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Chapter 18: Evolution and The Origin of Species

18.1: Introduction

• What does evolution mean to you?

• All species of living organisms, from bacteria to baboons to blueberries, evolved at some point from a different species.

• Evolution is an ongoing process for living things.

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Figure 18.1

• All organisms are products of evolution adapted to their environment. (a) Saguaro (Carnegiea gigantea) can soak up 750 liters of water in a single rain storm, enabling these cacti to survive the dry conditions of the Sonora desert in Mexico and the Southwestern United States. (b) The Andean semiaquatic lizard (Potamites montanicola) discovered in Peru in 2010 lives between 1,570 to 2,100 meters in elevation, and, unlike most lizards, is nocturnal and swims. Scientists still do no know how these cold-blood animals are able to move in the cold (10 to 15°C) temperatures of the Andean night. (credit a: modification of work by Gentry George, U.S. Fish and Wildlife Service; credit b: modification of work by Germán Chávez and Diego Vásquez, ZooKeys)

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• Evolution– Definition: How an entity changes through time

“Through time, species accumulate differences; as a result, descendants differ from their ancestors. In this way, new species arise from existing ones.”– Charles Darwin

Genetic Variation and Evolution

Figure 18.2

• Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources.

• Darwin was not the first to propose a theory of evolution, but what WAS new was the mechanism he proposed.

– Darwin proposed “natural selection” as the mechanism of evolution.

– Alfred Wallace studied wildlife in South American and Asia and decided to seek Darwin’s help in publishing his own ideas on evolution.

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Figure 18.3

• Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858.

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Reproduction

Reproduction

Individuals pass on their traits to next generation.

Darwin’s theory: natural selection leads to evolutionary change.

Over many generations, longer-necked individuals are moresuccessful, perhaps because they can feed on taller trees, andpass the long-neck trait on to their offspring.

Naturalselection

Some individuals born happen to have longer necks due togenetic differences.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 18.4

• A field biologist tranquilizes a polar bear for study. (credit: Karen Rhode)

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Genetic variation– Definition: Differences in alleles of genes found within

individuals in a population– Variation must be already present before natural

selection can occur

How does genetic variation arise in a population?– New alleles arise from existing alleles by mutation– Sexual reproduction creates new combinations of

alleles in organisms • recombination of alleles in meiosis and fusion of 2 unique

gametes in fertilization

Processes and Patterns of Evolution

Processes and Patterns of Evolution

• Adaptation: a heritable trait that helps the survival and reproduction of an organism in its present environment.– How have we as humans adapted to our

environments?• Divergent evolution: when two species evolve

in diverse directions from a common ancestor. (i.e. Figure 18.5).

• Convergent evolution: when similar traits evolve independently in a species that do not share a common ancestor (i.e. flying).

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Figure 18.5

• Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing star (Liatrus spicata) and the (b) purple coneflower (Echinacea purpurea) vary in appearance, yet both share a similar basic morphology. (credit a: modification of work by Drew Avery; credit b: modification of work by Cory Zanker)

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5 agents of evolutionary change

Selection

– Some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype, which is affected by genotype

– Two types of selection• Artificial selection• Natural selection - the main driver of

adaptive evolution!

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Evidence of Evolution

• Fossils

• Anatomy and Embryology

• Biogeography

• Molecular Biology

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Fossil Evidence of Evolution• Fossils are the preserved remains of once-

living organisms– The age of fossils can be estimated

• Rock fossils are created when three events occur– Organism buried in sediment– Calcium in bone or other hard tissue

mineralizes– Surrounding sediment hardens to form rock

• Process of fossilization is rare event 16

Figure 18.6

• In this (a) display, fossil hominids are arranged from oldest (bottom) to newest (top). As hominids evolved, the shape of the skull changed. An artist’s rendition of (b) extinct species of the genus Equus reveals that these ancient species resembled the modern horse (Equus ferus) but varied in size.

