EVOLUTION


Excerted from C. Colby


Evolution is the cornerstone of modern biology. It unites all the fields of biology
under one theoretical umbrella. It is         not a difficult concept, but very few
people   have a satisfactory grasp of it.   Misconception s permeate popular science
expositions of evolutionary  biology.  Mistakes even  appear into biology  texts. For
example, Lodish, et. al., in their cell biology text, proclaim,   "It was Charles
Darwin's great insight that organisms are all related in a great chain of being..." In
fact, the idea of a great  chain of being, which traces to Linnaeus, was overturned by
Darwin's idea of common descent. 
   Misunderstandings about evolution are damaging to the study of evolution and biology
as a whole. People who have a   general interest in science are likely to dismiss
evolution as a soft science after absorbing the pop science nonsense that   abounds.
The impression of it being a soft science is reinforced when "scientists" in unrelated
fields speculate publicly about  evolution. 

   What is Evolution?

   Evolution is a change in the gene pool of a population over time. A gene is a
hereditary unit that can be passed on unaltered  for many generations. 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. [evolution: a change in the gene   pool] In this moth there are two color
morphs, light and dark. H. B. D. Kettlewell found that dark moths constituted less
than 2% of the population prior to 1848. The frequency of the dark morph increased in
the years following. By 1898, the 95% of the moths in Manchester and other highly
industrialized areas were of the dark type. Their frequency was less in  rural areas.
The moth population changed from mostly light colored moths to mostly dark colored
moths. The moths' color  was primarily determined by a single gene. [gene: a hereditary
unit] So, the change in frequency of dark colored moths  represented a change in the
gene pool. [gene pool: the set all of genes in a population] This change was, by
definition,  evolution. 
   The increase in relative abundance of the dark type was due to natural selection.
The late eighteen hundreds was the time of England's industrial revolution. Soot from
factories darkened the birch trees the moths landed on. Against a sooty background,
birds could see the lighter colored moths better and ate more of them. As a result,
more dark moths survived until reproductive age and left offspring. The greater number
of offspring left by dark moths is what caused their increase in  frequency. This is an
example of natural selection. 

   Populations evolve. [evolution: a change in the gene pool] In order to understand
evolution, it is necessary to view populations as a collection of individuals, each
harboring a different set of traits. A single organism is never typical of an  entire
population unless there is no variation within that population. Individual organisms do
not evolve, they retain the same genes throughout their life, although mutaions may
occur.  Only if mutations are inherited will they affect the gene pool.  When a
population is evolving, the ratio of different genetic types is changing -- each
individual   organism within a population does not change. For example, in the previous
example, the frequency of black moths increased; the moths did not turn from light to
gray to dark in concert. 
**The process of evolution can be summarized in threesentences: 
           Genes mutate. [gene: a hereditary unit] 
           Individuals are selected. 
           Populations evolve. 

   Evolution can be divided into microevolution and macroevolution. The kind of
evolution documented above is microevolution. Larger changes, such as when a new
species is formed, are called macroevolution. Some biologists feel that the mechanisms
of macroevolution are different from those of microevolutionary change. Others think
the distinction between the two is arbitrary -- macroevolution is cumulative
microevolution. 

   The word evolution has a variety of meanings. The fact that all organisms are linked
via descent to a common ancestor is  often called evolution. The theory of how the
first living organisms appeared is often called evolution. This should be called
abiogenesis. And frequently, people use the word evolution when they really mean
natural selection -- one of the many mechanisms of evolution. 

   Common Misconceptions about Evolution
   Evolution can occur without morphological change; and morphological change can occur
without evolution. Humans are larger now than in the recent past, a result of better
diet and medicine. Phenotypic changes, like this, induced solely by changes in
environment do not count as evolution because they are not heritable; in other words
the change is not passed on to the organism's offspring. Phenotype is the
morphological, physiological, biochemical, behavioral and other properties  exhibited
by a living organism. An organism's phenotype is determined by its genes and its
environment. Most changes due to environment are fairly subtle, for example size
differences. Large scale phenotypic changes are obviously due to genetic changes, and
therefore are evolution. 

