The Life System: Lectures 10 and 11 - Mechanisms of Evolution.

Over the next few lectures we'll be looking at evolutionary and ecological processes. I'll commence today by giving a brief overview of evolution - especially natural selection - and how we collect data and use it to discern evolutionary patterns.

I. Charles Darwin (1809-1882) (Figure 1).

Darwin published Origin of Species by Means of Natural Selection in 1859, and one could use this milestone to date the beginning of our understanding of the temporal relationships of species. (Internet Infidels has this and two more of Darwin's most influential books available online at their Darwin Page.)

In 1831 at the age of 22, Darwin traveled extensively throughout the world. For 5 years he was the ship's naturalist on the H.M.S. Beagle  which sailed from Europe to South America to Australia to Africa to South America again and back to Europe. His travels were summarized in The Voyage of the Beagle.

During his journey, Darwin made several important observations:

  1. Overproduction - Species produce far more offspring than survive to maturity.
  2. Competition - There are not enough resources for everyone to survive, so there is competition among individuals.
  3. Heritable Variation - Individuals of a population are variable - some have traits that assist its chances of survival and reproduction and some have traits that hinder the chances. Importantly, these traits are heritable across generations.
  4. Differential Reproductive Success - Those individuals that posses favorable combinations of traits are most likely to survive and reproduce (survival of the fittest - actually a misnomer). So individuals differ from each other in the numbers of offspring they produce.

Darwin then used these observations to formulate two important insights:

  1. Traits that are passed on can be thought of as adaptations - evolutionary modifications that improve the chances of survival and breeding.
  2. Organisms with better sets of adaptations will survive and reproduce. This is the basis of evolution by natural selection.

Interestingly, Darwin's work sat unpublished for several years until he received a letter in 1858 from Alfred Russel Wallace (1823-1913), who based on his work on the Malay Archipelago (1854-1862), had arrived at almost exactly the same general conclusions.

There is an interesting sociological story of the relationship between these two men and the papers that formed the crux of the debate are all online from James L. Reveal, F.L.S., Paul J. Bottino and Charles F. Delwiche of the University of Maryland. Additionally, a description of Wallace's travels in the Malay Archipelago is available from Tim Severin, associated with the University of Limerick.

II. What sort of data are used to study evolution?

  1. The fossil record - Fossils are the mineralized remains or signs of organisms preserved in layers of rock. By identifying where in the rock the fossil is found we can estimate the relative age of the specimen. In some cases these dates can be made more precise (a better estimate of absolute age) by using techniques such as radiocarbon dating.
  2. Comparative Anatomy and Development - This technique involves comparing the structure and ontogeny (development) of species to delineate which are more similar and why. Structures can be similar due to homology (common evolutionary origin - for example frog legs and dog legs) or analogy. Analogous structures (for example bird wings and insect wings) are not derived from a common ancestor, and demonstrate convergent evolution.
  3. Historical Biogeography - the study of the geographical distribution of plants and animals. Species distributions are non-random, but are understandable in the context of evolution. In other words, if each species only evolved once, it must have come from elsewhere. One hint to that elsewhere, and the ancestral progenitor, is the center of origin for the species or group of organisms.
  4. Molecular Data - By examining the similarity of DNA from different organisms we can estimate the phylogeny (family tree - the lines of direct descent) for a group of organisms. Generally speaking, the sections of DNA examined are thought to change by mutation at the same, predictable rate in the different organisms. That is, the organisms posses the same molecular clock (based on Motoo Kimura's Neutral Theory of Evolution), so more recently related species have more similar sequences of DNA.

    Note the difference between a phylogeny and a taxonomy:

    Taxonomy: the process of naming, classifying and describing a group of organisms, which may or may not follow the phylogeny of the group. Our current system of classification is the Binomial System of Nomenclature based on the work of Carl Linnaeus (1707-1778).

    Phylogeny: the outcome of the process of describing how these organisms are related to each other. Usually this is shown in the form of a tree of relationships.

