Evolution and a Common Ancestor

What would be evidence of evolution?

Is it just fossils? Just fossils do provide tremendous evidence of evolution. But then so do many other pieces of scientific information. Especially in this era of molecular biology.

Evidence of Evolution

To get started, please take the time to watch this video from Stated Clearly. It does an excellent job of laying out the basics and it will be covered on your quiz. (Video not displaying properly? Try this direct link: https://youtu.be/lIEoO5KdPvg)

Monkey Business

So you are probably aware that the concept of evolution still generates controversy today, despite its wide acceptance.

In On the Origin of Species, the man considered the "Father of Evolution," Charles Darwin mentioned humans only once, predicting, "Light will be thrown on the origin of man and his history." Nevertheless, some people immediately distorted its far-reaching message about the unity of life into near-sighted shorthand, mistakenly stating that humans “came from” monkeys (Figure below).

Darwin never said this. His theory states that they, like all species, share common ancestry. Evolution relates to all of life – not just humans and monkeys. Biological evolution, like all scientific theories, is much more than just an opinion or hypothesis, it is based on evidence.

The Fossil Record

Structural Changes Through Time

Few would argue that dinosaurs roamed Earth in the past, but no longer exist. The fossil record is a revealing window into species that lived long ago.

Although the fossil record is far more detailed today than in Darwin’s time, Darwin was able to use it as powerful evidence for natural selection and common descent. Throughout geological history, species that appear in an early rock layer disappear in a more recent layer. Darwin argued that a species’ appearance recorded its origin and that its disappearance showed extinction. Moreover, he noted remarkable similarities among structures in differing species, supporting common ancestry. Finally, he could often correlate environmental conditions with structures, supporting his idea that natural selection led to adaptations which improved survival within certain habitats.

The fossil record for relatives of the modern horse is unusually complete, allowing us to select a few which show major changes over time. These changes can be correlated with environmental changes, supporting the ideas of evolution and natural selection.

As an example, let’s analyze a relatively complete set of fossils which record the evolution of the modern horse.

The Figure above sequences five species which show major evolutionary changes. The oldest fossil shows a fox-sized animal with slender legs and splayed digits: Hyracotherium bit and chewed soft leaves in wooded marshlands. Geology and paleontology suggest that the climate gradually dried and grasslands slowly replaced the marshes. Mesohippus was taller, with fewer, stronger digits, making it better able to spot and run from predators, thus making it more likely to survive and reproduce in the new grasslands. Merychippus was taller still and kept only one, enlarged digit, a hoof, to run fast on the hard ground. By the time Pliohippus walked the Earth, molar teeth had widened and elongated to grind the tough grasses. These fossils show gradual structural changes which correspond to changes in the environment. They appear to show a smooth, linear path directed toward the “goal” of the modern horse, but this is deceiving. These five fossils are merely “snapshots” of a bushy family tree containing as many as 12 genera and several hundred species. Some transitions are smooth progressions; others are abrupt. Together, they support natural selection and descent with modification from common ancestors. Similar fossil evidence demonstrates the evolution of the whale, moving from the land into the sea.

Biogeography

Biogeography is the study of how and why plants and animals live where they do. It provides more evidence for evolution.

The biogeography of islands yields some of the best evidence for evolution. Consider the birds called finches that Darwin studied on the Galápagos Islands (see Figure below). All of the finches probably descended from one bird that arrived on the islands from South America. Until the first bird arrived, there had never been birds on the islands. The first bird was a seed eater. It evolved into many finch species. Each species was adapted for a different type of food. This is an example of adaptive radiation. This is the process by which a single species evolves into many new species to fill available niches.

In the 1970s, biologists Peter and Rosemary Grant went to the Galápagos Islands. They wanted to re-study Darwin’s finches. They spent more than 30 years on the project. Their efforts paid off. They were able to observe evolution by natural selection actually taking place.

While the Grants were on the Galápagos, a drought occurred. As a result, fewer seeds were available for finches to eat. Birds with smaller beaks could crack open and eat only the smaller seeds. Birds with bigger beaks could crack and eat seeds of all sizes. As a result, many of the small-beaked birds died in the drought. Birds with bigger beaks survived and reproduced (seeFigure below). Within 2 years, the average beak size in the finch population increased. Evolution by natural selection had occurred.

Molecular Biology or DNA Comparison

Your genes are over 95% identical to those of the great apes. DNA Comparison is a newer, and extremely powerful line of evidence supporting evolution and a common ancestor. The closer genes from two species are to each other, the closer they are related evolutionarily. Molecular biology has played a significant role in determining these relationships.

Unknown in Darwin’s time, the “comparative anatomy” of the molecules which make up life has added perhaps the most convincing set of information to the evidence for evolution. All living organisms have genes made of DNA. The order of nucleotides – As, Ts, Cs, and Gs – in each gene codes for a protein, which does the work or builds the structures of life. Proteins govern the traits chosen (or not) in natural selection. For all organisms, a single Genetic Code translates the sequence of nucleotides in a gene into a corresponding chain of 20 amino acids. By itself, the universality of DNA genes and their code for proteins is strong evidence for common ancestry. Yet there is still more.

If we compare the sequence of nucleotides in the DNA of one organism to the sequence in another, we see remarkable similarities. For example, human DNA sequences are 98-99% the same as those of chimpanzees, and 50% the same as a banana’s! These similarities reflect similar metabolisms. All organisms have genes for DNA replication, protein synthesis, and processes such as cellular respiration. Although metabolic processes do not leave fossils, similar DNA sequences among existing organisms provide excellent evidence for common ancestry.

The differences in DNA sequences are even more intriguing. Many are single base substitutions resulting from mutations accumulated through time. Assuming mutations occur randomly, the number of differences in bases between any two species measures the time elapsed since two organisms shared a common ancestor. This type of molecular clock has confirmed traditional classification based on anatomy. Most scientists consider it sufficiently powerful to clarify or correct our understanding of evolutionary history. For example, human DNA differs 1.2% from chimpanzees, 1.6% from gorillas, and 6.6% from baboons; we can infer from this data that humans and chimpanzees share a relatively recent common ancestor, and that the common ancestor we share with gorillas lived much longer ago. Figure below shows a cladogram depicting hypothetical evolutionary relationships constructed with this data. Similarities and differences in the sequences of amino acids in proteins support common ancestry in the same way, because they are determined by DNA. See the History of Life: Molecular Clocks (Advanced)concept for additional information on molecular clocks.

Are We All Related?

This video from "It's Okay To Be Smart" does a good job of applying these concepts to us, humans... and it's pretty mind blowing. (Video not displaying properly? Try this direct link: https://youtu.be/mnYSMhR3jCI)

Summary

• Evolution is not “just a theory”; it is a scientific theory which explains and unifies the entire field of biology and has a great deal of evidence supporting it.

• Fossil species and their adaptations support the idea of natural selection occurring over time as environmental pressures changed.

• Biogeography is the study of how and why plants and animals live where they do. It also provides evidence for evolution; on island chains, such as the Galápagos, one species may evolve into many new species to fill available niches.

• The universality of DNA for genes, use of amino acids to build protein enzymes, and the Genetic Code (all of which fall under the field of Molecular Biology) are all strong pieces of evidence for common ancestry.

• A Cladogram is a tree-like diagram showing evolutionary relationships which can be constructed from one or multiple kinds of comparison data like DNA sequence comparisons.