Lecture 25Evolution

What is Evolution? Change over time, building on past & current features

Products evolve Knowledge evolves Beliefs evolve

What is Evolution? In other cultures and religions,

for example Taoism, evolution plays a central role

Evolutionary patterns in biology have been noted as far back as Aristotle

Darwin initially used the phrase “descent with modification” to explain the concept of evolution

Patterns of biological evolution have been observed in three major areas: Fossil records Anatomical features Molecular distances

Evolution: Getting from There to Here

Macroevolution Evolutionary change on a

grand scale

Encompasses the origins of new species and major episodes of extinction

Microevolution Evolutionary change on a

small scale

Encompasses the genetic changes that occur within populations over time

These changes are the result of changes in gene frequencies

Evolution: Getting from there to Here

In contrast, Darwin and Wallace proposed that: variation is an inherent characteristic of all biological populations It is not created by experience

This is readily observable in all populations – just look around this room

Prior to Darwin and Wallace it was widely thought that biological evolution occurred by inheritance of acquired characteristics Individuals passed on to offspring body

and behavior changes acquired during their lives

The Pace of Evolution

The standard view since Darwin was that evolutionary change occurred extremely slowly Imperceptible changes accumulate such that, over thousands or

millions of years, major changes could occur This is termed gradualism

In 1972, Niles Eldredge and Stephen Jay Gould proposed the Punctuated Equilibrium hypothesis Evolutionary change occurs in bursts separated by long periods of

little or no evolutionary change (termed stasis)

Examples of both gradualism and punctuated equilibrium exist So speciation clearly occurs in different ways

However, the idea that speciation is necessarily linked to phenotypic change has not been supported Speciation can occur without phenotypic change Phenotypic change can occur within a species in the absence of speciation

Different kinds of organisms do evolve at different rates Bacteria evolve much faster

than eukaryotes

The rate of evolution also differs within the same group of species In punctuated equilibrium,

evolution occurs in spurts In gradualism, evolution

occurs in a gradual, uniform way

The Rate of Evolution

The Evidence For Evolution

Evidence for evolution comes from the following

Fossil record

Anatomical record

Molecular record

Evolution in the titanotheres

Small bony protuberance

Large blunt horns

Hoofed mammals

Fossil Record

Provides the most direct evidence for macroevolution

Fossils are the preserved remains, tracks, or traces of once-living organisms They form when

organisms become buried in sediment and calcium in hard surfaces mineralizes

Arraying fossils according to age often provides evidence of successive evolutionary change

Fossil Record

Fossils have been found linking all the major groups

The forms linking mammals to reptiles are particularly well known

Ontogeny Recapitulates Phylogeny All vertebrates share a basic set of developmental instructions

Anatomical Record: Ontogeny

Homologous structures Have different structure and function but are all derived from the

same part of a common ancestor

A common ancestor possessed this 1-2-5 structure

Adaptations to different environments modified this structure in different ways

Anatomical Record: Homology

Analogous structures Resemble each other as

a result of parallel evolutionary adaptations to similar environments

Anatomical Record: Analogy

Analogous structures are the result of convergent evolution

Different animals often adapt in similar fashion when challenged by similar adversities or opportunities

Convergent evolution: many paths to one goal

Vestigial organs Structures that are no longer

in use such as the human appendix

Apes have a much larger appendix that is involved in digestion

Anatomical Record

New alleles arise by mutations and they come to predominance through favorable selection

Thus, evolutionary changes involve a continual accumulation of genetic changes Distantly-related

organisms accumulate a greater number of evolutionary differences than closely-related ones

This divergence is seen among vertebrates in the 146-amino acid hemoglobin chain

Molecular Record: Evolutionary Distance

Molecular Record: Molecular Clock

This same pattern of divergence is seen with DNA sequences, such as that of the cytochrome c gene

The changes appear to accumulate at a constant rate

This phenomenon is referred to as a molecular clock

Note: Different proteins evolve at different rates

Molecular Record: Homology

The eyes of these organisms are NOT homologous

The genes controlling eye development ARE homologous

500 million years ago an ancestor evolved the “switch” for eye development Probably simplest type of eye

structure

1 pigment cell & 1 photoreceptor

The gene was passed on to all its descendents

These descendents evolved different types of complex eyes Each switched on by a

descendent of the original gene

Evolution’s Critics

Critics of evolution raise seven principal objections

1. Evolution is not solidly demonstrated

2. There are no fossil intermediates

3. The intelligent design (irreducible complexity) argument

4. Evolution violates the 2nd law of thermodynamics

5. Proteins are too improbable

6. Natural selection does not imply evolution

None of these objections has held up to scientific scrutiny

Evolution & Religion

“Today, almost half a century after the publication of the Encyclical, new knowledge has led to the recognition of more than a hypothesis in the theory of evolution.

