Evolution

Lecture 10

Copyright © 2012 Joe Marx.

Evolution Page 2

Topics for This Lecture• What is biological evolution?

– On what material does evolution act?– How does evolution proceed?– What are the products of evolution?

• What kinds of evidence initially suggested the idea of evolution?

• What large evolutionary patterns can be seen in the fossil record?

• What is the importance of evolution to biology and paleontology?

EvolutionImage courtesy of University of Michigan

Exhibit Museum of Natural History. Page 3

What is Evolution?• Descent with modification.

– All biological species are the modified descendants of previously existing species.• Evolution explains how populations change over time,

not how individuals change during their lifetimes.

No plant species in this artist’s conception of a Pennsylvanian

coal swamp exists today, but their modified descendents can be

found today in the Mid-Atlantic forest. ►

Image courtesy of National Institutes of Health.Evolution Page 4

Genes• The raw material of evolution.

– Genes are chemical structures packaged into lineal sequences, called chromosomes, within cells.• Genes serve as the basic units for the transmission and

development of life’s hereditary characteristics.• When copied, genes sometimes undergo random

variations called mutations.

◄ Schematic of a gene in relation to the double

helix structure of DNA and to a chromosome.

Evolution Image courtesy of National Institutes of Health. Page 5

Sexual Recombination• The mixing of

chromosomes between generations.– Occurs when a male

and a female each contributes half of the genetic material passed to their offspring.

– Continuously creates new genetic combinations on which natural selection can act.

Schematic of sexual recombination.

Evolution Page 6

Natural Selection• The mechanism of evolution.

– Natural selection is the process by which advantageous traits spread through a reproductive population:1. Genetic variability occasionally produces traits that

make their possessors more likely to survive and reproduce.

2. These favorable traits increase in frequency with each generation.

3. Given enough time, this passive process results in adaptation and speciation.

– Natural selection results from the struggle for existence—the competition among organisms for a limited supply of resources within ecosystems.

Levels of Selection• Natural selection operates at many levels:

– Obvious levels include:• Ecological selection—Does the individual survive long

enough to reproduce?• Sexual selection—Does the individual attract a partner?

– Less obvious levels include:• Viability selection—Are an individual’s gametes viable?• Compatibility selection—Are an individual’s gametes

compatible with its partner’s?• Survival selection—Does an individual reproduce more

than once?– The combined effect of all selection pressures

determines the overall fitness of an individual, and hence the outcome of natural selection.

Evolution Page 7

Evolution Page 8

Adaptation• The modification, through natural selection, of a

group of organisms so that it becomes better fitted to persist within its environment.– Adaptations—structural or behavioral—enable an

organism to:• Get air, water and food.• Cope with its physical and biological environment.• Acclimatize itself to new conditions.• Reproduce.

– Natural selection does not necessarily lead to adaptation.• By eliminating the less fit, selection often results simply

in the maintenance of the status quo.

Evolution Page 9

Speciation• The process, operating through natural

selection, by which new biological species arise.– Varies in mode, depending on the degree of

geographic separation between populations:1. Complete isolation (e.g., island endemics).

2. Partial isolation by ecological niche (e.g., ring species).

3. Isolation only by behavior (e.g., the 19th-century development in North America of apple-feeding maggots from hawthorn-eating maggots).

– May be induced artificially, through plant breeding (e.g. wheat for bread making), animal husbandry (domestic sheep) or laboratory experiments (new Drosophila species).

Evolution Page 10

• An English naturalist who became convinced, during a five-year scientific expedition on the surveying and research vessel H.M.S Beagle, that evolution by natural selection had produced all the species that have lived on Earth.– Until the middle of the 19th century,

most scientists considered biological species to be essentially immutable.

– I

Charles Darwin

Darwin in his middle 60’s.

– In 1859, after nearly thirty years of further research and consideration, Darwin published On the Origin of Species by Natural Selection.

Image in the public domain.

