September 24, 2011 | 2
As you may know, I have been teaching BIO101 (and also the BIO102 Lab) to non-traditional students in an adult education program for about twelve years now. Every now and then I muse about it publicly on the blog (see this, this, this, this, this, this and this for a few short posts about various aspects of it – from the use of videos, to the use of a classroom blog, to the importance of Open Access so students can read primary literature). The quality of students in this program has steadily risen over the years, but I am still highly constrained with time: I have eight 4-hour meetings with the students over eight weeks. In this period I have to teach them all of biology they need for their non-science majors, plus leave enough time for each student to give a presentation (on the science of their favourite plant and animal) and for two exams. Thus I have to strip the lectures to the bare bones, and hope that those bare bones are what non-science majors really need to know: concepts rather than factoids, relationship with the rest of their lives rather than relationship with the other sciences. Thus I follow my lectures with videos and classroom discussions, and their homework consists of finding cool biology videos or articles and posting the links on the classroom blog for all to see. A couple of times I used malaria as a thread that connected all the topics – from cell biology to ecology to physiology to evolution. I think that worked well but it is hard to do. They also write a final paper on some aspect of physiology.
Another new development is that the administration has realized that most of the faculty have been with the school for many years. We are experienced, and apparently we know what we are doing. Thus they recently gave us much more freedom to design our own syllabus instead of following a pre-defined one, as long as the ultimate goals of the class remain the same. I am not exactly sure when am I teaching the BIO101 lectures again (late Fall, Spring?) but I want to start rethinking my class early. I am also worried that, since I am not actively doing research in the lab and thus not following the literature as closely, that some of the things I teach are now out-dated. Not that anyone can possibly keep up with all the advances in all the areas of Biology which is so huge, but at least big updates that affect teaching of introductory courses are stuff I need to know.
I need to catch up and upgrade my lecture notes. And what better way than crowdsource! So, over the new few weeks, I will re-post my old lecture notes (note that they are just intros – discussions and videos etc. follow them in the classroom) and will ask you to fact-check me. If I got something wrong or something is out of date, let me know (but don’t push just your own preferred hypothesis if a question is not yet settled – give me the entire controversy explanation instead). If something is glaringly missing, let me know. If something can be said in a nicer language – edit my sentences. If you are aware of cool images, articles, blog-posts, videos, podcasts, visualizations, animations, games, etc. that can be used to explain these basic concepts, let me know. And at the end, once we do this with all the lectures, let’s discuss the overall syllabus – is there a better way to organize all this material for such a fast-paced class.
Today, we introduce the concept of evolution, mainly via natural selection (sexual selection will come later in the course, and neutral selection etc. are too much for this level). Note that I tend to do a lot of drawing on the whiteboard in this lecture, which is not seen in these notes.
Imagine a small meadow. And imagine in that meadow ten insects. Also imagine that the ten insects are quite large and that the meadow has only so much flowers, food and space to sustain these ten individuals and not any more. Also imagine that the genomes of those ten insects are identical, except for one individual: that one has a mutation in one gene (due to an error in DNA replication, or due to crossing-over during meiosis, or due to chemicals in the environment, or due to getting hit by rays coming from outside Earth, etc). That mutation, during development, led to the induction of the production of more mitochondria in each muscle cell.
Normally, that mutation is not obvious – the insect flutters from flower to flower just like anyone else. However, if the situation arises, the mutant individual is just a tiny little bit faster because the additional mitochondria in muscles allow it to switch from aerobic to anaerobic sources of energy later than in other individuals. Thus, the “normal” individuals can fly one yard in one second, while the mutant can fly one yard plus one inch in one second.
Now imagine that, over some time period, a bird comes by the meadow four times. Each time, the bird chases the insects and catches the one that is the closest to her. Which individual is, statistically speaking, least likely to get caught and eaten? The mutant, as the little extra speed may give it just enough edge in comparison to other individuals. This comparative “extra edge” is called increased fitness.
After four insects have been eaten, six remain – three males and three females. They pair up, mate, lay eggs and die. Each pair lays, let’s say eight eggs, which all hatch, proceed normally through the larval development and become adults. This makes a total of 24 insects in a meadow that can support only ten individuals. At the same time, the bird has laid eggs, the eggs hatched and the hatchlings sometimes come to the meadow to hunt.
Let’s look at the genetics of this population for a moment. Two pairs of “normal” insects produced a total of 16 offspring, all of them “normal”. The offspring of one “normal” and one “mutant” each got one of the chromosomes from the mother, the other one from the father. All of them will have the mutation on one, and not on the other chromosome. Let’s say that having a mutation on only one chromosome adds a half-inch to the yard-per-second flaying speed. The full mutant is homozygous for this mutation. The half mutant is heterozygous for this mutation. The heterozygous individuals are still relatively more fit than the “normals”. As the hatchling birds hunt down the insects and cut down the population to ten individuals, the half-mutants are more likely to be present in the remaining population than the non-mutants.
Let’s call the “normal” variant of the gene A and the “mutant” variant of the same gene a. A and a are alleles of the same gene.
In the next generation, some normals will breed with normals, producing normal offspring. Some half-mutants will mate with normals and produce a mix of normals and half-mutants. Some half-mutants will mate with some half-mutants and the resulting eight offspring will consist of 2 normals (AA), two mutants (aa), and four semi-mutants (Aa).
As the a allele confers relative fitness to its carriers, this allele will spread through the population over several generations and either completely eliminate allele A, or attain some stable balanced ratio in the population.
When one compares the genetic composition of this population over generations, one notices that it changes over time, from preponderance of A in the first generation, through a series of intermediate stages, to the preponderance of a in the last generation.
The change of genetic composition of a population over multiple generations is called evolution.
