NATURAL NATURAL SELECTIONSELECTION

Chapter 7 in KardongChapter 7 in Kardong

INTRODUCTIONINTRODUCTION

          Organisms must survive the biotic and Organisms must survive the biotic and abiotic factors within their environment if they abiotic factors within their environment if they are going to survive to sexual maturity.  But are going to survive to sexual maturity.  But evolutionary success is not just dependent on evolutionary success is not just dependent on survival, evolutionary fitness is known to be survival, evolutionary fitness is known to be associated with reproductive success.  The associated with reproductive success.  The more offspring that an individual leaves for the more offspring that an individual leaves for the next generation, the more fit the individual.  At next generation, the more fit the individual.  At the level of the gene, fitness is measured by the the level of the gene, fitness is measured by the success of one genotype (or individual gene - success of one genotype (or individual gene - allele) over another genotype (or individual allele) over another genotype (or individual gene - allele). gene - allele).

SELECTION WORKS ON SELECTION WORKS ON THE PHENOTYPE THE PHENOTYPE

          The biotic and abiotic factors within The biotic and abiotic factors within the environment apply the environment apply selection selection pressurepressure to the phenotype, not the to the phenotype, not the genotype.  However, the environmental genotype.  However, the environmental factors working on the phenotype will factors working on the phenotype will result in certain phenotypes having result in certain phenotypes having greater reproductive success than other greater reproductive success than other phenotypes, thus the genotype phenotypes, thus the genotype ultimately changes.  Therefore, the gene ultimately changes.  Therefore, the gene pool changes - thus evolution will occur pool changes - thus evolution will occur as a result of selection pressures. as a result of selection pressures.

ARTIFICIAL SELECTIONARTIFICIAL SELECTION           Darwin studied the artificial selection practiced by Darwin studied the artificial selection practiced by

breeders of animals and plants to determine if and breeders of animals and plants to determine if and how particular traits (admired by humans, for how particular traits (admired by humans, for agricultural purposes, beauty, etc.) were passed on to agricultural purposes, beauty, etc.) were passed on to offspring (inherited).  Of course, as animal and plant offspring (inherited).  Of course, as animal and plant breeders had demonstrated, certain desirable traits breeders had demonstrated, certain desirable traits of animals and plants (as perceived by humans) were of animals and plants (as perceived by humans) were heritable and passed on to offspring, thus undesirable heritable and passed on to offspring, thus undesirable traits could be weeded out.  traits could be weeded out.  "This weeding out of "This weeding out of organisms by humans for human purposes is organisms by humans for human purposes is artificial selection."artificial selection." (Kardong, 2005). (Kardong, 2005).

Dogs and Cats Dogs and Cats Agriculture - corn, cabbage family, etc. Agriculture - corn, cabbage family, etc. Flowers (like the tulip, rose, etc.) Flowers (like the tulip, rose, etc.)

Diversity of dogs Diversity of dogs Artificial selection has produced Artificial selection has produced

numerous breeds of dogs, which diverged numerous breeds of dogs, which diverged from wolves about 15,000 years ago.from wolves about 15,000 years ago.

Corn Corn Artificial selection through the centuries evolved the Artificial selection through the centuries evolved the

modern male tassel and female ears of corn from wild grass. modern male tassel and female ears of corn from wild grass.

Corn varieties Corn varieties From left to right: popcorn, sweet corn, From left to right: popcorn, sweet corn,

flint corn, dent corn, and pod corn.flint corn, dent corn, and pod corn.

Tulips Tulips Tulips come in a variety of colors and stripes, but Tulips come in a variety of colors and stripes, but

never in a deep color of completely black. This never in a deep color of completely black. This variety does not exist, at least so far, within the variety does not exist, at least so far, within the species. species.

NATURAL SELECTIONNATURAL SELECTION             "The weeding out of organisms by biological "The weeding out of organisms by biological

processes, without deliberate or directed human processes, without deliberate or directed human intervention, is intervention, is natural selectionnatural selection."." (Kardong, 2005). (Kardong, 2005). Generally, natural selection is an average process that Generally, natural selection is an average process that works on the phenotypes in the population, ultimately works on the phenotypes in the population, ultimately resulting in the survival and reproductive success of resulting in the survival and reproductive success of phenotypes that are more fit for their environment. phenotypes that are more fit for their environment. Thus natural selection leads to a change in the Thus natural selection leads to a change in the genotypic frequencies in a population over time.  genotypic frequencies in a population over time. 

