1. AN INTRODUCTION TO EVOLUTIONARY ECOLOGY Print Version
What is Evolution?
• Evolution occurs when the frequencies of different alleles in a population change over time [see, Research Connection on Population Genetics]. Although sometimes referred to as the "Theory of Evolution", evolution is actually an observation that species change over time. These changes are often observable within our lifetimes.
• Contemporary Evolution describes observable evolutionary changes during less than a few hundred generations. One of the best-documented examples is the evolution of antibiotic resistant bacteria. In most cases, such resistance evolves within just 3 years of a new antibiotic being approved. Thus, the pharmaceutical industry has been forced into an evolutionary arms race trying to develop new antibiotics just one-step ahead of the evolution of drug-resistant microbes. As the authors of Research Connection 1 state: understanding how resistance evolves and spreads in bacterial populations is required for effective prevention or delay of further antibiotic resistance.
Mutations and Genetic Variation
• Evolution requires genetic variation. The ultimate source of all genetic variation is mutation. A mutation is any change in the nucleotide sequence of DNA and it can involve large regions of a chromosome or just a single nucleotide. Advances in molecular biology during the past 20 years have given us insight into the causes and nature of mutations.
• Mutations are divided into two general categories: 1) base substitutions, and 2) base insertions/deletions. A base substitution is the replacement of one nucleotide in the DNA sequence with another nucleotide. For example, the allele that causes sickle cell disease in humans resulted from a single substitution of the nucleotide thymine (T) with the nucleotide adenine (A) in the haemoglobin DNA. We now know that the alternate alleles of many genes have resulted from such base substitutions. Base insertions or deletions involve one or more nucleotides being inserted into or deleted from the DNA sequence.
• What causes mutations? Spontaneous mutations result from errors during DNA replication or recombination. Mutations can also result from mutagens: chemical or physical agents that alter the DNA sequence. Some of the most powerful mutagens include ultraviolet light, high-energy radiation, and X-rays. The researchers in Research Connection 2 used mutagens to assess experimentally whether a particular microbial allele has the potential to evolve into an allele that confers resistance to cefepime, one of the ß-lactam antibiotics.
• Mutations give rise to alternate alleles of genes, which are one source of phenotypic variation within a population. A phenotype is the expression of an individual's genotype (combination of alleles) subject to modifications by the environment. A good example of phenotype is human height. Individuals inherit genes for height from their parents (genetic component), but the nutrition (environmental component) they receive during key growth periods also influences heights. Phenotype includes an organism's anatomy, physiology, behaviour, and life history.
Phenotypic Variation
• Not only are there phenotypic differences between individuals within a population but populations themselves can differ phenotypically. For example, Song Sparrows (Melospiza melodia) in Alberta look quite different from those you would see during a visit to Vancouver with respect to body size, bill shape, overall colouration, and amount of streaking. This is an example of geographic variation within a species caused by genetic differences between the two populations.
• On the same trip to the West Coast, you may also notice that the Red Squirrels (Tamiasciurus hudsonicus) in Alberta are replaced by Douglas Squirrels (Tamiasciurus douglasii), similar yet distinct species (Figure 1). Why do Douglas Squirrels resemble the geographically adjacent Red Squirrels? Why are there so many cases of adjacent species resembling one another?
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Photo Credit © Olin Lathrop |
Photo Credit © Phil Myers
The Animal Diversity Web
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Figure 1. Similarity between geographically adjacent species. Left: Red Squirrel (Tamiasciurus hudsonicus); Right: Douglas Squirrel (T. douglasii) (Adapted with permission from Myers et al. (2006) The Animal Diversity Web (online)
© The Regents of the University of Michigan and its licensors). |
• Charles Darwin (1809-1882) was puzzled by the same phenomenon of geographic species resembling one another after he started sorting through the vast number of geological and biological specimens he had collected during his voyage around the world aboard the H.M.S. Beagle (1831-1836). Not only did Darwin realise that he had many examples of geographically adjacent species resembling one another, but he also noted the similarity between living and fossil specimens from the same area.
• These observations lead to Darwin's theory of Evolutionary Descent with Modification. This theory states that: 1) Species are not stagnant and can change over time, and 2) New species evolve from existing species (they do not arise spontaneously).
Natural Selection
• Charles Darwin was not the first evolutionist. Many others were intrigued by the idea that one life form could change into another life form and some, such as Jean-Baptiste Lamarck and Robert Chambers, had even proposed mechanisms by which such a change could occur. However, many of the arguments in support of these mechanisms were flawed or weak.