Anatomical Evidence for Evolution

• Homologous structures

– Structures with different appearances and functions that all derived from the same body part in a common ancestor

– The bones in the forelimb of mammals are homologous structures

– May have different functions (or the same), but evolved from the same ancestral structure 18

Figure 18.7

• The similar construction of these appendages indicates that these organisms share a common ancestor.

• Vestigial structures– Have no apparent function, but resemble

structures their ancestors possessed• Human post-anal tail• Hip bones in boa constrictors and whales• Wings on flightless birds; leaves on cacti 20

• Early embryonic development– Strongest anatomical evidence

supporting evolution comes from comparisons of how organisms develop

– Embryos of different types of vertebrates, for example, often are similar early on, but become more different as they develop

– Early vertebrate embryos possess pharyngeal pouches that develop into

• In humans: glands and ducts• In fish: gill slits

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TurtleChick

Mouse Human

Convergent evolution

• Organisms that share similar environments and similarities occur because of similar selective pressures—benefits of not being seen by predators.

– For example: Artic fox and Ptarmigan both live in the Artic region and have been selected for white phenotypes.

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Figure 18.8

• The white winter coat of the (a) arctic fox and the (b) ptarmigan’s plumage are adaptations to their environments. (credit a: modification of work by Keith Morehouse)

Biogeography• Study of the geographic distribution of

species

• Reveals that different geographical areas sometimes exhibit groups of plants and animals of strikingly similar appearance, even though the organisms may be only distantly related

• Natural selection appears to have favored parallel evolutionary adaptations in similar environments

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Molecular Biology

• Evidence of common ancestor for all life is found in DNA.

• DNA sequences have also shed light on some of the mechanisms of evolution.

– i.e. new functions of proteins.

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Misconceptions of Evolution• Evolutionary theory is nearly universally

accepted by biologists• Source of controversy for some in the general

public (Often caused by lack of knowledge regarding what the theory of evolution states, and what it does not state!)

• You need to visit this site (hyperlink below) and read it all the way through to the “Quick Quiz.” Some basic questions about this will appear on Exam 1.

• http://www.evolution.berkeley.edu/evosite/misconceps/index.shtml

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18.2: Formation of New Species

• A species is composed of populations whose members mate with each other and produce fertile offspring– One specie is distinguished from another

when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

• The more DNA they have in common

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Figure 18.9

• The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo. (credit a: modification of work by Sally Eller, Tom Reese; credit b: modification of work by Jeremy McWilliams; credit c: modification of work by Kathleen Conklin)

Hybrids

• Hybrids are sterile

– Abnormal sex organs

– Failure to form gametes

(i.e. African fish eagle and bald eagle appear similar in appearance but if humans were to artificially intervene these two species, the offspring would be a hybrid).

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Figure 18.10

• The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species. (credit a: modification of work by Nigel Wedge; credit b: modification of work by U.S. Fish and Wildlife Service)

Speciation• Speciation is a 2-part process

– The formation of two species form one original species.• They must evolve in a way that it

becomes impossible for individuals from the two new populations to interbreed.

– Darwin envisioned this process as a branching event (see Figure 18.11a).

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Figure 18.11

• The only illustration in Darwin's On the Origin of Species is (a) a diagram showing speciation events leading to biological diversity. The diagram shows similarities to phylogenetic charts that are drawn today to illustrate the relationships of species. (b) Modern elephants evolved from the Palaeomastodon, a species that lived in Egypt 35–50 million years ago.

Allopatric speciation

• Geographically separated, or allopatric, populations appear much more likely to have evolved substantial differences leading to speciation

• The northern spotted owl and the Mexican spotted owl

– Isolated populations are strikingly different from each other

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Figure 18.12

• The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of incipient speciation. (credit “northern spotted owl”: modification of work by John and Karen Hollingsworth; credit “Mexican spotted owl”: modification of work by Bill Radke)

Adaptive radiations• Closely related species that have

recently evolved from a common ancestor (founder species) by adapting to different parts of the environment

• Occurs – In an environment with few other

species and many resources• The Hawaiian honeycreeper birds

– Catastrophic event leading to extinction of other species

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Figure 18.13

• The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics.

