Darwin’s Origin of Species Recognition of evolution as an explanation of life’s unity and diversity Natural selection- the mechanism of evolution

Darwin’s Origin of Species

• Recognition of evolution as an explanation of life’s unity and diversity

• Natural selection- the mechanism of evolution

Facts and Inferences

FACT #1

All species have the potential for fertility


FACT #2

Most populations are stable


FACT #3

Natural resources are limited


FACT #4

No two individuals are the same (variation in genetic make up)


FACT #5

Most differences are heritable (genes passed to offspring)


INFERENCE #1

There is a struggle for existence, with the survival of only a few individuals


INFERENCE #2

Survival is NOT random

The best adapted will leave more offspring (fitness)


INFERENCE #3

Populations will change

Populations evolve: NOT individuals

Natural Selection works at the individual level, not population or group

Natural Selection

• the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution

Questions

• Is behavior of a species driven by natural selection?

* Explain how the behavior in the video demonstrates natural selection.

Evidence for Evolution

For Darwin the proof was in the Finches!

Other Evidence

Artificial Selection

Other Evidence

Fossil Record

Other Evidence

Comparative Anat/Phys

• Homologous Structures

Other Evidence

Comparative Anat/Phys

• Vestigial Structures

Other Evidence

Embryology

Other Evidencehttp://news.nationalgeographic.com/news/2005/12/1207_051207_dog_genome.html

• Comparative study of genetic code between species reveals an extremely common genetic code – evidence that supports being copied from original ancestor (same genes for same proteins)

• Ex. Humans, mice and dogs –coding genome sequences that are shared by all three species are virtually identical

Other Evidence

• Biogeography – the geographical distribution of species

• Biogeographical evidence of the Galapagos provided Darwin with observations leading to his Common Descent (nice way of saying evolution) theory

» Explains why island species resemble species from nearest mainland versus species from similar looking islands found in different part of world.

» Australia has diverse marsupial population but could support placental species.

Related Term

• Adaptive radiation – populations spread, the new environmental conditions lead to adaptations

• Ex. – Darwin’s finches, Marsupials - colonization of island (or other isolated environment) allows rapid variation due to a decrease in environmental stress

Adaptive Radiation

Microevolution

• Population – localized group of individuals belonging to the same species

• Species – group of populations whose individuals have potential to interbreed

• Gene pool – all alleles in a population at any one time, all alleles at all loci in all individuals

• Allele frequency – the fraction or percentage that the allele occupies within the population

Sample Problem #1Total of 500 individuals

20 = white320 = homozygous pink160 = heterozygous pink

Allele frequency:a = 2(20) + 160 = 200 = 0.2

1000 1000

A = 2(320) + 160 = 800 = 0.81000 1000

Hardy-Weinberg Theory

• Theorem: Frequency of alleles and genotypes in a population’s gene pool remain constant over generations unless acted upon by agents other than sexual recombination.

Hardy-Weinberg Equilibrium

• Allele frequency is constant from generation to generation

Hardy-Weinberg Equation

p2 + 2pq + q2 = 1

p + q = 1

Hardy-Weinberg Equation

p = frequency of dominant allele

q = frequency of recessive allele

p2 = frequency of homozygous dominant genotype (AA)

2pq = frequency of heterozygous genotype (Aa)

q2 = frequency of homozygous recessive genotype (aa)

5 Required Conditions for Hardy-Weinberg Equilibrium to be true:

#1 Very Large Population

5 Required Conditions for Hardy-Weinberg Equilibrium to be true:#2 Isolation From Other Populations

5 Required Conditions for Hardy-Weinberg Equilibrium to be true:#3 No Net Mutations (lethal or altering)


#4 Random Mating


#5 No Natural Selection (all alleles must be equally viable)

Why do these 5 conditions need to be met?

• Frequency of alleles will not change

• Frequency of genotypes will not change

Are these conditions met in nature?

• No, this is the ideal

• Nature will cause changes in allele frequency – causing changes in the gene pool

• This will cause variation – natural selection acts upon variation!

Sample Problem #2• Refer to the flower population in the previous sample problem.

What are the frequencies of the three possible genotypes for the next generation of flowers.

• Determine what you know and want to know.• Know p = 0.8 and q = 0.2 (check it with p+q=1)• Want to know frequencies of 3 genotypes

– AA = p2 = ?– Aa = 2pq = ?– Aa = q2 = ?