Evolution Page 11

Darwin’s Evidence• In his book, Darwin amassed numerous strands

of evidence to support his evolutionary hypothesis, for example:– Humans have been able to selectively breed plants

and animals into a multitude of different forms.– Closely related species often occupy discrete

geographic regions separated by barriers.– Animal species whose adults are dissimilar often

share features as embryos.– Organs of similar form but varying function occur

in many animal species.– Some animals possess useless organs that

resemble working organs in other species.•

EvolutionLeft-hand image courtesy of Wikimedia.

Right-hand image courtesy of U.S. Department of Agriculture. Page 12

Artificial Selection• Humans have created dramatic variations in

domesticated plants and animals by breeding for desirable characteristics.

Chihuahua and Great Dane. Carrots bred for color.

Evolution Image from Darwin, C., 1845, The voyage of the Beagle. Page 13

Geographical Separation• Similar species often occupy discrete territories

that are separated by land or water barriers.– Darwin collected thirteen

species of Galápagos Island finches, all of similar size and color. The birds differed in behavior, melody, and the size and shape of their beaks, reflecting differences in food and habitat. All the finches were shown by a zoologist to be simple variations of a single South American species.

Four species of finch from the Galápagos.

Evolution Page 14

Embryology• Many stages in the development of vertebrate

embryos are similar, even when the adult forms differ greatly.– For example, the embryos of all placental

mammals—including humans—form a yolk sac. In egg-laying animals, yolk sacs contain food to support embryonic development. In placental mammals, the yolk sacs are empty.

– Similarly, all vertebrate embryos develop gill slits. The slits develop into gills in animals whose adult forms breathe water. In non-aqueous vertebrates, they develop into such organs as jaws, throat and ears.

Image courtesy of Wikipedia.Evolution Page 15

Homology• A fundamental similarity between structures used

for different purposes in different organisms.

EvolutionLeft-hand image from G.Mützel, 1927: Brehms Tierleben.

Right-hand image courtesy of Wikipedia. Page 16

Vestigial Structure• An anatomical feature which has lost most or all

of its original function.– Typically persists in a degenerate, atrophied, or

rudimentary condition or form.

The blind mole rat has tiny eyes completely covered by skin.

Stress-induced goose bumps in humans are a vestigial reflex.

Originally, the bumps lifted hair to make our ancestors look more

formidable to predators.

After 150 Years• Not a single piece of evidence collected by later

scientists has contradicted Darwin’s contention that organisms evolved over geologic time.– Newly discovered fossils at all locations have

shown a continuous, consistent, unidirectional progression of life forms.

– Genetics and molecular biology, which did not exist in Darwin’s day, have revealed the biological and chemical mechanisms that undergird natural selection.

– Genomics has revealed that genes are not static objects but rather records of accumulated change.

– New species have been seen to appear in the wild.

Evolution Page 17

Evolution Page 18

Evolutionary Patterns• From Darwin’s time to the middle of the 20th

century, most biologists and paleontologists envisioned evolution as a gradual process.– Gradualistic evolution asserts that evolutionary

change is continuous, leading by small steps to eventual speciation.

• Today most scientists consider evolution to be a somewhat jumpy or episodic process.– Punctuated evolution asserts that new taxa appear

relatively quickly following major adaptational shifts but that, once evolved, taxa tend to remain relatively unchanged for long periods.

Image by Brian Gratwicke, courtesy of Wikipedia.Evolution Page 19

Living Fossil• An animal or plant, presently alive, known by

fossil evidence to have undergone relatively little modification for a considerable geologic time.– Examples include the horseshoe crab, ginkgo,

dawn redwood and the bowfin fishes.

◄ Horseshoe crabs, Chesapeake Bay, St. Michaels, Maryland. Horseshoe crabs are one of the oldest classes of marine

arthropods and have changed very little in 445 million years.

