BD  4

4. Evolution – the Evidence

IQ: What is the evidence that supports the Theory of Evolution by Natural Selection?

4.1   investigate, using secondary sources, evidence in support of Darwin and Wallace’s Theory of Evolution by Natural Selection, including but not limited to:  

a) biochemical evidence, comparative anatomy, comparative embryology and biogeography (ACSBL089)

b) techniques used to date fossils and the evidence produced 

4.2 explain modern-day examples that demonstrate evolutionary change, for example:  

a) the cane toad

b) antibiotic-resistant strains of bacteria 

Modern evolutionary theory states that all living organisms share a common origin that dates back to around 3800 million years ago.  These earliest organisms were bacteria, but  over millions of years they diverged. Some became extinct, others continued to diverge in ever-branching lines of descent to those we have today.

Many different forms of evidence have come together to provide the modern theory of evolution. These include 

a) biochemical evidence, comparative anatomy, comparative embryology and bio-geography 

b) techniques used to date fossils and the evidence produced   

Overview and Review

Read: 

a) EVIDENCE FOR EVOLUTION

i)  Biochemical Evidence

Summary: 

Certain parts of our DNA sequence called genes each code for a unique sequence of amino acids called a polypeptide chain. These polypeptides fold into proteins that ultimately regulate our cellular functions thereby determining our characteristics.

Evolution relies on mutations that alter the DNA sequence producing a new protein with an altered function. If the new function coveys some adaptive advantage it will be selected for (see natural selection)

However, not all mutations actually alter the amino acid sequence or structure of a protein. Therefore not every difference in the DNA sequence of two species represents an evolutionary change. Comparing the amino acid sequence or protein structures of two organisms gives a more accurate idea of their evolutionary relatedness.

Detail:

Biologists use the DNA sequences of modern organisms to reconstruct the tree of life and to figure out the likely characteristics of the most recent common ancestor of all living things — the "trunk" of the tree of life. In fact, according to some hypotheses, this "most recent common ancestor" may actually be a set of organisms that lived at the same time and were able to swap genes easily. In either case, reconstructing the early branches on the tree of life tells us that this ancestor (or set of ancestors) probably used DNA as its genetic material and performed complex chemical reactions. But what came before it? We know that this last common ancestor must have had ancestors of its own - a long line of forebears forming the root of the tree of life - but to learn about them, we must turn to other lines of evidence. 

There are certain key molecules and biochemical mechanisms shared by incredibly different organisms. Many of the chemical reactions occurring in human cells, in the cells of a fungus, and in a bacterial cell are quite different from one another; however, many of them are exactly the same and rely on the exact same molecules - photosynthesis and cellular respiration are identical or very similar in quite different types of organisms, all organisms use DNA and/or RNA for their genetic code; ATP,  essential for powering cellular processes, is used by all modern life. 

Because these molecules are widespread and are critically important to all life, they are thought to have arisen very early in the history of life.  

Read  https://evolution.berkeley.edu/evolibrary/article/0_0_0/origsoflife_06 

The current view is that an RNA world existed on Earth before modern cells arose. According to this hypothesis, RNA stored both genetic information and catalysed the chemical reactions in primitive cells. Only later in evolutionary time did DNA take over as the genetic material and proteins become the major catalyst and structural component of cells. If this idea is correct, then the transition out of the RNA world was never complete; RNA still catalyzes several fundamental reactions in modern-day cells, which can be viewed as 'molecular fossils; of an earlier world. 

"Experiments can help scientists figure out how the molecules involved in the RNA world arose. These experiments serve as "proofs of concept" for hypotheses about steps in the origin of life — in other words, if a particular chemical reaction happens in a modern lab under conditions similar to those on early Earth, the same reaction could have happened on early Earth, and could have played a role in the origin of life."

In the 1950's, biochemists Stanley Miller and Harold Urey, conducted an experiment which demonstrated that several organic compounds could be formed spontaneously by simulating the conditions of Earth's early atmosphere.

They designed an apparatus which held a mix of gases similar to those found in Earth's early atmosphere over a pool of water, representing Earth's early ocean. Electrodes delivered an electric current, simulating lightning, into the gas-filled chamber. After allowing the experiment to run for one week, they analyzed the contents of the liquid pool. They found that several organic amino acids had formed spontaneously from inorganic raw materials. These molecules collected together in the pool of water, suggesting complex molecules could have formed in early Earth.

ii) Comparative Anatomy

Comparing the body structures (anatomy) of different species also supports the notion (idea) of a common ancestor. Closely related species have more anatomical (structural) similarities. Even less closely related species show evidence of underlying anatomical similarities, with common structural features that have been modified for a different function / purpose.