• AA = p2 = (0.8)(0.8) = 0.64 = 64% of the next generation of flowers will be homozygous dominant

• Aa = 2pq = 2(0.8)(0.2) = 0.32 = 32% of the next generation of flowers will be heterozygous

• aa = q2 = (0.2)(0.2) = 0.04 = 4% of the next generation of flowers will be homozygous recessive

Sample Problem #3The actual results of the second generation (F1) of white and pink

flowers are listed below. Was Hardy-Weinberg equilibrium achieved? Why or why not?

GenotypesActualAA 245Aa 210aa 45

• No, HW equilibrium was not achieved from P to F1. • Expected results were:• AA = .64(500) = 320• Aa = .32(500) = 160• aa = .04(500) = 20• The expected and actual results vary, showing that HW equilibrium

did not occur. This means that at least on of the five conditions were not met.

Sample Problem #4A simple Mendelian trait has two alleles, D and d. If a population is

49% homozygous dominant, what percentage is heterozygous? What are the allele frequencies in the population?

What you know:DD = p2 = 0.49 so… p = 0.7 and q= .3 ( remember: p +q = 1)

Want to know:• Allele frequencies are:

• D = 70% or 0.7• d = 30% or 0.3

Percentage heterozygous:Dd = 2pq = 2(0.7)(0.3) = 0.42 or 42% of population will be heterozygous

III. Microevolution

• Definition:– Change in the gene pool of a population– Change in relative frequencies of alleles in

population– Evolution on the smallest scale

Causes of Microevolution

1. Mutations– In structural genes– In regulatory genes– Introduction of a new allele, variation for other

forces to act upon

Ex. Gene sequence for sickle cell anemia (structural gene mutation)

• Ex 2 – mutations in regulatory genes

http://www.findarticles.com/p/articles/mi_m1200/is_22_168/ai_n15931496

DNA clues to our kind: regulatory gene linked to human evolution

Science News, Nov 26, 2005 by B. Bower• A gene that exerts wide-ranging effects on the brain works harder in people than it

does in chimpanzees and other nonhuman primates, a DNA disparity that apparently contributed to the evolution of Homo sapiens, according to a new study.

• The gene participates in production of prodynorphin, an opiumlike protein that serves as a building block for chemical messengers in the brain known as endorphins. Studies have implicated endorphins in the anticipation and experience of pain, in the formation of intimate emotional bonds with others, and in learning and memory:

• All primates possess a virtually identical prodynorphin gene, say geneticist Matthew V. Rockman of Princeton University and his colleagues. However, a separate stretch of DNA regulates the extent to which the gene generates prodynorphin. This regulatory DNA displays a handful of mutations in people that must have evolved by natural selection and aided human survival, the scientists propose. The new findings appear in the December PLoS Biology.

• "This is the first documented instance of a neural gene that has had its regulation shaped by natural selection during human origins," says geneticist and study coauthor Matthew W. Hahn of Indiana University in Bloomington.


2. Gene Flow– Migration (immigration and emigration)– Genes move into and out of the gene pool– New alleles, changes frequecies– Hey – this actually decreases differences in

a population

Ex 1 – human populations “blending”Ex 2 – organic vs. genetically engineered crops


3. Genetic drift - change in gene pool of small population due to random chance

• Founder Effect

• Bottleneck Effect

Closer look: Founder effect

-definition – The founder effect occurs when populations are started from a small number of pioneer individuals of an original population. Due to small sample size, the new population could have a much different genetic ratio than the original one.

Ex. Today’s Amish are descendants of 30 people who migrated from Switzerland in 1720. The 30 original individuals carried a higher than normal % of genes for Dwarfism.

Closer look: Bottleneck effect

Definition: bottleneck effect – population reduced by outside forces (over-hunting, catastrophe, genocide) then population increases in number again – but all descendants of smaller representative group Ex. elephant seal off the Baja coast, Jewish population (Tay Sachs),

• Bottleneck Effect• This is a severe reduction in population size that causes

pronounced drift. For example, the elephant seal population was hunted down to just 20 individuals. Then the population rebounded to 30,000. However, electrophoresis has revealed that there is now no allele variation at 24 genes.

