Biology 115, Lecture 25: Evolution

Spring 1998 T. Irving; Revised April 2000 A.Howard

Goals of this section:

Students should be able to:

1. Describe what is meant by evolution
2. Describe how theory of evolution differs from creationism
3. Describe what is meant by fossils and how they are formed
4. Describe how the fossil record supports theory of evolution
5. Describe how biogeography can give rise to new species
6. Describe what is meant by parallel evolution
7. Describe how embryology and comparative anatomy support theory of evolution
8. Describe what is meant by a species.
9. Describe how humans are similar and different from other primates. What are the distinghuishing features of humans?
10. Describe two parallel tracks of evolution of genus Australopithecus and Homo
11. What is the real significance of the various races?
12. Describe to what extent humans are still evolving

Evolution

• If a mutation in a particular trait has results which are beneficial, individuals with that trait will produce more offspring on average than individuals without that change.
• Those with beneficial mutations are said to be evolutionarily more fit or to have increased fitness.
• After many generations this mutation and its resulting trait will predominate in a population.
• On a larger scale, all alleles and combinations of alleles which underlie beneficial traits will come to predominate. The species is said to evolve.
• Actual difference in fitness between individuals are generally small, but over many generations significant changes can occur.

How quickly can a small difference in fitness change allele frequencies/phenotypes?

Example 1

A population consisting of two pennies:
One earns interest at 1% a year the other at 0.5% a year.
(first penny more fit):

time (years)	amount (% of total) at 1.0 % interest	amount (% of total) at 0.5% interest
0	1 (50%)	1 (50%)
100	2.7 (63%)	1.6 (37%)
200	7.2 (73%)	2.7 (27%)
500	143 (92%)	12 (8%)
1000	20752 (99.3%)	146 (0.7%)

Example 2

Consider a phenotype produced by genotype aa which has a level of fitness only 1% less than that of phenotypes produced by genotypes Aa or AA.

If the frequencies of alleles A and a alleles in the population are initially each 0.5 ( time 0 is when aa phenotype is only 99% as fit as Aa or AA phenotypes), in about 10,000 generations allele frequencies of a = 0.01 and A =0.99 in the population.

Evolution is still 130 years after Darwin's Origin of the Species somewhat controversial

• Opponents of evolution, creationists claim that it is "just a theory" that cannot be proved.
• They are absolutely right
• You cannot prove that evolution by natural selection has been the driving force behind life on earth for 2 - 3 billion years; This would require time travel.
• In fact, philosophers of science say that you can't prove any theory, only disprove them.
• Evolution is an established theory because a substantial amount of evidence exists which strongly supports it.
•  
• Creationism as a scientific hypothesis (the earth and everything in it that exists now was created at a particular (fairly recent) time in history) does not have anywhere near this level of support, in fact the bulk of evidence is against it.
• Creationism as a religious truth (God created the universe, and all in it including humans) is as valid as it ever was to many religious people who also fully accept evolution as a scientific theory.
• Hypotheses concerning religious truths are generally not experimentally testable so are inappropriate questions to try and resolve using the scientific method

Darwin's Theory of Evolution and Natural Selection

Natural selection = forces of nature or changes in these forces

Summary of Darwin's Theory of Evolution

1. There are inheritable variations among the members of a population
2. Many more individuals are produced each generation than can survive and reproduce
3. Individuals with adaptive characteristics are more likely to be selected to reproduce by the environment
4. Over time, a population can become well adapted to a particular environment
5. End result of organic evolution is many different species each adapted to specific environments

Fossil Evidence for Evolution

• Fossils occur most often in bodies of water where dead organisms are covered with silt etc.
• Soft parts decay but skeletons, shells or wood can be preserved
• Preservation either because material forms a mold in hardened sediment or if it becomes petrified i.e. biological material gradually replaced by minerals.
• Thus most fossils are from creatures that lived in and around water.
• Bird fossils might be rare because they seldom die in a body of water (also their bones are fragile)
• Nonetheless a fairly continuous fossil record exists.

Evolutionary Relationships among organisms are indicated by similarity of fossils

• For comparison of higher organisms (e.g. is it a mammal or is it a reptile?) have to be based on skeletal features since soft features do not survive
• Evolutionary relationships also indicated by sequence of fossil record in geologic times
• Different fossils are deposited in different geologic strata in sedimentary rocks.
• Assumption is that earlier fossils are found deeper than later ones.