III. How do species originate?

Species - a group of morphologically similar organisms that interbreed in nature and are reproductively isolated from all other such groups. This definition is based on Ernst Mayr's Biological Species Concept. A recent interview with Mayr is available from PBS.

However, there are many other definitions of species concepts that were discussed during Lecture 2. There is a great deal of debate about what is a species and how one recognizes them.

It seems like a good way to summarize the consensus viewpoint is to recognize that no one single species concept will work for all organisms. Most likely, the best approach will be to have species concepts, each of which are most useful for a different lineage. Operationally, this is what taxonomists do currently anyway.

Nonetheless, we will use the biological species concept as the definition of choice, as it is what most non-taxonomists are most confortable with.

Subspecies are usually geographically isolated but have the ability to occasionally interbreed with neighboring subspecies. Note that this interbreeding is not abnormal, it is simply rare.

So one can find widely distributed "gradient" species.

Two examples:

  1. The Rana species complex.

    The Rana species complex (the leopard frog, Rana pipiens, is one member of this complex) is widely distributed from Wisconsin to Mexico. Yet there is a problem when we attempt to classify these organisms and delineate subspecies and species geographical boundaries.

  2. Ensatina eschscholtzii; also see this excellent site that discusses the formulation of speciation in progress.

    The distribution of this species forms a ring (ring species) around the dryer Central Valley of California. It is mainly found in the Pacific Coast Range and the Sierra Nevadas.

    Where the ring comes together near San Diego the populations are so distinct that they do not interbreed.

These two examples are evidence of the importance of geographical isolation. In fact, new species are usually the result of geographic isolation, a process termed allopatric speciation.

Rarer is sympatric speciation - production of two new species in the same area. This may be perhaps due to isolation in time or activity patterns, variation in microhabitat use, or polyploidy. Polyploidy occurs when a species possess more than two sets of chromosomes. This can lead to an inability to produce viable offspring with all members of the population. The concept of sympatric speciation is currently hotly contested among evolutionary biologists.

When new species evolve, they must have either a new habitat or a new way to use the old habitat. Newly formed species must also have reproductive isolation, usually due to geographic or reproductive isolation. If they were not completely separated, they would come together again, their genes would mix, and the barriers between the species would break down, and the two incipient specieswould likely go back to being a single species.

If two groups do come back together after evolving along divergent evolutionary lines, there are two ways to nonetheless retain the divergence and keep genetic integrity/reproductive isolation. The isolation mechanisms will have had to evolve prior to the removal of the barrier between the species. They are:

  1. Prezygotic Reproductive Isolation.
    1. functional or morphological change preventing coupling;
    2. behavioral change in one of the two groups that would prevent coupling;
    3. gamete inviability - no zygote formed if they do mate;
    4. temporal isolation - two species may mate at different times;
    5. habitat isolation - breed in different places or on different substrates or hosts.
  2. Postzygotic isolation.
    1. hybrid inviability - a zygote is formed but a hybrid is never born;
    2. developmental hybrid sterility - hybrid dies before reaching reproductive age;
    3. segregational hybrid sterility - hybrid is sterile (e.g. mule);
    4. F2 breakdown - hybrids can mate and produce offspring, but they usually die.

Obviously, post-zygotic strategies are quite costly to the organism and are therefore selected against.

IV. Proximate mechanisms of speciation.

The actual speciation process occurs because mutations (permanent changes to the DNA sequence or chromosomal make up of a cell) occur, are incorporated into the genetic complement of gametes, and thus create slightly different gene pools for two populations. Most mutations are neutral or deleterious, but occasionally one is positive (beneficial) and selection acts to keep it in the gene pool.

Over time, genetic drift (random fluctuations in the make-up of a gene pool) will lead to different populations having different frequencies of particular alleles. Small populations and those that have gone through a genetic bottleneck have more rapid genetic drift and decreased variation because alleles are lost at a rate that is greater than what new mutations can supply.

