Evolution: A fundamental question in biology is: “How can living organisms exhib

Question

Evolution: A fundamental question in biology is: “How can living organisms exhibit a common set of fundamental characteristics, yet exhibit tremendous diversity in form and function?” Evolution, the gradual change in populations from one generation to the next through natural selection, is the answer to this question (and others), and there is substantial scientific evidence that living organisms have and continue to evolve.

1. Final Question: Above it is stated that “there is substantial scientific evidence that living organisms have and continue to evolve.” Present three specific examples to support or refute this statement based on what you have learned this semester. Make sure to describe your specific examples in detail and explain how it defends or refutes the above statement.

Explanation / Answer

The statement “there is substantial scientific evidence that living organisms have and continue to evolve.” can be proved by these evidences.

The Homologous structures:-

If two or more species have a common unique physical feature, like a complex bone structure, it is thought that they might have inherited the feature from a common ancestor. So physical features similar in structure and position, shared due to evolutionary history ( or by having a common ancestor) is called homologous.

Example,of homologous structure are - The forelimbs of whales, humans, birds, and dogs, when we see them they are different in look, because they're made to function in different environments. But if the bone structure of forelimbs seen , it is found that the pattern of bones is very similar among the species.

It is not possible that such similar structures were evolved independently in each species, and the basic pattern of bones in each species is similar to their ancestors as observed in the ancestor of whales, humans, dogs, and birds.

Some homologous structures are seen only in embryos. as found in all vertebrate embryos (including humans) . They have gill slits and a tail at the time of early development. The developmental patterns of these species became more different later on (That is why in human embryonic tail is converted to tailbone, and the gill slits got converted into jaw and inner ear). Homoogous embryonic structures shows that the developmental process of vertebrates shows the variations in a similar pattern, as found in their last common ancestor.

Example- The small leg-like structures of some snakes species, eg. The boa constrictors, are vestigial structures. These are the remnant structures having no significance in present snakes, but they were used to be in snakes' tetrapod ancestor (which were walking on four limbs).

2. Fossil evidences:-

Example - The evolution of Modern day horse.

Fossils evidences shows that the existence of now-extinct species, It shows that different organisms have lived on earth during different periods of history. These fossil evidences, helps the scientists in reconstructing the evolutionary histories of present-day species.

In case of horse lineage, by using these fossils, scientists were able to reconstruct a large, branching "family tree" of horses and their now-extinct relatives. The changes in their lineage lead to the modern-day horses, like the reduction of toed feet to hooves, shows the adaptation to the changing environment.

52 million years ago Eohippus were found, they were the forest dwelling species of the size of a fox, having  limbs longer than there body, showing the beginning of adaptations for running. But all the major leg bones were unfused, as a result the legs were flexible and rotatable. Its wrist and hock joints were low to the ground. The forelimbs had developed five toes, among them four were having small proto-hooves, the large fifth "toe-thumb" was above the ground. The hind limbs had small hooves on three among the five toes, the vestigial first and fifth toes were not able to touch the ground. Its feet were padded, like a dog's, but having small hooves in place of claws.

Eohippus remained with few significant evolutionary changes.The most significant change was in their teeth, which started adopting to its changing diet, because they shifted from a mixed diet of fruits and foliage to browsing foods. During the Eocene period , Eohippus species branched out into various new types of Equidae.

Orohippus :- The next stage of Eohippus .

Eohippus transformed into Orohippus by a gradual series of changes. Orohippus was not a true horse and were not living in the mountains. It resembled Eohippus in size, but were having a slimmer body, an elongated head, slimmer forelimbs, and longer hind legs, these are all features of a good jumper. Orohippus was still pad-footed,but the vestigial outer toes of Eohippus were not present in Orohippus, there were four toes on each fore leg, and three on each hind leg.

The most significant changes toom place from Eohippus to Orohippus was in the teeth, the first of the premolar teeth were dwarfed, the last premolar shifted in shape and function into a molar, and the crests on the teeth became more prominent .These factors gave the teeth of Orohippus greater grinding ability, so it is thought that Orohippus used to eat tougher plant material.

Epihippus:- The next stage is Epihippus , in the mid-Eocene, around 47 million years ago, Epihippus originated , they were having efficient grinding teeth, evolved from Orohippus. Epihippus were having, five grinding, low-crowned cheek teeth with well-formed crests.

Meoshippus :- In late Eocene and the early Oligocene period, the climate of North America became drier, and the earliest grasses began to evolve. The forests were becoming flatlands , a home to grasses .

In response to the changing environment, the then-living species of Equidae also began to change. In the late Eocene, they began developing tougher teeth and becoming slightly larger and leggier, as a result able to run faster in open areas. In the early oligocene, Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front and hind feet (the first and fifth toes were there but were small and not used for walking).

Mesohippus was slightly larger than Epihippus, Its back was less arched, and its face, snout, and neck were longer. It was having larger cerebral hemispheres, and had a small, shallow depression on its skull called a fossa, which is also found in modern horses in more developed form. The fossa is used as an important marker to identifying an equine fossil's species.

Modern horses or Eqqus were developed from diniohippus . This fossil evidence shows that there is gradual change in populations from one generation to the next through natural selection,leading to evolution of new species.

Evidence from direct observation of Microevolution-

Microevoultion is small scale changes that affect just one on a few genes over short time scale.

The emergence of DDT resistance mosquitoes is an example of evolution by natural selection.

Before application of DDT a small fraction of mosquitos in the population had naturally occurring genes or (alleles) which made them resistant to DDT. These versions or alleles appeared by random mutation, or changes in DNA sequence. Without the presence of DDT around, the resistant alleles might not have helped the mosquitoes to survive or reproduce. so they might have remained rare.

When the DDT spraying began, most of the mosquitos were killed by the pesticide. And the mosquitos who survived were the rare individuals who were having DDT resistance alleles .These surviving mosquitoes are able to reproduce and have offsprings.

Over generations, more and more DDT-resistant mosquitoes might have born in the population. Because the resistant parents are supposed to survive more and reproduce than the non-resistant parents. Gradually the mosquito populations will increase but having mostly DDT-resistant strains.

In parts of the world where DDT has been used widely, many of the mosquitoes are now resistant. DDT can no longer be used to control these mosquito populations (and reduce malaria) in these regions.

Two important factors are responsible for the development of DDT resistant varieties, they are the large population size( by this some individuals in the population will, be by random chance, have mutations that makke them resistant) and short lifecycle.

This development of DDT resistant mosquito varieties in case of also supports the statement that living organisms have and continues to evolve.

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