processes of evolution chapter 18. processes of evolution 2 microevolution microevolution pertains...

Processes of Evolution

Chapter 18

2Processes of Processes of EvolutionEvolution

Microevolution

Microevolution pertains to the evolutionary changes within a population.

Populations are all the members of a single species occupying a particular area.

Population genetics - study of genetic changes within a population

- The various alleles at all the gene loci in all individuals make up the gene pool of the population.

- It is customary to describe the gene pool of a population in terms of gene frequencies.


Gene Frequencies

Suppose in a Drosophila population there are:36% flies homozygous dominant for long wings

48% heterozygous for long wings 16% homozygous recessive for short wings

In population of 100 flies there would be:36 LL, 48 Ll and 16 ll

Number of L alleles would be (2 X 36) + 48 = 120Number of l alleles would be (2 X 16) + 48 = 80

There are 120 L alleles and 80 l alleles


Gene Frequencies

To determine frequency of each allele:Calculate its percentage from total # of alleles in population.

For dominant allele L = 120/200 = 0.6For recessive allele l = 80/200 = 0.4

The sperm & eggs produced by this population should have the same frequencies. You can calculate the expected ratios of genotypes in the next generation by using a Punnett square.


Gene Frequencies

Punnett Squareeggs

0.6 L 0.4 lsperm 0.6 L 0.36LL 0.24 Ll

0.4 l 0.24 Ll 0.16 ll

Genotypes frequencies = 0.36 LL + 0.48 Ll + 0.16 ll = 1


Gene Frequencies

Note that the frequency of each allele in the next generation is the same as it was in the previous generation.

Sexual reproduction alone cannot bring about a change in allele frequencies.

Also, the dominant allele does not increase from one generation to the next. It does NOT become more common.


Hardy-Weinberg

The Hardy-Weinberg principle - mathematics:

p + q = 1 p2 + 2pq + q2 = 1

p = frequency of dominant allele

q = frequency of recessive allele

p2 = frequency of homozygous dominant individuals

q2 = frequency of homozygous recessive individuals

2pq = frequency of heterozygous individuals

8Calculating Gene Pool FrequenciesUsing the Hardy-Weinberg Equation

9Calculating Gene Pool FrequenciesUsing the Hardy-Weinberg Equation

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Do Practice Problems 18.1 (p. 303) now


Hardy-Weinberg

The Hardy-Weinberg principle:

Allele frequencies in a population will remain constant assuming five conditions are met:

No Mutations - alleles do not change

No Gene Flow - no migration in or out of population

Random Mating - individuals pair by chance

No Genetic Drift - populations are large such that gene frequencies don’t change by chance alone

No Selection - particular genotypes not selected


Hardy-Weinberg

In real life, the Hardy-Weinberg conditions are rarely, if ever, met.

Thus, allele frequencies in a population DO change from one generation to the next.

The significance of the Hardy-Weinberg principle is that it tells us what factors cause evolution:

Those that violated the conditions listed.

- Evolution can be detected by noting any deviation from a Hardy-Weinberg equilibrium.


Microevolution

The accumulation of small changes in the gene pool over a relatively short period of time is called microevolution.

Example:

Industrial Melanism:• Pepper moths in Great Britain - Before Industrial Revolution, light-colored moths more

common than dark-colored moths (< 10% dark) - After Industrial Revolution, dark-colored moth more common than light-colored moths. (By 1950s > 80% dark & 94% dark in 1960) - After Clean Air Act of mid-1950s: By 1994 only 19% dark

13Industrial Melanism and Microevolution

When vegetation is light-colored, dark moths are seen & eaten by birds

When vegetation is dark due to pollution, light

moths are seen & eaten by birds


Causes of Microevolution

1. Genetic MutationsThe raw material for evolutionary changeProvides new combinations of allelesSome might be more adaptive than othersMany traits are polymorphic - - Two or more distinct phenotypes are present in

a population. - Examples: Human freckles

ABO blood types



2. Gene Flow (Gene Migration)Movement of alleles between populations when:

Gametes or seeds (in plants) are carried into another population

Breeding individuals migrate into or out of population

Gene flow can increase variation in a population by introducing novel alleles

Continual gene flow makes gene pools similar & reduces differences among populations. This can prevent speciation from happening.

16Gene Flow

There is interbreeding between populations; thus gene flow occurs

among the populations. So they are sub-species

of species Elaphe obsoleta



3. Nonrandom MatingWhen individuals do not choose mates randomly - Inbreeding: •Mating with relatives. Increases frequency of recessive abnormalities.

Assortative mating: Individuals select mates with their phenotype Individuals reject mates with differing phenotypeCauses population to subdivide into two phenotypic classes

Homozygotes increases; heterozygotes decrease



3. Nonrandom Mating (cont’d) Sexual selection:

Males compete for the right to reproduceFemales choose to mate with males possessing a particular phenotype

Example:

Elaborate tail of peacocks may be due to female peahens choosing males with grander tails.

All of these mechanisms can cause an increase in homozygotes



4. Genetic DriftRefers to changes in allele frequencies of a gene pool due to chance.

More likely to have a large effect on smaller populations where the sampling error is a larger part of the population.

Can cause the gene pools of two isolated populations to become dissimilar

Some alleles are lost and others become fixed (unopposed)

20Genetic Drift


Genetic Drift

Bottleneck EffectSometimes a natural disaster, or humans, might cause a near extinction event.

