31 Evidences of Evolution

Dr. Subhendu K Acharya

Contents:

Introduction
The concept of Evolution
How Do We Know That Evolution Has Occurred?: Evidences of evolution
Case studies for evolution
Summary

INTRODUCTION

The Nobel Prize winning scientist Linus Pauling aptly described science as the search for truth. Science does this by continuously comparing its theories objectively with evidence in the natural world. When theories no longer conform to the evidence, they are modified or rejected in favor of new theories that do conform. In other words, science constantly tries to prove its assumptions to be false and rejects implausible explanations. In this way, scientific knowledge and understanding grow over time.

EVOLUTION: THE CONCEPT

Evolution is change in heritable traits of biological populations over successive generations. Evolutionary processes give rise to diversity at every level of biological organisation, including the level of species, individual organisms, and at the level of molecular eviolution.

There are different theories of evolution

Lamarckism

Darwinism

Pangenesis theory

Synthetic theory of evolution

The synthetic theory synthesized various concepts such as Darwin’s natural selection, Mendelian genetics, mutation theory and population genetics to understand the course and mechanism of evolution.

How Do We Know That Evolution Has Occurred?

The evidence for evolution has primarily come from four sources:

1. fossil record of change in earlier species

2. chemical and anatomical similarities of related life forms

3. geographical distribution of related species

4. recorded genetic changes in living organisms over many generations

Fossil Record

Remains of animals and plants found in sedimentary rock deposits give us an indisputable record of past changes through vast periods of time. This evidence attests to the fact that there has been a tremendous variety of living things. Some extinct species had traits that were transitional between major groups of organisms. Their existence confirms that species are not fixed but can evolve into other species over time.

The evidence also shows that what have appeared to be gaps in the fossil record are due to incomplete data collection. The more that we learn about the evolution of specific species lines, the more that these so-called gaps or “missing links in the chain of evolution” are filled with transitional fossil specimens. One of the first of these gaps to be filled was between small bipedal dinosaurs and birds.

Just two years after Darwin published On the Origin of Species, a 150-145 million year old fossil of Archaeopteryx was found in southern Germany. It had jaws with teeth and a long bony tail like dinosaurs, broad wings and feathers like birds, and skeletal features of both. This discovery verified the assumption that birds had reptilian ancestors.

Since the discovery of Archaeopteryx, there have been many other crucial evolutionary gaps filled in the fossil record. Perhaps, the most important one, from our human perspective, was that between apes and Homo sapiens. Since the 1920’s, there have been literally hundreds of well-dated intermediate fossils found in Africa that were transitional species leading from apes to humans over the last 6-7 million years.

The fossil record also provides abundant evidence that the complex animals and plants of today were preceded by earlier simple ones. In addition, it shows that multicelled organisms evolved only after the first single-celled ones. This fits the predictions of evolutionary theory.

Living things on earth are fundamentally similar in the way that their basic anatomical structures develop. No matter whether they are simple single-celled protozoa or highly complex organisms with billions of cells, they all begin as single cells that reproduce themselves by similar division processes. After a limited life span, they also all grow old and die.

All living things on earth share the ability to create complex molecules out of carbon and a few other elements. In fact, 99% of the proteins, carbohydrates, fats, and other molecules of living things are made from only 6 of the 92 most common elements. This is not a mere coincidence.

All plants and animals receive their specific characteristics from their parents by inheriting particular combinations of genes. Molecular biologists have discovered that genes are, in fact, segments of DNA molecules in our cells.

These segments of DNA contain chemically coded recipes for creating proteins by linking together particular amino acids in specific sequences.

All of the tens of thousands of types of proteins in living things are mostly made of only 20 kinds of amino acids. Despite the great diversity of life on our planet, the simple language of the DNA code is the same for all living things. This is evidence of the fundamental molecular unity of life.

In addition to molecular similarities, most living things are alike in that they either get the energy needed for growth,repair, and reproduction directly from sunlight, by photosynthesis , or they get it indirectly by consuming green plants and other organisms that eat plants.

