table of contents
November 2000

Twenty one Questions

Recently I was asked for an e-mail interview by some students in a Junior High project. They posed twenty one questions which I answered, sometimes at great length. Here are the questions and my answers. See how you would have answered them.

1. What is Darwin's theory of evolution?

Very briefly: Organisms produce more potential offspring than can survive and reproduce. On average those best adapted to their environment survive. Organisms differ within populations; the variations which are based in heredity and which are better at surviving will with time predominate in the population. This is called natural selection. The heritable variations will continue to occur and over time species will change.

2. How would you say Darwin's theory affect the present scientific thought about creation?

Young Earth creationism, the notion that the Earth is a few thousand years old, was discredited 200 years ago by geological evidence and in the past 150 years by astronomical evidence. Darwin's theory was not relevant for that. Darwin's theory did affect and replace the theory of special creation in which each species or group of species was specially created and remained unchanged as a fixed type over time.

3. About how long would it take to evolve a separate species from another?

The amount of time is quite variable. It can happen in one generation with polyploidy (doubling of chromosome count). This is common in plants; it is rare in animals but has been seen. Mosquitos in the London subways evolved into a separate species in about 100 years. A rule of thumb is that it takes about 1000 to 10000 generations. However there are species that are stable for millions of years.

4. How would you separate one species from another?

This is complicated. The difficulty is that the forms of life are quite varied and have several different ways that they reproduce. There are about 20 different species concepts, i.e, criteria for distinguishing species. One of the best know is the biological species concept which says that two populations are separate species if they don't interbreed naturally. This definition only works for sexually reproducing species. Some other concepts are morphological (shape and presence of specific features) and niche (its place in its ecology). It turns out that the different methods of classification yield similar results. All classification methods break down somewhat because species are often in a state of splitting.

5. If a species evolves into another one, would the original species die out because of the competition with the better-equipped species?

Not necessarily. Most speciation occurs because the sub-populations of the species become isolated from each other and go their separate ways. They continue to exist in different environments. Sometimes species don't split but just change. In that case the original species disappears because it has become something different.

6. What are mutations and adaptations?

Mutations are changes in the hereditary material (the DNA) of an organism. There are are quite a few different ways this can happen. I summarize them in my web page on mutations. Adaptations are specific features of organisms that fit with the environment they live in. For example animals which live in cold climates tend to be well insulated.

7. What types of adaptations are there?

There is no definite answer to that question. Rather generally, one can distinguish between adaptations which principally affect the internal physiology, those which affect response to the non-biological environment, and those which reflect response to the biological environment. An example of the latter are figs and the fig wasps. The fig wasp can only live within the fig; in turn the fig can distribute its pollen via the fig wasp. Each has features that are coordinated with those of the other.

8. How do mutations and adaptations work?

Mutations are changes in the genome; they are continually occurring. Most organisms have a few mutations. Mutations supply the raw material of genetic variation. Natural selection (the survival of the individuals better adapted to their environment) ensures that favorable changes become more prevalent.

9. How are the adaptations passed from one generation to another?

Adaptations which are heritable are inherited. It is important to remember that many adaptations are not heritable. Thus, for example, calluses are not inherited. On the other hand the propensity to grow calluses is.

10. What are gene frequencies?

A gene can have several variants. These are called alleles. As an oversimplified example there is a gene for eye color (actually there are several genes involved). This gene has some variants, one for blue eyes, one for brown eyes, et cetera. The ratio of the different alleles in a species is called the gene frequency (this is a bit of a misnomer). Thus, if 90% of the alleles are for blue eyes and 10% are for brown eyes, we would speak of the gene or allele frequencies being .9 and .1. The relative frequencies of the alleles don't necessarily match the expression of the genes in the species; in species in which genes come in pairs (this includes plants, animals, fungi, and protists but not bacteria) the two copies may interact.

11. Are mutations and adaptations random?

Mutations are random in the sense that they are unrelated to the fitness of the organisms. Natural selection is partly random because survival and reproduction are a matter of chance as well as fitness. On average, however, natural selection is not random.

12. How does Natural Selection work?

It works negatively by preferentially eliminating ill-adapted individuals and postively by well-adapted individuals having relatively more offspring.

13. Can you give an example of Natural Selection?

Bacteria are selected for resistence to antibiotics.

14. What are Selection Pressures?

Continuing features of the environment that favor adaptation.

15. Can you give some examples of Selection Pressures?

Winter is a selection pressure for adapatations to cold weather. The continuing use of antibiotics is a selection pressure for resistance to antibiotics.

16. How does the evolution of some species affect the population of the original?

This isn't a well phrased question. If a species splits there often is no immediate effect because they are in different environments. If, at a later point, they interact the results depend on the extent to which they are competitors.

17. How do environmental changes affect the evolution of a species?

The effects are quite varied. There are natural limits as to how fast a species can change. If the environmental changes are too rapid the species must either move to a better environment or go extinct. If the changes are fairly regular, e.g., the seasons in the temperate zones, species can adapt to handle a variable life-style in response to the changes. If the tempo of environmental change is very slow the rate of evolution tends to slow down.

One thing to remember is the majority of mutations are neutral; that is, they are neither favorable nor unfavorable. These changes slowly accumulate over time by random chance. This is known as genetic drift. Most of the evolution of the genome is due to genetic drift. However most adaptive change is in response to natural selection.

18. How do newly evolved species affect the environment?

The effects are quite varied. In part it depends on how successful the species is. A species which is very successful alters the environment for every species that it feeds upon and every species that feeds upon it. A successful species can even alter the climate by changing water retention and erosion of the soil. This was a consequence of the evolutions of grasses. The evolution of flowers made for a more favorable environment for flying insects. Evolution of large herbivores increases the partial clearing of forests. A new parasite may eliminate species or force them to change their feeding habits. Et cetera.

19. How does Natural Selection affect the population of a species?

The effects of natural selection are more sharply seen in adverse circumstances. It is then that differences between individuals matter more. Typically, though, population size is a function of the immediate environment. Natural selection determines which members of the species survive and reproduce; population size is determined by available resources.

20. Why do species produce more children than needed to replace parents?

If they don't the population size will decrease because some children die without reproducing. That, however, is not an answer to your question.

Individuals produce as many successful offspring as they can. There is a very strong selection pressure to do so. The lineages which survive are the ones which are best at successful reproduction. The resources which are available to organisms are limited. There are two general strategies that species use. In some species the individuals produce large numbers of potential offspring and invest very in each one. Most plants do this. In other species the parents produce relatively few offspring and invest a great deal of effort in the ones that they do produce. Humans are an example. 21. How does Lamarck's theory differ from Darwin's?

I'm going to give you the right answer rather that the popular belief. In LaMarck's theory species originated as very primitive organisms and then underwent changes over time. The species didn't split; instead there was a separate origins for each species. In Darwin's theory species began as a single (or possibly very few) primitive organism. Over time the species diversified and split. Thus Darwin's is one of common descent of species whereas LaMarck's theory is one of separate origins for each species.

Nowadays LaMarck is better known for a feature of his theory known as LaMarckian inheritance. This is popularly described as the inheritance of acquired features. His actual theory of inheritance was more subtle but can be presented in that way. Darwin had several different theories of inheritance which included inheritance of acquired features. Inheritance wasn't really worked out until the twentieth century when genetic inheritance became understood.

This page was last updated November 3, 2000.

table of contents
November 2000