• In bottlenecks, some stressful situation greatly reduces the size of a population leaving a few (typical or atypical) individuals to reestablish the population.http://trc.ucdavis.edu/biosci10v/bis10v/week6/05bottleneck.html


4. Nonrandom mating

- Self –pollination in plants

- Sexual selection in animals (mating calls, plumage, male combat, twig selection)


5. Natural Selection

* Not all alleles are equally viable

* some alleles have better success in the environment, the phenotypes are selected for while others are selected against

NATURAL SELECTION – major cause of microevolution

• Adaptation of a population to the biotic and abiotic environment– Requires:

• Variation - The members of a population differ from one another

• Inheritance - Many differences are heritable genetic differences

• Differential Adaptiveness - Some differences affect survivability

• Differential Reproduction – Some differences affect likelihood of successful reproduction

V. Types of Selection

• Most traits are polygenic - variations in the trait result in a bell-shaped curve

• Three types of selection occur:

– (1) Directional Selection

• The curve shifts in one direction

• Ex - when bacteria become resistant to antibiotics

Directional Selection

Types of Selection

• Three types of selection occur (cont):– (2) Stabilizing Selection

• The peak of the curve increases and tails decrease• Ex - when human babies with low or high birth

weight are less likely to survive

– (3) Disruptive• The curve has two peaks• Ex – When Cepaea snails vary because a wide

geographic range causes selection to vary

Stabilizing Selection

Disruptive Selection

Sexual Selection

• Sneaky Male Iguanas

IV. Preserving Genetic Variability

• Industrial Melanism and Microevolution

Maintenance of Variations

• Genetic variability

– Populations with limited variation may not be able to adapt to new conditions

– Maintenance of variability is advantageous to population

• Only exposed alleles are subject to natural selection

Maintenance of Variations

• Recessive alleles:– Heterozygotes shelter recessive alleles from selection

– Allows even lethal alleles to remain in population at low frequencies virtually forever

– Lethal recessive alleles may confer advantage to heterozygotes

• Sickle cell anemia is detrimental in homozygote

• However, heterozygotes more likely to survive malaria

• Sickle cell allele occurs at higher than expected frequency in malaria prone areas

Sickle-cell Disease

B. Ways to get variation

• Sex

• Mutations

• Diploidy – two possible alleles for trait

• Polymorphism – Heterozygote advantage– Hybrid vigor– Frequency dependent advantage

Examples of Polymorphism

Species Definitions

• Species Definitions– Morphological

• Can be distinguished anatomically• Specialist decides what criteria probably represent

reproductively isolated populations• Most species described this way

Species Definitions

• Species Definitions– Biological

• Populations of the same species breed only among themselves

• Are reproductively isolated from other such populations

• Very few actually tested for reproductive isolation

Biological Species Definition

Species Definitions

• Species Definitions– Phylogenetic

• Can be shown to have genetic differences• Usually based on DNA sequence analysis• Very few species determined this way, but growing

in use

Modes of Speciation

• Speciation:– The splitting of one species into two, or– The transformation of one species into a new

species over time• Two modes:

– (1) Allopatric Speciation• Two geographically isolated populations of one

species• Become different species over time• Can be due to differing selection pressures in

differing environments

Allopatric Speciation

North Rim and South Rim

Modes of Speciation

• Two modes:– (2) Sympatric Speciation

• One population develops into two or more reproductively isolated groups

• No prior geographic isolation• Tetraploid hybridization in plants

– Results in self fertile species– Reproductively isolated from either parental species

HAWTHORNS – native to North America

APPLES – introduced by immigrants over 200 yrs ago

Gene flow has been reduced between flies that feed on different food varieties, even though they both live in the

same geographic area.

Adaptive Radiation

• Adaptive Radiation

– When members of a species invade several new geographically separate environments

– The populations become adapted to the different environments

– Many new species evolve from the single ancestral species

• This is an example of allopatric speciation

Reproductive Isolating Mechanisms

• Reproductive isolating mechanisms inhibit gene flow between species

• Two general types:– (1) Prezygotic Mechanisms - Discourage

attempts to mate • Habitat Isolation• Temporal Isolation• Behavioral Isolation• Mechanical Isolation• Gamete Isolation

Temporal Isolation

Reproductive Isolating Mechanisms

• Two general types:

– (2) Postzygotic Mechanisms - Prevent hybrid offspring from developing or breeding

• Zygote Mortality

• Hybrid Sterility

• Reduced F2 Fitness

Documents

Darwin’s Origin of Species Recognition of evolution as an explanation of life’s unity and diversity Natural selection- the mechanism of evolution