Adaptations of Toothed Whales Long Beaked Common Dolphins, Essay Example

1. The life histories of insects demonstrate strategies that increase utilization of habitat resources and maintenance of high population densities.
   Describe the features of paleopteran and neopteran (both exopterygote and endopterygote) life histories that
   1.) increase fitness,
   2.) increase population sizes and
   3.) increase niche specialization allowing for higher biodiversity.

One of the most diversified and successful Phyla in the animal kingdom are the Phylum Arthropoda. Within Phylum Arthropoda is Class Insecta, the insects. Insects share many common features such as two pair of wings, three pair of legs, compound eyes, antennae and a segmented body composed of ahead, thorax and abdomen. In addition, they are distributed throughout all different types of habitats, such as freshwater, terrestrial and ocean habitats. The way in which these animals have increased their success in all different habitats is through their life history strategies. Their life history strategies increase utilization of habitat resources which maintain their high population densities.

The Arthropods branch into several subphylums, one of which is the Hexapoda. The Hexapoda have a vast variation and an extremely successful taxon and are classified under Class Insecta. During the late carboniferous period and early Devonian period these insects had already begun diversifying. The evolution of the insect’s wings is shown in the fossil records and date back about 350 million years ago. The features of the Orders Paleopteran and Neopteran illustrate how their life histories have made the Insects such a successful group. The evolution of the insect Class occurred in four stages. The first stage is referred to as the Apterygote stage. These insects were wingless with no developed legs or body parts. The second stage, which illustrates part of the success of Insects, is the Paleoptera stage. Here is where the development of wings is shown. The ability to fly resulted in four things: an increase in fitness, population size, and niche specialization for biodiversity. The evolution of wings allowed insects to fly to far distances to locate new habitats, mates, food, as well as escape predators. In addition, flying increased pollination and dispersion of plant life.

The primitive development in Insect is ametaboly. This is the hatching of the offspring from the egg into a juvenile with the genitalia. This is seen in the wingless orders of insects. The pterygote insects, on the other hand undergo metamorphosis. Insects undergoing metamorphosis are divided into two different patterns, hemimetaboly (partial metamorphosis) and holometaboly(full metamorphosis). Hemimetabolous undergoes an incomplete metamorphosis in which the young nymph resembles its adult physical characteristics and diet. It will go through a series of molts and emerge as an adult. The mayfly slightly defies the typical order. As it is the only insect to molt after it receives its wings this is called the subimago stage. For holometabolous development the insect metamorphosis is not partial it fully changes and its appearance also changes. The stages of development start with an egg. Unlike the hemimetabolous, this insect’s larvae stage does not look like the adult. The insect will move into the pupal stage by undergoing a series of molts. During the pupation stage the insect is incapable of eating and the insect transforms into adulthood.

The developing wings are visible on the dorsal side of the nymph of hemimetboly insects. These types of insects are referred to as exopterygote, because there is an external wing growth. The holometabolus, on the other hand, have a unique evolutionary mechanism in development in which there is a resting stage or pupal instar, in which the structural differences between the larval and adult are formed. The clade that shares this development is known as the Endopterygota. In this group the wings are present inside the larvae and this type of wing development is known as endopterygote. The evolution of holometaboly or complete metamorphosis results in the immature and adult insect stages to specialize to different types of resources, increasing fitness and survival, as well as increasing population sizes and diversity.

The Endopterygota includes the orders Diptera, Lepdioptera, Hymenoptera, and Coleoptera. All of these orders have a high species richness or high species diversity. This can be explained, again through metamorphosis. The adult and larval stages are different; therefore, different food resources and habitats can be used by the two. This can increase survival and decrease intraspecific competition in the species. In addition, if there is a change in the environment, such as extreme temperatures, no food, no water, the pupa stage can withstand this type of environmental fluctuation, thereby increasing survival and population size.

A great, specific example of insect success is from the dragonfly. Fossils found from the late Carboniferous period (300 mya) illustrate the first winged archaic insects. These insects were placed in the order Paleodictyoptera. The Paleodictyoptera are unique because they were the precursors to the modern day Order called, Odonata. Odonata are more commonly known as the Dragonflies. This species is unique because they have not changed much from their ancestors, indicating that they have a high fitness. All species in Odonata have similar characteristics for sight, life cylces, morphology, locomotion, mating, habitat and predation.