Anatomical features that are derived from a common ancestor but have been adapted to a different purpose are called homologous structures.

The pentadactyl (5-digit) limb found in most vertebrates (animals with a backbone) has the same general bone structure / pattern. However, the size and shape of each bone has modified to serve a slightly different function.

These "homologues" indicate that all of these species diverged from a common ancestor (adaptive radiation) and that the basic limb plan has adapted to meet the needs of different niches.

Some animals possess inherited features that they no longer need. 

Whales still have the remains of a hip bone. It is significantly reduced (smaller), but serves no known function. This is evidence that whales have evolved from a once four-legged ancestor. The hind legs and hips  have steadily become smaller and may one day be eliminated entirely. For now, whales are stuck with this "evolutionary baggage".

A major problem in determining evolutionary relationships based on comparative anatomy can be seen when we look at a common structure: the wing. Wings are present in a number of very different groups of organisms. Birds, bats and insects all have wings, but what does this say about how closely related the three groups are? It is tempting to say that the three groups must have had a common winged ancestor. However, the wings of bats and birds are both derived from the forelimb of a common, probably wingless, ancestor. Both have wings with bone structures similar to the forelimbs of ancestral and current tetrapod, or four-legged, animals. These are homologous traits. 

Structurally speaking, though, the wings of bats and birds have little in common with those of insects. Bird wings and insect wings are an analogous trait, or a trait that has developed independently in two groups of organisms from unrelated ancestral traits. (Analogous Structures are features that have a very similar function but completely different anatomy. They normally occur when distantly related species occupy a similar environment.)

iii) Comparative Embryology

All species start out as single celled organisms, zygotes. Many species develop into much larger, more complex organisms after conception. If we compare the embryos of animals as they develop, we often find they are much more similar than their fully developed counterparts. Many of the anatomical differences between species only arise during our embryonic development. Different species often start with the same basic tissues or structures but they develop differently and are re-purposed into different structures as the organism develops. The more closely two species are related the later in development these differences usually emerge. This too supports the idea that we are descendants with modified structures that were inherited form a common ancestor.

Another difficulty in comparing traits between species rests on the fact that homologous structures not present in the adult organism often do appear in some stage of embryonic development. In this way, the embryo serves as a microcosm for evolution, passing through many of the stages of evolution to produce the current state of the organism. Species that bear little resemblance in their adult form may have strikingly similar embryonic stages. For example, in humans, the embryo passes through a stage in which it has gill structures like those of the fish from which all terrestrial animals evolved. For a large portion of its development the human embryo also possesses a tail, much like those of our close primate relatives. This tail is usually reabsorbed before birth, but occasionally children are born with the ancestral structure intact. Tails and even gills could be considered homologous traits between humans and primates or humans and fish, even though they are not present in the adult organism. 

If you were to compare the embryos of these animals at what point do you think you could pick which one is human?

iv) bio-geography

The field of biogeography is concerned with the distribution of species in relation both to geography and to other species - historical biogeography is concerned with the origins and evolutionary histories of species on a long time scale.

Historical biogeographers depend heavily on evidence from other disciplines. Fossil records provide a large part of the information needed to determine distributions and past interactions. Molecular Biology furnished historical biogeographers with molecular clocks, metabolic molecules whose change over time help track the relatedness of species.

The geographic distribution of organisms on Earth follows patterns that are best explained by evolution, in combination with the movement of tectonic plates over geological time. For example, broad groupings of organisms that had already evolved before the breaking apart of the supercontinent Pangaea (about 200 million years ago) tend to be distributed worldwide. In contrast, broad groupings that evolved after the break tend to appear uniquely in smaller regions of Earth: there are unique groups of plants and animals on northern and southern continents that can be traced to the split of Pangaea into two supercontinents (Laurasia in the north, Gondwana in the south). 

