As we have discussed, we are going to focus on cell biology, heredity, evolution and diversity in this course. These concepts are relevant to all biological systems. As such, these concepts form a unifying core of principles that apply to all of life. However, although you might think that ‘diversity’ applies only to species, we see this characteristic across all scales of life, too: from the diversity of proteins in a cell, to the diversity of cells in a multicellular organism, to the diversity of species on a coral reef. Let’s examine these properties of unity and diversity, and see why they are so important.
A. Structural Unity
1. All living systems are made of the same stuff, arranged in largely the same way at a molecular level:
Biomolecules: lipids, carbohydrates, proteins, nucleic acids, in water.2. All life is cellular – even the largest living things are made of tiny living cells.
Why? Well, first, cells are much different from their environment. In order to maintain these differences and not just 'leak' their stuff out, a barrier is needed that separates the living thing from the non-living environment. This cell membrane is the barrier, and the living 'stuff' is trapped in this cell.
Why are they small? As a three-dimensional object gets larger, the volume increases at a cubic rate of linear dimension. Chemicals are VERY VERY small; they must run into one another to interact. If you have 100 protein molecules in a cell that is 1 cubic um, concentration is 100/nm3. If the cell doubles in length, it’s volume increases to 2 x 2 x 2 = 8 um3. So, to maintain the same concentration of reactants (proteins) a cell that doubles in “size” must make or accumulate 8 times as much stuff! Small cells can increase the concentration of reactants in that small volume more easily, and the concentration of reactants affects reaction rates. But these are just physical constraints. There could be entirely different types of cells that could take advantage of these physical properties.
B. Functional Unity
What is even more remarkable about life is not only the structural similarity—it is the functional similarity: cells work in the same way.The primary thing all cells do is make proteins. Some proteins are structural (collagen in your skin, actin and myosin in muscles), but others are enzymes. All chemical reactions in living cells—including those that make lipids, make carbohydrates, make other proteins, and make DNA—require protein enzymes. So, in order to make anything, cells must make particular proteins. And all cells make proteins the same way… they read a ‘recipe’ encoded by the sequence of ATCG in DNA, and a particular sequence of amino acids is put together to make a particular protein. All life. And the process isn’t just the same, the recipe is written in the same language. So, when TTT is read in the DNA of a bacterium, or an oak tree, or your muscle cells, the amino acid proline is placed into the protein sequence. THAT is UNITY.
And cells replicate in the same way to make more cells.
C. Why is life so similar?
All life is so structurally and functionally similar that it is inconceivable that this similarity arose independently and separately. Life replicates. This extraordinary similarity is a consequence of common ancestry… living cells are similar because they inherited genes and characteristics from the same ‘parents’.A. Types of Life/Cells
When we look at the diversity of living things, they can be categorized into three major groups:B. Types of Species
When we look a big closer, at the number of described species in each group, and in different types of eukaryotes, we find an astounding range of different types of life forms; approximately 2 million different species! Now, each of these species is, more or less (barring a small possibility of hybridization between closely related species), a discrete biological entity. Like you! You are a discrete biological entity that has a unique set of genes that no one in the history of the human race will ever have (unless you have an identical twin). Like an individual, a species is a discrete biological entity, too; it has sets of genes which, because it can’t breed with other species, is different and unique. And wow, when we look with the animals (which collectively comprise about 75% of all species of life), rough ¾ of all animal SPECIES (not individuals) are insects. Indeed, insects represent half of all the species of life on the planet. Huh! Why? We will address that later in the course.A. Overview
The question we have before us, now, is how and why has life become so diverse—why does it come in so many different flavors?? The answer is this: populations of an ancestral species become separated from one another. Genetic differences accumulate, eventually making them so different genetically and biologically that they can’t breed with one another. They have become two different biological entities, that have diverged from their common ancestral species. Some of these differences have arisen just by chance. Other differences have accumulated because they have been adapting to a particular environment. Random differences that accumulate between population have occurred by ‘genetic drift’, whereas differences that accumulated because of a functional benefit --"adaptation"-- have occurred by ‘natural selection’. All these changes, the changes of a descendant from the ancestor, and the divergence between descendants, is evolution: a change in the genetic structure of a population through time.
Interestingly, it is because of the unity of life that we can explain the diversity of life. All life works the same way, suggesting a single origin and a common ancestry to all life. (It is exceedingly unlikely that life would have arisen twice, or multiple times, using the same molecules and chemical reactions in the same way, with DNA encoding proteins using the same language. That is so unlikely that we can discount it, for now, while we examine the other hypothesis: that life arose once, and all life on the planet, today is descended from that initial population of living things.) Because all life uses DNA as it’s genetic system (coding for proteins), we can compare the sequence similarity in DNA from different species to determine who is more closely related (shares a more recent common ancestor) with who. For instance, African lions have DNA that is more similar to a house cat than to polar bear. This leads us to hypothesize that the lion and cat are more closely related to one another than either is to the polar bear. Well, when we say "they are more closely related", we mean that they have a more recent common ancestor. (Siblings are more closely related than second cousins, and they share a more recent ancestor with one another (parents) than with second cousins (great-grandparents). So, based on DNA similarity, we can test hypotheses of common ancestry.