Examples: cheetah, northern elephant seal.

Look at how drift works and the influence of population size and allele frequency:

Genetic drift can be overwhelmed by gene flow - the movement of individuals and their genes between populations, which can result in increased genetic variation.

Of course, we are talking about neutral mutations/variation. In this case it is possible to mathematically predict the genetic make-up of a population by using Hardy-Weinberg equations:

In a 2 allele system (allele b and allele B)

p = frequency of dominant allele B
q = frequency of recessive allele b

p + q = 1

So p and q give the allele frequencies in the population.

This can be expanded to look at the frequency of actual genotypes in the population. The potential genotypes are: BB (homozygous dominant), Bb (heterozygous), bB (heterozygous), and bb (homozygous recessive).

p2 + 2pq + q2 = 1 (1 = 100% of the individuals in the population)

So let's say that 9% of a population has a homozygous recessive trait.

q2 = 9%
q = 3
p = 7
p2 = 49% homozygous dominant
2pq = 42% heterozygous

H-W makes certain assumptions:

  1. the population is randomly mating;
  2. there is no mutation;
  3. the population size is large (so no effect of drift);
  4. no migration (gene flow);
  5. no natural selection (the traits are neutral).

One of the main uses for the H-W equilibrium is that it provides a null hypothesis which we can test. If we find that a population has changed through time, then we know that one or more of the above 5 assumptions have been violated. An interesting animation of the Hardy-Weinberg Equilibrium is available from the McGraw-Hill Online Learning Center.

It should be remembered, however, that selection acts directly on the phenotype (the expression of an organism's genes), not on the genotype (the genes). There are three types of selection:

  1. Stabilizing selection (e.g. birth rate, bird tail length);

  2. Directional selection (e.g. pepper moth coloration, cat brain case volume);

  3. Disruptive selection (e.g. beak size in Galapagos finches).

V. Evolution on a geological time scale.

On a geological scale, evolution may occur gradually (gradualism) or it may occur in sudden bursts (punctuated equilibrium - an idea developed by Stephen Jay Gould of Harvard and Niles Eldredge of AMNH during their graduate work at Columbia University).

Gradualism suggests that evolutionary change occurs gradually over time at a relatively constant rate, with missing links or intermediate forms in the fossil record not seen because of the incomplete nature of the record. In contrast, punctuated equilibrium suggests that the appearance of a new species in the fossil record is a punctuation following long periods of equilibrium, or stability. For instance, the fossil record for a particular group of organisms may indicate that the ancestral species changed little for millions of years and then changed so rapidly that it is difficult to identify each step in the speciation process. Thus, change in organisms can be considered a jerky, or episodic, rather than smoothly gradual. Punctuated equilibrium accounts for the abrupt appearance of new species in the fossil record with little evidence of intermediate (missing links) forms.

Whether evolution occurs following the traditional gradualist pattern or the punctuated equilibrium has been a source of contention among evolutionary biologists. Most biologists would now likely concur that both processes are operating and that the distinction between the two is a facile definition, as they are probably both extreme ends of a continuum.

Punctuated Equilibrium at Twenty: A Paleontological Perspective by Donald R. Prothero, from Skeptic Magazine goes through the history of the thought and its impact on the scientific community.

VI. Extinction.

Extinction - the end of a taxonomic unit; generally thought of as occurring when the last individuals die. Extinctions occur continuously - they are the eventual fate of all lower-order taxa (e.g. sub-species, species, genera, and families).

It may be useful to think of extinctions as occurring at two rates (although these rates may in reality be more of a continuum than a dichotomy):

Background extinction - Continual low-level extinction that occur over time.
Mass Extinctions - A sudden extreme increase in extinction rates.

Mass extinctions are usually followed (on a geological time scale!) by adaptive radiations - a sudden diversification of a few species to fill open niches.

Updated March 22, 2005
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