The vision in the dragonfly is conducted through compound eyes. These eyes allow for great eyesight. Within the compound eye are 30,000 individual eyes referred to as ommatidia, which provides the insect with a 360 degree field of view. In addition, it provides the dragonfly with a motion detector that it can use for predation and the avoidance of predation. This characteristic also helps it find mates. This feature, therefore, is another feature that helps improve fitness and increase population of their species.

Another selected feature in the dragonfly that illustrates the success of insects is their flying capability. It was already discussed about the ability for winged insects to move to different habitats, find mates, food and avoid predation; however, the dragonflies are exceptional in that they can reach speeds of 97km/hr. In addition, dragonflies have wings that are elongated and translucent with complex wing venation. Their venation adds to the strength and flexibility of their wings. Dragonflies also have a distinctive notch at the front edge of each wing. The dragonfly has a more robust base at the rear wing and the front wings are smaller than the rear wings.

Insects that do not undergo complete metamorphosis, so not have as good as a chance in regard to fitness compared to the complete metamorphosis individuals, they still have features that increase their fitness, diversity and population as well. For instance, these individuals still have wings and are able to fly to different locations, increasing their habitat and enabling them to find the perfect niche. In addition, they can fly to find mates, food and avoid predation. All of the types of features contribute to their evolutionary success.

Overall, the evolution of wings and complete metamorphism in Class Insecta has increased species fitness, diversity and niche development. This can be supported through the analysis of the dragonfly and its success for over 300 million years.

2. Compare and contrast adaptations of Sea Otters and toothed whales, such as Long-beaked Common Dolphins (Delphinus capensis), for life in the ocean.
   Fossil records dating back 40 million years ago show that a 16 meter long whale had external hind limbs that were less than one meter long, indicating that they were too short to be used for locomotion. Another fossil record dating back 57 million years ago, illustrates another whale having larger, but still weak hind limbs. This shows evidence that whales evolved from four-legged animals. The sea otter is a marine mammal that has not evolved into the ocean environment until about 1-3 million years ago. One can evaluate different features in marine mammals in order to see the adaptations that exist. In addition, a comparison between two different types of marine mammals can illustrate the similarities and differences in adaptations. For instance, the sea otter, which spends a lot more time above the surface and in shallow waters versus a toothed whale, which spends most of its time in deeper waters and below the surface.

Toothed whales are from the suborder Odontocenti. These include the sperm, killer and beluga whale and all dolphins and porpoises. The largest of the toothed whales is the sperm whale (Physeter macrocephalus) that reaches a length of 15 meters. The smallest toothed whale, apart from the dolphins and porpoises is the narwhal (Monodon monoceros) that reaches of length of 5 meters. The dolphins and porpoises are much smaller. For instance, the long-beaked common dolphin is a small dolphin that reaches lengths around 6-8.5 ft.

Marine mammals are warm blooded animals. A whale, like humans, has a high body temperature, actually two degrees higher than a human. This adaptation is due to the fact that heat loss is greater in water than in air. This brings us to the next adaptation of whales, insulation. Whales have adapted insulation through blubber, which prevents the movement of heat out of their body. The blubber is a thick layer of fat that conserves the body heat. The degree of thickness of the blubber can vary from two inches in small whales to one foot in larger whales.

Whales have also adapted teeth in order to feed in their environment. The toothed whales contain peglike teeth on their jaws which they use for predation on fish, seals, penguins and squid. These whales are predators and usually swallow their prey whole. Their stomachs have evolved compartments that actually chew the food. The interesting thing about this group of whales is that each whale has a unique tooth. The sperm whale, for instance, have large cone-like teeth on its narrow lower jaw. The narwhal has an elongated tooth on the upper jaw. In addition, some of the toothed whales have adapted specialized feeding techniques, such as stunning prey with sonar or slapping the water using their tails. Toothed whales have also been observed herding prey onto sloping beaches in order for the water to push their prey right into them.

The Cetaceans, another name for whales and dolphins, spend all of their time in the water. Their fertilization process is internal, as in other mammals, and the gestation period is 11 to about 18 months in the larger whales. For instance, the small long-beaked common dolphin has a gestation period at about 11 months. In addition, whales breed once every three years, and like humans they usually give birth to only one calf at a time. The parental involvement or care of mother whales is great and a bond is developed between the mother and the calf. The mother whale nurses her calf anywhere from six to 10 months.