By human standards, natural selection is a very slow By human standards, natural selection is a very slow process, acting over hundreds of generations.  process, acting over hundreds of generations.  However, viruses and bacteria (because of very rapid However, viruses and bacteria (because of very rapid reproduction) may evolve significantly in human reproduction) may evolve significantly in human lifetimes (examples, the AIDs virus, antibiotic lifetimes (examples, the AIDs virus, antibiotic resistance in certain strains of bacteria).  There are resistance in certain strains of bacteria).  There are also examples of more complex organisms that have also examples of more complex organisms that have recently evolved by natural selection, such as the recently evolved by natural selection, such as the following: following:

DARWIN’S FINCHESDARWIN’S FINCHES

Darwin reasoned that the finches of the Darwin reasoned that the finches of the Galapagos Islands had evolved from an Galapagos Islands had evolved from an ancestral species from the mainland of ancestral species from the mainland of South America and with the passage of South America and with the passage of time the descendants on each of the time the descendants on each of the islands had been modified by natural islands had been modified by natural selection to occupy distinctive niches.  In selection to occupy distinctive niches.  In particular, the beaks of the various particular, the beaks of the various finches are specialized for a specific diet, finches are specialized for a specific diet, such as soft or hard seeds, insects, etc. such as soft or hard seeds, insects, etc.

DARWIN’S FINCHESDARWIN’S FINCHES

MODERN STUDIES OF MODERN STUDIES OF DARWIN’S FINCHESDARWIN’S FINCHES

A modern study of the natural selection A modern study of the natural selection of beak size and beak depth of one of of beak size and beak depth of one of Darwin's Finches, Darwin's Finches, Geospiza fortisGeospiza fortis, by , by Peter and Rosemary Grant of Princeton Peter and Rosemary Grant of Princeton University on a small island, Daphne University on a small island, Daphne Major, in the Galapagos Archipelgo Major, in the Galapagos Archipelgo shows how a changing enivironment shows how a changing enivironment can cause natural selection over a can cause natural selection over a relatively short period of time.relatively short period of time.

MODERN STUDIES OF MODERN STUDIES OF DARWIN’S FINCHESDARWIN’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 Galápagos finches—natural selection on

beak shapebeak shape Finch beaks are their tools for opening seeds. Different seeds open Finch beaks are their tools for opening seeds. Different seeds open best with different beak size and features, which are inherited best with different beak size and features, which are inherited characteristics. When all seed sizes were easily available in normal characteristics. When all seed sizes were easily available in normal seasons, ground finches (Geospiza fortis), shown here, with large and seasons, ground finches (Geospiza fortis), shown here, with large and small beaks survived about equally well. But in dry seasons, small, small beaks survived about equally well. But in dry seasons, small, easy-to-open seeds were scarce. Large, hard-to-open seeds were more easy-to-open seeds were scarce. Large, hard-to-open seeds were more plentiful. Predominantly, finches with large, strong beaks could open plentiful. Predominantly, finches with large, strong beaks could open the large seeds and survive (red), but those with small beaks did less the large seeds and survive (red), but those with small beaks did less well (green). well (green).

THE PEPPERED MOTHTHE PEPPERED MOTH((Biston betularia) Biston betularia) and and Industrial Melanism 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 Natural selection on snails snails Birds, such as this song thrush, hunt snails and break their Birds, such as this song thrush, hunt snails and break their

shells open against “anvil rocks” where debris collects. The shells open against “anvil rocks” where debris collects. The snail (snail (Cepaea normalisCepaea normalis) has several distinct color morphs, ) has several distinct color morphs, which are camouflaged against different natural which are camouflaged against different natural backgrounds.backgrounds.

Snail SelectionSnail Selection The shell of the snail The shell of the snail Cepaea Cepaea occurs in three color phases: occurs in three color phases:

brown, pink, and green. In different habitats-beech brown, pink, and green. In different habitats-beech woodlands, meadows-different-colored shells are common or woodlands, meadows-different-colored shells are common or rare. In deciduous woodlands, the frequency of the color rare. In deciduous woodlands, the frequency of the color phases changes from spring to summer.phases changes from spring to summer.

