California’s Pacific coastline has created a unique evolutionary arena for plants adapted to its salty, abrasive conditions. Regions which are not occupied by coastal prairie grasslands, Redwood trees, or mixed evergreen forests are dominated by this iconic shrub community. Chaparral often comes to mind when associating shrubs with California. Although chaparral dominates a significant range of territory both inland and coastal, an additional shrub community is found in our State, and it is distinct from any other; California’s coastal scrub.
Stretching from southern Oregon to the Mexican border, coastal scrub is intermixed with strands of chaparral. Chaparral’s hard, sclerophyllous leaves and evergreen habit distinguishes itself from the distinctive coastal species. These coastal species are divided between the northern and southern regions based on key differences in morphology and species diversity. Southern Coastal Scrub embodies a high degree of species diversity where plants are commonly drought deciduous and produce soft leaves that emit strong aromatic compounds. Both northern and southern species overlap in Monterey County as their boundaries fade between the two. Species of the Northern Coastal Scrub, strictly found north of San Francisco, are not drought deciduous like their southern counterpart. This phenomena can be attributed to high levels of coastal fog that provide year round moisture in addition to winter rains. Floristically, northern coastal scrub is an assemblage of varying chaparral species and plants endemic to Northern California’s coastline. A distinction between the two communities is subtle and is often not distinguishable as a shifting mosaic between the two is constantly occurring.
Fig A: a commonly observed coastal scrub/chaparral mosaic along the Pacific coastline
The Pacific coastline, in combination with California’s Mediterranean climate, has selected for plants that thrive in harsh conditions. It is a prime example of evolution at work, resulting in a significant degree variation among members of plant communities occupying our coastline. Unfortunately, these biodiverse regions are threatened by urbanization and poor fire management strategies. Current distributions of conservation areas do not support suitable habitat for species of conservation concern. Regions that are host to source populations of SCS are threatened by development, habitat destruction, and invasive species. The source patches are of high interest in conservation management and deserve public consideration. Appreciation of these resources is a crucial first step to widespread support for preservation of these natural systems.
Fig B: Lupinus albifrons glowing in the coastal California Sunshine
II. The Processes of Evolution
Many understand evolution as a change through time. This ambiguous definition is accurate but nowhere near adequate. There are countless misconceptions as to what evolution really is, what it means, and why it is happening. Pundits, inept educators, and officials responsible for public policy often reinforce these misconceptions. It is the goal of this text is to attain a broad, yet accurate understanding of the evolutionary process, as well as how it relates to our world, and most importantly, how it illustrates the diversity seen among coastal plants in California!
Evolution is the change of inherited characteristics in populations through successive generations. More specifically, it is the change in gene frequencies of a population through time. To understand the concept of a gene, it is important to rewind and attain a sound understanding of heredity and how it relates to this “change through time” we previously addressed.
A. The Molecular Basis of Heredity:
Reproduction is the basis of life. Acquired traits are the product of inherited genetic material (DNA; deoxyribonucleic acid), Prior to death, an organism attempts to maximize their chance of leaving a mark on the gene pool. Strategies to maximize the fitness of an individual often explain the acquired traits found in that organism. This acquired trait was attained from the previous generation, from individuals who possessed desired characters that enable one to successfully pass their genes to the subsequent generation. Individuals that lack desirable traits are limited in their reproductive abilities, thus unable to contribute to the gene pool in a manner significant as the individual with the desired trait.
As generations pass, this differential selecting of advantageous genes results in the “change through time” we previously addressed. Depending on environmental influences, new traits (resulting from the gene) arise, ultimately selected for or eliminated from the gene pool.
B. Driving the Change in Gene Frequencies
Natural selection, described in section A, often comes to mind when approaching the concept of evolution. Although natural selection plays a significant role in the process, it is not the sole cause of evolutionary change. The following highlights a few, of numerous, factors that also drive the change in gene frequencies in a population through time.
1. Mutations: mutations arise in the chemical structure of DNA. If the mutation is a disadvantage to the organism, reproduction is unlikely (negative selection). In the occurrence an advantageous mutation, the mutated gene may become selected for and remain the gene pool (positive selection). Mutations are the source of novel genes in a population. But! Mutation rates are so low that other factors more than likely account for the change in gene frequency of the population.
2.Gene flow: the immigration or emigration of individuals into or out of a population. In the case of plants, pollen and seed distribution is a classic example of this phenomenon. Gene flow brings novel genes to a population of closely related individuals (who differ slightly in their genetic composition). The migrating individual then contributes to the new gene pool, thus altering the gene frequencies.
3. Genetic drift: changes in the gene frequencies that occur due to chance. Genetic drift is more common in smaller populations where changes in gene frequencies are more pronounced among the entire population; less genes being shared among fewer individuals.
-Bottleneck effect: when population size is drastically reduced (geologic activity, flood, fire, urbanization, etc.) causing certain genes to be overrepresented in the gene pool.
-Founder effect: select individuals from a population colonize a new region. Much like the bottleneck effect, certain genes will be overrepresented or completely absent from the pioneering individuals.
