4.4 Evidence of Evolution

4A: Evidence for Evolution

In The Origin of Species, Darwin put together a group of facts that prior to this time had seemed unrelated. While Darwin was not the first person to decide that lifeforms changed over long periods of time, he was the first to publish these ideas in a comprehensive manner. Prior to Darwin's publication of The Origin of Species, scientists from many disciplines - biology, geology, geography and paleontology - had provided lots of research that supported and strengthened Darwin's work - The theory of evolution by natural selection.

Fossils:  Evidence for the History of Life

Sedimentary rock that contains fossils, creates a fossil record of the history of life by showing different species that have lived on Earth in the past. In British Columbia, the Burgess Shale fossil beds contain fossils of animals that lived in an ancient ocean over 500 million years ago, in the Cambrian Period. The Burgess Shale fields contained not only micro-organisms and soft-bodied animals, but also some of the earliest fossils of animals with hard parts found in the fossil record. Some of these animals are ancestors of animals that are common today. Others have long been extinct and are unlike any species found on Earth today. 

The image above, taken from the Pearson Biology 11 textbook (2009), shows how organisms in the fossil record have changed overtime.  Notice how compared to many of the organisms, humans have been on the Earth for a very short amount of time?  This image demonstrates when certain organisms first appeared in the fossil record.

Evidence from the Fossil Record Supporting The Theory of Evolution by Natural Selection

1. Fossils found in young layers of rock (closer to the surface) are much more similar to species alive today than fossils found in older (deeper) layers of rock. 

2. Fossils appear in chronological order in rock layers.  Probable ancestors for species are found in older rocks, usually lying beneath the strata in which the species is found.

3. Not all organisms appear in the fossil record at the same time.  For example the fossil history of vertebrates shows that fish are the oldest vertebrates.  In newer layers, the fossils of amphibians, reptiles, mammals and birds appear - reinforcing that amphibians evolved from ancestral fish, reptiles evolved from ancestral amphibians, and both mammals and birds evolved from different groups of reptiles, mammals first, then birds.  Remember that this was a slow process, and took millions of years.

The image above demonstrates how modern horses have evolved, using fossils and comparing them to the bones of a modern horse.  The Eocene and the Miocene are both divisions of the Cenozoic Era, and the Eocene was 54 million years ago, demonstrating how slow evolutionary change really is.

Evidence from Transitional Fossils

The original fossil record contained "gaps", and the ongoing discovery of hundreds of "transitional fossils" - fossils that show intermediary links between groups of organisms - has helped scientists understand the evolutionary process and relationships between groups of organisms.  Transitional fossils help to link the past with the present. 

For example, scientists have found fossilized whales that lived 36 - 55 million years ago.  These fossils connect modern day whales to their ancestors.  Basilosaurus and Dorudon were ancient whales that had tiny hind limbs, but led an entirely aquatic life.  Dorudon was the size of a large dolphin, about 5 m long, and a tiny pelvis with tiny legs (about 10 cm long).  Structures that are the reduced form of structures that were functional in the organism's ancestors are called vestigial structures.  The pelvic bone in the Dorudon whale - and in some modern whales such as the Baleen Whale - is called a vestigial pelvic bone.

Ambulocetus, a transitional form that was discovered more recently (1994), had heavier leg bones.  Scientists hypothesize that it could live on both land and water.

The image above, taken from www.nature.com, shows a comparison between modern whales and their ancestors from the fossil record. 

The fossil record also shows evidence of birds evolving from reptiles through Archaeopteryx.  Fossils of Archaeopteryx show a transitional stage in the fossil record because they exhibit characteristics of both reptiles (dinosaurs) and birds.  Archaeopteryx had feathers, but, unlike any modern bird, it also had teeth, claws on it's wings and a bony tail.

Evidence from Biogeography

Biogeography is the study of the past and present geographical distribution of organisms.  Both Darwin and Wallace used examples from biogeography to develop their theories. Darwin and Wallace hypothesized that species evolve in one location, and then spread out to other regions.  Biogeography supports this hypothesis with examples such as:

1. Geographically close environments (example - desert and forest habitats in South America) are more likely to be populated by related species than locations that are geographically separated, but environmentally similar (example - a desert in Africa and a desert in Australia).  Cacti are native to deserts of North, Central and South America, and are not found naturally in deserts in other areas of the world such as China, Australia or Africa.

2. Animals found on islands often closely resemble animals found on the closest continent.  This suggests that animals on islands have evolved from animals that migrated from nearby land masses, becoming adapted over time to the environmental conditions of the island.  For example, although they are different species, the lizards found on the Canary Islands, off the northwest coast of Africa, are very similar to the lizards found in west Africa.

3. Fossils of the same species can be found on the coastline of neighbouring continents.  For example, fossils of the reptile Cynognathus have been found in Africa and South America.  How can this be explained?  The location of continents is not fixed - plate tectonics moves the plates underneath the continents slowly away from each other.  About 510 million years ago, the continents of Africa and South America were joined in one super-continent called Gondwana (see image above). 

4. Closely related species are almost never found in exactly the same location or habitat.

Evidence from Anatomy

Vertebrate forelimbs can be used for various functions, such as flying (birds and bats), running (horses and dogs) and swimming (whales and sea lions).  Despite their different functions, all vertebrate forelimbs contain the same bones, organized in similar ways.  The best explanation of this is found using phylogeny, and that is that the basic vertebrate forelimb evolved from a common ancestor. 

Homologous Structures are those that have similar physiological features and origin, but have different functions. 

The image above shows how each of these vertebrates has the same bones in their forearms (distinguished by colour), yet each of their forelimbs performs a different function.  These are homologous structures.

Functional similarity in anatomy does not necessarily mean that organisms are closely related.  The wings of insects and birds are similar in function, but not in structure.  Bird wings are made of bones, while insect wings are made out of chitin, the same material that makes up the cell walls of bacteria and fungi.  Body parts that perform similar functions, even though the organisms do not have a close evolutionary origin are called analogous structures.  Analogous structures evolve in species of different origins that have similar environmental selective pressures. 

Evidence from Embryology

Embryology is the study of early, pre-birth stages of an organism's development.  The embryos of different organisms exhibit similar stages of embryonic development.  Recall from the diversity unit that all embryos of sub-phylum vertebrata contain pharyngeal gill slits, a post anal tail and a hollow dorsal nerve tube at some point in their development.  The similarities between embryos in related groups points to a common ancestral origin.  It follows that related species share both adult features and embryonic features. 

Evidence from DNA

As you learned in the genetics and diversity units, evolutionary relationships between species are reflected in their DNA.  Since DNA carries the genetic code, scientists can compare how closely related two different species are to each other by comparing their DNA.  If species have similar patterns in DNA, it indicates that those patterns were inherited from a common ancestor.  For example, humans are the most closely related to bonobos and chimpanzees as per genetic similarities. 

Use of modern technology has led to many discoveries that support Darwin's theory.  Scientists now know that species pass on their traits to their offspring, and how the genes for these traits could change by mutation.  Current evolutionary theory connects genetics with the theory of natural selection, and how natural selection operates on populations.  Therefore, genetic evidence and our understanding of heredity and mutations lend support to hypotheses that stem from observations of fossils, anatomy, biogeography, embryology and DNA relationships.