   Evolution is not ‘progress’. Populations simply adapt to their current surroundings.
They do not necessarily become better in any absolute sense over time. A trait or
strategy that is successful at one time may be unsuccessful at another. Paquin and
Adams demonstrated this experimentally. They founded a yeast culture and maintained it
for many generations.   Occasionally, a mutation would arise that allowed its bearer to
reproduce better than its contemporaries. These mutant   strains would crowd out the
formerly dominant strains. Samples of the most successful strains from the culture were
taken   at a variety of times. In later competition experiments, each strain would
outcompete the immediately previously dominant  type in a culture. However, some
earlier isolates could outcompete strains that arose late in the experiment.
Competitive ability of a strain was always better than its previous type, but
competitiveness in a general sense was not increasing. Any organism's success depends
on the behavior of its contemporaries. For most traits or behaviors there is likely no
optimal design or strategy, only contingent ones. Evolution can be like a game of
paper/scissors/rock. 

   Organisms are not passive targets of their environment. Each species modifies its
own environment. At the least, organisms remove nutrients from and add waste to their
surroundings. Often, waste products benefit other species. Animal dung is fertilizer
for plants. Conversely, the oxygen we breathe is a waste product of plants. Species do
not simply change to fit  their environment; they modify their environment to suit them
as well. Beavers build a dam to create a pond suitable to  sustain them and raise
young. Alternately, when the environment changes, species can migrate to suitable
climes or seek out   microenvironments to which they are adapted. 



Genetic Variation

   Evolution requires genetic variation. 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 increase or create genetic variation and
mechanisms to decrease it. Mutation is a change in a gene. These changes are the source
of new genetic variation. Natural  selection operates on this variation. 
   Genetic variation has two components: allelic diversity and non- random associations
of alleles. Alleles are different versions of the same gene. For example, humans can
have A, B or O alleles that determine one aspect of their blood type.   Most animals,
including humans, are diploid -- they contain two alleles for every gene at every
locus, one inherited from  their mother and one inherited from their father. Locus is
the location of a gene on a chromosome. Humans can be AA,  AB, AO, BB, BO or OO at the
blood group locus. If the two alleles at a locus are the same type (for instance two A
alleles) the individual would be called homozygous. An individual with two different
alleles at a locus (for example, an AB individual) is called heterozygous. At any locus
there can be many different alleles in a population, more alleles than any  single
organism can possess. For example, no single human can have an A, B and an O allele. 

  If two alleles were found together in organisms more often than would be expected by
normal inheritance patterns, the alleles are in  disequilibrium.  Disequilibrium can be
the result of physical proximity of the genes. Or, it can be maintained by natural
selection if some combinations of alleles work better as a team. 
   Natural selection maintains the  disequilibrium between color and pattern alleles in
Papilio memnon.  In this moth species, there is a gene that determines wing morphology.
One allele at this locus leads to a moth that has a tail; the other allele codes for a
untailed moth. There is another gene that determines if the wing is brightly or darkly
colored. There are thus four possible types of moths: brightly colored moths with and
without tails, and dark moths with and without tails. All four can be produced when
moths are brought into the lab and bred. However, only two of these types of moths are
found in the wild: brightly colored moths with tails and darkly colored moths without
tails. The non-random association is maintained by natural selection. Bright, tailed
moths mimic the pattern of an unpalatable species. The dark morph is cryptic. The other
two combinations are neither  mimetic nor cryptic and are quickly eaten by birds. 

   Assortative mating causes a non-random distribution of alleles at a single locus.
[locus: location of a gene on a
chromosome] Non-random mating results in a deviation from expected allele frequency in
a population. Humans mate assortatively according to  race; we are more likely to mate
with someone of own race than another. In populations that mate this way, fewer
heterozygotes are found than would be predicted under random mating. [heterozygote: an
organism that has two different  alleles at a locus] A decrease in heterozygotes can be
the result of mate choice, or simply the result of population  subdivision. Most
organisms have a limited dispersal capability, so their mate will be chosen from the
local population. 

