Download Continuous Variation Most traits in a population. eg height and body

Continuous Variation
Most traits in a population. e.g. height and body weight vary in a continuous way from one
extreme to the other.
A plot of the distribution of the trait in a population often produces a bell-shaped curve like this
one that shows the distribution of heights among a group of male secondary-school seniors.
Such a distribution could arise from
a. environmental factors - perhaps the continuous height variation in the boys is simply a
result of variation in their diet as they grew up.
b. genetic factors - tall parents tend to have tall children
or - most likely - both.
Types of Natural Selection
You might expect that natural selection would continually "improve" or make a species more fit
for its environment by weeding out unfit traits. For example, we might expect that giraffe neck
size would continue to increase generation after generation if the environment was selecting this
trait (ie. if food supply was limited and unable to sustain the entire population).
The pressures of natural selection can affect the distribution of phenotypes in a population in
several ways.
Stabilizing Selection
Natural selection often works to weed out individuals at both extremes of a range of phenotypes
resulting in the reproductive success of those near the mean. In such cases, the result is to
maintain the status quo. It is the continual elimination of extreme phenotypes, continually
selecting a particular trait. Therefore, this trait is said to be stable because it is maintained
generation after generation.
It is not always easy to see why both extremes should be handicapped; perhaps sexual selection
or liability to predation is at work. In any case, stabilizing selection is common. In humans, for
example, the incidence of infant mortality is higher for very heavy as well as for very light
babies.
Explaining Stabilizing Selection
Observation shows that their is very little variation in giraffe neck size, most have about the
same size of neck. In stabilizing selection, short necked giraffes are eliminated from the
population because this trait is a disadvantage. It is possible to suggest that having too long a
neck may also be a disadvantage, perhaps hindering the giraffes survival because it does not
have the developed musculature to hold the head up. Therefore, extremely long and short necks
are eliminated leaving a stable trait (very little neck size variation) that is found in most
individuals.
Directional selection - selects for the most extreme phenotype and so is likely to result in
the gradual replacement of one trait by another in the population. (giraffe's necks continue to
get longer)
A population may find itself in circumstances where individuals occupying one extreme in the
range of phenotypes are favored over the others.
Example: Darwin's finches in the Galapagos Islands.
When rainfall, and thus food, are plentiful, the birds tend to
1.
have a varied diet, e.g., eat seeds of a range of sizes
2.
show considerable variation in body and beak size (large beaks are better for large
seeds but can handle small seeds as well as small beaks)
From 1976 through 1977, a severe drought struck the islands, with virtually no rainfall for
over a year. This caused a large decline in the production of the seeds that are the dietary
mainstay of Geospiza fortis, the medium ground finch. The population declined from 1400 to
200 on the island of Daphne Major, a tiny (10-acre) member of the Galapagos Islands.
One of the plants to make it through the drought produces seeds in large, tough fruits that are
virtually impossible for birds with a beak smaller than 10.5 mm to eat.
Sampling the birds that died as well as those that survived showed that
1.
the larger birds were favored over the smaller ones
2.
those with larger beaks were favored over those with smaller ones.
Beak length
Beak depth
Dead birds
10.68
9.42
Survivors
11.07
9.96
Here, then was natural selection at work.
But did it produce evolution?
The answer turned out to be yes. As the population of G. fortis recovered after the rains
returned, the average body size and beak depth of their offspring was greater than before (an
increase of 4-5% for beak depth). The bell-shaped curve had been shifted to the right.
Disruptive Selection
In some circumstances, individuals at both extremes of a range of phenotypes are favored over
those in the middle. This is called disruptive selection.
An example:
The residues ("tailings") of mines often contain such high concentrations of toxic metals (e.g.,
copper, lead) that most plants are unable to grow on them. However, some hardy species (e.g.
certain grasses) are able to spread from the surrounding uncontaminated soil onto such waste
heaps. These plants develop resistance to the toxic metals while their ability to grow on
uncontaminated soil decreases. Because grasses are wind pollinated, breeding between the
resistant and nonresistant populations goes on. But evidently, disruptive selection is at work.
Higher death rates of both
1.
2.
less resistant plants growing on contaminated soil and
more resistant plants growing on uncontaminated soil
leads to increasing divergence of the populations into two sub populations with the extreme
manifestations of this trait.
The evolutionary significance of disruptive selection lies in the possibility that the gene pool
may become split into two distinct gene pools. This may be a way in which new species are
formed.
The formation of one or more species from a single precursor species is called speciation.