Determining Fossil Age:

1. Stratigraphical dating:
   Use the strata in sedimentary rock which contain the fossils. The approximate rate of deposition of the rock is known, so the distance from the surface to a stratum determines that fossils age.
2. Radiodating:
   
   • Assume that we have radioactive element A that decays to non-radioactive element B at a known rate.
   • You find fossils in a particular stratum in a spot where you find some element A
   • Any element B in the same spot probably came from radioactive decay of A, starting at the point of time the stratum was formed

   20 million yrs	10 million yrs	Now
   All A-->	0.5 B-->	0.5 B
   	0.5A-->	0.25B
   		0.25A

Now 3 parts B to one part A.

Phylogenetic Evidence

• Relationships among species or comparative anatomy
• If evolution did not exist you might not expect such relationships to exist.
• Relationships may be close e.g. all echinoderms have five-fold radial symmetry
• Some not so close e.g. some mollusks have 2 external shells, some one external shell, some (squid) have an internal shell.
• Skeletons of different vertebrates show the same bones but adapted to the different habitats of the organisms (Fig. 21.2)
• Table 21.1 shows the taxonomic classification of humans. Taxonomy assumes implicitly that organisms classified similarly are related i.e. have a common ancestor somewhere

Biogeography

• Geographic distribution of both live animals and fossils
• Compare animals from two separate continents with at least partially similar climates (S. America & Africa)
• If no evolution should expect same animals in both regions but opposite occurs
• South American fossils are more similar to living South American creatures than they are to are to living African creatures
• Living South American creatures from various climates are more similar to each other than are South American to African Animals from similar climactic regions
• Parallel Evolution: in similar habitats has frequently resulted in similarly adapted but different animals in geographically separated regions.

Continental drift and Evolution

• About 200 million years ago all the continents were joined into one large continent
• Subsequently, the continents separated and have been slowly drifting apart ever since.
• The resulting geographic barriers produce disruptive selection

Past	one continent with species A
soon after	S. America A	Africa A
later	A1 A2	A3 A4
Present	A1’ A1" A2’ A2"	A3’ A3" A4’ A4"

A₁' A₁" A₂' A₂" are more closely related to each other than to A₃' A₃" A₄' A₄" and vice versa

Also fossils A₁ A₂ are more closely related to A₁' A₁" A₂' A₂" than to A₃' A₃" A₄' A₄"

Embryology

• Embryology studies the steps from zygote to baby
• One concept is that if evolution does not occur, the steps in embryonic development in various organisms would show only random similarities
• The more similar the adults of two species are, the more similar their embryonic development

Comparative Anatomy

• Another concept is that of vestigial characteristics
• Humans have a tail bone (the coccyx) even though it is of no use to us
• Some snakes show a pelvis and vestigial rear limbs in their skeletons
• These features only make sense if they are remnants of once useful features that are in the process of being evolved out of existence

Experimental Evidence for Evolution

Start with a single population of fruit flies:

• Flies show geotaxis i.e. tendency to move towards earth i.e. gravity. May be positive or negative
• Choose individuals who show +ve geotaxis and allow them to breed only with each other
• Choose a second group of individuals who show -ve geotaxis and allow them to breed only with each other
• Follow expression of geotaxis over time repeating the selection process each time
• Eventually offspring will show the one phenotype predominately
• If the selection pressure removed. Fly population will gradually revert to wild type.

Similar experiments have been done with animal and plant breeding

Natural History Examples of Evolution

• Over past 150 years wing color in a certain species of moth in England have evolved to match the color of the background for camouflage
• Recent book The Beak of the Finch suggests that natural selection has been acting very quickly indeed in Galapagos finch populations.
• Drug resistance in bacteria, insecticide resistance in insects, herbicide resistance in weeds
• We have inadvertently been applying intense selection pressure on select populations of organisms

Evolution and Speciation

For current species, species are considered to be separate that do not interbreed in nature or that , if they do, produce sterile offspring:
Horses & donkeys, lions & tigers

