copyright © 2009 pearson education, inc. ◦ adaptations: inherited traits that enhance an...

◦ Adaptations: inherited traits that enhance an organism’s ability to survive and reproduce

– Behavioral adaptations– Structural adaptations– Biochemical and physiological adaptations

◦ Evolution: descent with modification

DARWIN’S THEORY OF EVOLUTION

◦ Aristotle viewed species as perfect and unchanging

◦ Jean Baptiste de Lamarck (early 1800s) suggested that life on Earth evolves

Mechanism of change was use and disuse

Inheritance of acquired characteristics

A sea voyage helped Darwin frame his theory of evolution

NorthAmerica

ATLANTICOCEAN

GreatBritain

Brazil

TheGalápagosIslands

PACIFICOCEANPinta

MarchenaGenovesa

Santiago

Fernandina Pinzón

Isabela

SanCristobal

EspañolaFlorenza

DaphneIslands

SantaCruz Santa

40 miles

Equator

Europe

Africa

SouthAmerica

Argentina

Cape Horn

Cape ofGood Hope

PACIFICOCEAN

Equator

NewZealand

Australia

Tasmania

◦ 1831- Sets sail around the world at age 22

◦ 1840s- Writes first of essays describing evolution

◦ 1850s- Alfred Wallace writes theory identicle to Darwin

◦ 1859- publication of Darwin’s On the Origin of Species by Means of Natural Selection

◦ Theory:

Over time, present day species have arisen from ancestral species by natural processes (natural selection) or what is called descent by modification

Natural selection- the differential survival and reproduction of individuals within a population

Artificial selection= the selective breeding of plants and animals that possess desired traits

Terminalbud Lateral

Leaves

Brussels sprouts

Cauliflower

Cabbage

Kohlrabi

Wild mustard

Flowerclusters

Flowersand stems

Broccoli

Note these important points #1 Individuals do not evolve: populations

evolve

#2 Natural selection can amplify or diminish only heritable traits; acquired characteristics cannot be passed on to offspring

#3 Evolution is not goal directed and does not lead to perfection; favorable traits vary as environments change

Ex: Development of antibiotic resistance in bacteria

Methicillin-resistant S. aureus (MRSA)

Vancomycin-resistant Enterococci (VRE)

Invasive Group A Streptococcus (GAS)

E. coli and Salmonella

What evidence do we have for evolution?

Fossils: preserved remnants or impression of organism that lived in past

The fossil record shows that organisms have evolved in a historical sequence

– The oldest known fossils are what organism?– The oldest eukaryotic fossils are a billion years

younger– Multicellular fossils are even more recent

Example of geological strata- can be used to create a fossil record

Dickinsonia costata 2.5 cm

◦ Many fossils link early extinct species with species living today

Pelvis andhind limb

Rhodocetus (predominantly aquatic)

Pakicetus (terrestrial)

Dorudon (fully aquatic)

Balaena (recent whale ancestor)

Pelvis andhind limb

◦ Biogeography: the geographic distribution of species

Ex. Galapagos

◦ Comparative anatomy is the comparison of body structures in different species

◦ Homology is the similarity in characteristics that result from common ancestry

Humerus

Radius

Carpals

MetacarpalsPhalanges

Human Cat Whale Bat

Some homologous structures are vestigial organs

Structure of marginal or no importance to organism

Comparative anatomy and vestigial organs

Pelvis andhind limb

Rhodocetus (predominantly aquatic)

Pakicetus (terrestrial)

Dorudon (fully aquatic)

Balaena (recent whale ancestor)

Pelvis andhind limb

◦ Comparative embryology is the comparison of early stages of development among different organisms

– When you were an embryo, you had a tail and pharyngeal pouches (just like an embryonic fish)

Pharyngealpouches

Post-analtail

Chick embryo Human embryo

Molecular biology: Comparisons of DNA between different organisms reveal evolutionary relationships

– All living things share a common DNA code for the proteins found in living cells

– We share genes with bacteria, yeast, and fruit flies

– 96% same DNA between us and chimps!

Darwin was the first to represent the history of life as a tree

Tetrapod limbs

Amnion

Lungfishes

Feathers

Amphibians

Mammals

Lizardsand snakes

Hawks andother birds

Ostriches

Crocodiles

Tetrap

THE EVOLUTION OF POPULATIONS

Population: A group of individuals of the same species living in the same place at the same time

Evolution is the change in heritable traits in a population over generations

Gene pool: the total collection of genes in a population at any one time

Microevolution is a change in the relative frequencies of alleles in a gene pool over time

What is necessary for evolution to happen???

Mutation: change in the nucleotide sequence of DNA the ultimate source of new alleles

– Point mutation (single nucleotide change) or frameshift (can be multiple nucleotide change)

– Results in missense, nonsense or silent mutation

– Every organism has spontaneous mutation rate

– Mutagens increase this mutation rate• Chemicals (caffeine, AZT, aflatoxin)• UV radiation• Ionizing radiation

Chromosomal duplication : If a gene is duplicated, the new copy can undergo mutation without affecting the function of the original copy

– Example: olfactory receptors in mammals

Sexual reproduction shuffles alleles to produce new combinations

How does sexual reproduction (including meiosis) lead to genetic

variation?

