Download NATURAL SELECTION

NATURAL
SELECTION
Chapter 7 in Kardong
INTRODUCTION
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Organisms must survive the biotic and abiotic
factors within their environment if they are going to
survive to sexual maturity. But evolutionary success is
not just dependent on survival, evolutionary fitness is
known to be associated with reproductive success. The
more offspring that an individual leaves for the next
generation, the more fit the individual. At the level of
the gene, fitness is measured by the success of one
genotype (or individual gene - allele) over another
genotype (or individual gene - allele).
SELECTION WORKS ON THE
PHENOTYPE
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The biotic and abiotic factors within the
environment apply selection pressure to the
phenotype, not the genotype. However, the
environmental factors working on the
phenotype will result in certain phenotypes
having greater reproductive success than other
phenotypes, thus the genotype ultimately
changes. Therefore, the gene pool changes thus evolution will occur as a result of selection
pressures.
ARTIFICIAL SELECTION
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Darwin studied the artificial selection practiced by breeders of
animals and plants to determine if and how particular traits
(admired by humans, for agricultural purposes, beauty, etc.) were
passed on to offspring (inherited). Of course, as animal and
plant breeders had demonstrated, certain desirable traits of
animals and plants (as perceived by humans) were heritable and
passed on to offspring, thus undesirable traits could be weeded
out. "This weeding out of organisms by humans for human
purposes is artificial selection." (Kardong, 2005).
Dogs and Cats
Agriculture - corn, cabbage family, etc.
Flowers (like the tulip, rose, etc.)
Diversity of dogs
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Artificial selection has produced numerous breeds of
dogs, which diverged from wolves about 15,000 years
ago.
Corn
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Artificial selection through the centuries evolved the modern
male tassel and female ears of corn from wild grass.
Corn varieties
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From left to right: popcorn, sweet corn, flint
corn, dent corn, and pod corn.
Tulips
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Tulips come in a variety of colors and stripes, but never
in a deep color of completely black. This variety does
not exist, at least so far, within the species.
NATURAL SELECTION
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"The weeding out of organisms by biological processes,
without deliberate or directed human intervention, is
natural selection." (Kardong, 2005). Generally, natural
selection is an average process that works on the phenotypes in
the population, ultimately resulting in the survival and
reproductive success of phenotypes that are more fit for their
environment. Thus natural selection leads to a change in the
genotypic frequencies in a population over time.
By human standards, natural selection is a very slow process,
acting over hundreds of generations. However, viruses and
bacteria (because of very rapid reproduction) may evolve
significantly in human lifetimes (examples, the AIDs virus,
antibiotic resistance in certain strains of bacteria). There are also
examples of more complex organisms that have recently evolved
by natural selection, such as the following:
DARWIN’S FINCHES
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Darwin reasoned that the finches of the
Galapagos Islands had evolved from an ancestral
species from the mainland of South America
and with the passage of time the descendants on
each of the islands had been modified by natural
selection to occupy distinctive niches. In
particular, the beaks of the various finches are
specialized for a specific diet, such as soft or
hard seeds, insects, etc.
DARWIN’S FINCHES
MODERN STUDIES OF
DARWIN’S FINCHES
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A modern study of the natural selection of beak
size and beak depth of one of Darwin's Finches,
Geospiza fortis, by Peter and Rosemary Grant of
Princeton University on a small island, Daphne
Major, in the Galapagos Archipelgo shows how
a changing enivironment can cause natural
selection over a relatively short period of time.
MODERN STUDIES OF
DARWIN’S FINCHES
"Evidence that natural selection alters beak size in Geospiza fortis. In dry years,
when only large, tough seeds are available, the mean beak size increases. In wet years,
when many small seeds are available, smaller beaks become more common. " (from:
http://www.txtwriter.com/Backgrounders/Evolution/EVpage06.html)
Galápagos finches—natural selection on beak shape
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Finch beaks are their tools for opening seeds. Different seeds open best
with different beak size and features, which are inherited characteristics.
When all seed sizes were easily available in normal seasons, ground finches
(Geospiza fortis), shown here, with large and small beaks survived about
equally well. But in dry seasons, small, easy-to-open seeds were scarce.
Large, hard-to-open seeds were more plentiful. Predominantly, finches with
large, strong beaks could open the large seeds and survive (red), but those
with small beaks did less well (green).
THE PEPPERED MOTH
(Biston betularia) and Industrial
Melanism
(From
(From: http://www.txtwriter.com/Backgrounders/Evolution/EVpage07.html)
The moth occurs in two color phases, peppered and melanic. (a) Both phases are displayed
against an unpolluted, lichen-covered tree. (b) Both phases are displayed against a dark tree,
on which the lichen were killed by pollution.
Natural selection on snails
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Birds, such as this song thrush, hunt snails and break their shells
open against “anvil rocks” where debris collects. The snail
(Cepaea normalis) has several distinct color morphs, which are
camouflaged against different natural backgrounds.
Snail Selection
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The shell of the snail Cepaea occurs in three color phases: brown, pink, and
green. In different habitats-beech woodlands, meadows-different-colored
shells are common or rare. In deciduous woodlands, the frequency of the
color phases changes from spring to summer.
Natural Selection of Northern Water Snakes
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The northern water snake (Nerodia) occurs
throughout eastern North America. It is darkly
patterned, except on some islands in Lake Erie
where many are light colored.
Water Snake Differential Survival
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(a) Scored color phases A (light) to D (banded). In young born on the islands, most are
C or D (dark and banded). But by the time the snakes reach adult stage, most are A or
B (light colored). (b) On the islands, predaceous gulls feed on young snakes, usually
spotting and eating the more conspicuous banded snakes, producing differential
survival of mostly unbanded snakes. Occasional immigrants from the mainland return
some of the genes for banded color.
TYPES OF NATURAL
SELECTION
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Stabilizing Selection: The extremes of certain
characters (traits) within a population are
disadvantageous and eliminated.
Directional Selection: One extreme for certain
characters within a population are disadvantageous and
are selected against. (The Giraffe neck length example
we talked about before.)
Disruptive Selection: Selection favors the extremes
for particular traits rather than the central tendency,
which is selected against.
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The bell-shaped curve represents the distribution of a character in a
population. The shading indicates where in that variation selection acts to
eliminate individuals. (a) In stabilizing selection, extremes are eliminated,
leading to a narrowing of the variation. (b) In directional selection, one
extreme is eliminated, shifting the curve. (c) In disruptive selection,
individuals with intermediate variation are eliminated, producing two bell
shaped curves at the extremes.
SEXUAL SELECTION
Secondary sexual characteristics—kudu
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This male kudu, a resident of Africa, sports spiraled horns which
are part of its social display during the reproductive season.
Sexual dimorphism
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The large, male California sea lion, is distinctive
from the surrounding, smaller females.
Red winged blackbird
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This male red winged blackbird illustrates the
colorful shoulder feathers, which are displayed
during courtship and territorial defense.
Peacock
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The peacock has a luxuriant tail and bright body
used to attract the attentions of the female, the
peahen.
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The solid boxes represent the average number of young each group of males
produced during a summer; the vertical lines express the range of variation of the
results (standard deviation). The control groups did not differ significantly from each
other, but both differed from shortened males, which produced significantly fewer
offspring, and from elongated males, which produced significantly more offspring.
(After MØller, 1988)
Barn Swallows