• Evolution can change a species without necessarily creating a new species.
• Eventually enough changes accumulate so that a new species can be said to have evolved
• With geographic barriers, one species may evolve into two or more new species

Human Evolution

Human Evolution

• This area of study started in the mid 1800’s
• Darwin: the Descent of Man (1871)
• Findings of Human or human-like fossils (beginning 1856)
• Neanderthals a bit more primitive and more massively built than modern humans
• Fossils found from Spain to China, Germany to Zambia
• Cro-Magnon man is identical to modern humans
• First fossils found in France (oldest about 26,000 years old)

Evolution of humans started long before Neanderthals

• Humans are primates, as are apes, monkeys, lemurs and tarsiers
• Earliest fossil primates date to ~65 million years ago
• Appeared to evolve from a group of shrew-like animals
• These creatures in turn had as ultimate ancestor a simple cell 3.5 - 4 billion years ago.
• Many different branching schemes exist for when primates diverged from other mammals, apes, from monkeys etc. differing slightly in their details
• These schemes are based on the "fossil record" as well as comparisons of DNA sequences between modern species

How good are family trees derived from the fossil record?

• Total number of fossils are small
• Fossils are often incomplete yet paleontologists draw conclusions from them
• Often totally conflicting conclusions can be drawn from the same fossils by different scientists!
• It often isn't clear whether differences between various fossil skeletons are due to real differences between species or to normal variation within a species
• A professional wrestler , a jockey, and a marathon runner may have radically different shapes and sizes of skeleton but be the same species
• Speculations as to the behavior of extinct species are made by comparison to living primates
• Overall outline of human evolution as we know it is probably generally correct.
• Details very much subject to change

Much of human evolution occurred in Africa

• From Lucy to Homo habilis (up to ~ 2 million years ago) all occurred in Africa
• Homo habilis still relatively small brain, ape-like features, first to use tools
• Homo erectus had a large brain, hunted, used fire
• migrated to Asia, Eurasia ~ 1.9 million years ago
• H. erectus has been found in Siberia so probably understood how to live in the cold (fire, boots, mitts etc.)

Neanderthal and Cro-Magnon man

• "Classic" cave men
• H erectus evolved to H sapiens neanderthalis in Asia, Africa ~100,000 years ago
• Neanderthals had slightly larger brains than modern humans, were shorter and more massive
• Made tools, buried their dead (funeral rites)
• Cro-Magnon man are same as modern humans.
• Responsible for cave paintings showing animals and hunting in Spain and France

Modern humans appear 30-40,000 years ago.

It is controversial whether:

1. There was gradual evolution from Neanderthals to modern humans
2. Modern humans emerged from Africa and somehow abruptly replaced Neanderthals

• The latter idea is supported by a recent study comparing mitochondrial DNA sequences from various modern populations.
• These results have been challenged on grounds of serious methodological flaws in this study
• Thus whether the Neanderthal --> modern human evolution occurred all over the world or in one place still not resolved.
• It is also uncertain whether there is only one or more than one lineage back to the various Homo erectus populations as well

How are humans different from Apes?

• Humans share more than 95% of genetic information with chimpanzees (an ape)
• Fossils of hominids (human like animals) distinguished by:
  • Bipedalism
  • Flat face
  • Brain size (400cc for apes, 1330 for humans)
  • Opposable thumbs
  • Toolmaking
  • Language
  • More complex culture

Modern human "races"

• Modern races represent different populations of humans with different characteristics
• Result from reproductive isolation
• Skin color appears to be adaptations to climate
• No consensus of how many races exist, schemes of 3 -30 exist
• Differences in characteristics between groups are differences in average characteristics
• Overall genetic differences between races are no greater than those within races.

All humans are the same species

• Differences are way too small to make different species
• All humans are fertile in cross-group marriages
• Geographic isolation between groups was never complete in the past.
• Isolation is getting less and less all the time so we can expect differences to be less and less with time.

Future Evolution of Humans

• Will probably be increasingly slow because technological advances (medicine, food, shelter) circumvent many of the forces of natural selection
• Like all evolution, human evolution is driven by natural selection of advantageous traits
• Advantageous traits being the result of certain alleles at many genes.
• As evolution proceeds, allele frequencies & traits in populations change.