Sea otters, another type of marine mammal, are from the Mustelid family, which is the weasel family; however, this is the only species that lives in the water. Sea otters are also the smallest marine mammal with an average length of 4 ft. Sea Otters do not have blubber ; however, they have adapted other techniques in order to survive in the ocean environment. For instance, sea otters that live further from the equator have a larger body mass then those closer to the equator this is called Bergmann’s Rule. Likewise, Allen’s Rule accounts for shorter appendages in colder climates and longer appendages in warmer climates. They are able to survive in cold water at 3-4 degrees Celsius without the added blubber tissue as seen in the other marine mammals, like the toothed whales. This is due to the adaptation of dense hair and an increased metabolism. In addition, sea otters are able to regulate their body temperature, through the use of the feet, fur and lungs. This is comparable to humans sweating and dogs panting. They regulate their temperature for cooling by extending their feet outward to increase surface area under water, which also decreases their lung capacity. To prevent heat loss, they float of their backs in order to keep their appendages out of the cold water. This also allows them to absorb heat from the sun. In addition, their backs contain the thickest portions of hair which helps protect from the cold water. They have also adapted methods for tucking their feet inward in order to retain heat and then can increase buoyancy by retaining air in their lungs. They also go to shallow waters in order to forage so that they are able to breathe and feed at the same time.

The Sea Otters pups have a shorter gestation period compared to the toothed whales at around six months in length. The pups are born without the ability to swim; however, they are extremely buoyant. The otter’s usually only give birth to one pup at a time. This is also true for the toothed whale. The births normally occur in the sea and the pups are able to see at birth. The pups start eating solid foods a couple of days after birth. They begin swimming around four weeks. The mother provides parental care for about 6 months and nursing her young while resting on her back and allowing the baby to feed above water on her abdomen.

Sea otters have also adapted methods for feeding in the ocean environment. They have 32 canine molar teeth in sea otters that are used for crushing shells. Sea otter do contain two pair of incisors in the lower jaw that are used to tear fish apart. In addition, these teeth function in scraping the parts of the mollusc shells off as well. Furthermore, Teeth in the sea otter vary with their diet. For instance, fish-eating sea otters use their mouths to catch their prey with their sharp teeth. There are other types of otters that eat shellfish. These types have more blunt teeth that are used in order to crush the shells. Compared to the whales, the feeding process in the sea otter versus the toothed whale have evolved similar aspects in that they are both predators and need teeth, but differs in the fact that the whales just consume their prey whole, while the sea otter tears it apart using its teeth.

Like fish and whales, sea otters are believed to exhibit a countercurrent heat exchange system. In the countercurrent exchange, the arterial system heats the venous system by being parallel to each other. In addition, the arteries and veins in all of the extremities are much closer together. Therefore, the heat in the blood moving through the arteries can be easily transferred to the veins instead of the surrounding environment. This also helps the otters and whales retain body heat. The sea otters feet are webbed, which increases their speed and agility. This is different than the whales and dolphins, which exhibit streamlined bodies. They also have thick tails that tapper down flattening at the tip further facilitating the otter’s speed.

Toothed whales, and other whales, have adapted blowholes on top of their head, which are directly linked to the lungs through the trachea. This allows the whale to get air by only exposing the top of its head above the surface. This also prevents water from entering the lungs since some whales can spend up to one hour underwater. Sea otters, on the other hand, put their entire heads above water to obtain oxygen.

Both the toothed whale and sea otter must dive to different depths. They have adapted ways to do this. The toothed whales have a large surface area and dive to extreme depths of up to 3km in some species. Therefore, their organ systems change while diving. For instance, the respirator and circulatory system slows the heart down in order to use less oxygen and less energy. The oxygen that is available moves towards the needed tissues to help keep it at the deep depths. In addition, whales have adapted the ability to have a higher concentration of red blood cells versus white blood cells compared to land animals. This helps store more oxygen. Otters also have adapted the ability to dive. Sea otters forage on the benthic floor and usually bring their prey to the surface to eat. Therefore, they are usually found in shallow water since they have to reach the sea floor in order to find food. For instance, Alaskan sea otters can reach depths of 40m and the California sea otters can reach depths around 20 m. They can also remain underwater between 52-90 seconds.

Toothed whales and otters have visual acuity in common. They are both able to see both underwater and on land. In both the whale and the otter, the use of vision helps protects them from predators as well as facilitates finding their favorite foods such as shellfish, urchins, crabs, and fish. In addition, the otter’s jaws and skull are reinforced to accommodate crushing such hard-shelled creatures. Muscles articulate at the crest of the otter’s skull as well as wrap the zygomatic arches inhibiting the condylar process from disarticulating making their crushing power sensational. Their strong bite coupled with the adapted molars of this otter make their hard-shelled pray easy targets.

