Current understanding of cetacean transitions back to sea

Macroevolutionary Transition of Cetaceans Back to the Sea

Today, one of the best known examples of macroevolution is that which can be speculated upon and observed in relation to marine mammals. Wales, porpoises and dolphins, members of the Catacean order, share a number of distinctions in the marine ecosystem, not the least of which is their high intelligence. Additionally, that these species are mammals that must ascend to the surface for respiration has underscored long-standing zoological speculation as to their origins. As the question of macroevolution suggests, these origins may well denote that the species in question originated on land.

According to the research by Bajpai et al. (2009), the speculative nature of the macroevolutionary theory was given some of its strongest evidence to date by fossil finds in the Indian and Pakistan region. These have suggested that whales in particular can be shown to have evolved into aquatic creatures from an array of land-dwelling prehistoric species. The research by Bajpai et al. indicates that "the first steps of whale evolution, i.e. The transition from a land mammal to obligate marine predators, documented by the Eocene cetacean families of the Indian subcontinent: Pakicetidae, Ambulocetidae, Remingtonocetidae, Protocetidae, and Basilosauridae, as well as their artiodactyl sister group, the Raoellidae. We also discuss the influence that the excellent fossil record has on the study of the evolution of organ systems, in particular the locomotor and hearing systems." (Bajpai et al., p. 673)

Perhaps more telling than these organ, locomotor and hearing systems are the vestigial hind flippers that are observable on cetaceans. These, our research denotes, point directly to a feature descendent from the order's ungulate origins and implies that at early points in its evolution, these hind flippers had been hind legs.

2. Explain the process of phylogenetic reconstruction including types of data and analyses.

Understanding the process of evolution for a single species or of an array of related species requires a mode of inquiry that combines both fossil evidence and speculative evaluation in order to devise the likeliest sequence of evolutionary steps and correlations. This mode of evaluation is called phylogenetic reconstruction and employs a wide array of variables relating the phylogenetic and taxonomic units represented by different species to draw apparent connections between their paths of development. This is a useful process for helping to understand the way that different contextual and environmental circumstances will have led to critical points of differentiation. Moreover, phylogenetic reconstruction can provide us with information regarding the particular events that might have led to a particular differentiation. As Barton et al. (2007) indicate, this kind of reconstruction "can shed light on past evolutionary events, such as gene duplications and lateral gene transfers, as well as how it can be used for other purposes, such as predicting gene function and resolving RNA secondary structures." (Barton et al., p. 1)

A wide variance exists in the type of data gathered and the type of analysis rendered. As the research on this subject demonstrates, like many subjects relating to the science of evolution, phylogenetic reconstruction does rest on some measure of speculative science. The 'parsimony' method, for example, relies heavily on observation of that which seems to obviate an evolutionary relationship, as do some other approaches. As the Barton text notes, "both parsimony and likelihood methods must scan through tree space; thus, they take much longer to process the same dataset than a distance method would. Likelihood methods are often slower yet due to their typically more complex calculations." (Barton et al., p. 19)

3. Compare-contrast the different types of natural selection and explain the changes in the offspring in terms of genotype frequencies.

The process of evolution centers around the notion of natural selection and suggests that those species which are most fit for survival will gradually develop the biological capabilities to flourish in a given environmental context. According to the text by Meek (1996), there are three primary types of natural selection that invoke our interest. Meek identifies these as stabilizing selection (in which extremity becomes an outlier and most animals in a species represent a certain evolutionary mean), directional selection (in which a specific genetic extreme becomes the ideal and therefore the norm) and disruptive selection (in which the species will alter in a specific trait as a matter of longterm survivability). (Meek, p. 1)

In terms of genotype frequency, stabilizing selection suggest that the majority population of a species will coalesce toward specific 'average' features. For instance, Meek indicates, human evolution denotes that it there are physiological disadvantages to being either short or tall in extremity. Therefore, most human beings will tend to fall on the spectrum within an 'average height' range. This mid-range height genotype is the most commonly selected and implies relevance to strategic survivability. This differs from directional selection, which instead denotes that the extremity is the more frequently represented genotype, such as in the extreme speed represented by the cheetah. Failure to attain this genetic extreme will prevent long-term survival, thus reducing the genotypic frequency of such an occurrence. Disruptive selection may be evidences in the butterfly or fish which gradually evolves to present spots and markings intended to deceive natural predators, a selective change aimed at improving viability.

4. Explain genetic drift as an evolutionary force and the aspects that contribute to its stochasticity.

The discussion above on natural selection suggests that the genetic changes which may occur across generations are inherently a response to environmental conditions and the need for survival. However, as an examination of genetic drift reveals, evolution is not always as precisely organized as natural selection would imply. Instead, genetic drift suggests that these types of changes in allele frequency can occur at random and will inevitably be the consequences of procreation amongst complex, multi-cellular organisms.

This randomness, or stochasticity, is far more observable over a short sample of time than are the effects of natural selection. Indeed, these can be observed even from one generation to the next. As the text by Moran (1993) denotes, "One aspect of genetic drift is the random nature of transmitting alleles from one generation to the next given that only a fraction of all possible zygotes become mature adults. The easiest case to visualize is the one which involves binomial sampling error. If a pair of diploid sexually reproducing parents (such as humans) have only a small number of offspring then not all of the parent's alleles will be passed on to their progeny due to chance assortment of chromosomes at meiosis. In a large population this will not have much effect in each generation because the random nature of the process will tend to average out. But in a small population the effect could be rapid and significant." (Moran, p. 1)

This denotes that the process of evolution is only in part a reflection of Darwin's ideas on natural selection. Genetic drift illustrates that in fact, the evolution of species occurs only partially on a continuum of adaptability. The process is also at least partially random and impossible to project.