Modern Biology II

Who Are You?

• Your major• Your year, part time or full time• Your plans• Your objectives• Your interests

Evolution• Evolution as fact and theory• Evolution is change; “descent with

modification”• Small scale - changes in gene frequency from

one population to the next• Large scale - descent of different species from

a common ancestor• What are we trying to explain?

– Adaptation: the good fit”of organisms to their environment

Evolution• Please read Chapter 20

(Genes Within Populations) if you have not done so already

• Know who Charles Darwin is…(naturalist on the HMS Beagle in 1831; The Origin of the Species; Galapagos Islands, etc)

Descriptions of the history of life on Earth

1. Separate Creation: no mechanism involved; independent creation (‘nothing to explain’)

2. Transformism: Lamarck; inheritance of acquired characteristics (e.g., giraffe’s neck); internal forces/unknown mechanisms cause organisms to produce offspring slightly different from themselves

3. Evolution: Darwinian Natural Selection; Survival of the Fittest

Descriptions of the history of life on Earth

Name Descent with modification? Common ancestor? Extinction?

Creation (A) No No No

Transformism (B) Yes No No

Evolution (C) Yes Yes Yes

change changechange

extinctiontime

And let’s not forget about Intelligent Design…

• Created by a subset of Creationists; offers an alternative to Creationism

• Avoids specifying the identity of a ‘designer’ • “Certain features of living things (and the

universe) are best explained by an intelligent cause, not an undirected process”

• Very Controversial!

…or the Church of the Flying Spaghetti Monster (FSM)

• Satirical protest to the Kansas Board of Education’s decision to require the teaching of Intelligent Design in public schools

• Founded in 2005• The FSM is invisible and undetectable; evidence

of evolution is planted by FSM to test the Pastafarian’s faith

Global Warming & Pirates

Evolution

• Evolution is descent with modification; living species are descendents of ancestral species that were different from present-day ones

• Evolution describes the genetic changes in a population over time

Evolution

• Organisms are not perfectly fit – a good fit, but not perfect

• As descendents of a remote ancestor spread into various habitats over millions and millions of years, they accumulated diverse modifications (adaptations) that fit them to specific ways of life; descent with modification (evolution!)

Evolution

• Natural variation among individuals is based on heredity (and mutation). These variations enable organisms to become adapted to their environment over time

Natural Selection

• Natural selection is the process by which favorable, inherited traits become more numerous in successive generations of a population of reproducing organisms

Natural Selection

• Over time, natural selection leads to species that are well adapted (highly evolved) to their environments

The Principles of Natural Selection

King Penguin Rookery © Momatiuk - Eastcott/Corbis

• Struggle for existence/Competition – More offspring are produced than can be supported

by resources

The Principles of Natural Selection• Variation

– Some individuals, due to heredity or mutation, possess characteristics which make them better

adapted to their environment

The Principles of Natural Selection• Inheritance of Traits

– Best-suited organisms will survive to produce more individuals that share same adaptation

Survival of the Fittest• Organisms are adapted to their environment

through natural selection• Natural selection is a pessimistic process

Population with varied inherited traits1

Elimination of individuals with certain traits2

Reproduction of survivors3

Certain individuals with a distinct, inherited characteristic will be selected against, while others with a (different) distinct, inherited trait will survive

Evidence for Evolution

• Microevolution – can be observed in nature; small changes in organism, generations changing over time

• Artificial selection – evolution can be experimentally produced

Microevolution• A well-known example of microevolution

involves the peppered Moth, Biston betularia in England during the industrial revolution

• Prior to the industrial revolution, light variants of the peppered moth survived better than dark variants because they blended well with the light colored trees– caused by the presence of a

light-colored lichen on the dark- colored bark

Microevolution• Pre-industrial era – only light variant known• During the industrial revolution, poor air

quality killed the lichens which covered the (otherwise dark) trees and camouflaged the light moths against predation

• In 1848, the first dark variant collected• By mid-1900’s, the dark variant made up 90%

of population in industrial areas!

Microevolution

Artificial Selection• Darwin got idea of natural selection by

artificial selection!• Modern corn looks very different from its

ancestor• Tumbler pigeons!!!

www.flickr.com/photos/terryandchristine/2399227035/www.flickr.com/photos/rinalia/

3285371111/

Artificial Selection• All dogs are domesticated breeds of the Gray

wolf, Canis lupus; “Fido” is actually a subspecies of the wolf!