Toothed whales and otters are both excellent swimmers. The whales are fast swimmers and are even known to surf waves and boats. Their streamlined bodies allow for them to move quickly. The otters swim by moving their hind legs and tail. Researchers have found that sea otters can reach speeds up to 9 mph. This is an advantage for chasing prey and avoiding predators. In addition it allows for the sea otter to find new habitats if necessary.

Although the toothed whales and otters can see, they use other means in order to locate food and avoid predation. The toothed whales use echolocation, in which they send out signals through sound and analyze the echoes that it produces when it hits and object. The toothed whales have an organ called a melon in their foreheads which serves as a sonar that can focus sound waves. This helps them find food and avoid predators. The adaptation sea otters have that is similar to whale echolocation is its long whiskers. They use the whiskers around their snout to detect fish from vibrations of the water caused by movement of the fish. These whiskers improve the sea otters hunting capabilities even in unfavorable water condition.

Lastly, the sea otter is able to drink salt-water. The otter’s can drink roughly one cup a day, though not necessary. Most of their water is retained via their unique nasal passages and from eating. They have large kidneys, which remove the toxic salt and urea from their systems concentrating it and excreting it as urine.

3. Describe the adaptations of plants for life on land. Include in your descriptions the life histories of algae, moss, ferns and gymnosperms and the structural features that allow growth and reproduction in terrestrial environments.

Plants were there first organism to colonize on land. Plants emerged onto land around 420 million years ago. It is theorized that the terrain was inhabited by cyanobacteria, algae and fungi, allowing for multicellular green algae to move onto the land and spread. The early plants provided food that initiated the evolution of fungi and the beginning of animals on land. Plants; however, most likely evolved from green algae that were growing near the edge of water features, where tidal fluctuations left the algae exposed. In addition, weather conditions, such as rain and wind that would sweep the algae away. These types of conditions could have selected for several different adaptations that we see in plants today, such as an anchoring body, division of labor in cells, the ability to supply cells in areas that lack sunlight with food from photosynthesis, water storing compartments, and a lipid coating that decreases evaporation. All of these emerging features were pre-adaptations that made it possible for plants to emerge onto land. These pre-adaptations were selected for due to the advantages in the existing environment.

The main sources of challenge for plants are acquiring sufficient light, water, carbon dioxide, minerals and oxygen in order for photosynthesis and metabolism to occur. The advantage of land was the increasing amount of light available for plants. In addition, the availability of carbon dioxide and oxygen for photosynthesis and respiration are also increased on land. Furthermore, early plants beginning adaptation on land had little competition for resources and predation was small due to the small amount of animals that were evolved for land at that time.

There are some disadvantages that occurred during the evolution of aquatic plants to land plants. Firstly, was of course, the decrease in water on land. Therefore, it became harder for plants to obtain water. In addition, if water is obtained, evaporation can occur increasing the rate of water loss on land as well. Secondly, the dense air and the lack of support to the plant’s body are much less compared to a plant in an aquatic environment. The aquatic environment provides support to the body and buoyancy. Thirdly, the air also does not have a protective layer from ultra violet radiation, which can damage DNA in the plant cells. Water serves as a barrier to ultraviolet radiation. Lastly, water is no longer available for reproduction. The eggs and sperm cannot disperse in the water column for fertilization.

Therefore, with these disadvantages the plants had to evolve new adaptations for life on land. For one, the essential needs for the plant’s survival were separated. For instance, water and minerals were available at or below the soil surface and sunlight and air were above the surface. This segregation of resources selected for a segregation in the plant body. The structures of the plant that were underground began to serve as the anchors and also capable of absorbing water and minerals in the soil. The plants evolved vascular tissue so that they could transport material within the plant so that the all plant parts would receive food and water. The walls of the plant also evolved lignin, which was a thickening of the wall that provided the needed support for the plant. There are some plants; however that never evolved the vascular tissue, such as mosses. The mosses are unable to be more than a few centimeters in height because it obtains it food and nutrients through diffusion and diffusion would be extremely slow at a large height.