This prevents a majority of individuals, and their genotypes, from entering the next generation

Example: - Cheetahs: Extreme genetic similarity is believed to be due to a bottleneck

They suffer from infertility because of intense inbreeding after that time


Genetic Drift

Founder EffectWhen rare alleles occur at higher frequency in a population isolated from general population

Happens when a new population is started from just a few individuals

The alleles carried by population founders are dictated by chance

Examples: - Amish of Lancaster County, PA have high

incidence of dwarfism & polydactylism - Lake Maracaibo, Venezuela has high incidence

of Huntington disease

23Founder Effect

Rare recessive form of dwarfism linked to

polydactylism is very common in Amish of

Pennsylvania

1/14 individuals carries recessive allele


Natural Selection

Process that results in adaptation of a population to the biotic and abiotic environments

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


Natural Selection

Results in:A change in allele frequencies of the gene pool Improved fitness of the population

Natural selection is the major cause of microevolution


Types of Selection

Most traits are:• polygenic - controlled by more than one pair of

alleles located at different loci• variations in such traits result in a bell-shaped

curves

Three types of selection occur:1. Directional Selection2. Stabilizing Selection3. Disruptive Selection


Types of Selection

1. Directional SelectionOccurs when one extreme phenotype is favored The curve shifts in one direction Examples: •When bacteria become resistant to antibiotics •Human struggle against malaria Plasmodium & mosquito evolution of resistance to treatments

•Gradual increase in size of horse

28Directional Selection


Types of Selection

2. Stabilizing Selection

Occurs when an intermediate phenotype is favored

Can improve adaptation of population to a relatively constant environment

The peak of the curve increases and tails decrease

Examples:

•When human babies with low or high birth weight are less likely to survive

•Swiss starlings clutch size

30Stabilizing Selection

Swiss starlings optimal clutch size is 4-5 eggs


Types of Selection

3. Disruptive Two or more extreme phenotypes are favored over any intermediate phenotypes

The curve has two peaks

Examples:

•When Cepaea snails vary because a wide geographic range causes selection to vary

32Disruptive Selection

Dark shells more prevalent in forested

areas

Light shells near low-lying vegetation


Maintenance of VariationsGenetic variability

Populations with limited variation may not be able to adapt to new conditions & become extinct

Maintenance of variability is advantageous to population

Only exposed alleles are subject to natural selection:

● Thus, heterozygotes can be a protector of recessive alleles that might otherwise be weeded out.Allows even lethal alleles to remain in population at low frequencies virtually forever


Maintenance of Variations

Heterozygote Advantage:Lethal recessive alleles may confer advantage to heterozygotes Sickle cell disease is detrimental in homozygote However, heterozygotes more likely to survive malaria than homozygous dominants because malaria parasite is unable to live in their red blood cells while it destroys the RBCs of homozygotes.

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

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35Sickle-cell Disease


Macroevolution

Macroevolution

- Any evolutionary change at or above the level of the species

● Speciation

- Splitting of one species into two or more species

- Transformation of one species into a new species over time.


Definition of a Species

Morphological

Can be distinguished anatomically

Physical traits differ

Specialist decides what criteria probably represent reproductively isolated populations

Most species described this way


Species Definitions

Biological Species Concept

A group of populations that can breed among themselves to produce fertile offspring

Are reproductively isolated from other such populations; unable to reproduce with members of other groups

The organisms share a gene pool

Very few actually tested for reproductive isolation

Cannot be applied to asexual organisms or those only known from the fossil record

39Biological Species Definition

These three species of flycatchers are

reproductively isolated since they do not

reproduce with each other


Reproductive Isolating Mechanisms

Reproductive isolating mechanisms are any structural, functional or behavioral characteristics that prevent successful reproduction from occurring between different groups of organisms.

Two general types: Pre-zygotic Mechanisms - Discourage attempts to mate ● Post-zygotic Mechanisms - Prevent hybrid offspring from developing or breeding


Prezygotic Mechanisms

1. Habitat Isolation

- Occupy different habitats & are less likely to meet & reproduce

2. Temporal Isolation

- Species live in same habitat but reproduce at different times

3. Behavioral Isolation

- Species have their own courtship rituals

- Firefly flashes; cricket chirping; chemical signals

42Temporal Isolation


Prezygotic Mechanisms

4. Mechanical Isolation

- Reproductive parts are incompatible

5. Gamete Isolation

- If gametes meet, they do not fuse to become a zygote.


Postzygotic Mechanisms

1. Zygote Mortality - Hybrid zygote might be created but dies2. Hybrid Sterility - Hybrid zygotes develop into sterile adults - Example: Mule is a cross between a horse & a donkey.

They are usually sterile

3. Reduced F2 Fitness

- If hybrids can reproduce, their offspring cannot.


Two Modes of Speciation

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

Examples:

California salamanders separated by Central Valley

Green iguanas in Galapagos Islands are thought to be ancestors of marine & rhinoceros iguanas

46Allopatric Speciation


Two Modes of Speciation

2. Sympatric Speciation

One population develops into two or more reproductively isolated groups

No prior geographic isolation

Examples:

Tetraploid hybridization in wheat (polyploidy)Results in self fertile species

Reproductively isolated from either parental species

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Adaptive Radiation

When many new species evolve from a single ancestral species.

Occurs when members of species become adapted to the different environments

This is an example of allopatric speciation

Examples:1. Hawaiian honeycreepers 2. Galapagos finches

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