All of these major chemical and anatomical similarities between living things can be most logically accounted for by assuming that they either share a common ancestry or they came into existence as a result of similar natural processes.

Geographical Distribution of the Related Species

The geographic distribution of related species is another example for the past evolution. This is also niw clear that many of the major isolated lands and group of islands hve evolved their distinctive plants and animals communities. For example, in Australia, before humans arrived their before 60-40,000 years ago, had no advanced terrestrial placental mammals like cats, dogs, bears, horses rather had more than 100 species of kangaroos, koalas, and other marsupials. It is also reported that the terrestrial mammals were entirely absent from the even more isolated islands that make up Hawaii and New Zealand. All of these had a larger number of plant, insect, and bird species that were found nowhere else in the world. This can be explained through evolution in isolation of Australia’s, New Zealand’s and Hawaii’s biotic environment from the rest of the world for million of years

Genetic Changes over Generations

The environment of earth is continuously changing, usually in subtle and complex ways. The changes when go beyond a threshold when it is beyond the toleration of a population of organisms mass deaths occur. This also goes with the observation of Chales Darwin that not all the organisms would peish after this. Fortunately, natural populations have genetic diversity. Only the organisms whose internal characteristics allow them to survive an environmental crisis are likely the only ones able to reproduce.

Lately these traits will be more common in the next generation–evolution of the population will have occurred.

The process of this selection (natural) resulting in evolution can be demonstrated over a 24 hour period in a laboratory Petri dish of bacteria living in a nutrient medium. Whenever higher or lethal dose of antibiotic is added, majority of the bacteria will die-off. While majority of these die only few of the bacteria usually are immune and survive. The next generation is mostly immune because they have inherited immunity from those who have survived. The purple bacteria in the Petri dishes shown below in the figure showing that the bacterial population has evolved.

Similar phenomenon is observed in human bodies as well where the body reacts when an antibiotic drug is administered and its unable to completely eliminate a bacterial infection. Due to this the medical doctors recommend the antibiotic to be taken even after the disease doe not persists as a course. They do not want to allow any potentially antibiotic resistant bacteria to survive.

Selective breeding has paved way for the development of new species of planyts and animals. The process is similar to the bacterial experiment discussed above. The organisms are selected on the basis of the traits required for the development of particular breed. All those individuals who lack the desirable characteristics are not allowed to breed. Therefore, the following generations more commonly have the desired traits.

Species that mature and reproduce large numbers in a short amount of time have a potential for very fast evolutionary changes. Insects and microorganisms

often evolve at such rapid rates that our actions to combat them quickly lose their effectiveness. We must constantly develop new pesticides, antibiotics, and other measures in an ever escalating biological arms race with these creatures. Unfortunately, there are a few kinds of insects and microbes that are now significantly or completely resistant to our counter measures, and some of these species are responsible for devastating crop losses and deadly diseases.

If evolution has occurred, there should be many anatomical similarities among varieties and species that have diverged from a common ancestor. Those species with the most recent common ancestor should share the most traits. For instance, the many anatomical similarities of wolves, dogs, and other members of the genus Canis are due to the fact that they are descended from the same ancient canine species and still share 99.8% of their genes. Wolves and dogs also share similarities with foxes, indicating a slightly more distant ancestor with them.

Evolution can be a very slow process. But actually we now have numerous examples where long term studies of populations document evolution happening within decades. Through meticulous measurement and attention to detail we can watch evolution in action. Evolution within populations is called microevolution and the changes are often small, but adding up these small differences through time is how evolution works.

Case study 1: Darwin’s Finches

One of the best studied examples of evolution in action comes from work done by Prof. Peter Grant and his wife Dr. Rosemary Grant on the beak size of Galapagos finches (aka Darwin’s finches).

These birds live on the Galapagos Islands, off the coast of Ecuador, right on the equator. For almost 40 years researchers have been living on these islands and taking detailed measurements of the birds. They focus on a few islands in particular, one is called Daphne. Daphne is small enough to be able to count all the birds on the island, and catch them often enough to keep track of their growth and size.