   Natural Selection 

   Some types of organisms within a population leave more offspring than others. Over
time, the frequency of the more   prolific type will increase. The difference in
reproductive capability is called natural selection. Natural selection is the only
mechanism of adaptive evolution; it is defined as differential reproductive success of
pre- existing classes of genetic variants in the gene pool.   The most common action of
natural selection is to remove unfit variants as they arise via mutation. [natural
selection:   differential reproductive success of genotypes] In other words, natural
selection usually prevents new alleles from increasing  in frequency.   
  Sometimes a situation occurs where heterozygotes have an advantage, termed balancing
selection.  An example of this is the maintenance of sickle-cell alleles in human
populations subject to malaria.   Variation at a single locus determines whether red
blood cells are shaped normally or sickled. If a human has two alleles for sickle-cell,
he/she develops anemia -- the shape of sickle-cells precludes them carrying normal
levels of oxygen. However,   heterozygotes who have one copy of the sickle-cell allele,
coupled with one normal allele enjoy some resistance to malaria   -- the shape of
sickled cells make it harder for the plasmodia (malaria causing agents) to enter the
cell. Thus, individuals homozygous for the normal allele suffer more malaria than
heterozygotes. Individuals homozygous for the sickle- cell are anemic.  Heterozygotes
have the highest fitness of these three types. Heterozygotes pass on both sickle-cell
and normal  alleles to the next generation. Thus, neither allele can be eliminated from
the gene pool. The sickle-cell allele is at its highest frequency in regions of Africa
where malaria is most pervasive.    Balancing selection is rare in natural populations.
[balancing selection: selection favoring heterozygotes] Only a handful of  other cases
beside the sickle-cell example have been found. 

   At one time population geneticists thought balancing selection   could be a general
explanation for the levels of genetic variation found in natural populations. That is
no longer the case.   Balancing selection is only rarely found in natural populations.
And, there are theoretical reasons why natural selection  cannot maintain polymorphisms
at several loci via balancing selection.    Individuals are selected. The example given
earlier was an example of evolution via natural selection. [natural selection:
differential reproductive success of genotypes] Dark colored moths had a higher
reproductive success because light   colored moths suffered a higher predation rate.
The decline of light colored alleles was caused by light colored individuals   being
removed from the gene pool (selected against). Individual organisms either reproduce or
fail to reproduce and are hence the unit of selection. One way alleles can change in
frequency is to be housed in organisms with different reproductive rates. Genes are not
the unit of selection (because their success depends on the organism's other genes as
well); neither are groups of organisms a unit of selection. There are some exceptions
to this "rule," but it is a good generalization. 

   Natural selection favors traits or behaviors that increase a genotype's inclusive
fitness. Closely related organisms share   many of the same alleles. In diploid
species, siblings share on average at least 50% of their alleles. The percentage is
higher if the parents are related. So, helping close relatives to reproduce gets an
organism's own alleles better represented in the  gene pool. The benefit of helping
relatives increases dramatically in highly inbred species. In some cases, organisms
will completely forgo reproducing and only help their relatives reproduce. Ants, and
other eusocial insects, have sterile castes  that only serve the queen and assist her
reproductive efforts. The sterile workers are reproducing by proxy. 

  The opportunity for natural selection to operate does not induce genetic variation to
appear -- selection only distinguishes   between existing variants. Variation is not
possible along every imaginable axis, so all possible adaptive solutions are not   open
to populations. Here is an example of natural selection. Geospiza fortis lives on the
Galapagos islands along with fourteen other finch species. It feeds on the seeds of the
plant Tribulus cistoides, specializing on the smaller seeds. Another species, G.
magnirostris, has a larger beak and specializes on the larger seeds. The health of
these bird populations depends on seed  production. Seed production, in turn, depends
on the arrival of wet season. In 1977, there was a drought. Rainfall was well  below
normal and fewer seeds were produced. As the season progressed, the G. fortis
population depleted the supply of  small seeds. Eventually, only larger seeds remained.
Most of the finches starved; the population plummeted from about  twelve hundred birds
to less than two hundred. Peter Grant, who had been studying these finches, noted that
larger beaked  birds fared better than smaller beaked ones. These larger birds had
offspring with correspondingly large beaks. Thus, there was an increase in the
proportion of large beaked birds in the population the next generation. To prove that
the change in bill size in Geospiza fortis was an evolutionary change, Grant had to
show that differences in bill size were at least partially  genetically based. He did
so by crossing finches of various beak sizes and showing that a finch's beak size was
influenced  by its parent's genes. Large beaked birds had large beaked offspring; beak
size was not due to environmental differences  (in parental care, for example). 

