead to the co-evolution of other forms of herbivory, such as sap-sucking, leaf mining, gall forming and nectar-feeding.
Our understanding of herbivory in geological time comes from three sources:
Long thought to be a Mesozoic phenomenon, evidence for herbivory is found almost as soon as fossils which could show it. Within under 20 million years of the first fossils of sporangia and stems towards the close of the Silurian, around 420 mya, there is evidence that they were being eaten. Animals fed on the spores of early Devonian plants, and the Rhynie chert also provides evidence that organisms fed on plants using a "pierce and suck" technique. Many plants of this time are preserved with spine-like growths, which may have had a defensive role.
During the ensuing 75 million years, plants evolved a range of more complex organs – from roots to seeds. There was a gap of 50 to 100 million years between each organ evolving, and it being fed upon. Hole feeding and skeletonization are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.
Herbivores depend on plants for food, and have evolved mechanisms to get this food despite the evolution of many plant defences. Herbivore adaptations to plant defence have been likened to "offensive traits". They are adaptations which allow increased feeding and use of a host plant. Relationships between herbivores and their host plants often results in reciprocal evolutionary change, called co-evolution.
When a herbivore eats, it eats plants that do not mount a sufficient defensive response. This creates the opportunity for plants which "can" mount a defence. So plants with defences become the more common. In cases where this relationship demonstrates "specificity" (the evolution of each trait is due to the other), and "reciprocity" (both traits evolve), the species are thought to have co-evolved.
The "escape and radiation" mechanism for co-evolution is the idea that adaptations in herbivores and their host plants have been the driving force behind speciation. The adaptations have played a role in the radiation of insect species during the age of angiosperms. Some herbivores have evolved ways to hijack plant defences to their own benefit, by sequestering (keeping) these chemicals and using them to protect themselves from predators.
Most of the plants which defend their leaves so vigorously nevertheless do need animals. Up to 98% of flowering plants in tropical lowland rain forests rely on animals for pollination and seed dispersal.
Wheeling Tunnel
The Wheeling Tunnel is a pair of tunnels in Wheeling, Ohio County, West Virginia. The tunnels allow Interstate 70 and U.S. Route 250 to go through Wheeling Hill. Each tunnel has two lanes of each road.
The tunnel is run by the West Virginia Division of Highways. Opened in 1966, the Wheeling Tunnel cost $6.9 million to build. It was rebuilt between 2007 and 2010 for $13.7 million. In 2009, an average of 59,600 cars used the tunnel every day.
The Wheeling Tunnel is between exits 1A and 1B on Interstate 70 and U.S. Route 250. These two highways use the same road through the city of Wheeling to go through Wheeling Hill. People who need to use exit 1A at the western end of the tunnels use the tunnels' right lanes as acceleration (speeding up) and deceleration (slowing down) lanes. Changing lanes is not allowed inside the tunnel.
Several accidents involving semi-trailer trucks caused local politicians to try to ban trucks from the tunnel. Their plan would have the trucks use the nearby I-470.
A total of 13,000 square feet (1,200 m) of industrial tile was used to cover the inside of the tunnels. The tunnels cost $6.9 million (which is the same as $ million in 2021) to build. The then-governor of West Virginia, William Wallace Barron, and the state roads commissioner started construction on the tunnel on August 22, 1963. Barron said it was the largest single building project in the state's Interstate Highway program. The state gave the construction contract to C.J. Langenfelder & Son, Inc. in Baltimore, Maryland. During construction, several workers became sick after breathing a lot of carbon monoxide. As a result, the company stopped construction in 1964 until fans were installed. The tunnel officially opened to traffic on December 7, 1966.
The West Virginia Division of Highways began planning to rebuild the tunnel in 2005. Rebuilding began in January 2007. The plan allowed three months for the work. However, rebuilding of the eastbound tunnel took over eleven months. There were problems with the glue that attached the tiles to the tunnel wall. Problems with drains in the tunnel also caused delays. Workers found a coal mine in the tunnel. This also delayed the project.
Local officials suggested closing the tunnels because of these problems. They planned to just build a road over Wheeling Hill instead. Building the new road would have removed at least fifty families from their homes. The National Association for the Advancement of Colored People (NAACP) was against this plan because those people's houses would be destroyed. The state canncelled the plan for the new road. Rebuilding of the eastbound tunnel started again in July 2008.
During this rebuilding of the eastbound tunnel, the westbound tunnel was only closed overnight for rebuilding. New guard rails, fire sprinklers, and security cameras were added. The whole tunnel was also cleaned very well. Workers put a new road surface in the tunnel. A German tile maker replaced the tiles. The German company got the contract because no other company could finish the work on time. The company finished rebuilding the eastbound tunnel on October 31, 2008. The state closed the westbound tunnel in February 2010. It opened a month ahead of schedule in September 2010. The total cost of the project was over double the original plan. It totaled $13.7 million because of the delays.
Postgenderism
Postgenderism means 'beyond gender'.