Artificial Selection

Evidence for Evolution

• “Ring species” – series of neighboring populations that can

interbreed with closely-related populations, but two “dead ends” exist in the series that are too distantly-related to interbreed

Ring Species• Larus gulls – forms a ‘ring’

around the North Pole; European gull can breed with American, which can breed with Vega and so on…but the European gull cannot breed with the Lesser blacked-back gull at ‘end’ of the ring

Evidence for Natural Selection

• Darwin’s fishes of the Galapagos

14 species of finch; 1 common ancestor

(from the mainland); different beaks

Evidence for Evolution

• Homologous similarities: similarities between 2 species that is NOT functionally necessary

• Provide clues to common ancestry• Constraint is not there, may look the same, but

doesn’t have to• Example: Tetrapods; do not need 5 digits to

make flying wings, swimming structure, etc. • Common pentadactyl ancestor; limb adapted

into various ‘means’

Humerus

Radius

Ulna

Carpals

MetacarpalsPhalanges

Human Cat Whale Bat

Evidence for Evolution• Homologous structures – similar characteristics

that result from a common ancestry

The Panda’s Thumb

• The panda’s thumb is a homologous trait; modification of the wrist bone, not anatomically a finger (or thumb) at all

• Constructed from the radial sesamoid, normally a small component of the wrist

• May have originated from a single genetic change (mutation)

The Panda’s Thumb

Homologies

• Fossil evidence of evolution: Whale ‘missing link”

• Vestigial structures – no apparent function, but resemble structures ancestors possessed

Homologies

Vestigial structures of a whale

http://www.edwardtbabinski.us/mpm/mpm_whale_limb.html

Evidence for Evolution

Evolutionary View of Homology

• Use the starting materials and processes already available

• Fashion adaptations, rather than starting from scratch

• Evolution is a tinkerer, not a master engineer

Evolutionary View of Homology

• Adaptations are NOT perfect – it is the imperfection of adaptation that, instead, gives evidence for evolution (a tinkerer uses tools already there to improve)

• Different homologies are correlated – similar patterns between human, ape, and monkey for many proteins

• Similarities stem from common ancestor

Anatomical Evidence of Evolution• Orchid petals - used as pollinator lure• Snake with 2 leg bones• Manatee fingernails• Humans – muscles for wiggling ears

Anatomical Evidence of Evolution• Developmental similarities

reflect descent from a common ancestor

• “Ontogeny recapitulates phylogeny”!

• (Ontogeny = growth and development of an organism; Phylogeny = evolutionary history of a species)

Evidence for Evolution

• In contrast to homologous structures, analogous structures also provide evidence for evolution

• Analogous structures are structures that share similarities by a way of life, not by a common ancestry

• Analogous structures arise among unrelated organisms in response to similar needs or similar environmental factors

Analogous structuresExamples: wings of insects (a) and birds (b); flippers of seals (c) and penguins (d)

Origin of New Species

• Individuals do NOT evolve• Populations are the smallest units that can

evolve• A population is a group of interbreeding

individuals belonging to a particular species sharing a common geographic area

Origin of New Species

• What is a species?• Biological species concept: a population or

group whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot interbreed with other such groups

• Species are based on ability to interbreed – NOT on physical similarities

Origin of New Species• Example: Eastern and Western Meadowlarks – 2

different species with similar shape and coloration, but differences in song help prevent interbreeding

• Barriers to breeding can be behavioral

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Origin of New Species

• In contrast, humans have considerable diversity, but we all belong to the same species because of our ability to interbreed

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Speciation happens• It takes a reproductive barrier to keep

individuals of closely-related species from interbreeding

• Reproductive barriers may be behavioral, geographical, anatomical, or temporal

Allopatric Speciation

• How do reproductive barriers arise?• One of the clearest forms of species is

allopatric speciation caused by a geographic barrier

• When a geographic barrier occurs, the isolated populations each become adapted to their own environment, such that over time, they may no longer interbreed ( speciation)