It is indeed remarkable that this theory has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge.

The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of the theory.”

Pope John Paul II November 19, 1996 Address to the Pontifical Academy of Sciences

The Evolutionary Path to Apes

The Earliest Primates had two distinct features that allowed them to succeed in the arboreal insect-eating environment Grasping fingers and toes

Allow them to grip limbs, hang from branches & use tools

Overlapping binocular vision Lets the brain judge distance precisely

~ 40 mya, the earliest primates split into two groups: Prosimians and Anthropoids

Prosimians (“before monkeys”) Primarily nocturnal and herbivorous Only a few survive today

The story of human evolution begins around 65 mya with the Archonta A group of small, arboreal mammals that were primarily

insectivorous They underwent an explosive radiation that gave rise to different types of

mammals including bats and primates

Higher primates include monkeys, apes and humans

Origin of the Anthropoids

Anthropoids are almost all diurnal and herbivorous Evolved a bigger brain

and improved senses to adapt to daytime foraging

Live in groups with complex social interactions

Tend to care for their young for prolonged periods of time

Early anthropoids, now extinct, evolved in Africa

Their direct descendants are:

Old World MonkeysStayed in AfricaSplit into two lineages

i. Old world monkeysii. Hominoids

Many are ground dwellersNone have prehensile tails

New World MonkeysMigrated to South AmericaDeveloped in isolationAll are arborealMost have prehensile tails

Comparing Apes to Hominids

Hominoids evolved from anthropoids Apes Hominids (humans and their

direct ancestors) Chimpanzees are the closest living

human relatives Chimpanzees and humans

share 98.4% of their nuclear DNA

Gorillas and humans share about 97.7%

The common ancestor of apes and hominids is thought to have been an arboreal climber Hominids became bipedal (walking upright) Apes evolved knuckle-walking

Anatomical differences between the two are related to bipedal locomotion Considerable controversy exists about the identity of the first hominoid

Attention is now focused on an early Miocene ape, Proconsul Has many of the characteristics of Old World Monkeys It lacks a tail and has apelike hands, feet and pelvis

There are two major groups of hominids

The genus Homo 3-7 species

depending on how you count them

The genus Australopithecus

7 species Older Smaller-brained

A Hominid Evolutionary Tree

Bipedalism seems to have evolved as our ancestors left dense forests for grasslands and open woodland

Did bipedalism precede or succeed brain enlargement? Fossils unearthed in Africa demonstrate

that bipedalism extended back 4 mya Substantial brain expansion, on the other

hand, did not appear until about 2 mya

Australopithecine characteristics Bipedal ~ 1 m tall and ~ 18 kg in weight Hominid dentition Brains not any larger than those

of apes Fossils only found in Africa

Out of Africa: Homo erectus

The first humans evolved from australopithecine ancestors about 2 mya The exact ancestor is thought to be A.

afarensis

In the 1960s, hominid bones were found near stone tools in Africa This early human was called Homo

habilis (Latin for “handy man”) It closely resembled Australopithecus

but had a larger brain

In 1891, in Java, a Dutch anatomist named Eugene Dubois found a 500,000 year old skull cap and thighbone which he called Java man, The thigh bone indicated that he was

bipedal The skull indicated a brain twice the

size of that of Australopithecus Now recognized as Homo erectus:

Taller than H. habilis with a larger brain

In 1976, a 1.5 million year old H. erectus skull was found in East Africa This suggests that H. erectus originated

there and migrated into Asia and Europe

H. erectus survived for over a million years - longer than any other species of humans

The Last Stage of Hominid Evolution

Modern humans first appeared in Africa about 600,000 years ago Three human species are thought to have evolved

Homo heidelbergensis Evolved in Africa about 600,000 years ago Migrated to Europe and Western Asia

Homo neanderthalensis Appeared in Europe about 130,000 years ago as H. heidelbergensis

was becoming rarer Likely branched off of the ancestral line leading to modern humans

Homo sapiens (“wise man”) Evolved in Africa about 130,000 years ago then migrated to Europe

and Asia This is called the Recently-Out-of-Africa model This model is supported by a variety of gene studies

Human mitochondrial DNA Y and X chromosomes Autosomes

Our Own Species: Homo sapiens

Neanderthals (H. neanderthalensis) Named after the Neander Valley of Germany

where their fossils were first discovered in 1856 Evolved in Europe, then migrated to Asia Abruptly disappeared about 34,000 years ago

Cro-Magnons (H. sapiens) Named after the Valley in France where their

fossils were first discovered Evolved in Africa, then migrated to Asia Eventually spread to N. America and Australia

We humans are unique animals and the product of evolution Our evolution has been marked by a progressive

increase in brain size Refined and extended conceptual thought Symbolic language Cultural evolution