22 April 2005

Lecture 41

Reading, Chapter 16, 19, and 30

B. Eukaryotes

4. Kingdom Animalia

This is the animal kingdom. Animals are all multicellular and heterotrophic. Animal cells have no cell wall. The biggest distinctions between animals and other multicellular organisms are:

1. Animals are capable of rapid movements. Many have muscles and nerves specialized for this purpose. (Plants can't move much).

2. Animals eat things. They ingest other organisms or pieces of them as sources of energy and raw materials. (Plants don't do this either).

Movement and eating are consequences of animal cell structure. Unlike the cells of plants and fungi, animal cells have no walls. This means they can be highly flexible and allow dramatic movements, such as muscle contraction. Lack of cell walls also allows animals cells to envelope and eat other organisms or relatively large chunks of them. Absence of a chloroplast makes eating all the more necessary for animal cells.

Movement and eating define Animalia. What we perceive as the complexity of animal bodies consists largely of adaptations for eating and the avoidance of being eaten.

There are two major groups of animals, the invertebrates and the vertebrates. Invertebrates lack a backbone and spinal cord while vertebrates have these structures.

35 of the 36 animal phyla, a total of roughly ten million species, are invertebrates. Most of these are aquatic or marine but some groups are highly successful land organisms, especially the insects.

The vertebrates are a single phylum, the phylum Chordata. Phylum Chordata has about 50,000 species. They are divided into familiar groups: the fishes, amphibians, reptiles, birds, and mammals.

Ecology is the study of how groups of organisms interact with each other and with their environment. We will finish Introductory Biology with this general topic area. Because humans exert an enormous impact on all organisms and their environment, we will focus on what are known as "environmental issues" in the light of some simple ecological principles.

A. Population Ecology

A biological population is an interbreeding group of organisms. Members of a population are said to share a common "gene pool" because the different versions of their genes (alleles) are shared among the members of the population by sexual reproduction. Some examples of populations include:

all the brook trout in a lake

all the mice on a small island

all people on Earth (modern transportation has removed the barriers to reproduction between different groups of humans).

A point to re-emphasize here is that populations are the biological entities on which evolution acts. If a population is divided into two groups that cannot interbreed, two forces cause them to eventually become different species:

1) Mutations and other genetic variations occur randomly and independently in each group. Because these variations are not shared by interbreeding, different variations occur in the two groups.

2) Selective factors may be different in the two groups, e.g. because they are in different places. This means that different kinds of offspring may tend to survive in the two different places.

In humans, different races are the result of the geographic isolation of human populations that existed for about 100,000 years following migrations of the earliest Homo sapiens out of Africa. The different races were slowly evolving into what might eventually have become different human species. Geographical barriers to interbreeding between races were shattered by technology in the last hundred years, however. The very small genetic differences between races are now being distributed throughout the human population by sexual reproduction. Already, many of us are the descendants of multiple races. In time, for obvious biological reasons, race will disappear and we will have a diversity of individuals and regional cultures in a large human population.

1. Population growth

One of the biological characteristics of populations is that they grow. If their are more births than deaths in a population, it exhibits "exponential growth". This is a growth pattern in which the rate of growth is continuously accelerating, since the number of organisms reproducing continually increases.

Populations of bacteria in a test tube or in a potato salad exhibit exponential growth. Assuming warm temperatures and a cell division rate of once every 30 minutes:

Time	Number of cells
0 hours	1 cell
4 hours	256 cells
8 hours	65,536 cells
12 hours	> 4 million cells

This is why you shouldn't leave your potato salad in the sun and also why an infected wound should not be neglected.

Exponential growth only occurs when there are no limits to growth. There are always limits so exponential growth never continues indefinitely. Deaths eventually increase and births decrease because of "density-dependent" factors or "density-independent" factors.

Density-dependent factors are those that result from the number of individuals in the population. They grow stronger as the population grows. Density-dependent factors include:

Communicable disease, which is spread more quickly when individuals are close together.

Predation and parasitism. Predators and parasites tend to prosper as the numbers of their prey increase.

Lack of food, water, or other resources.

Accumulation of waste.

 

Density-independent factors that increase the death rate are not influenced by population size. They include:

Climate variations

Catastrophes like floods or fires

 

"Logistic growth" is the pattern in which the population grows for a while and then stabilizes at the "carrying capacity" of the environment. The carrying capacity is the size of a particular population that can be sustained by a particular environment for a long period of time. It is important to note that the carryng capacity of an environment for a particular population changes if the envrionment changes. It is not a static parameter.

If exponential population growth greatly exceeds the carrying capacity of an environment, a population "crash" results. Exponential growth continues until the population is much larger than the carrying capacity of the environment. At some point, deaths increase rapidly from density dependent factors and the population drops well below the carrying capacity. In some cases, the population dies out altogether. A test tube of bacteria shows this pattern, getting very dense and then becoming completely clear as the population crashes from accumulated waste. Populations of deer and other mammals have also shown population crashes.

2. Human population growth

This is a topic of great importance to all of us. At present, the human population of the globe is growing exponentially. Our numbers have tripled in the last hundred years and there are now over 6 billion humans on the planet. The rate of human population growth has accelerated dramatically in the last hundred years because the death rate has declined, owing to the benefits of modern medicine and agriculture. Birth rate has not yet declined proportionally.

As we have seen, biological populations do not grow indefinitely. At some point, density-dependent factors increase the death rate, stabilizing the population at the carrying capacity of the environment or precipitating a population crash. In the case of the human population, a desirable alternative is to decrease the birth rate rather than wait for the death rate to rise. In recent years, rich nations have shown a pattern of decreased birth rate and population stabilization that may be possible to replicate around the globe. There is hope that we can avoid the enormous human suffering of a population crash.

An issue relevant to human population growth is the carrying capacity of the Earth for humans. At present, the production of grain, area of farmable land, annual fish catch, area of rangeland, and area of forested land on Earth are all declining annually per person. Energy supplies and waste (pollution) may also place limits on human population that cannot be easily predicted. The carrying capacity of the Earth for humans has been estimated at about 12 billion. At present rates of growth, your children and you yourself will live in a world population of this size. What will their quality of life be?

The "replacement rate" for human beings is a little more than two children per woman. Two approaches have been taken to achieve this in different parts of the world:

Government control

In China, couples are limited to one child. There are significant tax penalties and other disincentives for having more than one child. As a result, population growth has slowed dramatically, though it has not stopped. This approach may seem draconian and undemocratic but it was an emergency measure that prevented the starvation of tens of millions of children.

Education and economic development

In Europe, growth rates are slower than in China but there are no strict government controls on family size. Similarly in the United States and Canada, much population growth is now the result of immigration rather than births. Over the last 30 years, we have observed that wealth, economic opportunity, and the education of women appear to generate a powerful limitation on family size. 

World population growth is a grave concern. All other "environmental problems" could be solved by reducing world population. The U.S. military has formally identified world overpopulation as a threat to our national security.

Fortunately, recent history suggests that promoting economic development and the education of women in developing countries may be the best strategy for coping with this threat.