Survival isn’t easy on these islands; it’s baking hot during the day, freezing cold at night and there isn’t much food. Because of this, the birds have very specialised feeding behaviours. We see them now as 13 species all feeding on different things in different ways. But all these 13 species evolved from one common ancestor. Originally a South American finch colonised the island and became distinct from the mainland population (peripartic speciation). Different sub-populations of the birds also began to occupy new habitats and specialise on different ways of feeding. Selection kept these groups apart because hybrids were less fit (parapatric and sympatric speciation).

Each ground finch species has evolved a beak adapted to target different categories of seeds. The small ground finch has a small beak (3-4mm deep) and eats mostly small seeds, the medium ground finch has a medium sized beak (5mm deep) and eats moderately sized seeds and the large ground finch targets larger seeds that the others can’t crack open; it has a heavy duty beak (8mm deep) to allow it to do so.

The weather affects the abundance of seeds on the island. When it’s very hot and dry few seeds are produced and competition for food increases amongst the finches. The Grant’s research team were on the island during one dry spell that lasted much longer than usual. The island became very dry, vegetation died back and plants stopped producing seeds. They watched and recorded measurements about the seeds and beak sizes as the birds ate their way through the last of the easily eaten nuts and many starved and died. After many months without rain only the biggest, toughest and most spine covered seeds were left.

The researchers found that the lack of food had increased competition among the birds. The smaller beaked birds fared worst and amongst the large ground finch there was very strong selection for larger beaks, because the individuals with the largest beaks were the only ones able to eat. In fact, compared to the average depth of beak at the start of this dry period, by the end of it the average beak depth was 1mm bigger.

Another example comes from Prof. David Reznick’s work on guppies which live in rivers in a Caribbean Island called Trinidad. The guppies either live with or without the presence of a predator fish, a pike-like thing. So some populations grow up having to avoid being eaten, others don’t. The presence of the predator changes the selective pressure acting on the guppies, so they have evolved different traits.

The ones living with predators grow up faster, mature at smaller sizes and reproduce earlier; they need to breed as soon as possible because they could get eaten any time. In rivers without predators the guppies can grow up at a more leisurely pace and put more energy into their offspring.

Prof. Reznick’s team have done a series of experiments where they took fish from each population and put them into rivers which previously did not contain guppies. Some of these new rivers had predators in them, others didn’t. The experiment was set up so populations from rivers with predators were moved to rivers without predators and vice versa. As a control they also moved populations from rivers with predators to different rivers with predators, and the same for populations from rivers lacking predators.

The team left their fish in their new homes and came back after just ten years. Their prediction was that evolution should be replicated: the fish would evolve larger body sizes and delayed maturation in rivers without predators, and evolve smaller body sizes and earlier maturation in rivers with predators. Sure enough that’s exactly what they found! This proved the natural populations were adapted to maximise their reproductive output by adopting different strategies depending on their circumstances.

Another fish example is armor plating in sticklebacks. Since the last ice age, three-spined sticklebacks have been very successful, spreading from marine populations up into rivers and lakes across the world. As they have spread, sticklebacks have evolved a number of adaptations to help them evade predators.

Some populations have large pelvic spines, which make them harder to swallow so protect them against big predator fish. Some populations lack the spines, these fish usually swim quite close to the bottom of the water bed, within reach of crustacean predators which can use their claws to reach up and grab the pelvic spines of the sticklebacks. For these populations, lacking the pelvic spines helps them to avoid a different sort of predator. Researchers have even been able to pinpoint the gene involved in pelvic reduction, it’s called Pitx1 and seems to be involved in multiple, independent evolutionary events when spines have been lost.

Another defensive trait is armor plating. Sticklebacks which live in marine environments have a series of bony plates along either side of their body. These make it difficult for predators to eat the fish once they’ve caught it, so often the stickleback will get away. The problem with armour plating is that it slows the fish down, but since in marine environments water tends to be quite clear it’s unlikely the stickleback could escape it’s predator by swimming away.

