From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: microevolution (a t.o. FAQ -- part one)
Date: 12 Nov 92 17:10:03 GMT
Organization: animal -- coelomate -- deuterostome

AN INTRODUCTION TO EVOLUTIONARY BIOLOGY -- BY CHRIS COLBY
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INTRODUCTION
------------

Evolution is one of the most powerful theories science has ever known. 
For a variety of reasons, however, it is also one of the most 
misunderstood. One common misunderstanding is that the phrase 
"survival of the fittest" summarizes evolutionary theory. In fact, it does 
not. The phrase is both incomplete and misleading. Two other common 
misinterpretations are that evolution is progress and organisms can be 
arranged on an evolutionary ladder from bacteria to man. 

This post is an outline of the basics of evolutionary biology. It is intended 
to be a brief overview of the concepts and mechanisms of evolution and 
dispel pervasive misunderstandings about the theory. Creationist 
arguments are not addressed directly here; nor is a "laundry list" of 
reasons to believe in evolution provided. Many interesting topics in 
evolutionary biology are not covered (symbiosis and endosymbiosis, 
origins of life, evolution of sex, human evolution and much more) 
because I can't include everything and keep this down to a readable 
length. 

WHAT IS EVOLUTION?

Evolution is a change in the gene pool of a population over time. The gene 
pool is the set of all genes in a species or population. The English moth, 
_Biston__betularia_, is a frequently cited example of observed evolution. 
In this moth, rare black variants spread through the population as a 
result of their habitat becoming darkened by soot from factories. Birds 
could see the lighter colored moths more readily and ate more of them. 
The moth population changed from mostly light colored moths to mostly 
dark colored moths. Since their color was determined by a single gene, 
the change in frequency of dark colored moths represented a change in 
the gene pool. This change was, by definition, evolution. 

The kind of evolution documented above is called "microevolution". 
Larger changes (taking more time) are termed "macroevolution". Some 
biologists feel the mechanisms of macroevolution are different from 
those of microevolutionary change. Others, including myself, feel the 
distinction between the two is arbitrary. Macroevolution is cumulative 
microevolution.

In any case, evolution is defined as a change in the gene pool. This means 
that evolution is a population level phenomena. Only groups of organisms 
evolve. An individual organism does not evolve, nor do subunits of 
organisms evolve (with limited exceptions). So, when thinking of 
evolution, is necessary to view populations as a collection of individuals. 

I have defined evolution, here, as a process and that is how I will use the 
term in this essay. Keep in mind, however, that in everyday use evolution 
often refers to a variety of things. The fact that all organisms are linked 
via descent to a common ancestor is often called evolution. The theory 
that life arose solely via natural processes is often called evolution 
(instead of abiogenesis). And frequently, people use the word evolution 
when they really mean natural selection -- one of the many mechanisms 
of evolution. 

WHAT ISN'T EVOLUTION? 

For many people evolution is equated with morphological change, i.e. 
organisms changing shape or size over time. An example would be a 
dinosaur species slowly evolving into a species of bird. It is important to 
note that evolution is often accompanied by morphological change, but 
this need not be the case. Evolution can occur without morphological 
change; and morphological change can occur without evolution. For 
instance, humans are larger now than in the past few hundred years, but 
this is not an evolutionary change. Better diet and medicine brought 
about this change, so it is not an example of evolution. The gene pool did 
not change -- only its manifestation did.

 An organism's phenotype is determined by both its genes and its 
environment. Phenotype means the morphological, physiological, 
biochemical, behavioral and other properties exhibited by a living 
organism. Phenotypic changes induced solely by changes in environment 
do not count as evolution because this change is not heritable; in other 
words the change is not passed on to the organism's offspring. Most 
changes due to environment are fairly subtle (e.g. size differences). Large 
scale phenotypic changes (such as dinosaur to bird) are obviously due to 
genetic changes, and therefore are evolution. 

WHAT EVOLUTION ISN'T 

Evolution is not progress. Organisms simply adapt to their current 
surroundings and do not necessarily become "better" over time. A trait or 
strategy that is successful for an organism at one time may be deleterious 
at another. Studies in yeast have shown that "more evolved" strains of 
yeast can sometimes be competitively inferior to "less evolved" strains. 
An organism's success or failure depends to a great deal on the behavior 
of its contemporaries; for most traits or behaviors there is likely no 
optimal design or strategy, only contingent designs or strategies. 

HOW DOES EVOLUTION WORK? 

If evolution is a change in the gene pool; what causes the gene pool to 
change? Several mechanisms can bring about a change in the gene pool, 
among them: natural selection, genetic drift, gene flow, mutation and 
recombination. I will discuss these in more detail later. It is important to 
understand the difference between evolution (change in the gene pool) 
and the mechanisms that bring about this change. 

GENETIC VARIATION
 -----------------

Bringing about a change in the gene pool assumes that there is genetic 
variation in the population to begin with, or a way to generate it. Genetic 
variation is "grist for the evolutionary mill". For example, if there were no 
dark moths, the population could not have evolved from mostly light to 
mostly dark. In order for continuing evolution there must be mechanisms 
to both increase genetic variation, or create it, (e.g. mutation) and 
decrease variation (e.g. natural selection and genetic drift). 

HOW IS GENETIC VARIATION DESCRIBED? 

Genetic variation has two components: allelic diversity and non-random 
associations of alleles. Alleles are different versions of the same gene at a 
given locus. For example, at the blood group locus (locus means location) 
humans can have an A, B or O allele. There are subtypes of these alleles 
as well. Most animals, including humans, are diploid. This means they 
contain two alleles for every gene at every locus. If the two alleles are 
the same type (for instance two A alleles) the individual would be 
termed "homozygous" for that locus. An individual with two different 
alleles at a locus is called "heterozygous". Allelic diversity is simply the 
number of alleles at each locus scaled by their frequency in the gene 
pool. At any given locus there can be many different alleles in the gene 
pool. It is important to realize that there can be more alleles in the gene 
pool at any locus than any single organism can possess. 

Linkage disequilibrium is a measure of association of alleles at different 
loci. If each gene assorted entirely independently, the gene pool would 
be at linkage equilibrium. However, if some alleles were often found 
together in organisms (i.e. did not assort randomly) these alleles would 
be in linkage disequilibrium. Linkage disequilibrium can be the result of 
physical proximity of the genes or maintained by natural selection if 
some combinations of alleles work better as a team. 

HOW MUCH GENETIC VARIATION IS THERE? 

Considerable variation has been detected in natural populations. At 45 
percent of loci in plants there is more than one allele in the gene pool. 
Any given plant is likely to be heterozygous at about 15 percent of it's 
loci. Levels of genetic variation in animals range from roughly 15% of loci 
having more than one allele (polymorphic) in birds, to over 50% of loci 
being polymorphic in insects. Mammals and reptiles are polymorphic at 
about 20% of their loci - - amphibians and fish are polymorphic at around 
30% of their loci. Most loci assort independently (i.e. they are at linkage 
equilibrium). In most populations, there are enough loci and enough 
different alleles that every individual (barring monozygotic (identical) 
twins) has a unique combination of alleles.