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https://allyouneedisbiology.wordpress.com/2015/05/07/evolution-amphibians/
https://courses.lumenlearning.com/boundless-biology/chapter/amphibians/
http://chem.tufts.edu/science/evolution/fish-amphibian-transition.htm
https://www.thoughtco.com/300-million-years-of-amphibian-evolution-1093315
https://www.nationalgeographic.com/animals/amphibians/r/red-eyed-tree-frog/
http://digimorph.org/specimens/Dermophis_mexicanus/
https://www.nationalgeographic.com/animals/amphibians/t/tiger-salamander/
http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amphibfr.html
https://www.ncbi.nlm.nih.gov/books/NBK10113/
https://www.ncbi.nlm.nih.gov/books/NBK10113/
http://www.michellecaldwell.com/linkfiles/scaredcricket/articles/other/evolution.html
https://www.biology-online.org/dictionary/Divergent_evolution

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American Bullfrog
European Green Toad
Blue-tailed Newt
Giant Marine Toad
American Toad
Alligator Newt
Fire Salamander
Hellbender
Tiger Salamander
Caecilian

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Habitat:
Red-Eyed Tree Frogs can be found in tropical lowlands. They can be found in rain forest canopies throughout Southern Mexico and Central America.
Physical Characteristics:
-Long, hind legs
-Smooth, moist skin
-Disk shaped, adhesive pads on fingers and toes to assist with climbing
-Red eyes
-Sticky, long tongues
Behavioral Characteristics:
Red-eyed tree frogs are carnivorous creatures and nocturnal hunters. Also, they use startle coloration to void off predators.
Taxonomy:

Kingdom | Animalia
Phylum | Chordata
Class | Amphibia
Order | Anura
Family | Hylidae
Genus | Agalychnis
Species | Callidryas

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Habitat:
Tiger Salamanders can be found in forests, grasslands, or marshes. They are known as "mole salamanders" because they live underground for most of their life. A large amount of tiger salamanders can be found across the United States.

Physical Characteristics:
-17-33 cm
-Thick-blooded
-Most common salamanders have yellow, "tiger" spots on a black background
-Webbed feet
-Develop air breathing lungs once they leave the water
-Small eyes
-Tails contain poison
Behavioral Characteristics:
Tiger Salamanders are burrowers who dig underground to avoid extreme temperatures on the surface. They feed and hunt small insects as carnivores. During mating season, tiger salamanders will all meet at a pond or body of water and mate. Also, they use their poisonous tails against predators.
Taxonomy:

Kingdom| Animalia
Phylum| Chordata
Class| Amphibia
Order| Caudata
Family|Ambystomatidae

Genus| Ambystoma

Species| Tigrinum

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Habitat:
Mexican-Burrowing Caecilian can be found in tropical climates, especially swamy areas underground. Most of them are located in Mexico.
Physical Characteristics:
-7in to 4.5 ft in length
-Limbless
-Legless
-Worm-like appearance
-Have jaws and teeth
-Tentacles
-Lungs
Behavioral Characteristics:
These worm like creatures enjoy to bury themselves in soft soil and use their tentacles to probe their way through the darkness. They use their sharp teeth and jaws to eat other underground worms and termites (carnivorous).
Taxonomy:

Kingdom| Animalia

Phylum| Chordata
Class| Amphibia
Order| Gymnophiona
Family| Dermophiidae
Genus| Dermophis
Species| Mexicanus

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Major Characteristics of the Class Amphibia

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Major Characteristics of the Class Amphibia

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"Amphibia" refers to the double life amphibians live in water and then on land, this gives them very unique shared characteristics throughout the class.
Vertebrates
Permeable Skin
Cold-Blooded
Lungs and Gills
All amphibians are vertebrates meaning they are big enough creatures that need a backbone to support them. Their permeable skin allows it to take in oxygen and water in order to stay hydrated. Also, because amphibians spent some of their land in water and then migrate to land, being cold-blooded allows them to regulate their body temp to the harsh, hot conditions on land. In addition, they have gills from the water and develop lungs on land to breathe out of. Amphibians are unique due to adapting to life on land from water. This change gave them their limbs that characterize them as amphibians. These characteristics are shared by all amphibians and make life on land livable, however each species has more specific traits to help them survive.

