AP Bio Unit 7: Evolution

Evolution

The processes that have transformed life on Earth from its beginning to today’s diversity

Theory

Hypothesis supported repeatedly by data; Makes testable predictions

theory

Layman’s and TV use of the word; confused with ‘hypothesis’ in science

James Hutton

Belief: Big change is the product of slow, continuous precesses
- Changes on Earth are gradual
- Earth must be VERY old
- Slow processes can equal big changes

Lamarck

Belief: Life changed from simple to complex over time
- Fossils are remains of past life forms
- Evolution did occur
- Use and Disuse/Acquired Characteristics

Alfred Wallace

Wrote paper on Natural Selection identical to Darwin’s ideas

Charles Darwin

Naturalist whose training/travel opportunities allowed him to formulate and support his ideas on Natural Selection
Theory: Decent with Modification

Darwinian View/Decent with Modification

History of life is like a tree with branches over time from a common source; Current diversity of life is caused by forks from common ancestors

Origin of Species

Documented the occurrence of evolution; Suggested that the mechanism for evolution was natural selection

Observations made in Origin of Species (4)

1. Members of a population often vary greatly in their traits

2. Traits are inherited from parents to offspring

3. All species are capable of producing more offspring than their enviornment can support

4. Owing to lack of food or other resources, many of these offspring do not survive

Inferences made from Darwin’s observations (2)

1. Individuals who inherit advantageous traits are more likely to survive and reproduce

2. The unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations

Natural Selection

When nature determines when characteristics are favorable and who survives

Artificial Selection

When man determines the characteristics that survive and reproduce
- Various breeds of animals and plants were developed

Ex: Mustard Plant → broccoli, cauliflower, kale, cabbage, brussels sprouts

Evolution success is measured by

Survival & Reproduction

Requirements for Natural Selection

Variation within a population, Long periods of time (according to Darwin)

Subtleties of Natural Selection

Populations are the units of evolution; Only inherited characteristics can survive

Evidence for Evolution

1. Direct Observations

2. Fossils

3. Homology

4. Convergent Evolution

5. Biogeography

6. Molecular Biology

Evidence for Evolution: Direct Observations

Color pattern in guppies, drug resistance HIV, beak size in birds

Direct Observations: HIV Drug Resistance

Drug resistance strains selected for by treatments
Result: Resistant strains became 100% dominant in 4-5 weeks

Evidence for Evolution: Fossils

Relics or impressions of organisms from the past; Shows changes over time from simple to complex; Many fossils don’t have descendants

Evolution relevance of fossils

Life has changed over time; Many species failed to survive & became extinct

Comments about fossilization

1. Fossilization is a rare event

2. Only hard parts fossilize well

3. Problem in finding fossils

4. Interpretation

5. Missing links

Evidence for Evolution: Homology (Homologous Structures)

Same structure, different function

Homology: Vestigial Organs

Rudimentary structures of marginal, if any, use; Ex: Wisdom Teeth in humans

Evolution Relevance of Homology

Remodeling of ancestral structures as their functions or adaptations changed

Homology in Embryos

Closely related organisms go through similar stages in their embryonic development

Evidence for Evolution: Convergent Evolution

Unrelated organisms show similar adaptations; Cause- lived in a similar environment with similar selection processes

Convergent Evolution: Analogous Structures

Features of different species that are similar in function but not necessarily in structure

Evidence for Evolution: Biogeography

The geographical distribution of species: Species mixtures in islands, Marsupials in Australia

Evolution Relevance of Biogeography

Biogeographical patterns reflect decent from the ancestors that colonized the area

Evidence for Evolution: Molecular Biology

Study of evolution at the DNA or protein levels; Related species have similar DNA sequences

Evolution Relevance of Molecular Biology

Related species share a common ancestral DNA; The closer the relationship, the more similar the DNA sequences are

Population Genetics

The study of genetic variation in populations; Use population genetics as the means to track and study evolution

Sources of Variation

Sexual Reproduction & Mutations

Sexual Reproduction

Random assortment of chromosomes, Random fertilization, Crossing over

Mutations

Inherited changes in a gene, (rare)

