The Core Theme of Biology: Evolution Accounts for Unity & Diversity of Life
This post is part of the ‘Learn Biology’ series
There is consensus amongst biologists that evolution is the core theme of biology. To quote one of the founders of modern evolutionary theory, Theodosius Dobzhansky:
Nothing in biology makes sense except in the light of evolution.”
Organising the Diversity of Life
Diversity is one of the most notable characteristics of life.
About 1.8 million species have been identified and named by scientists.
This includes at least:
6,300 species of Prokaryotes (organisms made of prokaryotic cells)
100,000 Fungi
290,000 Plants
52,000 Vertebrates (animals with backbones)
1 million Insects
Thousands of additional species are identified by researchers every year. The estimates for the total number of species vary from 10 million to 100 million.
Biology is the study of all life; from the level of the biosphere right down to the level of individual molecules inside cells.
So that the diversity of life can be organised biologists group closely related species (e.g. black bear and polar bear) together in a category called ‘genus’. Genera are grouped into families, and so forth.
Grouping Species: The Basic Idea
Life is grouped according to the similarities between organisms and different categories of organisms.
Whales have the same basic forelimb structure as humans. This correctly indicates that whales are more closely related to us than, say, sharks.
Sometimes similarities can be confusing though. For example; the wings of bats and birds, though similar in form and function, actually evolved independently from each other and do not indicate a close relationship between the different bird and bat species (bats are mammals, whereas birds evolved from the dinosaurs).
The Three Domains of Life
Up to a few decades ago biologists grouped life into 5 kingdoms: plants, animals, fungi, single celled eukaryotic organisms, and prokaryotes. However, due to new methods like those comparing DNA sequences, there have been many revisions of the number of kingdoms and what they include.
There is consensus now, though, that the kingdoms of life can themselves be grouped at a higher level into domains, of which there are 3:
Domain Bacteria
Domain Archaea
Domain Eukarya
The organisms making up the domains bacteria and archaea are all organisms made up of prokaryotic cells (most are single celled and microscopic)!
The reason that prokaryotic organisms are divided in their two higher order domains is that there is much evidence that supports bacteria and archaea representing two very different branches of prokaryotic life. There is also evidence that archaea is at least as closely related to eukaryotic organisms as they are to bacteria.
Multicellular eukaryotic life, to which we belong, is divided into 3 kingdoms:
Plantae – produce own sugars and foods from photosynthesis (conversion of light energy to chemical energy)
Fungi – absorb dissolved nutrients from surroundings (e.g. leaf litter, feces)
Animalia – ingest nutrients (consume through putting in mouth)
Unity in the Diversity of Life
Similarities can be found in the skeletons of animals that most people might think of as being very different, such as in the mammalian forelimb:
Unity can also be seen in many features of eukaryotic cell structure:
Charles Darwin and the Theory of Natural Selection
In 1859, when Darwin published his book On the Origin of Species by Means of Natural Selection, much evidence, in the form of fossils found, research on geology, and the comparing of different species, indicated that the Earth had been changing for billions of years and with it so had a multitude of different species.
Marine fossils (fossils of organisms that live in the ocean) had been found in rock at the top of mountains. Different layers of rock contained their own unique collections of fossils, many of which represented species that were no longer alive, and creatures that first appeared in one layer were never found again in lower (older) layers.
Darwin’s work made sense of much of this information. There were two main points in his book:
That the history of life was that of decent with modification
This point highlights both the unity of life, that different species descended from common ancestors, and the diversity of life, that modifications arose through the generations.
That natural selection was the mechanism for this descent with modification
His reasoning for drawing this conclusion was thus:
• Individuals in a population vary in their traits (characteristics) and traits often seem to be passed on from parents to offspring
• A population can produce more offspring than can survive to have offspring of their own
• With more individuals than the environment can support competition for resources is inevitable
• Species generally seem to suit their environments (e.g. birds living where tough seeds are a good resource may have exceptionally strong beaks)
Since individuals in a population vary in their traits some individuals must be better suited to their environment than other individuals.
Darwin reasoned that individuals that inherited traits best suited to the local environment would be more likely to survive and reproduce than individuals that didn’t inherit the traits best suited to the local environment. The logic is compelling; it’s like saying that sports teams with excellent players are more likely to get into the next round of a tournament than sports teams that don’t have such good players.
Darwin wrote that over many generations the proportion (relative number) of the population with such advantageous traits will have increased. He concluded that it was the unequal reproductive success of individuals that adapts (makes suitable) a population to its environment. He called this mechanism natural selection because the natural environment “selects” certain traits to increase in frequency (rate at which something occurs) in a population.
Creationists viewed the exquisite compatibility of organisms with their environments as being evidence that the structure of all organisms had been designed by some supernatural ‘god’ to be compatible with their functions. However, through the work of scientists like Darwin we have learned that it is natural selection that is responsible for such a fit between a species’ form and its function.
Here’s an example of natural selection acting on a population of beetles and increasing the frequency of the dark colour characteristic
The Tree of Life
Have another look at the above image comparing the mammalian forelimb of different species. The wing of a bat shares the same basic bone structure with the arm of a human, foreleg of a horse, and flipper of a whale. It also shares with all other mammalian forelimbs the same joints, nerves, and blood vessels. Despite the obvious differences between a flipper and an arm all mammalian forelimbs are anatomical variations of a common architecture.
Such examples highlight the unity and diversity of life that have arisen through descent with modification. Descent from a common ancestor produces the unity. Modification through the generations produces the diversity.
Modern mammals have a common forelimb structure because they have all inherited it from a common ancestor, a ‘prototype’ mammal. Variation arose through modifications as natural selection operated over millions of generations on populations living in different environmental contexts. Such a view has been supported by fossil and other evidence.
Darwin proposed that over long periods of time natural selection could cause an ancestral species to give rise to 2 or more descendant species. This could happen if one population were separated into isolated groups in different environments. A trait that is adaptive and increases an individual’s chance of surviving and reproducing successfully in one environment may not be ‘selected’ for in another environment. Some traits may increase in frequency in one population but not the other and over time that one population could radiate into multiple species.
Here’s an example of an adaptive radiation:
The finches pictured above are found on the Galapagos Islands located 900 kilometres from the South American Pacific coast. The relatively young islands, having formed several million years ago due to volcanic activity, are home to many species of plants and animals found nowhere else on Earth, though most are clearly related to species on the South American mainland (the finches are thought to come from the Caribbean).
It is likely that the finches are all descendants of an ancestral species of finch that made it to the islands and diversified there after the islands were formed. Research into sorting out the relationships among the finches continues using anatomical and geographical data, and more recently with DNA sequence comparisons.
Tree diagrams (without the trunk) make sense for thinking about evolutionary histories in the same way that family trees make sense for diagramming a person’s relationship with the other members of their family.
Each twig branching from the tree of life represents one species. Species that are very similar, like the Galapagos finches, share a common ancestor at a relatively recent branching point. Finches share a more distant common ancestor and branching point with hawks, penguins, and all other birds. Birds, mammals, and all other vertebrates share an even more distant and ancient common ancestor. We can see similarities at the level of all eukaryotic cells, for example in the identical construction of their cilia as per the image above. Go back far enough and the oldest fossils recovered are of prokaryotes that lived on Earth over 3.5 billion years ago. We can still recognise inherited remains from the prokaryotes in structures within our own cells, and in the universal genetic code.
Life on Earth shares a very long evolutionary history.
(Campbell, Reece, et al, 2008, “Biology”)