   Natural selection may not lead a population to have the optimal set of traits. In
any population, there would be a certain  combination of possible alleles that would
produce the optimal set of traits (the global optimum); but there are other sets of
alleles that would yield a population almost as adapted (local optima). Transition from
a local optimum to the global optimum may be hindered or forbidden because the
population would have to pass through less adaptive states to make  the transition.
Natural selection only works to bring populations to the nearest optimal point. This
idea is Sewall Wright's   adaptive landscape. This is one of the most influential
models that shape how evolutionary biologists view evolution. 

   Natural selection does not have any foresight. It only allows organisms to adapt to
their current environment. Structures or   behaviors do not evolve for future utility.
An organism adapts to its environment at each stage of its evolution. As the
environment changes, new traits may be selected for. Large changes in populations are
the result of cumulative natural   selection. Changes are introduced into the
population by mutation; the small minority of these changes that result in a  greater
reproductive output of their bearers are amplified in frequency by selection. 
Complex traits must evolve through viable intermediates. For many traits, it initially
seems unlikely that intermediates would   be viable. What good is half a wing? Half a
wing may be no good for flying, but it may be useful in other ways. Feathers   are
thought to have evolved as insulation (ever worn a down jacket?) and/or as a way to
trap insects. Later, proto-birds   may have learned to glide when leaping from tree to
tree. Eventually, the feathers that originally served as insulation now   became
co-opted for use in flight. A trait's current utility is not always indicative of its
past utility. It can evolve for one purpose, and be used later for another. A trait
evolved for its current utility is an adaptation; one that evolved for another  utility
is an exaptation. An example of an exaptation is a penguin's wing. Penguins evolved
from flying ancestors; now they  are flightless and use their wings for swimming. 


Mutation:   The cellular machinery that copies DNA sometimes makes mistakes. These
mistakes alter the sequence of a gene. This is called a mutation. There are many kinds
of mutations. A point mutation is a mutation in which one "letter" of the genetic code
is changed to another. Lengths of DNA can also be deleted or inserted in a gene; these
are also mutations. Finally,  genes or parts of genes can become inverted or
duplicated. Typical rates of mutation are between 10-10 and 10-12 mutations per base
pair of DNA per generation. 
   Most mutations are thought to be neutral with regards to fitness. Only a small
portion of the genome of eukaryotes contains coding segments. And, although some
non-coding DNA is involved in gene regulation or other cellular functions, it is
probable that most base changes would have no fitness consequence. 
   Most mutations that have any phenotypic effect are deleterious. Mutations that
result in amino acid substitutions can change the shape of a protein, potentially
changing or eliminating its function. This can lead to inadequacies in biochemical
pathways or interfere with the process of development. Organisms are sufficiently
integrated that most random changes will not produce a fitness benefit. Only a very
small percentage of mutations are beneficial. The ratio of neutral to deleterious to
beneficial mutations is unknown and probably varies with respect to details of the
locus in question and environment. 

   Mutation limits the rate of evolution. The rate of evolution can be expressed in
terms of nucleotide substitutions in a lineage  per generation. Substitution is the
replacement of an allele by another in a population. This is a two step process: First
a  mutation occurs in an individual, creating a new allele. This allele subsequently
increases in frequency to fixation in the  population. The rate of evolution is ‘k =
2Nvu" (in diploids) where  k  is nucleotide substitutions, 
N  is the effective population size,   v  is the rate of mutation and  u  is the
proportion of mutants that eventually fix in the population.   This formula has been
validated and is often used by systematists to document species relatedness.

