Unit III Evolution

Learning Goal 2

Describe how genetic variation in populations leads to microevolution

Variation in Natural Populations

• Phenotypic VariationQuantitative Variation – individuals differ in small, incremental ways. If you measured the height of everyone in this classroom you would see a continuous variation from shortest to tallest.Graphing this data would result in a bell-shaped curve. A narrow bell would indicates little variation. A broader bell represents more variation.

• Qualitative Variation

Traits that exist in two or more discrete states. This is called polymorphism; such as in snow geese which have either blue or white plumage.

• Genetic and Environmental Causes

Sometimes phenotypes (the appearance of an organism) can be influenced by genetics (genotypes) or environmental factors.

Only genetically based variation is subject to evolutionary change

• Processes That Generate Genetic VariationNew alleles (members of gene pairs) can arise from mutations in DNA. These can result from changes in a few letters of the code or large scale changes in chromosomes during crossing over.Shuffling of existing alleles through normal genetic recombination during fertilization.

Population Genetics

• Genetic Structure

Individuals in populations contain pairs of chromosomes in their cells called homologous chromosomes. They have two alleles at each gene locus. The sum total of all alleles in a population is called the gene pool.

Scientists study the genetics of populations by calculating genotype frequencies, the percentage of individuals possessing each genotype.

Allele frequencies can then be calculated since there are two alleles for every genotype. The letter p is used to represent the frequency of one alleles and the letter q is used for the other.

Snapdragons can have red, pink, or white flower phenotypes.In this example:p = 50%q = 50%The sum of the two allele frequencies must equal 1 (100%)

The Hardy-Weinberg Principle

• Principle developed to specify conditions under which a population would reach genetic equilibrium, a situation in which neither allele frequencies nor genotype frequencies change in succeeding generations. This can only happen if the following conditions are met:

1. No mutations

2. No migration

3. Size is infinite

4. All genotypes survive

equally

5. Random mating

• This model is called a null model because it shows a situation where there is no change.

• If a population’s genotype frequencies do not match predictions of this model and allele frequencies are changing, then microevolution may be occurring.

Agents of Microevolution

• MutationsSpontaneous changes in DNA that can occur when cells divide. Those that occur in the sex cells (gametes) can be inherited by the next generation.

• Mutations can be:Deleterious – Alter individual in harmful waysLethal – Cause death of an individualNeutral – Don’t affect the individual.Advantageous – Benefit the individual.

• Gene Flow

As organisms move from one population to another it can change the gene pool as new alleles are introduced.

Genetic Drift

• Population BottlenecksOccur when an event such as drought or disease dramatically decreases a population thus reducing genetic variability.

• Founder EffectOccurs when a small group of individuals move to a new area. The genetic make up of the new population may be different than in the original group.

Natural Selection

• Process by which traits that enable individuals to survive better and reproduce more become more common in subsequent generations.

• Directional SelectionIndividuals near one end of the phenotypic spectrum are favored and that trait becomes more common.

• Stabilizing Selection

Individuals expressing intermediate phenotypes have the highest relative fitness (leave more offspring).

• Disruptive Selection

Extreme phenotypes have higher relative fitness than intermediate phenotypes.

Sexual Selection Results in Nonrandom Mating

• Believed to be responsible for showy structures in males and sexual dimorphisms, the difference in appearance between males and females of a species.Intersexual – females choose males based on certain showy features.Intrasexual – males use otherwise useless structures to fight or intimidate other males.

Adaptation

• Adaptive traits are products of natural selection that increase the relative fitness of organisms in their environment.

• Adaptation is the accumulation of adaptive traits over time.

Species

• Morphological Species ConceptIndividuals of the same species share measurable traits that distinguish them from other species.Used to classify species since the time of Linnaeus.

• Biological Species Concept

Species consist of groups of interbreeding natural populations that produce fertile offspring.

• Phylogenetic Species ConceptUses both morphological and genetic sequence data to classify species.Applies to organisms that reproduce asexually such as microorganisms as well as sexually reproducing organisms.Also extinct organisms whose reproductive habits are not known.

Reproductive Isolation

• Prezygotic Isolating Mechanisms

Exert their effects before the production of a zygote (fertilized egg). There are five types:

Ecological Isolation – species live in the same geographical region but occupy different habitats.

Temporal Isolation – Species live in the same habitat but mate at different times of the year.

Behavioral Isolation – Mating signals for one species are not recognized by another.

Mechanical Isolation – Species have differences between reproductive organs or other body parts.

Gametic Isolation – There is a mismatch between the sperm of one species and the eggs of another.

Postzygotic Isolating Mechanisms

Hybrid Inviability – Hybrids (offspring of mixed species) die as embryos or at an early age.

Hybrid Sterility – Hybrids develop into adults but cannot reproduce.

Hybrid Breakdown – First generation hybrids are healthy and fertile, but subsequent breeding produce chromosomal abnormalities in future generations.

Geographic Speciation

• Allopatric Speciation – A population becomes separated by a geographic barrier such as a mountain range or a river.

• Mutations, genetic drift, and natural selection can lead to differences that prevent reproduction if they encounter each other in the future.

Parapatric Speciation

• A species is distributed in one area where parts of the environment differ substantially. Individuals may experience natural selection in different ways which could lead to reproductive isolation.

Sympatric Speciation

• Occurs in species where some members of the population occupy different areas of the same habitat, then stick to themselves for mating purposes. Enough genetic difference may arise to lead to reproductive isolation.

The Genetics of Speciation

• Genetic Divergence – When populations are separated geographically, gene flow stops and genetic differences accumulate.

Polyploidy

Occurs in plants, and some animals when errors in gamete formation (sex cells) result in extra sets of chromosomes.

Chromosome Alterations

If chromosomes become rearranged it can lead to reproductive isolation because members of the population are no longer genetically compatible.

Learning Goal 2 Vocab

1. Genetic Equilibrium –

2. Genetic Drift –

3. Directional Selection –

4. Stabilizing Selection –

5. Disruptive Selection –

6. Biological Species Concept –

7. Reproductive Isolation –

8. Prezygotic vs Postzygotic Isolating Mechnisms –

9. Parapatric vs Sympatric Speciation –

10. Genetic Divergence -