Evidence for Evolution – Explanation & Evidence

Natural selection and evolution  

Introduction

Evolution is a biological theory that states that all organisms on earth– plants and animals have a common origin. Their differences today result from modifications that occurred in successive generations. 

Explanation

As we know, reproduction brings about some changes in the offspring. These differences are not easily spotted because of the similarities of offspring with their parents. Still, some differences exist due to the unequal distribution of chromosomes during cell division in sexual reproduction in animals. 

We know that there are many similarities between our parents and us. 

Also, there are a lot of dissimilarities between our friends and us or anyone who is not a part of our family. 

All this is due to an interesting phenomenon called heredity. It is the phenomenon of the transfer of characters from parents to their offspring.  

Evolution is defined as a change in the inherited characteristics of biological populations over successive generations. It is a gradual, continuous process in which something changes into a different and usually more complex or better form. 

There is a variety of living organisms on earth. Every organism reproduces to bring more organisms like itself on earth. It is believed that all organisms have evolved from a common ancestor. 

Conventional religious texts tell us about the theory of special creation. This theory puts forward three ideas. One, that all living organisms that we see today were created as it is. Two, that the diversity on earth was always the same since creation and will be the same in future also. Three, that the Earth was created 4000 years ago. All these connotations were strongly challenged during the nineteenth century. 

Based on observations made by Charles Darwin, he stated that existing living forms share similarity in characters to varying degrees not only among themselves but also with life forms that existed ages ago. Many such life forms are extinct. There has been a gradual evolution of life forms. Any population has a built-in variation in their characteristics. Those characteristics which allows some to survive better in natural conditions would outbreed others that are less adapted to survive under such natural conditions. Hence, those who are a better fit in an environment leave more progeny than others. These, therefore, will survive more and thus are selected by nature. He called it natural selection and considered it as a mechanism of evolution. 

WHAT ARE THE EVIDENCES FOR EVOLUTION?  

Is Evolution just a theory? 

Evidence for evolution is strong proof in support of evolution that it is a continuous process that had occurred in the past from the time of the origin of life and is still continuing and will shape the diversity of life in future. 

Biologists sometimes classify evolution into two types based on a scale: 

• Macroevolution, which refers to large-scale changes that occur over extended time periods, such as the formation of new species and groups. Macroevolution cannot be seen easily because it happens over a long period.  
• Microevolution, which refers to small-scale changes that affect just one or a few genes and happen in populations over shorter timescales. Sometimes microevolution may turn out to contribute to macroevolution, but this does not necessarily occur. 

Biogeography: 

Biogeography can be defined as the study of the geographical distribution of animals and plants in different parts of the earth. The geographic distribution of all organisms on earth follows a pattern that is best explained by evolution along with the shift in tectonic plates over geological time. 

Darwin noticed that there are similarities between species of one biogeographical area, whereas species living in different regions tend to differ more, even if the species habitat a similar ecological niche. This led to the wondering of how very closely related species ended up in entirely different parts of the world. Similar species habituating in different parts of the world are due to geological changes, migrations, convergent evolution.  

One such evidence is the breakup of the supercontinent Pangaea. Groups that evolved after the separation appear uniquely in regions of the earth, like the unique flora and fauna of northern continents that originated from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana.  

The evolution of unique species on the islands is another example of how evolution and geography intersect. Let us take the example of the mammal species in Australia; these mammals are marsupials, whereas most mammalian species elsewhere in the world are placental. This is because Australia was isolated by water for millions of years; these species were able to evolve without competition from other mammalian species in the world. 

The marsupials of Australia, Darwin’s finches in the Galápagos, and many species on the Hawaiian Islands are unique to their island settings but have distant relationships to ancestral species on the mainland. This process is also termed adaptive radiation. 

Evidence from Fossils: 

Fossils are remains of living organisms that are preserved from the remote past. The study of fossils is known as paleontology. Fossil mainly preserves only a part of the dead organism like the skeleton, bone, teeth etc. These remains give us a record of past changes through vast periods of time and establish missing links between evolved species. 

Some extinct species had traits that were transitional between major groups of organisms. Their existence confirms that species aren’t fixed but can evolve into other species over time. The fossil record is not complete or unbroken: most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. Archaeopteryx is an iconic fossil; it is often thought of as the ‘missing-link between dinosaurs and birds. It was first described in 1861 by Hermann von Meyer (1801-1869), the German paleontologist. 