The evolution of unique species on islands is another example of how evolution and geography intersect. Most of the mammal species in Australia are marsupials (carry young in a pouch), while most mammal species elsewhere in the world are placental (nourish young through a placenta). Australia’s marsupial species are very diverse and fill a wide range of ecological roles. Because Australia was isolated by water for millions of years, these species were able to evolve without competition from (or exchange with) mammal species elsewhere 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 mainlands. This combination of features reflects the processes by which island species evolve. They often arise from mainland ancestors – for example, when a landmass breaks off or a few individuals are blown off course during a storm – and diverge (become increasingly different) as they adapt in isolation to the island environment.

OVERVIEW

Generally, we think of bones, shells, or teeth that are buried in rock, but fossils can also be outlines of leaves or footprints or trails. This second set of fossils, which are the outlines of items from the past rather than the items themselves, are called trace fossils. Fossils are formed when sediment covers some material, such as a piece of bone. Very gradually, the bone becomes impregnated with chemicals from the surrounding rock. Eventually all that remains is essentially a piece of rock in the shape of the original bone, or material.

The fossil record provides snapshots of the past that, when assembled, illustrate a panorama of evolutionary change over the past four billion years. The picture may be smudged in places and may have bits missing, but fossil evidence clearly shows that life is old and has changed over time.  Whereas molecular biology might be used to study microevolution, or the development of individual species, paleontology is used to study macroevolution, or large evolutionary trends. 

• Taken together, fossils can be used to construct a fossil record, which is a timeline of fossils reaching back through history. Several factors must be taken into account when constructing such a record. The strata of rock in which fossils are found give us clues about their relative ages. Similarly, technological techniques such as radioactive carbon dating help determine the absolute ages of fossils. In addition to supplying a fossil's relative age, rock strata can also give clues about the environments in which an animal or plant lived. The chemical make-up of these strata can tell us the balance of gases in ancient atmospheres. Major cataclysmic events such as eruptions and meteor strikes also leave their mark on the fossil record.

• There are, however, limitations on the information fossils can supply. Fossilisation is an improbable event. Most often, bones and other materials are crushed or consumed before they can be fossilised. In addition, fossils can only form in areas where sedimentary rock is formed, such as ocean floors. Organisms that live in these environments are therefore more likely to be fossilised. Erosion of exposed rock faces or through the crushing action of geological movements can destroy fossils even after they are formed. All of these conditions lead to large and numerous gaps in the fossil record.

Phylogenetic Trees

The evolutionry evidences, taken together, allow scientists to build phylogenetic trees. These are hypotheses, not laws or theories. 

• A phylogenetic tree is a diagram that represents evolutionary relationships among organisms. 

• The pattern of branching in a phylogenetic tree reflects how species or other groups most probably evolved from a series of common ancestors.

• In trees, two species are more related if they have a more recent common ancestor and less related if they have a less recent common ancestor.

• In a phylogenetic tree, the species or groups of interest are found at the tips of lines referred to as the tree's branches.  

• The pattern in which the branches connect represents our understanding of how the species in the tree evolved from a series of common ancestors. Each branch point (also called an internal node) represents a divergence event, or splitting apart of a single group into two descendant groups.

• At each branch point lies the most recent common ancestor of all the groups descended from that branch point. For instance, at the branch point giving rise to species A and B, we would find the most recent common ancestor of those two species. At the branch point right above the root of the tree, we would find the most recent common ancestor of all the species in the tree (A, B, C, D). Each horizontal line in our tree represents a series of ancestors, leading up to the species at its end. For instance, the line leading up to species E represents the species' ancestors since it diverged from the other species in the tree. Similarly, the root represents a series of ancestors leading up to the most recent common ancestor of all the species in the tree. 

Natural selection increases the frequency of characteristics that make individuals better adapted and decreases  the frequency of other characteristics, leading to changes within the species

The key components to the process of natural selection are ICE AGE:

• Inherited variation exists within the population

• Competition results from an overproduction of offspring

• Environmental pressures lead to differential reproduction

• Adaptations which benefit survival are selected for

• Genotype frequency changes across generations

• Evolution occurs within the population

a) antibiotic resistance: Darwinian

• The development of bacterial resistance to antibiotics is direct evidence for evolution by the process of natural selection.

Overview:

• Antibiotic resistance is one of the biggest threats to global health, food security, and development today.

• Antibiotic resistance occurs naturally, but misuse of antibiotics in humans and animals is accelerating the process.

• A growing number of infections – such as pneumonia, tuberculosis, gonorrhoea, and salmonellosis – are becoming harder to treat as the antibiotics used to treat them become less effective.