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The phenotype is the outward appearance and behaviour of an individual. The phenotype arises from an interaction between its genotype and its environment. Any change in the phenotype is called a modification and is not inherited. The genotype is the total genetic makeup of an individual. This is the genetic information which is located on the chromosomes and found in each cell. The genotype of the parents is passed to their offspring. When writing out genotypes the dominant letter is always stated first. Any change in the genotype is a mutation and may be found (Julie Brega 2005).
Homology is when a familiar structure is found in different species which can mean they all descend from at least one common ancestor. For example two species that share similar histological appearance are referred to as homologous organs (Oxford Zoology 2009).
Gastrulation involves extensive cell movements and in most animals, gastrulation converts the spherical blastula into a more complex arrangement of three layers. During cleavage the zygote undergoes rapid divisions with no significant growth, producing a cluster of cells that is the same size as the original zygote.
In 1859, Darwin wrote his evolution theory of natural selection to explain adaptation and speciation. His theory was simple but powerful: it basically stated that species who are best suited to their environment will more likely survive and to pass their genes on to the next generation. As long as there is variation, there will always be some individuals with the most advantageous genes. If these type of genes become inherited, then it could lead to a progressive evolution of a species and over time they can eventually become a separate species altogether.
Darwin's idea of evolution was greatly inspired when the Beagle Voyage stopped at the Galapagos Islands for supplies and he was able to walk on the islands to have a look at some species that he observed the deck of the boat.
Darwin thought of natural selection by comparing how farmers select their animals for breeding, which he named "artificial selection". During this period the concept of genetic drift was not formulated, although Darwin strongly believed that even though natural selection played a role in their evolution it could not be the exclusive reason to their modification. During Darwinââ‚¬â„¢s time natural selection was considered to be too weak for it to explain fully the characteristics of living things due to its unguided and no progressive nature. However, those who embraced the natural selection theory, like Herbert Spencer then introduced the theory of survival of the fittest, which became popular. This theory was later added to the fifth edition of On the Origin of Species that was published in 1869 this survival of the fittest phrase was used as an alternative to natural selection as it was more accurate and sometime convenient.
Describe the advantages and disadvantages both asexual and sexual reproduction?
All Organisms have to reproduce and, according to evolution, these organisms have to choose from a variety of methods to reproduce. There are two categories for reproduction and this is either sexual or asexual reproduction. Each reproduction methods have their own advantages and disadvantages. Humans are exclusively sexual in their reproduction where as many animals are asexual in their reproduction.
The first major category is called asexual reproduction this is where a new organism is reproduce with the presence of a single parent, in this process there is no joining of gametes for example a sperm fussing with an ovum. The new organism will have identical genes and chromosomes to its parent organism.
A good example of this would be the one-celled organism which reproduces by binary fission, where the cell simply divides itself in half. The end result will be a second cell that is identical to its parent cell, this type of reproduction has the benefit of being very quick and energy efficient. The cryptosporidium parvum splits into more than one organism simultaneously. The multicellular organisms use a similar asexual reproduction called fragmentation. This is where small pieces of the cell break off and grow into new organisms. However most organisms reproduce by budding, this is where a clone grows on the body of the parent cell, which then splits off.
All the variations that are explained above all perform asexual reproduction in one form or another and they have one thing in common, the end result is an offspring that is a direct clone of its parent cell. Asexual reproduction is an advantage to the parent as it is quick, simple, and the gene is not diluted by a second gene source. In addition, another advantage would be that no energy is spent from finding a mate. Also there is a high chance of survival for the offspring, if it stays near its parents in a stable environment. The disadvantage of asexual reproduction is that there is no genetic diversity, these organisms may not have the ability to adapt to changing environments and this type of reproduction can lead to overcrowding of populations and a struggle for survival.
The second major category is sexual reproduction which is more complex, as it requires the production of gametes for example a sperm and an ovum. When an organism needs to make sex cells, it will undergo a process called meiosis, which produces haploid cells from diploid cells. The main process of meiosis is that two copies of one chromosome come together to become a completely new chromosome which contains a completely different new set of genes from that of its parent chromosomes. The overall effect of this crossing-over is that certain genes on a specific chromosome can change position from one chromosome to the next. So genes from either parent may be next to each other on the same chromosome. This switching is a good way to keep active in a given population. When these gametes are made in both the female and male, they must fuse together in order to form an offspring. The male gamete provides one copy of its genome. The female gamete will also provide a copy of its genome. So therefore, the resulting offspring of two reproducing organisms has more than one chance to change genes around, crossing over and the union of the two parents.
The advantage of sexual reproduction is that there is a greater genetic variation in their offspring which means that there is a higher chance of survival in a changing environment.
The disadvantage of this type of reproduction is that there is a lot of energy is spent in looking for a mate. However though there are some animals that are able to reproduce sexually and asexually when the conditions are right.
Describe the reasons for differences between sexual cycles in vertebrate animals?
Before any sexual cycle can begin an animal must reach sexual maturity, which for most species is between the ages of two and three, except for some animals which have a longer immaturity. For example some fish tend reach maturity and spawn around the ages of 1 and 2, however some tropical fish can reach maturity and spawn as early as 2 months to 20 years of age. Amphibians like the frog reaches maturity around the ages of 2 and 4. In birds, the golden eagle, gannet has the longest record of immaturity of about 4 years. In mammals the length of immaturity really goes to the extreme, from the house mouse, which reaches their maturity and can become pregnant at only 6weeks to the elephant which may not reach maturity until 15 years.
After the onset of maturity, seasonal reproductive cycles are common in all species. These cycles can be clearly seen in animals living in temperate and sub-arctic regions. On considering these cycles, animals to do not just breed during the common moths of spring and summer where food is widely available; there are a few animals on earth that braves the polar region. The most well known animal is the emperor penguin, which develops their eggs in the darkest months of the Antarctic continent in temperatures of -55 C. when spring arrives their young have grown into downy chicks, and are ready to take advantage of the season for their growth to adult hood. These differences in the animal sexual cycles are designed so that the breeding season of each animal gives the maximum advantage to their young during their early period of growth.
The timing of all reproductive cycles must meet the needs of a given species, which can be achieved by natural selection. This natural selection still works in the out of season breeding that has been described above. Clearly it is an advantage that all this rapid growth and differentiation should occur at a season when food is most abundant and most succulent for that species. So far only the act of mating and becoming mature have been discussed above, for most mammals the situation is much more complicated by the habit of allowing the eggs to develop in the uterus for a longer or shorter period of pregnancy. So therefore it is not the act of mating but the act of birth which must be timed to take place in a season that provides optimum conditions for the survival and growth of their young. In mammals with only a short period of pregnancy, both mating and giving birth may take place in the spring. This seems to be the case with small species, for example, the wild rabbit, which their duration of pregnancy is 30 days, which means they can produce a series of litters between mid winter and midsummer. In larger species pregnancy is can be prolonged so that the breeding season becomes separated from the season of birth.