How do we know how old the fossils are? There are two components to this estimation which are as follows: 

Relative dating: It determines the approximate age by comparing it to similar rocks and fossils of known ages. 

Absolute dating: It is used to determine an exact age of a fossil by using radiometric dating. Radiometric dating measures the precise age of the fossil by measuring the decay of isotopes, either within the fossil or, more often, the rocks that are associated with it. One of the readily used and well-known dating techniques is carbon-14 dating. 

Embryological Evidence: 

It has been discovered that embryos of different organisms appear similar in the early stages of embryonic development. 

Von Baer had noticed remarkable similarities among vertebrate embryos that are quite different from each other as adults, and generalized these observations into a ‘law’. 

Ernst Haeckel further developed and popularized this concept into “the biogenetic law: ontogeny recapitulates phylogeny.” 

This notion holds that the development of an individual organism (ontogeny) is a replay of the evolutionary history of the species (phylogeny).  

For example, all vertebrate embryos go through a stage where they have pharyngeal (gills) arches on their throats. 

In fish, these gill arches develop into gills. In terrestrial vertebrates, these embryonic structures become modified for other functions, like the Eustachian tubes in humans that connect the middle ear with the throat. 

The embryonic development of the heart chambers in vertebrate animals as we move from simple to advanced vertebrates. 

The development of all triploblastic animals starts in a similar manner. 

Great ape embryos, including humans, have a tail structure during their development that is lost by the time of birth. 

Evidence from Comparative Anatomy: 

Comparative anatomy is one of the strongest forms of evidence of evolution, which compares structural similarities of organisms to determine their evolutionary relationships.  

Organisms with similar anatomical features are assumed to be relatively closely related evolutionarily, and they are assumed to share a common ancestor. 

• Homologous structures: organs that are similar embryologically but serve different functions in different organisms. An example of homologous structures is the forelimb of mammals. Each possesses the same number of bones arranged in almost the same way. While they have different external features and they function in different ways. This is divergent evolution. 

• Analogous structures: organs that have a different structure but perform the same function. The wings of a bird and butterfly both serve the same function (i.e., help the organism to fly) but are structurally and developmentally very different. 

The bird wing has bones inside and is covered with feathers, while the butterfly wing is missing both of these structures. 

Analogous structures result from convergent evolution. 

• Vestigial organs: Vestigial organs are some of the most interesting homologous structures. These organs are non-functional and rudimentary in nature. However, they were very functional in the ancestors of the organism. The presence of such structures in one and a related, although the vestigial structure in other suggest that the organisms have evolved from a common ancestor. Example- nictitating membrane, pinna muscles & vermiform appendix (man). 

Other Evidences: 

Like structural and functional homologies, similarities between biological molecules can suggest common evolutionary ancestry. All living organisms share DNA, ATP, many identical enzymes, DNA tripled code, 20 amino acids, etc., at the most basic level.  

The differences between the DNA sequence of a number of organisms helps us understand the evolutionary relationship between those organisms. 

For example, 2.5% difference in humans and chimpanzees, but a 42% difference between humans and chimpanzees. 

Such data helps us establish that humans and chimpanzees share a more recent common ancestor than humans and lemurs. 

Summary

• Evolution is defined as a change in the inherited characteristics of biological populations
  over successive generations. It is a gradual, continuous process in which something
  changes into a different and usually more complex or better form.
• There is a huge variety of living organisms on Earth. Every organism reproduces to bring
  more organisms like itself on Earth. It is believed that all organisms have evolved from a common ancestor.
• We have lots of evidence to prove the biological evolution, namely fossils, comparative anatomy and embryo development pattern. Biogeographical patterns provide clues about how species are related to each other. The presence of related organisms across continents indicates when these organisms may have evolved.
• Fossil evidence shows the changes in lineages over millions of years and helps establish
  connecting links between species.
• Embryological evidence for evolution was given by Ernst Heckel based upon his observation of certain features during the embryonic stage, which are common to all vertebrates but are absent in their adult forms.
• Organs with common origin & structure but different functions are termed homologous organs Organs that perform a similar function but have different origins & structures are termed analogous organs.
• Homologous and analogous organs tell us the evolutionary relationship between different species; maybe they originated from the same species with fore-limbs, gradually got changed as per their survival needs. So, we can say that they share a common ancestor.
• Finally, molecular biology provides data supporting the theory of evolution. DNA also provides clues into how evolution may have happened.