What are genes and what do they do?

Genes are made up of DNA and are found on chromosomes. Genes are considered to be the primary unit of heredity. Genes determine the different traits or characteristics that an organism displays, and alternative forms of a gene are called alleles. For example, if you inherited your mother’s curly hair you may have received her dominant allele for the gene that is expressing that trait.

To better understand the terminology associated with genes, we will use classic Mendelian genetics. Mendelian genetics is the standard pattern of inheritance for sexually reproducing organisms and was discovered by Gregor Mendel, an Austrian monk that used the garden pea to study patterns of inheritance. He determined that physical expression of traits (or the phenotype) was determined by the genetic make up (or the genotype) of the individual.

Mendel also determined that pairs of alleles were responsible for an organism’s genotypic make up, and the phenotypic expression of the traits associated with that genotype. For example, Mendel found that if two alleles were identical, the individual was homozygous for that gene. Let’s examine the peppered moth (Biston betularia) as a real world example of how genes might work in nature (Table 3.1). In the case of the peppered moth, the allele for light body color has a stronger phenotypic expression than that of the dark body color; therefore, the light color is considered to be dominant and the dark color is recessive. If two alleles are different, the individual is heterozygous, or has two different alleles for a particular gene. When moths are heterozygous, the dominant allele (in this case the light body color) has such a strong phenotypic expression that it conceals the presence of the weaker or recessive allele (the dark body color).

If we cross a dark bodied male (dd recessive) with a light bodied female (LL dominant) moth we will end up with all light colored moths in the F₁ or first generation of offspring (Table 3.1). This is because all of the offspring will be the heterozygous (Ld) genotype. Next, if we cross the two heterozygous moths (Ld x Ld) from the F₁ generation, we will end up with three light bodied moths (LL, Ld, Ld) and one dark bodied moth (dd) in the F₂ or second generation. This is because half of the moths will be homozygous, one dominant and one recessive (LL and dd) and half will be heterozygous (Ld) like the F₁ generation.

Moth Hybridization	Generation	# of Light Phenotype (Genotype)	# of Dark Phenotype (Genotype)
	F₀ (parents)	1 Light (LL)	1 Dark (dd)
	F₁	All Light (Ld)	0 Dark (Ld)
	F₂	3 Light (1 LL, 2 Ld)	1 Dark (1 dd)

Table 3.1

Phenotypic expression of genotypes in the peppered moth

http://www.wwnorton.com/college/anthro/bioanth/ch2/

Evolution is dependent on changes in the genetic frequencies in a population, not in the individual. Allele frequencies are important for studying genetic variation in the gene pool, or the total of genetic information present in a population. Genetic variation then, is central to understanding evolutionary change in populations. We can determine mathematically whether a population is changing, or whether it is at equilibrium using the Hardy-Weinberg equation. The Hardy-Weinberg rule tells us that if a population is at equilibrium it will have the same genotypic frequencies from generation to generation, and that allele frequencies will remain the same unless some selection pressure acts to change them. There are many types of selection pressures that may change the genotypic frequencies in a population. Some of these selection pressures include: genetic mutations, genetic drift as a result of bottlenecks or the founder effect, migration, and lastly natural selection.

What is natural selection and how does it work?

Natural selection is the differential selection of genotypes and is part of evolutionary theory, which was developed by Charles Darwin. Evolution can be considered as change in the genetic composition of a population with the passage of each generation. One of the mechanisms of evolutionary change that Darwin developed was natural selection.

There are three basic types of natural selection (Figure 3.2). Under diversifying (disruptive) selection, both extreme phenotypes are favored at the expense of intermediate varieties. This is uncommon in nature, but it suggests a mechanism for species formation without geographic isolation. The second type of natural selection is directional selection. Under directional selection, individuals at one end of the phenotypic distribution survive or reproduce better than those with the other phenotype. Thus, the frequency distribution of the trait in the evolved population (or the second generation) is shifted toward the more favorable trait. This is what we typically think of as natural selection. The third type of natural selection is stabilizing selection. Under stabilizing selection, extreme varieties from both ends of the frequency distribution are eliminated. This is one of the most common forms of natural selection. Human birth weight is a good example of this type of selection, with unusually large babies and unusually small babies being selected against and medium-sized babies being selected for in the population.

Figure 3.2

Types of natural selection

For natural selection to occur, two requirements must be met: First, traits must be heritable. For example, body color in moths or beak size in birds, which is what Darwin studied, are inherited traits. Second, possession of a trait must then produce differential survival or reproduction rates in the population of organisms. Unless both these requirements are met, adaptation by natural selection cannot occur. Let’s examine the peppered moth as an example of directional natural selection.