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Taxonomy is very helpful in showing relatedness between species. It shows where new species formed and the breakdown of a kingdom as it split due to environmental pressures to create more individualized creatures. For example, amphibians all share common characteristics, however their order difference shows the classified diversity of their uniqueness. The Anura order describes the frogs and toads. More specifically, the Red-eyed Tree Frog remains in rainforest canopies and therefore has adhesive pads on their feet to stick to the tree's leaves. The Caudata order describes the salamanders. These creatures are unique in living in grasslands and having tails, unlike others. The Gymnophiona order describes the final type of amphibians, caecilians. Unlike frogs or salamanders, caecilians are worm-like creatures with no limbs. They live mostly underground to escape the hot temperatures on land. Each species shows the diversity of amphibians, while sharing the major characteristics.

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The Fossil Record of amphibians shows their development from fish to present day, tetrapods living in water and land.  Fossils give scientists clues from physical features that show evolution from other species. The first amphibians (Elginerpeton) were found in the late Devonian period as tetrapods with limbs, no fins, and a full lengthy abdomen. They then developed broader ribs to hold organs while on land as shown in Ichthyostega fossil. After that, the Eryops fossil indicates amphibians became large carnivorous animals for a while with large bodies, hips, and especially short but wide, that allowed them to move along the land. The Tudianus shows the amphibians taking on a smaller, lizard like structure, with an overall smaller skull. Finally, the Seymouriamorpha fossil shows the continued development of the lizard like shape. The amphibian evolution branches off of fish like aquatic organisms.

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Gradualism describes the speciation of amphibians because there were NO abrupt changes throughout their evolution. The process was slow and contained cumulative steps. The first amphibian first evolved from the current lung fish called Sarcopterygii, about 400 million years ago. They grew legs and walked onto land, characterizing them as tetrapods. After this amphibians then split into 3 orders Gymnophiona, Caudata, and Anura that escaped to land. They all shared a common ancestor from the late Devonian period, the Ichthyostega. From there, each one adapted to the environment and developed their own, unique qualities. Some of these amphibians, however, developed gils and other characteristics classifying them as reptiles. True amphibians were known to man about 100 million years ago. Overall, the evolution of amphibans was slow and cummulative, including NO abrupt changes. They changed from fish, growing limbs, and forever created their own class, Amphibia.

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2919-251 million years ago- End of Triassic

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400 million years ago amphibians began to evolve and change in order to escape their aquatic envirnoments. To this day scientists are not positive of the exact reason why tetropods wanted to leave the water, but the main assumption is overpopulation. If amphibians stayed in the water they would soon run out of resources and it would be a competition based on survival of the fittest. This would cause extinctions and extreme gene frequency shifts. Instead, they left the water behind in hope of finding a more stable environment. They evolved from current day lungfish to tetropds. Over time, on land they had to develop lungs, nostrils, and backbones to allow them to breath and be supported. This shows decent with modification because amphibians evolved from lung fish into what they look like today. Natural Selection still plays a major role in amphibian's lives. When they first emmigrated from water onto land, it was a competition to see who physically changed the right way. The ones who negatively changed, soon died and the ones who positively changed, lived and reproduced. Today, amphibians have learned how to use behavioral characteristics to their advantage when it comes to natural selction. Many frogs and salamanders use startle coloration, camoflauge, and their unique skin patterns to scare predators and survive. This is an example of disruptive natural selction because it supports the extremes of the class and not just one dominant feature. Amphibians evolved to match their environment. For example, green tree frogs can blend in with the trees, salamanders can blend in with rocks, and caecillians can blend in with the ground. The species that survive due to evolving based on their environemnt show natural selection takes place.