Members of the same species occupy the same gene pool. Speciation, or the occurrence of a new species, is the separation of gene pool into distinct entities of their own, ultimately unable to exchange genes (reproduce) between the two. The following are two mechanisms that drive a speciation event.
1. Allopatric speciation: a given species is isolated from one another (geographic barrier), preventing gene flow, where the newly isolated population is acted on by a different set of selection pressures than the first.
2. Sympatric speciation: a speciation event occurring within a gene pool where individuals are not separated from one another. This phenomenon is commonly seen with plants given their ability to produced viable offspring when a doubling of their chromosomes occur. In many organisms, a doubling of the chromosome count will yield offspring unable to survive. But with plants, they survive but are inhibited from reproducing with the parent population. This situation creates for a speciation event.
This genetic anomaly, known as polyploidy, is not as uncommon as one may think. Seedless watermelons, for example, are the product of mating a polyploid individual with a non-polyploid individual causing the plant to produce fruit (the watermelon) that cannot make viable seeds. Other crops that man has utilized for their chromosomal anomalies are wheat, tobacco, cotton, apples, oats, peanuts, kiwi, bananas, and many more. This unique ability of plants is why we have been able to breed them for an assortment of characters useful to man.
The unfathomable degree of variation in the natural world has perturbed naturalists for millennia. Beginning in the days of Aristotle (fourth century B.C.), individuals attempted to classify organisms observed in nature. Thousands of years later, this innate curiosity developed into a science of its own where the days of grouping organisms based on physical features has long passed into an era of molecular classification based on an organism’s genetic makeup.
This unimaginable variation is driven by evolutionary processes and continual speciation events through time. The change in gene frequencies and ultimate modification of traits leads to a differentiation among new species in respect to common ancestors. Biologists track these speciation events and evolutionary lineages through the creation of a phylogeny to assess various groups of organisms and better understand their ancestral origin. Thus, one cannot truly discuss evolution without a hypothesis regarding the relationship among contemporary organisms and patterns of events leading to the observed diversity. This study of evolutionary relationships and classification falls under the discipline of systematics. The systematist’s role is to create and assess phylogenies, propose hypotheses regarding evolutionary events, create scientific names (taxonomy), provide descriptions, and amass collections of organisms.
Fig C: an example of a phylogenetic tree displaying the evolutionary relationship among a group of organisms with each name representing an individual species. Each branching point represents a common ancestor, where the point furthest to the left is the common ancestor for the entire group.
-Hierarchical Classification and Binomial Nomenclature
The discipline of naming and categorizing organisms, based on evolutionary relationships, is known as taxonomy. The taxonomic method is hierarchical, based on a systematic grouping of increasingly inclusive categories where the base unit is the organism’s binomial name.
The two words seen representing an individual species in Fig. C is the binomial name assigned to the species. The first word is the genus, whereas the second word is the species (more specifically, the specific epithet). As stated, the taxonomic system is hierarchical. An individual genus may have dozens of species. And that genus’ family may just as many genera (plural of genus) as the genus has species. So, it is an inclusive system where the names represent evolutionary relationship and taxonomic order.
-“So, why don’t they just use common names?”
-Douglas Fir (common name)
Pseudo=false & tsuga=hemlock
-Western Hemlock (common name)
-Red Fir (common name)
*Pseudotsuga menziesii is commonly referred to as Douglas
Fir. The genus name Pseudotsuga means false hemlock (hemlock for the genus Tsuga, or the flowering plant that killed Socrates!) where the common name, Douglas Fir, refers to the genus Abies (or the fictitious species Abies douglasii).
*Common names are far too ambiguous, arbitrary, and inconsistent to consider for taxonomic use.
Binomial name: The name used when referring to individual species. First word is the genus, second is the specific epithet (i.e. Sequoia sempervirens)
Common ancestor: the species from which a subsequent speciation event took place. An ancestral species shared by 2 or more contemporary species
Diversity: total amount and abundance of species in a collection of individuals.
Drought deciduous: losing leaves in response to drought.
Endemic: a species unique to a specific geographic location.
Evergreen: retaining leaves year round.
Mediterranean climate: hot and dry summers contrasted by wet winters.
Molecule: an electrically neutral grouping of two or more atoms that are held together by chemical bonds.
Phylogeny: a diagram showing evolutionary relationships among groups of organisms
Population: all organisms of the same species that live in a defined geographic region.
Sclerophyllous: tough, sometimes waxy, leaves with a high carbon to nitrogen ratio.
Species: the principle unit of evolutionary specialization
Systematics: the study of evolutionary relationship and the classification of species thereof.
Taxonomy: the nomenclatural system employed by one who practices systematics (the systematist)
VI. Species on Display:
- Peter Raven, Ray Evert, and Susan Eichhorn. Biology of Plants. Sixth. New York: W.H. Freeman and Company, 1999. Print.
- Campbell, Neil, Jane Reece, et al. Biology. 8th. San Francisco: Pearson Education, 2008. Print.
-Calflora: Information on California plants for education, research and conservation.
[web application]. 2008. Berkeley, California: The Calflora Database [a non-profit organization].
Available: http://www.calflora.org/ (Accessed: Oct 20, 2008).
-All photographs: Creative Commons Attribution