Natural Selection of Northern Natural Selection of Northern Water SnakesWater Snakes The northern water snake (The northern water snake (NerodiaNerodia) occurs ) occurs

throughout eastern North America. It is throughout eastern North America. It is darkly patterned, except on some islands in darkly patterned, except on some islands in Lake Erie where many are light colored.Lake Erie where many are light colored.

Water Snake Differential Water Snake Differential SurvivalSurvival (a) Scored color phases A (light) to D (banded). In young born on (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 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 snakes reach adult stage, most are A or B (light colored). (b) On the islands, predaceous gulls feed on young snakes, usually spotting and islands, predaceous gulls feed on young snakes, usually spotting and eating the more conspicuous banded snakes, producing differential eating the more conspicuous banded snakes, producing differential survival of mostly unbanded snakes. Occasional immigrants from survival of mostly unbanded snakes. Occasional immigrants from the mainland return some of the genes for banded color.the mainland return some of the genes for banded color.

TYPES OF NATURAL TYPES OF NATURAL SELECTIONSELECTION

Stabilizing SelectionStabilizing Selection: The extremes of : The extremes of certain characters (traits) within a population certain characters (traits) within a population are disadvantageous and eliminated.are disadvantageous and eliminated.

Directional SelectionDirectional Selection: One extreme for : One extreme for certain characters within a population are certain characters within a population are disadvantageous and are selected against. disadvantageous and are selected against. (The Giraffe neck length example we talked (The Giraffe neck length example we talked about before.)about before.)

Disruptive SelectionDisruptive Selection: Selection favors the : Selection favors the extremes for particular traits rather than the extremes for particular traits rather than the central tendency, which is selected against.central tendency, which is selected against.

The bell-shaped curve represents the distribution of a character in a population. The bell-shaped curve represents the distribution of a character in a population. The shading indicates where in that variation selection acts to eliminate The shading indicates where in that variation selection acts to eliminate individuals. (a) In stabilizing selection, extremes are eliminated, leading to a individuals. (a) In stabilizing selection, extremes are eliminated, leading to a narrowing of the variation. (b) In directional selection, one extreme is eliminated, narrowing of the variation. (b) In directional selection, one extreme is eliminated, shifting the curve. (c) In disruptive selection, individuals with intermediate shifting the curve. (c) In disruptive selection, individuals with intermediate

variation are eliminated, producing two bell shaped curves at the extremes.variation are eliminated, producing two bell shaped curves at the extremes.

SEXUAL SELECTIONSEXUAL SELECTION

Secondary sexual Secondary sexual

characteristics—kuducharacteristics—kudu This male kudu, a resident of Africa, sports This male kudu, a resident of Africa, sports spiraled horns which are part of its social display spiraled horns which are part of its social display

during the reproductive season.during the reproductive season.

Sexual dimorphism Sexual dimorphism The large, male California sea lion, is distinctive The large, male California sea lion, is distinctive

from the surrounding, smaller females. from the surrounding, smaller females.

Red winged blackbird Red winged blackbird This male red winged blackbird illustrates This male red winged blackbird illustrates

the colorful shoulder feathers, which are the colorful shoulder feathers, which are displayed during courtship and territorial displayed during courtship and territorial defense. defense.

Peacock Peacock The peacock has a luxuriant tail and The peacock has a luxuriant tail and

bright body used to attract the bright body used to attract the attentions of the female, the peahen. attentions of the female, the peahen.

Barn SwallowsBarn Swallows

The solid boxes represent the average number of young each group of The solid boxes represent the average number of young each group of males produced during a summer; the vertical lines express the range of males produced during a summer; the vertical lines express the range of variation of the results (standard deviation). The control groups did not variation of the results (standard deviation). The control groups did not differ significantly from each other, but both differed from shortened differ significantly from each other, but both differed from shortened males, which produced significantly fewer offspring, and from elongated males, which produced significantly fewer offspring, and from elongated males, which produced significantly more offspring. (After Mmales, which produced significantly more offspring. (After MØØller, 1988)ller, 1988)