• Darwin's most significant contribution to biology was to propose and develop the Theory of Natural Selection as a means by which evolution could proceed. Another naturalist, Alfred Russell Wallace (1823-1913) conceived the same idea and both are credited with the discovery. However, it was Charles Darwin who developed the theory thoroughly and his name alone is more often associated with Natural Selection.
• Natural Selection involves differential reproductive success. The three basic requirements of Natural Selection are: 1) More offspring are born than are needed simply to replace the parents; 2) Individuals differ phenotypically and some phenotypes survive and/or reproduce better than others in a given environment; and 3) Phenotypes are at least partially inherited (result from the individual's combination of alleles).
• The outcome of these conditions is that individuals possessing phenotypes best suited to the current conditions will contribute more genes to the next generation than will other individuals. Thus, the alleles encoding for those beneficial phenotypes will increase in frequency in the gene pool and the population will have evolved by natural selection.
• Natural Selection explains many observable phenomena. For example, the evolution of pesticide-resistant agricultural pests can be explained as follows: Within populations of pest species are some individuals with pesticide-resistant phenotypes. Pesticide application alters the environment in which these pests live such that resistant phenotypes now survive and reproduce better than non-resistant phenotypes. Over time, the descendants of the resistant individuals will become more prevalent in the population and they will possess the alleles that confer resistance. The pesticide will no longer be effective because the population has evolved.
• Humans often impose selection on species. For example, bacteria are subjected to an environment of antibiotics, fungi pests are subjected to an environment of fungicides, and dandelions growing in lawns are subjected to weekly mowing which selects for individuals with shorter flower stalks.
• The authors of Research Connection 3 examined the effects of trophy hunting on a big horn sheep (Ovis canadensis) population in Alberta. The authors asked: Are the alleles that contribute to "trophy" qualities in rams becoming rarer over time? What are the long-term consequences of this selection to the big horn sheep population?
• Whereas Research Connection 4 examines natural selection and heritability of two life history traits in red squirrels. Are mother red squirrels that produce larger litters more successful than other squirrels? Is litter size heritable? What influence does parturition (birth) date have on offspring survival?
Genetic Drift
• Natural Selection is not the only mechanism by which populations evolve. As mentioned earlier, mutation alters the genetic composition of a population, although it does so very slowly. Genetic Drift is the change in allele frequencies in a population due to random events.
• Such events include accidents in genetic segregation during meiosis, random variation in offspring production, and accidental deaths. Genetic Drift generally results in allele frequencies fluctuating up or down, but in a small population, these random fluctuations can result in the loss of alleles.
• This loss of genetic diversity is one of the concerns in trying to manage endangered species. The smaller the population, the more vulnerable it is to allele loss. Populations with low genetic variability (few alleles for each gene) have limited potential to evolve in response to changes in environmental conditions and are therefore, more vulnerable to extinction. Research Connection 5 examines the role of understanding genetic drift and contemporary evolution in conservation biology.
Speciation
• The formation of new species is called speciation. A species is a group of organisms that can interbreed and produce fertile offspring. Thus, speciation is the process by which populations become reproductively isolated from one another.
• Reproductive barriers or isolation between species include several prezygotic (before zygote formation) and postzygotic (after zygote formation) mechanisms. Prezygotic barriers prevent fertilisation or zygote formation, and postzygotic barriers prevent development of fertile adults.
• The reproductive isolating mechanisms include environmental, behavioural, mechanical, and physiological barriers (see Table 1 for example barriers and related mechanisms.
Table 1. Reproductive barriers between species (Adapted from pg. 27, Krebs 1994)
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| Prezygotic Barriers |
Mechanisms preventing fertilization and zygote formation: |
| Habitat |
Populations live in same region but occupy different habitats and do not meet |
| Seasonal or temporal |
Populations live in same region but mature sexually at different times |
| Behaviour (animals) |
Populations are isolated by different or incompatible mating behaviour |
| Mechanical |
Cross-fertilization is restricted or prevented by incompatible reproductive structures, or gametes fail to unite |
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| Postzygotic Barriers |
Mechanisms leading to inviable, sterile, or weak hybrids |
| Inviability |
Hybrids fail to develop or reach sexual maturity |
| Sterility |
Hybrids fail to produce functional gametes |
| Infertility |
Offspring of hybrids are weak or infertile |
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• Reproductive barriers can evolve in several ways. The most common pattern involves independent evolution in populations that are geographically isolated (allopatric speciation).
