Evolution of Life

Chapter 13

Origin of Life

• Age of Planet Earth - 4.6 billion years

• Oldest fossils - 3.5 billion years

• Possible Formation of the First Cells– Inorganic molecules reacted to

form organic molecules– Organic molecules polymerized

to become macromolecules– Plasma membrane formed– Protocells formed

Evidence of Evolution

Evidence of Evolution

• Evolution– All the changes that have occurred in living

things since the beginning of life• Due to differential reproductive success

• 4 main areas of evidence that lead us to believe in “some” evolution:

Evidence of Evolution

• Fossil Evidence

• Biogeographical evidence

• Anatomical evidence

• Biochemical evidence

1. Fossil Evidence • Fossils

– The remains and traces of past life or any other direct evidence of past life

• Deposited in layers called strata– Each stratum is older than

the one above and younger than the one below

• Transitional fossils– Especially significant– Represent evolutionary

links

Fossil Evidence - Geological Timescale

• Based on dating of fossil evidence– Relative Dating Method

• Determines the relative order of fossils and strata but not the actual date

– Absolute Method-• Radioactive dating techniques • Assign an actual date to a fossil• Technique based on the half-life of radioactive

isotopes

Absolute Method

Fossil Evidence - Mass Extinctions

• Large numbers of species become

extinct in a short period of time

– Remaining species may spread out

and fill habitats left vacant

• Five Major Extinctions have occurred

2. Biogeographical Evidence

• Biogeography – Study of the distribution of species throughout the

world

• Earth has six biogeographical regions– Each has its own distinctive mix of species

• Barriers prevented evolving species from migrating to other regions

• Continental Drift-– The positions of continents and oceans has shifted

through time– The distribution of fossils and existing species allows

us to determine approximate timeline

3. Anatomical Evidence• Common descent offers explanation

for anatomical similarities– Homologous Structures

• Same function and same basic structure, indicating a common ancestor

• Ex: human arm and whale forelimb

– Analogous Structures • Same basic function but different

origins• Ex: wing of bird and wing of an insect

– Vestigial Structures• Anatomical structures fully functional

in one group and reduced, nonfunctional in another

• Ex: Modern whales have a pelvic girdle and hind leg bones

4. Biochemical Evidence

• All organisms use same basic biochemical molecules– DNA– ATP– Identical or nearly identical enzymes

• Many developmental genes are shared• Degree of similarity between DNA base

sequences and amino acid sequences indicates the degree of relatedness

Process of Evolution

Process of Evolution

• Occurs at the population level• Evolution – genetic changes within

population – more generations – phenotypic changes

• Microevolution - evolution on a small scale– Gene pool of a population

• All the alleles in all the individuals making up the population

– When the allele frequencies for a population change, microevolution has occurred

Evolutionary Change

• Mutations

• Genetic drift

• Gene flow

• Nonrandom mating

• Natural selection

1. Mutations

• Genetic changes that provide the materials for evolutionary change

• Can cause allele frequencies to change in a gene pool – microevolution to occur

• Has the ability to create new alleles in a population

• Happen randomly • Does not have an adverse effect on the

individual’s reproductive state

2. Genetic Drift

• Changes in the allele frequencies of a gene pool due to chance

• Greater effects on smaller populations

2. Genetic Drift

• Two main mechanisms– Founder Effect

• Few individuals found a colony

• Their collective genes represent only fraction of the original gene pool

– Bottleneck Effect: • Population is subjected to

near extinction by a disaster

• Only a few genotypes contribute to next generation

3. Gene Flow

• Movement of alleles between populations

• Occurs when individuals migrate from one population to another

• Keeps gene pools similar

4. Nonrandom Mating

• Occurs when individuals pair up according to phenotype or genotype

• Inbreeding is an example

5. Natural Selection

• Natural Selection

– Process by which populations adapt to their environment

– Charles Darwin explained evolution through natural selection

– Why does this happen????

Charles Darwin and The Origin of Species

• Charles Darwin’s On the Origin of Species by Means of Natural Selection– Published on November 24, 1859

• Argued that contemporary species arose from ancestors– Through a process of “descent with

modification,” with natural selection as the mechanism

• Darwin made two main points in The Origin of Species:– Organisms inhabiting Earth today

descended from ancestral species– Natural selection was the mechanism

for descent with modification• Basic idea of natural selection is

that:– Organisms can change over

generations– Individuals with certain heritable

traits leave more offspring than others• The result of natural selection is

evolutionary adaptation

Natural Selection• Evolution by natural selection requires the

following– Variation

• members of a population differ

– Inheritance• differences are inheritable

– Overproduction• populations produce more offspring than the environment

can support (struggle for existence)

– Differential Reproductive Success• better adapted individuals survive to reproduce more

offspring

Natural Selection

• Fitness– Measured by the number of fertile

offspring produced by an individual

Natural Selection

• Three Main Types of Natural Selection:

– Stabilizing Selection

– Directional Selection

– Disruptive Selection

Natural Selection

• Stabilizing Selection

– Occurs when an

intermediate, or

average, phenotype is

favored

– Improves adaptation of

population to a stable

environment

– Extreme phenotypes

are selected against

Stabilizing Selection

Natural Selection

• Directional Selection

– One extreme phenotype is favored

– Distribution curve shifts in that

direction

– Can occur when population is

adjusting to a changing

environment

Directional Selection

Natural Selection

• Disruptive Selection

– Two or more extreme phenotypes are selected

– Two different habitats result in two different

phenotypes in a population

Disruptive Selection

Speciation

Chapter 14

What is a species?

• biological species

• All individuals of 1 species can interbreed

• Offspring are fertileSterile zebra-horse cross: not a

new species

Speciation

• Macroevolution

– observed best within the fossil record

• Speciation

– Splitting of one species into two or more

– the transformation of one species into a new species over time

Hybrid Animals

• Breeding of two closely related but distinct species

• Human activities

• Unnatural conditions

• Usually sterile

Reproductive Barriers between Species

• Prezygotic isolating mechanisms

• Postzygotic isolating mechanisms

Reproductive isolating mechanisms

• Prezygotic isolating mechanisms– Habitat isolation

– Temporal isolation

– Behavioral isolation

– Mechanical isolation

– Gamete isolation

Reproductive isolating mechanisms

• Postzygotic isolating mechanisms– Zygote mortality

– Hybrid sterility

– F2 fitness

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Process of Speciation

• Allopatric Speciation– Geographical barriers separate a

population into two groups

• Sympatric Speciation– Occurs without geographical barriers– Ex: Plants

• Self-reproduction can maintain a new species

Pace of Speciation

• How FAST does evolution occur?

• 2 main mechanisms to explain:– Phyletic Gradualism– Punctuated Equilibrium

Pace of Speciation• 1. Phyletic Gradualism

– Change is slow but steady before and after a divergence

– Proposes that speciation occurs after populations become isolated

• Reproductive isolation cannot be detected in fossils

Pace of Speciation

• 2. Punctuated Equilibrium– Some think species appear

suddenly• Remain essentially unchanged

phenotypically until they undergo extinction

– Long periods of stasis followed by rapid speciation

• Occurs relatively rapidly

Extinction…• Inevitable • Occurs all the time

– rates have not been steady

• Extinctions typically eliminate various species of organisms– followed by

explosive diversifications of organisms