Vascular plants also evolved the cuticle, which is a waxy covering that is impermeable to water and decreases loss of water through evaporation. The cuticle, does however, have small pores, called stomata that allow carbon dioxide and oxygen to exchange. Vascular plants also evolved different hormones which allowed the plants to respond to different environmental cues, as well as help the plant grow.

Before life on land, the plant reproduced though flagellated, swimming spores and sperm. Therefore, in order to adapt to land, there was some adaptations to their life cycles. For instance, the gametophyte (which is the haploid stage in the alternation of generations) and the sporophyte (which is the diploid stage in the alternation of generations) changed their size and relationship. The bigger and longer-lived type became the dominant generation. In nonvascular plants, such as mosses, liverworts and hornworts, the dominant generation became the haploid gametophyte. The diploid sporophytes are smaller, short-lived, and grow on the gametophyte. The vascular plants, such as the ferns, conifers, and plants, it is opposite, with the sporophyte generation dominant. In ferns, the gametophyte becomes completely independent of the sporophyte. Then we see in the seed-bearing plants, such as conifers the gametophyte is very small and grows either on the flower or within a cone.

Since plants cannot move, they have evolved mechanisms for bringing their gametes (sex cells) together for fertilization. Sperm move in moisture, spores, pollen and seeds travel in wind. Some even have structures on them that allow them to be swept easier in the air. For instance, the fern plant contains what are called sporangium. These sporangium’s act as catapults for the spores. As water evaporates through the cell wall of the plant, the surface tension increases and the sporangium opens and releases the spores. In addition, some plants have adapted symbiotic relationships with animals in order for their pollen or seeds to be transported to other plants.

The life cycles of plants also show have certain plants have adapted for life on land. For instance, the non-vascular plants do not contain transport tissue and with the lacking aquatic environment the sperm can dot disperse. For the sperm to be successful in reaching the egg, the non-vascular plants remain small and confined to moist habitats. In addition, the plants emerge above slightly above the ground in order to for their reproductive cells to disperse. Examples are the mosses. Sperm formed in the moss are splashed out in a raindrop and swim through a film of water in order to reach the egg. However, we do see that the sporophyte grows inside the female organ, the archegonium and is therefore supplied with nutrition and physically supported. In addition, we see evolution of recombination of genes to increase variation. There is also wind dispersal present in this life cycle that aids in the adaptation to the terrestrial habitat. Water is no longer the dependent factor for gamete transportation. The gametophyte in the life cycle also shows some adaptations, such as the phyllids, that press along the stipe to insulate and decrease evaporation. The stipe also evolves into a bundle of filaments that serve as storage for food and water.

Vascular plants, on the other hand, are more advanced and complex. These include the seedless vascular plants, gymnosperms (plants with naked seeds) and angiosperms (flowering plants). Each of these plants exhibits a new adaptation from one level to the next and illustrates significant evolutionary advances to adapting to land. Each of these plants was able to inhabit areas compared to the non-vascular plants. The first major advance, of course, was the vascular tissue, as mentioned previously aided in transport and support. The next major advance was the evolution of roots. Fossil evidence showed that the early vascular plants did not have roots, but rhizomes on the ground stem. The third major advancement was the evolution of leaves. The first sign of a leaf are the microphylls that are thin flaps on the stem. They became larger and eventually received a vein from the vascular system of the plant. Then Megaphylls evolved from the side branches. This led to dominant stem with smaller side stems. In addition, the megaphylls became connected with the photosynthetic tissues and the vascular system increased with veins throughout the leaves. The fern is a good example of the highly divided leaves that contain many veins.

The final evolution of plants is the seed plants. The key component in the seed plant life cycle is the gametophyte, sperm and sporophytes that are no longer released into the environment. Instead they are held within the plant itself. For instance, even in fern plants, which are a somewhat more specialized seedless plant, the gamete cells are dispersed into the air as a spore that drifts to the ground to grow into a gametophyte. The seed plants; however, had a much greater selection for the sporophyte becoming a part of the parent plant. The sporophyte had access to the resources and the survival was increased due to protection from the parent plant. In addition, pollen grains evolved the ability to withstand drying; therefore, no water was needed. Furthermore, the seed itself allowed for the increase in fitness of vascular plants on land. The seed contained an outer protective layer called the seed coat, the embryo of the new sporophyte and food supply.

Overall, the living seed plants become more efficient than the seedless plants, the gametophytes became dependent on the sporophyte for food and protection, pollen grains were able to withstand drying, and the dispersion occurred by seeds. These all increased the fitness, diversity and populations of plants that could survive on land.