That sentence is the most commonly used definition of evolution. The process that favored one allele over the other, resulting in evolution of flight speed in these insects, is called natural selection. The environment – the carrying capacity of the meadow plus the bird predators – was the selecting agent. The process that turns a genetic change (mutation) into a trait that can affect fitness of the whole organism is development. Thus, one can also define evolution as “change of development by ecology”.
For evolution to proceed, the trait must vary in a population, one of the variants has to confer greater fitness than the other variants, there has to be a limit on the fecundity (how many offspring can survive in each generation) leading to differential rate of reproduction, and the trait has to be heritable, i.e., the offspring have to be more like parents in respect to that trait than like other individuals in the population. The inheritance is usually, though not always, conferred by the genome (the DNA sequence).
The example we used is quite unrealistic. Populations are much more likely to number in thousands or millions than just ten individuals. Thus, instead of a few generations, it may take thousands or millions of generations for a new allele to sweep through the population. In annually breeding organisms, this means thousands to millions of years. In slow-breeding animals, like elephants, it will take even longer. In fast reproducers, like bacteria, this may only take several months or years, as in evolution of antibiotic resistance in bacteria or evolution of pesticide resistance in agricultural pests.
Another way that the example was unrealistic was the assumption that all the individuals were genetically identical to each other except for that one mutation in that one gene. In reality, there will be variation (two or more alleles) in every gene, and new mutations show up all the time. Some mutations decrease fitness, some are neutral and some increase fitness. Some alleles affect fitness depending on which other alleles of other genes are present in the same individuals, or depending on the environment it finds itself in at a particular time, as in the ‘norm of reaction’ phenomenon (see previous lecture). Due to this, some combinations of alleles may tend to move from one generation to the next together.
Finally, in many organisms, genes can be transmitted horizontally – not from parent to offspring but directly from one individual to another. This most often happens in bacteria, where individual bacteria may exchange bits and pieces of their DNA. Likewise, viruses are carriers of DNA sequences from one organism to another as well. Some of the sequences in our genome are of bacterial origin, transmitted some time in the past by viruses, and now fully integrated into our genome and even assuming an indispensable function. For instance, HERV genes are originally viral genes that are now parts of our genome and are necessary for the development of the placenta.
Thus, in the real world, the situation is more complicated than in our example. Still, the proportions of various alleles of many genes are constantly changing in populations over generations – evolution occurs all the time.
Let’s now assume that our insects live in a much larger area and that there are millions of them. The frequencies of various alleles fluctuate all the time, and there is quite a lot of genetic variation contained in the population. Natural selection may work on preserving the average phenotype as its fitness is high and outliers at each end have lower fitness. This is called stabilizing selection.
As the climate slowly changes, or other aspects of the environment change, the relative frequencies of alleles of various genes will track those changes. New conditions may, for instance select for larger body size. The largest individuals tend to leave most offspring, while the smallest individuals, on average, put the least of their genes into the next generation. The selection for large body size is an example of directed selection.
In some cases, selection may favor the extremes, but not the middle. Fast fliers may be selected for because they can escape the birds. The slowest fliers may be selected because they mostly walk or crawl and are thus not easily spotted by birds. They are also fit, but via a different strategy. The medium-speed fliers are selected against. This is an example of disruptive selection, forming two different morphs of the same species.
If those two morphs tend to, on average, be more likely to find each other and mate with each other within a morph than between two morphs, this may lead to splitting the species into two species – this is called sympatric speciation. As the gene flow between the two groups declines, more and more mutations/alleles will be found only in one morph and not the other. Those genes will also be under the influence of selection, and the selecting environment is different between crawlers and fliers. Soon enough, the individuals belonging to the two groups will not even recognize each other as belonging to the same species. Even if they recognize each other, they may not like each other (“mate-choice”) enough to mate. Even if they mate, their eggs may not be fertile. Even if their eggs are fertile, the resulting offspring may not be fertile (hybrids, like mules for instance). If, for whatever reason, two related populations do not, will not or cannot interbreed, they have became separate species – speciation occurred.
Imagine now that a small cohort of about ten individuals got blown away by wind from the mainland to a nearby island. The mainland population is huge. The island population is tiny. The ability of any mutation or any allele to spread fast through the population is much greater in a small group. The selective pressures are also different.
It may be better for the island insects to be small and for the mainland insects to be large, perhaps due to the types of flowers or kinds of predators that are present. The mainland insects may be selected for high flying speed because of bird predation. The island insects may not have any bird predators, but, those individuals who are the best fliers are most likely to be swept off the island by wind and drown in the ocean, never placing their genes into the next generation. Thus, they are selected not to fly, even to lose their wings.
If, after a number of generations, those two populations again get into contact – e.g., a land bridge gradually arises, or another cohort of mainland insects floats on a log onto the island, the two populations will not recognize each other as the same species (or not like each other enough to mate, or not having fertile eggs or offspring). Thus, they have also become reproductively isolated, thus, by definition, they have become two separate species. Speciation occurred. This type of speciation, where a geographic barrier separates two parts of a population preventing gene flow between them is called allopatric speciation, and is much better documented and much less controversial than sympatric speciation.
Billions of such speciation events, meaning branching of species into two or more species, resulted in the evolution of all species of organisms on Earth from a single common ancestor (a very primitive bacterium) over a period of more than 3.5 billion years.
Previously in this series:
Biology and the Scientific Method
BIO101 – Cell Structure
BIO101 – Protein Synthesis: Transcription and Translation
BIO101: Cell-Cell Interactions
BIO101 – From One Cell To Two: Cell Division and DNA Replication
BIO101 – From Two Cells To Many: Cell Differentiation and Embryonic Development
BIO101 – From Genes To Traits: How Genotype Affects Phenotype