Prior to the 1800’s in England, the typical peppered moth was light bodied with black spots. Dark colored moths were rare and were therefore collectors' items, with the first one being found in 1819. During the Industrial Revolution, soot and other industrial pollution darkened tree trunks and killed off lichen communities that lived on the trees. This made the light colored moths more visible to bird predators and caused the light-colored moths to become differentially selected by bird predators. The selective predation by birds, which favored camouflage coloration in the moth and was in turn dependent on the color of the tree-trunks, caused the light-bodied moths to become more rare and the dark-bodied moths to become more abundant. By 1886 dark-bodied moths were far more common than light-colored moths, illustrating rapid evolutionary change in the population. Eventually light moths were common in only a few locations, far removed from industrial areas.

How does speciation happen?

Historically there have been two main ways to differentiate between species. One way is the morphological species concept, which differentiates between species based on anatomical morphology, or the shape and look of the body and its parts. Another way to differentiate between species is based on the biological species concept, or the ability to interbreed. This methodology basically assumes that if organisms can interbreed and have biologically viable offspring they are the same species, and if they cannot produce viable offspring they are different species.

Distinct species appear when a single lineage splits into two populations that are no longer capable of inter-breeding. Usually this occurs as a result of geographic isolation, due perhaps to mountain building, climate change, or when a river cuts a deep chasm that prevents movement of individuals between the isolated populations. This is called allopatric speciation or speciation due to geographic isolation, and is the most common form of speciation (Figure 3.3). Genetic differences accumulate by chance and may be reinforced by different selection pressures in the two locations. Given sufficient time, the two populations will accumulate enough differences that they are unable to produce viable offspring if they come back into contact. Reproductive isolation is the key attribute of species-hood, and it can arise from prezygotic (those that prevent conception) or postzygotic (those that prevent the normal development and birth of a healthy individual) isolation mechanisms. Prezygotic isolation mechanisms prevent the breeding process from occurring between two populations in the first place. For example, if the two new species have different breeding seasons, say early and late spring, they may never be ready to breed at the same time and so experience a temporal prezygotic isolation mechanism. On the other hand, if two populations were not separated by their breeding period, they may occupy different habitats or have different courtship displays. Distinctive courtship behaviors allow individuals to ensure that they breed with other individuals of the same species, and thus avoid wasting a breeding opportunity.

Figure 3.3

Allopatric and Sympatric speciation

Postzygotic isolation mechanisms include the failure of sperm and egg to unite and begin development, the inability of hybrid progeny to develop to the point of birth, or the birth of individuals that are less able to feed, grow and compete than non-hybrids. One form of postzygotic isolation is hybrid inviability where the zygote develops abnormally and is aborted. Hybrid sterility is also a form of postzygotic isolation where the zygote may develop and be healthy; however, the individual born is sterile because the different chromosomes inherited from its parents do not pair and cross over properly during meiosis, thus the individual cannot produce viable offspring of their own. Another postzygotic isolation mechanism manifests as reduced reproductive fitness, which may occur directly with the hybrid individual or may occur over a few generations, eventually leading to inviable offspring in the hybrid lineage. Thus there are several ways in which reproductive isolation is achieved, and these speciation processes can occur rapidly in some organisms (like bacteria) or over hundreds or thousands of years in other organisms.

What is a terrestrial biome and why does the Earth have them?

When we combine all of the trophic levels and the web of their interactions with the abiotic environment, we refer to that as an ecosystem, and ecosystems are combined into biomes. Biomes are Earth’s major terrestrial ecosystems, or large biotic regions of Earth influenced by precipitation, temperature, humidity, latitude, and topography. Biomes are often grouped by the characteristics of the major vegetation types, animals, and climate that is associated with the biome location. The classification of terrestrial biomes is based upon the structure (or the physiognomy) of the vegetation, and the names generally give us information about the dominant vegetation type in the biome, as well as the type of climate that created and maintains the biome. For example, cold deserts, hot deserts, temperate rainforests, and tropical evergreen forests are all examples of biomes that give us information about the climate and dominant vegetation type associated with the biome. Note that the major units of aquatic ecosystems are not characterized by the term “biome”, mainly because the fundamental habitat is all water, whether it is salty or fresh. However, just as for the terrestrial ecosystems, aquatic systems are affected strongly by climate and their location on Earth.

The spatial structure of terrestrial biomes corresponds to the Earth’s major climatic zones (Figure 3.4). The Earth’s climate is central in determining the annual temperature ranges, the amount of precipitation and the potential evapotranspiration in different locations on Earth. Because soil is formed from the geologic parent material and from physical and chemical weathering (which is dependent on temperature and rates of precipitation) in a particular location, biomes are also found to be related to the soil types found on Earth.