Sexual reproduction shuffles alleles to produce new combinations

1. Homologous chromosomes sort independently as they separate during anaphase I of meiosis

2. During prophase I of meiosis, pairs of homologous chromosomes cross over and exchange genes

3. Further variation arises when sperm randomly unite with eggs in fertilization

Parents

Offspring,with newcombinationsof alleles

Gametes

Meiosis

Randomfertilization

A1 A2 A3

A3A1A2A1

◦ Sexual reproduction alone does not lead to evolutionary change in a population

– Alleles are shuffled, BUT the frequency of alleles and genotypes in the population does not change

◦ For a population to be in equilibrium for a specific trait, it must satisfy five conditions:

1. Very large population

2. No gene flow between

3. No mutations

4. Random mating

5. No natural selection

A population in equilibrium

◦ The Hardy Weinberg principle:

Allele and genotype frequencies within a sexually reproducing, diploid population will remain in equilibrium unless outside forces act to change those frequencies

Often referred to as Hardy Weinberg equilibrium

◦ EXAMPLE: Imagine that there are

two alleles in a blue-footed booby population: W and w

– W is a dominant allele for a nonwebbed booby foot

– w is a recessive allele for a webbed booby foot

Webbing No webbing

◦ Consider the gene pool of a population of 500 boobies

– 320 (64%) are homozygous dominant (WW)– 160 (32%) are heterozygous (Ww)– 20 (4%) are homozygous recessive (ww)

Phenotypes

320–––500

Genotypes

Number of animals(total = 500)

Genotype frequencies

Number of allelesin gene pool(total = 1,000)

Allele frequencies

WW Ww ww

320 160 20

= 0.64 160–––500 = 0.32 20–––

500 = 0.04

40 w160 W + 160 w640 W

8001,000 = 0.8 W 200

1,000 = 0.2 w

◦ Frequency of dominant allele (W) = 80% = p

– 80% of alleles in the booby population are W

◦ Frequency of recessive allele (w) = 20% = q

– 20% of alleles in the booby population are w

◦ Frequency of all three genotypes must be 100% or 1.0

◦ What about the next generation of boobies?

– Probability that a booby sperm or egg carries W = 0.8 or 80%

– Probability that a sperm or egg carries w = 0.2 or 20%

Gametes reflectallele frequenciesof parental gene pool

W eggp = 0.8

w eggq = 0.2

W spermp = 0.8

Allele frequencies

Genotype frequencies

Next generation:

w spermq = 0.2

WWp2 = 0.64

wwq2 = 0.04

wWqp = 0.16

Wwpq = 0.16

0.64 WW 0.32 Ww 0.04 ww

0.8 W 0.2 w

Allele frequencies don’t change

◦ For a population to remain in Hardy-Weinberg equilibrium for a specific trait, it must satisfy five conditions:

1. Very large population

2. No gene flow between populations

3. No mutations

4. Random mating

5. No natural selection

MECHANISMS OF MICROEVOLUTION

◦ IF five conditions for the Hardy-Weinberg equilibrium are not met gene pool may change

– Mutations are rare and random and have little effect on the gene pool

– If mating is nonrandom, allele frequencies won’t change much (although genotype frequencies may)

◦ The three main causes of evolutionary change are

1. Natural selection

2. Genetic drift

3. Gene flow

◦ Natural selection

Process in which organisms with certain inherited traits are more likely to survive/reproduce than others

– Consider the boobies: Would webbed or nonwebbed boobies be more successful at swimming and capturing fish?

– What about sickle cell disease?

◦ Genetic drift

– Genetic drift is a change in the gene pool of a population due to chance

– In a small population, chance events may lead to the loss of genetic diversity (let’s flip a coin)

◦ Types of genetic drift– The bottleneck effect leads to a loss of

genetic diversity when a population is greatly reduced

Originalpopulation

Bottleneckingevent

Survivingpopulation

◦ Bottlenecks can be due to human intervention Example: Hunting of northern elephant seal in

1890s reduced population to 30 individuals

Example: Breeders of pugs and Persians use so much inbreeding that gene pool has been dramatically reduced

o Bottlenecks can be due to catastrophic event Example: Toba catostrophe theory 70,000

years ago where hominid population was reduced to 5,000-10,000 breeding pairs

Types of genetic drift– Genetic drift produces the founder effect

when a few individuals colonize a new habitat (ex. Martha’s Vineyard heritable deaf population)

◦ Gene flow

– Gene flow is the movement of individuals or gametes/spores between populations and can alter allele frequencies in a population

◦ An individual’s fitness is the contribution it makes to the gene pool of the next and subsequent generations

1. Stabilizing selection favors intermediate phenotypes, acting against extreme phenotypes

2. Directional selection acts against individuals at one of the phenotypic extremes

Directional selection is common during periods of environmental change, or when a population migrates to a new and different habitat

3. Disruptive selection favors individuals at both extremes of the phenotypic range

Originalpopulation

sOriginalpopulation

Evolvedpopulation

Phenotypes (fur color)

Stabilizing selection Directional selection Disruptive selection

So why don’t we run out of different alleles due to natural selection against

“bad ones”?

◦ Sexual dimorphism: males and females show distinctly different appearance

◦ Intrasexual competition (or mate choice) involves competition for mates, usually by males

◦ Diploidy preserves variation by “hiding” recessive alleles (within the heterozygote)

– A recessive allele is only subject to natural selection when it influences the phenotype in homozygous recessive individuals

– Example: cystic fibrosis

◦ Balancing selection maintains stable frequencies of two or more phenotypes in a population

In heterozygote advantage, heterozygotes have greater reproductive success than homozygous

– Example: sickle-cell anemia

◦ In frequency-dependent selection, two different phenotypes are maintained in a population

– Example: African scale eating fish

“Right-mouthed”

“Left-mouthed”

01981’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90

Sample year

“lef

” in

◦ Some variations may be neutral variation, providing no apparent advantage/ disadvantage

– Eample: human variation in fingerprints

1. Selection can only act on existing variation– Natural selection cannot conjure up new

beneficial alleles

2. Evolution is limited by historical constraints

3. Adaptations are often compromises

4. Chance, natural selection and the environment interact

copyright © 2009 pearson education, inc. ◦ adaptations: inherited traits that enhance an...

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