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368 million years ago - Late Devonian Period

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359-299 million years ago - Carboniferous to early Permian era

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363 million years ago - Late Devonian Period

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Embryology can show relatedness because it is the beginnings of life in an animal.  In amphibians, especially, their embryology is alike to fish. This proves amphibians evolved from fish after establishing their limbs. In embryology, the commonly, shared characteristics of an organism appear first and then the unique qualities develop. All amphibians are alike in having a radical cleavage and a mesolecithal egg meaning there is a lot of yolk in the zygote. A radical cleavage means the entire egg is divided up into smaller cells.  Alike to fish,  they have the same structure however amphibians go through metamorphosis to develop their limbs and lungs later in the embryology stages after being a tadpole.

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299-276 million years ago - Early Permian period

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Figure shows females and males both having 22 chromosomes, but differences in their chromosomal types.

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Chromosomal differences found in amphibians lead to differences in their genes and alleles. The figure below shows the karyotype of 4 different tree frogs. Even though they are so closely related, their difference in chromosome numbers leads to speciation and unique characteristics. However, even though salamander males and females both have 22 chromosomes, they are still different when compared to tree frogs. Evolution is seen because humans also have 22 chromosomes with the female sex cell XX and male XY. Chromosome differences lead to speciation, but the alike numbers and sex chromosome characteristics can show evolution.

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Anatomy of a Frog, Salamander, and Caecilian

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Anatomy of a Frog, Salamander, and Caecilian

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Amphibans clearly went through divergent evolution. Divergent evolution is when multiple species develop from a common ancestor. This usually occurs due to different environmental pressures causing unique features to develop (speciation). Amphibians all have tetrapods as their common ancestor. However, tetropods also evolved into reptiles too. Not only is that an example of divergent evolution, but amphibians evolved into different species too. They continued to change into frogs with springy hind legs, and sometimes adhesive pads on fingers/toes. Also, snake-like caecilians with no limbs, but contained a backbone. Finally, into salamanders that contained tails, sometimes poisonous. These different feautures classified them as new species, evolved from the common ancestors of fish and tetrapods.

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The left of this picture shows the relatedness between amphibian's and their ancestor fish. Starting as tadpoles amphibians then develop limbs separating them from fish.

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Emmigration pmlayed a huge role in the evolution of amphibians. Some of the first amphibians stayed near or lived mostly on the top layer of aquatic regions, like swamps. However, they weren't getting enough food from their habitat. When they moved onto land completely they adapted based on the environment. At first the temperatures were harsh for the cold-blooded animals, so they found the coolest areas. For example, caecilians became burrowers, developing a hard skull to easily dig into the cool groud. Tree frogs spend thir days under the cool tree covering. Amphibians decided to emmigrate from the water and into new habitats and as a result they evolved into different species. To this day, they are still dependent on aquatic environments because that is where they mate to reproduce. Emmigration is the main cause of amphibian evolution.

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Microevolution is a change in the gene frequency of a population. A common ancestor of amphibians, tetrapods, were the first vertebrae species that could walk. Amphibans evolved from them as they evolved to their environment. Some amphibians in very dry locations began to grow scales and this caused other characteristic changes. Through reproduction, this trait's gene frequency became more common. With some time, these animals could no longer be classified as amphibians and were named reptiles, as shown in the phylogenic tree below. As a result of gene frequency, some amphibans became reptiles. Amphibians all had to change their skin due to the harsh conditions on land. That is why all amphibians have permeable skin, while reptiles that evolved from amphibians have gills. They share a common ancestor, but reactions to environmental conditions showed how different orders and species evolved from amphibians.

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The anatomy of amphibians is alike for the most part. After evolving from fish their class is defined as tetrapods. They underwent metamorphosis to develop limbs and are considered vertebrates, since they need a backbone to hold them together on land. Caecilians don't share this characteristic because they developed no limbs as an adaption to the environment. However, they have a backbone and skull like all other amphibians. Physiology, many of them enjoy living in cooler places or living underground. This is because their fish ancestry has caused land temperatures to be extremely hot and they maintain homeostasis by finding the coolest areas. They are carnivorous animals that use their unique qualities (poisonous tails, sticky tongues, and webbed feet) to trap smaller insects and eat them.