When Carl Woese did this for bacteria, arachaea, and eukarya species, he found something surprising: Although Archaeans are small, anuceate organisms that supervicially resemble bacteria, they are actually more similar, genetically, to the Eukarya (at least in nuclear genes). And, because mutations in genes occur randomly, differences accumulate at roughly a constant rate; so the amount of genetic difference between two living species can be used to create hypotheses about how long ago the common ancestor lived! Then, we can test these hypotheses with an entirely independent line of evidence… fossils. But before we do test, let’s look at the history of life first, as we know it from the fossil record.
B. The History of Earth and Life
1. Timeline
Go ahead and learn these dates. Tell yourself the story of life’s history. Here it is:
2. ‘Transitional’ Fossils
As the major patterns listed above show, there are patterns in the fossil record – everything isn’t present in the oldest strata. Rather, there is an accumulation of complexity over time, and a colonization of new environments by life: from the oceans to land to air. When we look at the fossil record of vertebrates, for instance, we see fish present in strata from 530 mya to the present.
Amphibians, however, are only present in strata from 370 mya to the present. Where did they come from? If life only comes from life, they must have ancestors… but the only vertebrates that could be their ancestors were fish! The hypothesis that amphibians evolved from fish leads to a prediction: there should have been animals along this lineage that were both fish-like and amphibians-like: what we call ‘transitional fossils”. We have found such fossils, just where evolution predicts they should be: after similar fish in the fossil record and before similar amphibians. Tiktaalik, to the right, had front feet but hind fins, and is sometimes called a "fishopod". It dates to 375 mya, right before four-footed amphibians appear in the fossil record.
So, the fossil record is also an incredible body of evidence that tests and supports the hypothesis of common ancestry.
C. Concordance
So, we have these two major ways of making trees: genetics and fossils. If they both have the SAME CAUSE and are truly describing the same phenomenon (common ancestry), then the trees should be largely the same. In other words, transitional fossils should be where genetic analyses of modern species predicts they should be. They are. This is the most compelling proof of the validity of common ancestry.
Think deeply about this: We can take two species, like a lion from the savannahs of Kenya and a white-tailed deer from the forests of Maine, and we can compare their DNA. Based on the number of differences that occur, and the known rate at which these differences accumulate, we can calculate the amount of time needed for this amount of genetic difference to accumulate. In other words, we can calculate when in the past the common ancestor to these species walked the Earth. This ancestral species was obviously neither a lion-like animal nor a deer-like animal; the lineages of carnivores and ungulates (hooved mammals) converge early in the Cenozoic, when primitive placental mammals were radiating. But we have evidence of this in the fossil record, too. And the proof of evolution is that the predictions from lion and deer match the location of these early mammals. Now, why is it that we can compare MOLECULES of species walking around on the planet today, and predict when, in the sedimentary strata of the Earth’s crust, a third separate species was alive? If these organisms were NOT related, there is no way this should work. But, it does. Time and time again.
A. Cellular Level
In multicellular organisms, different cells perform different functions. This allows them to focus their energy on a limited number of tasks, and perform those tasks more efficiently and productively. It is a “division of labor” that, through specialization, increases efficiency of the system.
What is doubly amazing is that all these different cells, that are performing different tasks because they are reading different genes, are all descended from the same ancestral cell (the zygote). Through development, cell lineages have diverged in FUNCTION by reading different subsets of genes (even though they have all the same genes).
So, through genetic divergence (in terms of what is READ, not what is PRESENT), functional diversity and a division of labor occurs.
B. Ecological Level
Similar species, like those that are recently derived from a common ancestor, are often biologically similar. They may use the same resources in roughly the same way. But because of this, they compete with one another. Those members of each population that use slightly different resources have these resources all to themselves. These organisms do better than those competing with others species, and within that population, there is selection to use this new resource. In this way, species diverge and use different resources. These different species are now more efficiently using the resources in the environment… using the full range of seeds available, not just competing for the same limited range of seeds. This divergence, called ‘niche partitioning’ results in more efficient use of resources in the community. It came about by genetic divergence of these species, adapting to different resources.
At all biological scales, genetic divergence leads to functional divergence and a division of labor which increases the efficiency of the system.
Things to Know (in your head, without your notes!):
1) Know the types of molecules that make up life. We will cover their structure and function in more detail later.
2) Understand the reasons why life is cellular, and why cells are small.
3) Know how cells work, and why making proteins is so fundamental to cell function.
4) Why are cells in the same organism similar? Where did they come from? Why are siblings similar? Where did they come from? Why are species of cat (tiger, lion, leopard) similar?
5) Know the timeline of Earth's history, as listed.
6) Know how can we use genetic similarity among living species to predict when, in the deep past, their common ancestor lived.
Study Question:
1) What does diversity do for biological systems? Consider both the cellular and ecological levels.