In freshwater environments however the water can be a lot murkier and there could be selective pressure to bolt quickly when attacked so as to disappear into the murk. This would require the armour plates to be lost. In fresh water sticklebacks around the world this is what is seen. The number of plates is reduced, and sticklebacks go for a quick escape strategy of defence. Scientists have also pinpointed the gene involved in this phenotypic change, its called ectodysplasin, and, like Pitx1, it’s responsible for similar changes in multiple independent cases.

The whole idea of natural selection rests on the gradual adaptation of organisms to their environment. If the environment changes, some individuals will have characteristics that make them better able to cope than others, and these individuals will be more likely to pass on their genes to future generations.

The environment is changing. It is now widely accepted that carbon dioxide levels are rising and that this is causing temperatures to increase via the greenhouse effect. Global warming is a reality, and it will have major effects on many aspects of our environment, including…

Temperature rises of between 2 and 12 °C by 2100
Changes to rainfall and humidity levels
The destruction of habitats due to flooding, droughts or extreme weather, plus secondary events like forest fires
Changing disease patterns
Ocean acidification (decreased pH of seawater due to the formation of carbonic acid when carbon dioxide dissolves in water)
Changes in ocean currents and sea levels

These changes will and are already affecting life on earth. We are already witnessing changes to the genomes of organisms resulting in shifts in metabolic rate, breeding season, migration patterns, disease dynamics and food availability. All of these changes will upset the delicate balance of ecosystems.

Recent studies suggest the possibility of mass extinctions as a result of these changes – this is because rapid changes in the environment are likely to be challenging for organisms as they have adapted over thousands of years to suit particular niches (roles within the community). Some may be suited to the new environment but others will be forced to evolve in new ways to survive in the new environment. As a result there is a possibility that the rate of evolution may rise as selection pressure increases, and as a result large changes to biodiversity may result.

The changing climate is already resulting in earlier springs, longer summers and milder winters. It is this, rather than direct temperature change, which many organisms are battling against.

During the past forty years, scientists have observed species migrating, reproducing and developing earlier. A good example of this is the European great tit. European great tits are evolving different breeding times to ensure maximum food supplies to increase offspring survival. This is because Winter Moth caterpillars are maturing earlier in spring due to rising temperatures and these caterpillars are a staple food source for these birds. So individuals which can bring forward their egg-laying ensure greater food availability for their chicks and so these individuals are more successful. Research has shown that birds lay their eggs two weeks earlier than they did in the 1970s, a clear example of how organisms are altering in response to global warming. Similarly, Canadian red squirrels are breeding earlier allowing them to gather more pinecones to tide them over during winter.

Another example is the mosquito Wyeomyia smithii. In response to changes to length of day insects are developing later and gradually the genomes of northern individuals are coming to resemble more closely those of insects living near the equator.

In fact, several species depend on day length to tell them when to migrate, mate or go into hibernation. Other cues such as daily length of warmth (thermo period), rainfall and food availability are also crucial in allowing organisms to decide when to perform these tasks. Global warming is affecting all these cues except for day length.

Summary

Evolution is change in heritable traits of biological populations over successive generations.
Evolutionary processes give rise to diversity at every level of biological organisation, including the level of species, individual organisms, and at the level of molecular evolution
There are different theories of evolution
The evidence for evolution has primarily come from four sources: fossil record, chemical and anatomical similarities, geographical distribution and recorded genetic changes.
Evolution within populations is called microevolution and the changes are often small, but adding up these small differences through time is how evolution works.
There is a tendency in all organisms to increase in geometric ratio.
Biological variations with regard to structural, physiological and behavioural traits occur among the individuals.
Those who have selective advantage tend to survive.
They reproduce more number of offspring than those who are less adaptive.
The accumulation of advantageous traits in future generations gradually bring changes in the species and eventually a new species may be evolved

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References & Further Reading

The Beak of the Finch by Jonathon Weiner, Vintage Books: 1995
Darwin’s Dream Pond by Tijs Goldschmidts, MIT Press: 1997
How and Why Species Multiply by Peter & Rosemary Grant, Princeton: 2008
The Making of the Fittest by Sean B. Carroll, Quercus: 2008
Why Evolution is True by Jerry Coyne, OUP: 2009
The Future of Life by Edward O. Wilson, Abacus: 2002