Population

A localized group of individuals of the same species

Species

A group of similar organisms; A group of populations that could interbreed

Gene Pool

The total collective genes in a population; If evolution is occurring, then changes MUST occur in the gene pool of the population over time

Microevolution

Changes in the relative frequencies of alleles in the gene pool

Hardy Weinberg Theorm

A mathematical way to measure evolution; Population is evolving if the allele frequency is changing (favoring one allele)

Hardy Weinberg Equation

Basic: p + q = 1
- p = % of dominant allele
- q = % of recessive allele

Expanded: p² + 2pq + q² = 1
- p² = Homozygous Dominant
- 2pq = Heterozygous
- q² = Homozygous Recessive

Importance of Hardy Weinberg

Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold (no evolution should occur)

Hardy Weinberg Assumptions

1. Large Population

2. Isolation

3. No net mutations

4. Random Mating

5. No Natural Selection

If Hardy Weinberg Assumptions hold true…

Gene frequencies will not change over time; Evolution will not occur

What is microevolution?

Violations of the 5 Hardy Weinberg assumptions

Causes of Microevolution

1. Genetic Drift

2. Gene Flow

3. Mutations

4. Non-Random Mating

5. Natural Selection

Genetic Drift

Changes in the gene pool of a small population by chance

Bottleneck Effect

Loss of the population by disasters; Surviving population may have a different gene pool than the original population

Bottleneck Effect: Result & Importance

Result: Some alleles are lost; Other alleles are over-represented; Genetic variation is usually lost

Importance: Reduction of population size may reduce gene pool for evolution to work with; Ex: Cheetah

Founder Effect

Genetic Drift in a new colony that separates from a parent population

Founder Effect: Result & Importance

Result: Genetic variation reduced; Some alleles increase in frequency while others are lost (as compared to parent population)

Importance: Very common in islands and other groups that don’t interbreed

Gene Flow

A movement of genes in/out of a population; Ex: Immigration/Emigration

Result: Changes in gene frequencies within a population; Immigration often brings new alleles into populations- increasing genetic diversity

Mutations- Result

May change gene frequencies (small population); Source of new alleles for selection

Non-Random Mating

Failure to choose mates at random from the population; Causes: Inbreeding within the same “neighborhood”; Assortative Mating (like with like)

Microevolution: Natural Selection

Differential success in survival & reproduction; Result: Shift in gene frequencies

Rate of Selection

Differs between dominant & recessive alleles; Selection pressure by the environment

Modes of Natural Selection

1. Stabilizing

2. Directional Selection

3. Diversifying/Disruptive

4. Sexual Mate Selection

Stabilizing Selection

Selection toward the average and against the extremes; Ex: Birth weight in humans

Directional Selection

Selection towards one extreme; Ex: Running speeds in race animals/Galapagos Finch beak size & food source

Disruptive/Diversifying Selection

Selection towards both extremes & against the norm; Ex: Bill size in birds; Can split a species into several new species if it continues for a long enough period of time & the populations don’t interbreed

Sexual Mate Selection

May not be adaptive to the environment, but increases reproduction success of the individual; Very Important

Sexual Mate Selection: Result & Comment

Result: Sexual dimorphism- secondary sexual features for attracting mates
Comment: Females may drive sexual selection and dimorphism since they often “choose” their mate

Preserving Genetic Variation

1. Diploidy: Preserves recessives as heterozygotes

2. Balanced Polymorphisms: Preservation of diversity by Natural Selection

Heterozygote Advantage

When the heterozygote or hybrid survives better than the homozygotes- Also called Hybrid Vigor

Heterozygote Advantage: Result & Comment

Result: Can’t breed “true” and the diversity (both alleles) of the population is maintained; Ex: Sickle Cell
Comment: Population genetics believe that ALL genes that persist in a population must have had a selective advantage at one time

Why doesn’t evolution result in perfect organisms?

1. Historical Constraints

2. Compromises

3. Non-Adaptive Evolution (Chance)

4. Available Variations- most come from using a current gene in a new way

Macroevolution

Refers to larger changes over a long time scale. Macroevolution can result in speciation or the emergence of new species

Speciation

How a new kind of plant or animal species is created

Two Species Concepts (What differentiates populations as different species?)