4. Compare and contrast the advantages of asexual and sexual reproduction. Include in your answer the “adaptation of sexual reproduction”, the features of sexual selection that improve fitness, and the various mechanisms that determine sex in different organisms.

Most animals and plants use sexual reproduction as the means of reproducing and passing on their genes; however, asexual reproduction is common among smaller organisms, such as the protozoa, algae and fungi. Asexual reproduction is the multiplication of an organism that does not involve meiosis and fertilization as in sexual reproduction. For instance, most unicellular organisms reproduce asexually by dividing into two smaller cells, or from forming new organisms attached to the parent and then budding off. In addition, some organisms produce asexually from unfertilized eggs. This is referred to as parthenogenesis and in plants, agamospermy.

There are disadvantages and advantages of both sexual and asexual reproduction. Comparing the different aspects of each of the reproductions and its energy cost, as well as its genetics in regard to evolution can further describe these. One of the good advantages to asexual reproduction is the energy used is more efficient. For example, one or few offspring are produced at a time in a large size, such as a unicellular organism dividing into two. A multicellular organism that reproduces asexually usually has the offspring attached to them until they bud off. They are using a good amount of energy; however, they are using it efficiently because it is all going to the offspring. In sexual reproduction, energy is wasted. For instance, sperm or pollen that is released into a water column or the air has to find a mate in order to fertilize. This is by chance. If they do not find a mate, that energy that was spent making the sperm, pollen or eggs is wasted. In internal fertilization, the sperm are placed near the egg; however, more energy is wasted other ways, such as courting, producing nutrients, or attracting animals to carry pollen. In addition, genetically if an organism is reproduced with mutated genes that are not viable, the energy spent making it is wasted.

The genetics behind asexual and sexual reproduction is also different. Asexual reproduction reproduces via Mitosis and sexual reproduction reproduces via Meiosis. In Mitosis, the parent passes all of the genes to the offspring. In regard to fitness, this is good because the fact that the parent organisms was able to survive and reproduce indicates that the offspring has a good chance as well. In Meiosis, the offspring produced contains half the parent’s genes. The other half comes from the other individual organism. In this case, there is also the possibility of offspring receiving homozygous deleterious recessive genes that cause death. I addition, some of the genes received from the offspring may be less suited to survive the environment of the parent.

Overall, asexual reproduction wastes less energy and the evolutionary success is twice as great as an organism in sexual reproduction; however, there are positive adaptive successes to sexual reproduction. For one, there is more genetic variation in the population. Organisms from asexual reproduction are all the same genetically, like clones. The only sort of genetic variation comes from mutations. In sexual reproduction, genes undergo recombination and form new assortments at fertilization. This can produce combinations of genes that are better suited to adapt to new environmental conditions.

In the first sexual organisms, the entire haploid organism was the gamete. The evolution of sexual reproduction resulted in specialized gametes. There are different types of gametes. Organisms with gametes that are identical are called isogamy. Oogamy is the type of sexual reproduction that involves a large, nonmotile gamete and a smaller motile gamete. The larger gamete is the female and the smaller one the male. This type of sexual reproduction is the most common. The third type is anisogamy. Anisogamy occurs when a large flagellated gamete merges with a smaller gamete. The selective advantage of having an egg containing the maternal chromosomes, as in oogamy, and storing the food is that the embryo can be better developed. A non-motile egg in sexual reproduction is no use without the motile sperm; therefore, they both evolved to complement each other. In addition to the size of the gametes, the sex of the gamete can be determined through the chromosome size. The X chromosome of the female is large and the Y chromosome of the male is small.

In sexual reproduction there are different roles of the male and female and this evolved into having different appearances. This is referred to as sexual dimorphism. An example is the large antlers and horns seen in hoofed mammals, or the large size of male seals. This allows competition against other males when looking for a mate. Therefore, there are selective pressures for the males, but not for the females. Monogamous species are species that have only one mate at a time. There are no selective pressures for the male in this situation; however, their sexual dimorphism is different. Males usually are more colorful, like a male peacock. This evolved because the female is more vulnerable to predation while sitting on her eggs and therefore, being less colorful is advantageous. In addition, the female has to choose the mate and that results in the male’s evolutionary success dependent on his traits that are attractive to the female. Polgamous species are species that mate with more than one other mate. The males appearance, physique and courtship behavior depend on the males success.

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