• Reproductive barriers can also result from an error during meiosis in which the number of chromosomes carried by gametes is doubled (polyploidization). Such an error would be fatal in vertebrate animals; the resulting egg or sperm with the incorrect number of chromosomes would fail to make its contribution to a viable zygote and the error would not be passed on to the next generation.
• However, many plants can self-fertilise, such that polyploid pollen fertilises the polyploid eggs from the same plant. If these polyploid plants are unable to mate with the original population then they are reproductively isolated and, by definition, constitute a new species. In such a case, speciation has occurred between populations at a single location (sympatric speciation) and within a single generation. Approximately 50% of all plant species have evolved this way.
Relevance of Evolution
• Evolution influences many aspects of our lives. For example, food crops have evolved via artificial selection (i.e., selective breeding and genetic engineering) to produce plants with larger seeds (e.g., grains), tastier or fleshier fruit (e.g., strawberries, bananas), or even enhanced nutritional value (e.g., some varieties of corn and rice). Pesticides, herbicides, and fungicides used to increase crop yields are becoming less effective as resistance against them evolves in pest populations. Many fish populations are evolving so that they attain sexual maturity at smaller sizes because the larger individuals in their populations are vulnerable to fishermen’s nets. Antibiotics are becoming less effective as drug resistance evolves in more bacterial strains and many of us have had the experience of needing more than one course of antibiotics to rid ourselves of a bacterial infection.
• Therefore, if we desire to find ways to feed the human population, treat disease, and conserve endangered species for future generations, then we need to understand evolution and mechanisms by which it proceeds.
5. RESEARCH LITERATURE CONNECTION Print Version
Alberta Ord's Kangaroo Rat Recovery Team (2005) Recovery Plan for Ord's Kangaroo Rat in Alberta. Alberta Sustainable Resource Development, Fish and Wildlife Division, Alberta Species at Risk Recovery Plan No. 5. Edmonton, AB. 28 pp. [Online: http://www.srd.gov.ab.ca/fw/speciesatrisk/recovery_teams.html#b (Ord’s Kangaroo Rat)].
Barlow, M. and B.G. Hall. 2003. Experimental prediction of the natural evolution of antibiotic resistance. Genetics 163: 1237-1241.
Collignon, P.J. 2002. eMJ 11: Antibiotic resistance. The Medical Journal of Australia 177(6): 325-329 [Online: Accessed December 12, 2006 at http://www.mja.com.au/public/issues/177_06_160902/col10836_fm.html].
Coltman, D.W., P. O'Donoghue, J.T. Jorgenson, J.T. Hogg, C. Strobeck, and M. Festa-Bianchet. 2003. Undesirable evolutionary consequences of trophy hunting. Nature 426: 655-658.
Gummer, D.L. 1997. Ord's Kangaroo Rat (Dipodomys ordii). Alberta Environmental Protection, Wildlife Management Division, Wildlife Status Report No. 4. Edmonton, AB. 16 pp. [Online: http://www.srd.gov.ab.ca/fw/status/reports/mam/index.html (Ord’s Kangaroo Rat)].
Gummer, D.L. and S.E. Robertson. 2003. Distribution of Ord’s Kangaroo Rats in Southeastern Alberta. Alberta Sustainable Resource Development, Fish and Wildlife Division, Alberta Species at Risk Report No. 63. Edmonton, AB [Online: http://www.srd.gov.ab.ca/fw/speciesatrisk/reports.html (No. 63)].
Krebs, C.J. 1994. Ecology: The Experimental Analysis of Distribution and Abundance, 4th Edition. HarperCollins College Publishers: New York, NY. 801 pp.
Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web [Online: Accessed December 12, 2006 at http://animaldiversity.org].
Normark, B.H. and S. Normark. 2002. Evolution and spread of antibiotic resistance. Journal of Internal Medicine 252: 91-106.
Reale, D., D. Berteaux, D., A.G. McAdam, and S. Boutin. 2003. Lifetime selection on heritable life-history traits in a natural population of red squirrels. Evolution 57(10): 2416-2423.
Stockwell, C.A., A.P. Hendry, and M.T. Kinnison. 2003. Contemporary evolution meets conservation biology. Trends in Ecology and Evolution 18 (2): 94-101.
Zarnayova, M., E. Siebor, A. Pechinot, J-M. Duez, H. Bujdakova, R. Labia, and C. Neuwirth. 2005. Survey of Enterobacteriaceae producing extended-spectrum beta-lactamases in a Slovak hospital: Dominance of SHV-2a and characterization of TEM-132. Antimicrobial Agents and Chemotherapy 49(7): 3066-3069.