Speciation• Reasons for geographic

(allopatric) isolation• Example of allopatric isolation:

antelope squirrels on south and north rim of Grand Canyon

Speciation

Speciation

• Prezygotic vs. Postzygotic• Prezygotic – mechanisms preventing

formation of a zygote (ecological, behavioral, temporal, mechanical)

• Postzygotic – mechanisms preventing organisms from developing into a reproducing adult

Prezygotic examples• Ecological – Lions and

Tigers• Behavioral – blue-

footed boobies• Temporal – Wild

lettuce (different blooming periods)

• Mechanical - insects

Postzygotic example

• Hybrid inviability or infertility – mule• Mules are the reproductive result of a horse

and a donkey breeding• Mules are sterile • Therefore, a horse and a

donkey must represent 2 distinct species

Sympatric Speciation• Sympatric speciation – the process by which

new species arise within the range of another species

• More controversial• In this case, a new species does not arise from

geographic isolation• Instead, a new species may arise by accident

when errors during cell division resulted in organisms with extra sets of chromosomes (very common in plants)

Sympatric Speciation

• New species formed by having extra sets of chromosomes are considered to be polyploid

• Polyploid organisms have more than 2 complete sets of chromosomes

• For example, a polyploid containing 4 sets of chromosomes will produce diploid (2n) gametes! This species would be unable to mate with normal diploid species (which produce haploid gametes)

Parent speciesDiploid

Polyploid (“tetraploid”)

1

Diploidgametes

2

Viable, fertiletetraploidspecies

Self-fertilization

3

Error in cell

division

Polyploid cells undergo

meiosis

Polyploidy by error in cell division and self-fertilization

Species A2n = 4

Gameten = 2

1 2

Species B2n = 6

Gameten = 3

Sterile hybridn = 5

Chromosomes nothomologous(cannot pair)

Viable, fertilehybrid species

2n = 10

3

3. However, ‘sterile’ hybrid can reproduce asexually (as

many plants do), and if subsequent errors in cell division occur,

chromosome duplications can resultin a fertile polyploid species!

Polyploidy by errors in cell division of a ‘sterile’ hybrid

Polyploid speciation

• Remember, polyploidy is a type of sympatric speciation

• As many as 80% of all living plants today are believed to have arisen by polyploidy!

• A polyploid contains twice (or sometimes more) the genetic diversity as its diploid predecessors, which provides an adaptive advantage!

Polyploid speciation

• Many of the plants grown for food are polyploids– Oats and Barley– Potatoes– Bananas– Peanuts– Plums and Apples– Wheat– Coffee!

Speciation

Macroevolution

• Origin of taxonomic groups higher than the species level

• Evolutionary change substantial enough to view its products as new genera, families or phyla

• Has a random component• Considers major evolutionary innovations –

bird feathers, insect wings• Extinctions and Radiations

Macroevolution

Adaptive Radiation

• The evolution of many diverse species from a common ancestor is called adaptive radiation

• The adaptations of these species allow them to fill new habitats or roles in their communities (“niches”)

• New phenotypes arise in response to the environment, driven by natural selection

Adaptive Radiation

• Example: Development of a fourth cusp in mammalian tooth – increases range of food which can be utilized

• Adaptive radiation typically occurs when a few organisms colonize new, unexploited habitats, or when environmental changes open up new opportunities for the survivors

Adaptive Evolution

• The Galapagos Islands is one of the world’s greatest showcases of adaptive radiation

• Each island arose ‘naked’ from underwater volcanoes and were gradually clothed by plants, animals and micro-organisms which strayed from the South American mainland

Darwin’s finches are a prime example of adaptive evolution

Rise of the mammals• The extinction of the dinosaurs provided a

tremendous evolutionary opportunity to mammals, who once lived in their shadows

Macroevolution• Mass extinctions

Macroevolution

• Fossil record provides clues for the outline of macroevolution

• Extant (living) species also supply clues• What about soft-bodied organisms which do

not leave a fossil record?• Does evolution occur in ‘fits’ and ‘starts’ or is

our understanding of historical accounts skewed?

Suggested Readings

• The Selfish Gene, Richard Dawkins• The Blind Watchmaker, Richard Dawkins• Unweaving the Rainbow, Richard Dawkins• The Panda’s Thumb (and any other collection

of stories by), Stephen J. Gould

…some light reading for a spring night.