Evolution Change in allele frequency in a population

Evolution Change in allele frequency in a population over time.

Population • A localized group of individuals of the same species.

Importance of Hardy-Weinberg • Way to calculate allele frequencies through time. • Yardstick to measure rates of evolution.

Hardy-Weinberg Assumptions 1. Large Population 2. Isolation 3. No Net Mutations 4. Random Mating 5. No Natural Selection

If H-W assumptions hold true: • The allele frequencies will not change over time. • Evolution will not occur.

Causes of Evolution 1. Genetic Drift 2. Gene Flow 3. Mutations 4. Nonrandom Mating 5. Natural Selection

Genetic Drift • Changes in the gene pool a population due to random chance • Types: 1. Plain Genetic Drift 2. Bottleneck Effect 3. Founder's Effect

By Chance

Bottleneck Effect • Loss of most of the population by disasters. • Surviving population may have a different gene pool than the original population.

Result • Some alleles lost. • Other alleles are over-represented. • Genetic variation usually lost.

Importance • Reduction of population size may reduce gene pool for evolution to work with. • Ex: Cheetahs

Founder's Effect • The reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors. • Ex: Old-Order Amish

Result • Genetic variation reduced. • Some alleles increase in frequency while others are lost (as compared to the parent population). Genetic Drift has a larger impact on small populations

Gene Flow • 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 • May change allele frequencies (small population). • Source of new alleles for selection. • Often lost by genetic drift.

Nonrandom Mating • Sexual-Selection

Natural Selection • Differential success in survival and reproduction.

Fitness - Darwinian • The relative contribution an individual makes to the gene pool of the next generation.

Chapter 24 The Origin of Species

Question? • What is a species? • Comment - Evolution theory must also explain how species originate.

Question? • How many species of African Violets are here?

Two Concepts of Species 1. Morphological 2. Biological

Morpological Species Concept • Organisms with very similar morphology (physical form) are the same species

Problem • Where does extensive phenotype variation fit?

Biological Species Concept • A group of organisms that could interbreed in nature and produce fertile offspring.

Problems • Can’t apply BSC to: • Asexual organisms • Extinct species

Speciation (new species arising) Requires: 1. Variation in the population. 2. Selection/Genetic Drift 3. Reproductive Isolation. Speciation in lizards https: //www. youtube. com/watch? v=a DIQFQOCGa. I

Reproductive Barriers • Serve to isolate a populations from other gene pools. • Create and maintain “species”.

Main Types of Barriers Prezygotic - Prevent mating or fertilization. Postzygotic - Prevent viable, fertile offspring.

Prezygotic - Types 1. Habitat Isolation 2. Temporal Isolation 3. Behavioral Isolation 4. Mechanical Isolation 5. Gametic Isolation

Habitat Isolation • Populations live in different parts of the environment • 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 and dances in birds.

Mechanical Isolation • Structural differences that prevent gamete transfer. • 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 Types 1. Reduced Hybrid Viability 2. Reduced Hybrid Fertility

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 often “odd” so meiosis won’t work. • Ex – donkeys and horses produce mules

How do species occur? 1. Allopatric Speciation 2. Sympatric Speciation Both work through a block of gene flow between two populations.

Allopatric Speciation • Allopatric = other homeland • Ancestral population split by a geographical feature.

Sympatric Speciation • Sympatric = same homeland • New species arise within the range of parent populations.

What is the Speed of Speciation?

• Gradualism • Punctuated Equilibrium

Gradualism Predicts: • Long periods of time are needed for steady, continuous evolution • Should see plenty of “transition species” • Problem: not many “transition species” fossils found

Punctuated Equilibrium • Evolution has two speeds of change: • Gradualism or slow change • Rapid bursts of speciation

Predictions • Speciation can occur over short periods of time in a changing environment (natural disasters) • Fossils of transition species are rare

Summary • Be able to discuss the main theories of what is a “species”. • Know various reproductive barriers and examples.

Summary • Know allopatric and sympatric speciation. • Know adaptive radiation. • Be able to discuss gradualism and punctuated equilibrium theories.