    The Fate of Mutant Alleles- Mutation creates new alleles. Each new allele enters
the gene pool as a single copy amongst many. Most are lost from the  gene pool, the
organism carrying them fails to reproduce, or reproduces but does not pass on that 
particular allele. A mutant's fate is shared with the genetic background it appears in.
A new allele willinitially be linked to other loci in its genetic background, even loci
 on other chromosomes. If the allele increases in frequency in the population,
initially it will be   paired with other alleles at that locus -- the new allele will
primarily be carried in individuals heterozygous for that locus.   The chance of it
being paired with itself is low until it reaches intermediate frequency. If the allele
is recessive, its effect won't be seen in any individual until a homozygote is formed. 
The eventual fate of the allele depends on whether it is neutral, deleterious or
beneficial. 
   One example of a beneficial mutation comes from the mosquito Culex pipiens. In this
organism, a gene that was involved  with breaking down organophosphates - common
insecticide ingredients -became duplicated. Progeny of the organism with this mutation
quickly swept across the worldwide mosquito population. There are numerous examples of
insects developing resistance to chemicals, especially DDT which was once heavily used
in this country. And, most importantly, even though "good" mutations happen much less
frequently than "bad" ones, organisms with "good" mutations thrive while organisms with
"bad" ones die out. 
   If beneficial mutants arise infrequently, the only fitness differences in a
population will be due to new deleterious mutants  and the deleterious recessives.
Selection will simply be weeding out unfit variants. Only occasionally will a
beneficial allele  be sweeping through a population. The general lack of large fitness
differences segregating in natural populations argues  that beneficial mutants do
indeed arise infrequently. However, the impact of a beneficial mutant on the level of
variation at a   locus can be large and lasting. It takes many generations for a locus
to regain appreciable levels of heterozygosity following   a selective sweep. 

    Recombination:   (here used for translocations and duplications] creates new
combinations of alleles. Alleles that arose at different times and different places can
be brought  together. Recombination can occur not only between genes, but within genes
as well. Recombination within a gene can  form a new allele. Recombination is a
mechanism of evolution because it adds new alleles and combinations of alleles to the
gene pool. 

Genetic Drift:   Allele frequencies can change due to chance alone. This is called
genetic drift. Drift is a binomial sampling error of the gene   pool. What this means
is, the alleles that form the next generation's gene pool are a sample of the alleles
from the current  generation. When sampled from a population, the frequency of alleles
differs slightly due to chance alone. 
   Alleles can increase or decrease in frequency due to drift. The average expected
change in allele frequency is zero, since increasing or decreasing in frequency is
equally probable. A small percentage of alleles may continually change frequency in   a
single direction for several generations just as flipping a fair coin may, on occasion,
result in a string of heads or tails. A   very few new mutant alleles can drift to
fixation in this manner. 

   Gene Flow:     New organisms may enter a population by migration from another
population. If they mate within the population, they can   bring new alleles to the
local gene pool. This is called gene flow. In some closely related species, fertile
hybrids can result from interspecific matings. These hybrids can  move genes from
species to species. 
   Gene flow between more distantly related species occurs infrequently. This is called
horizontal transfer. One interesting  case of this involves genetic elements called P
elements. Margaret Kidwell found that P elements were transferred from  some species in
the Drosophila willistoni group to Drosophila melanogaster. These two species of fruit
flies are   distantly related and hybrids do not form. Their ranges do, however,
overlap. The P elements were vectored into D. melanogaster via a parasitic mite that
targets both these species. This mite punctures the exoskeleton of the flies and feeds
on the "juices". Material, including DNA, from one fly can be transferred to another
when the mite feeds. Since P elements  actively move in the genome (they are themselves
parasites of DNA), one incorporated itself into the genome of a   melanogaster fly and
subsequently spread through the species. Laboratory stocks of melanogaster caught prior
to the   1940's lack of P elements. All natural populations today harbor them. 

   Overview of Evolution within a Lineage

   Evolution is a change in the gene pool of a population over time; it can occur due
to several factors. Three mechanisms add  new alleles to the gene pool: mutation,
recombination and gene flow. Two mechanisms remove alleles, genetic drift and  natural
selection. Drift removes alleles randomly from the gene pool. Selection removes
deleterious alleles from the gene  pool. The amount of genetic variation found in a
population is the balance between the actions of these mechanisms. 
   Except in rare cases of high gene flow, new alleles enter the gene pool as a single
copy. Most new alleles added to the   gene pool are lost almost immediately due to
drift or selection; only a small percent ever reach a high frequency in the
population. Even most moderately beneficial alleles are lost due to drift when they
appear. But, a mutation can reappear   numerous times. 
   The fate of any new allele depends a great deal on the organism it appears in. This
allele will be linked to the other alleles near it for many generations. A mutant
allele can increase in frequency simply because it is linked to a beneficial allele at
a  nearby locus. This can occur even if the mutant allele is deleterious, although it
must not be so deleterious as to offset the  benefit of the other allele. Likewise a
potentially beneficial new allele can be eliminated from the gene pool because it was
linked to deleterious alleles when it first arose. An allele "riding on the coat tails"
of a beneficial allele is called a hitchhiker.   Eventually, recombination will bring
the two loci to linkage equilibrium. But, the more closely linked two alleles are, the
longer the hitchhiking will last. 
   The effects of selection and drift are coupled. Drift is intensified as selection
pressures increase. This is because increased  selection (i.e. a greater difference in
reproductive success among organisms in a population) reduces the effective  population
size, the number of individuals contributing alleles to the next generation. 
   Adaptation is brought about by cumulative natural selection, the repeated sifting of
mutations by natural selection. Small  changes, favored by selection, can be the
stepping-stone to further changes. The summation of large numbers of these  changes is
macroevolution. 