Sympatric speciation• Can divergence occur if no physical barriers

are in place to separate individuals who continue to live and reproduce in the same habitat?

• One species splits into two at a single locality, without the two new species ever having been geographically separated

• One type occurs commonly as the result of polyploidy– Individuals that have more than two sets of

chromosomes39

• Ways polyploidy occurs:– Aneuploidy

• Error in chromosome separation and the end cell product has too many or too few individual chromosomes.

– Autopolyploidy• Error resulting in two sets of chromosomes• Error in cell division produces a tetraploid (4n)

– Allopolyploidy

• Two species hybridize• Resulting offspring have one copy of the

chromosomes of each species• Infertile: cannot reproduce with either species –

can’t produce gametes (example: mule)• Can reproduce asexually OR can become

fertile if chromosomes spontaneously doubled (polyploidy) 40

Figure 18.14

• Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis. In the example shown here, the resulting offspring will have 2n+1 or 2n-1 chromosomes

Figure 18.15

• Autopolyploidy results when mitosis is not followed by cytokinesis.

Figure 18.16

• Alloploidy results when two species mate to produce viable offspring. In the example shown, a normal gamete from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring.

The biological species concept focuses on the ability to exchange genes.

Members of separate species remain separate because gene exchange between the 2 species is somehow blocked.

– Prezygotic isolating mechanisms• Mechanisms that prevent formation of a

zygote

– Postzygotic isolating mechanisms• Mechanisms that prevent development

into a fertile adult 44

• Reproductive isolating mechanisms

– Prezygotic isolating mechanisms• Habitat isolation-species are moved to a

new habitat and rarely encounter one another

• Behavioral isolation-species differ in mating rituals

• Temporal isolation-differences in breeding schedules

• Gamete barrier-prevent fertilization from taking place

– Postzygotic isolating mechanisms• Hybrid inviability or infertility 45

Figure 18.17

• These two related frog species exhibit temporal reproductive isolation. (a) Rana aurora breeds earlier in the year than (b) Rana boylii. (credit a: modification of work by Mark R. Jennings, USFWS; credit b: modification of work by Alessandro Catenazzi)

Figure 18.18

• Speciation can occur when two populations occupy different habitats. The habitats need not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, and the cricket (b) Gryllus firmus prefers loamy soil. The two species can live in close proximity, but because of their different soil preferences, they became genetically isolated.

Figure 18.19

• The shape of the male reproductive organ varies among male damselfly species, and is only compatible with the female of that species. Reproductive organ incompatibility keeps the species reproductively isolated.

Figure 18.20

• Some flowers have evolved to attract certain pollinators. The (a) wide foxglove flower is adapted for pollination by bees, while the (b) long, tube-shaped trumpet creeper flower is adapted for pollination by humming birds.

Figure 18.21

• Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric speciation. Lake Apoyeque, a crater lake, is 1800 years old, but genetic evidence indicates that the lake was populated only 100 years ago by a single population of cichlid fish. Nevertheless, two populations with distinct morphologies and diets now exist in the lake, and scientists believe these populations may be in an early stage of speciation.

18.3: Reconnection and Rates of Speciation

• Two species may recombine or even continue interacting indefinitely.

– Hybrid zone- an area where two closely related species continue to interact and reproduce, forming hybrids.

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Figure 18.22

• After speciation has occurred, the two separate but closely related species may continue to produce offspring in an area called the hybrid zone. Reinforcement, fusion, or stability may result, depending on reproductive barriers and the relative fitness of the hybrids.

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The Pace of Evolution• Gradualism

– Accumulation of small changes

– Standard view for a long time

• Punctuated equilibrium

– Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium

– Long periods of stasis followed by rapid change

• Gradualism and punctuated equilibrium are two ends of a continuum

Figure 18.23

• In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.