Biomes are generally found parallel to the lines of latitude on Earth, which also indicates that biome distribution is related to climate (Figure 3.5). One exception to this occurs in mountainous regions. Mountain ranges differ in their pattern of temperature and precipitation compared to lowland areas at the same latitude. As elevation increases, temperature decreases and precipitation is generally more readily available. Therefore, ecosystems in mountainous areas will be more similar to biomes found in polar-regions. This change in biome type will become more extreme as elevation increases. For example, Hopkin’s bioclimatic law tells us that if you are in the northern hemisphere and increase your elevation by just 1000 feet, you have essentially moved the ecosystem equivalent of 100 miles north in latitude (Figure 3.6).

Biome types can be broken down into a large number of specific categories. However, some of Earth’s classic biomes include tundra, taiga (coniferous forests), tropical rainforests, deciduous forests, grasslands, and deserts. Tundra occurs in the Arctic and in high mountainous regions at all latitudes. Tundra is characterized by subsoil that is permanently frozen, with high precipitation rates and low temperatures. Temperatures in tundra biomes are too cold for trees; therefore, the dominant vegetation types are mosses, lichens, grasses and low growing perennial herbs. The taiga or boreal forest biome is located south of the Arctic tundra and at lower elevations than the alpine tundra ecosystem. Boreal forests are characterized by long winters with heavy snow and short warm summers, and they tend to have low species diversity with coniferous evergreen trees as the dominant vegetation type. The deciduous forest biome is located south of the taiga on the eastern side of North America and Asia, and in Western Europe. Deciduous forests are dominated by broadleaf trees and have temperate climates with ample rainfall. Tropical Rain Forests generally are found in equatorial regions with abundant rainfall and warm temperatures. Tropical rainforests have the highest diversity of plant and animal species on the planet. Grasslands are found in many regions on Earth where rainfall is limited and unevenly dispersed throughout the year. Grasslands are also species rich. Many agricultural regions of the world were once grasslands. Deserts are found in areas with extreme temperature fluctuations and extremely low rainfall.

Figure 3.4

Temperature and precipitation related to biomes

http://www.csus.edu/indiv/l/loom/ecol/terrest%20biomes.JPG

Figure 3.5

Biomes and latitude

http://www.agen.ufl.edu/~chyn/age2062/lect/lect_28/40_31A.GIF

Figure 3.6

Biomes and altitude

http://www.agen.ufl.edu/~chyn/age2062/lect/lect_28/40_38.GIF

What effect does human consumption have on the life system?

Currently, the human global population is greater that 6.6 billion people and it is projected to reach 9 billion by 2050. Because the human population has been increasing at such a rapid rate for the past one hundred years or more, we are using greater quantities of resources to sustain our needs. Worldwide there exist fewer than 1.8 biologically productive hectares per person. According to NASA, humans alone now consume approximately 20% of all the net primary production (NPP) that the Earth produces every year (Figure 3.7).

Ecological footprint analysis calculates the amount of ecologically productive land area that it takes to sustain a population by accounting for the food, energy, and water used by the population, as well as the waste generated in the process. To be ecologically sustainable, each person living on the planet should require less than 1.8 hectares of land for their total ecological footprint. However, the average American’s ecological footprint is roughly eight times that amount! Human consumption rates are influenced by the number of people in a population, the per capita consumption rate, and the technology that is available, which varies greatly by country and region.

Globally, humans are consuming renewable resources at a faster rate than the Earth’s ecosystems can replenish them. Due to this over-consumption, approximately 60% of world’s ecosystems have been degraded. The 2005 Footprint of Nations report determined that the current global population is consuming 39% more of Earth’s biological capacity than is ecologically sustainable. This unsustainable rate of consumption has negative consequences for many of the plant and animal species with which humans coexist. For example, human consumption pressures have resulted in irrevocable losses in Earth’s biodiversity with approximately 10-30% of all amphibian, bird, and mammal species threatened with extinction. Furthermore, the current rate of species loss is 100 to 1,000 times greater than the natural extinction rate. In addition to directly consuming resources, humans affect biological diversity by disrupting and destroying habitats, introducing exotic species and pests, as well as contaminating ecosystems with waste and pollution.

Much of the ecosystem degradation and species loss on Earth is associated with high levels of deforestation to keep up with increasing population growth rates and consumption demands. Logging for timber, paper, and fuel, as well as land conversion for agriculture are the primary causes of deforestation. Forest ecosystems house nearly 70% of the world’s terrestrial biodiversity. Unfortunately, the World Resources Institute estimates that more than 80% of the Earth’s natural forests already have been damaged or destroyed with 12 million hectares of forest being cleared annually. In addition, the world’s tropical forests are highly endangered and at the current deforestation rate some impoverished African, Asian, and Central American countries are expected to completely destroy their remaining tropical forests in the next 5-10 years.

Figure 3.7

Human appropriation of global NPP

http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16586

Last updated: 10/23/2006 6:40 PM

Figure 3.1

Structure of DNA

Allopatric and Sympatric speciation

Biomes and latitude

Biomes and altitude

Human appropriation of global NPP