Common Misconceptions about Selection

   Selection is not a force in the sense that gravity or the strong nuclear force is.
Selection merely favors  beneficial genetic changes when they occur by chance -- it
does not contribute to their appearance. The potential for  selection to act may long
precede the appearance of selectable genetic variation. When selection is spoken of as
a force, it often seems that it is has a mind of its own; or as if it was nature
personified. This most often occurs when biologists are   waxing poetic about
selection. This has no place in scientific discussions of evolution. Selection is not a
guided or cognizant entity; it is simply an effect. 
   A related pitfall in discussing selection is anthropomorphizing on behalf of living
things. Often conscious motives are
seemingly imputed to organisms, or even genes, when discussing evolution. This happens
most frequently when discussing  animal behavior. Animals are often said to perform
some behavior because selection will favor it. This could more  accurately worded as
"animals that, due to their genetic composition, perform this behavior tend to be
favored by natural selection relative to those who, due to their genetic composition,
don't." Such wording is cumbersome. To avoid this, writers  often anthropomorphize.
This is unfortunate because it often makes evolutionary arguments sound silly. Keep in
mind this is only for convenience of expression. 

   The phrase "survival of the fittest" is often used synonymously with natural
selection. The phrase is both incomplete and  misleading. For one thing, survival is
only one component of selection -- and perhaps one of the less important ones in  many
populations. For example, in polygynous species, a number of males survive to
reproductive age, but only a few ever mate. Males may differ little in their ability to
survive, but greatly in their ability to attract mates -- the difference in
reproductive success stems mainly from the latter consideration. Also, the word fit is
often confused with ‘physically fit’.   Fitness, in an evolutionary sense, is the
average reproductive output of a class of genetic variants in a gene pool. Fit does not
necessarily mean biggest, fastest or strongest. 

   Sexual Selection

   In many species, males develop prominent secondary sexual characteristics. A few oft
cited examples are the peacock's
tail, coloring and patterns in male birds in general, voice calls in frogs and flashes
in fireflies. Many of these traits are a liability from the standpoint of survival. Any
ostentatious trait or noisy, attention getting behavior will alert predators as well
as potential mates. How then could natural selection favor these traits? 
   Natural selection can be broken down into many components, of which survival is only
one. Sexual attractiveness is a very   important component of selection, so much so
that biologists use the term sexual selection when they talk about this subset of
natural selection. 
   Sexual selection is natural selection operating on factors that contribute to an
organism's mating success. Traits that are a liability to survival can evolve when the
sexual attractiveness of a trait outweighs the liability incurred for survival. A male
who lives a short time, but produces many offspring is much more successful than a long
lived one that produces few. The  former's genes will eventually dominate the gene pool
of his species. In many species, especially polygynous species where only a few males
monopolize all the females, sexual selection has caused pronounced sexual dimorphism.
In these species  males compete against other males for mates. The competition can be
either direct or mediated by female choice. In species where females choose, males
compete by displaying striking phenotypic characteristics and/or performing elaborate
courtship behaviors. The females then mate with the males that most interest them,
usually the ones with the most outlandish displays. There are many competing theories
as to why females are attracted to these displays. 
   The good genes model states that the display indicates some component of male
fitness. A good genes advocate would say that bright coloring in male birds indicates a
lack of parasites. The females are cueing on some signal that is correlated with some
other component of viability.    Selection for good genes can be seen in sticklebacks.
In these fish, males have red coloration on their sides. Milinski and  Bakker showed
that intensity of color was correlated to both parasite load and sexual attractiveness.
Females preferred  redder males. The redness indicated that he was carrying fewer
parasites.  There are many examples of this model as well as variations on it.