Morphospecies & Biological Species

Morphospecies Concept

Organisms with similar morphology or physical form

Biological Species Concept

A group of organisms that could interbreed in nature & produce fertile offspring

Speciation Requires (FRQ)

1. Variation in the population

2. Selection

3. Isolation

Reproductive Barriers

Serve to isolate a population from other gene pools; Create and maintain “species”

Prezygotic Barriers

Prevent mating or fertilization

1. Habitat Isolation

2. Temporal Isolation

3. Behavioral Isolation

4. Mechanical Isolation

5. Gametic Isolation

Habitat Isolation

Populations live in different habitats or ecological niches (Can live in different parts of the same habitat/area);
Ex: Mountains vs Lowlands

Temporal Isolation

Breeding seasons or time of day different
Ex: Flowers open in morning or evening

Behavioral Isolation

Mating or courtship behaviors different; Different sexual attractions operating;
Ex: Songs/dances in birds

Mechanical Isolation

Structural differences that prevent gamete transfer (plants)
Ex: Anthers not positioned to put pollen on a bee, but will put pollen on a bird

Gametic Isolation

Gametes fail to attract each other and fuse
Ex: Chemical markers on egg and sperm fail to match

Postzygotic Barriers

Prevent viable, fertile offspring

1. Reduced Hybrid Viability

2. Reduced Hybrid Fertility

3. Hybrid Breakdown

Reduced Hybrid Viability

Zygote fails to develop or mature
Ex: When different species of frogs or salamanders hybridize

Reduced Hybrid Fertility

Hybrids are viable, but can’t reproduce sexually; Chromosome count is often “odd” so meiosis won’t work
Ex: Donkeys/Horses produce Mules (whose chromosome count is 63)

Hybrid Breakdown

Offspring are fertile but can’t compete as well as the “pure breeds” (plants)
Ex: Many plant hybrids

How do Species Occur?

Block gene flow between two populations

Allopatric Speciation

Ancestral population split by a geographical feature
Ex: Pupfish populations in Death Valley

Conditions that favor allopatric speciation

1. Founder Effect - with the peripheral isolate

2. Genetic Drift- gives isolated population variation as compared to the original population

3. Selection pressure on the isolate differs from the parent population (enviornment is different)
   
   Result: Gene pool of isolate changes from the parent population and new species can form

Sympatric Speciation

New species arise within the range of parent populations; Can occur in a single generation

Sympatric Speciation in Plants

Polyploids may cause new species because the change in chromosome number creates post-zygotic barriers

Adaptive Radiation

Rapid emergence of several species from common ancestors; Common in island and mountain top populations or other “empty” environments

Adaptive Radiation: Mechanism

Resources are temporarily infinite; Most offspring survive
Result: Little natural selection and the gene pool can become very diverse
Once Environment Saturates: Natural Selection resumes, new species form rapidly if isolation mechanisms work
Ex: Galapagos Finches

Speed of Speciation: Gradualism

Small, gradual changes over long periods of time

Gradualism: Predictions & Problems

Predictions: Long periods of time are needed for evolution; Fossils should show continuous links
Problem: Gradualism doesn’t fit the fossil record very well (too many gaps)

Speed of Speciation: Punctuated Equilibrium

Theory that deals with the “pacing” of evolution
Evolution has two speeds: Gradualism & Rapid Bursts of Speciation

Punctuated Equilibrium: Predictions

Predictions: Speciation can occur over a very short period of time- too fast for fossils to record; New species will appear in fossil record without connecting links or intermediate forms; Established species will show gradual changes over long periods of time

Punctuated Equilibrium: Possible Mechanism

Rapid: Adaptive Radiation, especially after a mass extinction event
Gradual: Saturated environments favor gradual changes

How many genes does it take to be a new species?

Can be as little as 1 gene which prevents interbreeding; Ex: Snails; (Probably a larger number of genes in many cases)

Mass Extinction

Any circumstance that results in the loss of 95% of Earth's living species within a relatively short period of geologic time

General Extinction

The dying out of a species

Differential Survival

Describes how animals, plants, and other living organisms manage to successfully survive changes to their environment or fail and die out