Evolution Page 20

Adaptive Radiation• Rapid speciation by a small number of species to

fill many ecological niches.– Occurs because:

• A key adaptation provides a distinct advantage.– Modern corals proliferated in the early Mesozoic because

their more porous skeletons and their symbiosis with algae gave them an edge over their slower-growing competitors.

• Extinction empties whole niches or a lineage reaches an island with unoccupied niches.

– Early Cenozoic mammals replaced the vanished dinosaurs.

– Galápagos finches evolved from a single mainland finch.• Specialization subdivides a single niche into many.

– Five hundred specialized species of cichlid fishes have evolved in Lake Victoria during the last 13,000 years.

Image courtesy of Wikipedia.Evolution Page 21

Evolution of the Horse• An example of a relatively

quick adaptive radiation.– Driven by a habitat

change from forest to grassland.

– Main trends included:• Increases in overall size.• Lengthened limbs and

feet.• Increased length and

thickness of the third digit, with a reduction of the other digits.

• Increases in tooth length.

Evolution Page 22

Molecular Clock• A technique used by genetic researchers to date

when two species diverged from each other.– Assumes that mutations accumulate at a fixed rate.– Deduces the time since divergence by counting the

number of differences between the genetic codes of the two species.

• Relies on statistical inference, so does not offer direct evidence of the divergence, unlike fossils.

• Useful for studying taxonomic relationships in groups with poor fossil records.– Indicated that the human family diverged from

ancestral apes earlier than formerly thought, a conclusion confirmed by subsequent fossil finds.

EvolutionMiddle image courtesy of U.S. Fish & Wildlife Service.

Other images courtesy of Wikipedia. Page 23

Convergent Evolution• A process whereby organisms that are not

closely related to each other evolve similar traits as adaptations to similar environments or niches.

◄ Euphorbia obesa (top) and Astrophytum asterias belong to different plant families but have evolved similar solutions for surviving in a hot, dry environment.

The Old World Nubian vulture (left) is in the hawk family. The New World turkey vulture is

related to herons and storks. ▼

Evolution Page 24

Extinction• The total, worldwide

disappearance of a species or a higher taxon.– Occurs when a taxon, unable to

survive in its current environment, cannot migrate to more favorable conditions.

– Pseudoextinction is the loss of a species because it has evolved into another species.

– Most species that have existed during Earth’s history are now extinct.

The passenger pigeon, a North American bird hunted to extinction.

Image courtesy of Wikipedia.Evolution Page 25

Rates of Extinction• Vary greatly by geologic age and by taxon.

– In Earth history, there have been a number of mass extinctions—sharp decreases in species numbers within a relatively short period of time.

– A human-caused Holocene mass extinction is in progress.

Genus extinction intensity for commonly preserved marine

genera. The intensity is the fraction of genera that are present during

an interval of time but do not exist in the following interval. The yellow line shows the long-term trend. The five major mass extinctions of the past are shown in large font. ►

Image courtesy of Wikimedia Commons.Evolution Page 26

Dollo’s Law• An evolving group of

organisms, or a part of an organism, cannot return to an ancestral condition.– This irreversibility of

evolution occurs because probability prevents the necessary complex sequences of genetic change from repeating in reverse order.

– Provides the principal basis for the relative dating of rock.

It’s just as well that Tyrannosaurus rex can’t re-evolve!

Image courtesy of Wikimedia Commons.Evolution Page 27

The Tree of Life• Assumes that life has developed over geologic

time from one or a few simple organisms to all the more complex organisms.– Supported by the fact that all known forms of life

have the same basic biochemical organization and employ a nearly identical genetic code.

Phylogenetic tree of living organisms.

Evolution Page 28

Evolution by Natural Selection• The conceptual foundation of modern biology.• A scientific theory supported by evidence as firm

as that which supports the theory of gravity.• The best explanation available for:

– The history of life.– The similarities and differences among organisms.– The geographic distribution of past and present

organisms.• Would be falsified simply by “the discovery of a

fossil rabbit in a Precambrian rock.”• Nothing in biology—or in paleontology—makes

sense without evolution.