Evolution Review

Chapters 22 - 26

Introduction

▪ Definition: Evolution is a change in allelic frequencies in a population

▪ Sometimes this can happen quickly, ex., antibiotic-resistant bacteria or it can take place over hundreds, thousands, or even millions of years.

▪ The theory of evolution is supported by evidence from many disciplines

History of Earth

▪ Earth’s history dated by several methods– Studying undisturbed sed. Rock

layers with fossils reveals the relative age of rocks and fossils

– Radiometric dating can reveal the absolute age of rocks – this is based on half life.

▪ The Earth is considered to be 4.6 byo.

Extinctions

▪ 5 major extinctions have occurred

▪ 4 bya there was no free O2

▪ Cyanobacteria, O2 generating organisms that form rocklike structures called stromatolites, provided free O2

▪ This lead to the Cambrian explosion or sudden appearance of many present-day animal phyla

▪ Permian extinction, 251 mya, 90-96% of all species lost

Evidence for Evolution

▪ Many areas of scientific study provide evidence for evolution. Here are six:– Fossil Record

– Comparative Anatomy

– Comparative Biochemistry

– Comparative Embryology

– Molecular Biology

– Biogeography

▪ Fossil Record: think radiometric dating & half life

▪ Comparative Anatomy: think homologous structures

Biogeography

▪ Think plate tectonics – giant plates of the Earth’s crust floating on the mantle.

▪ Convection currents in the mantle move the plates in what is called continental drift.

▪ The movement effect has changed the flora and fauna of the Earth – 250 mya, all continents together into one supercontinent – Pangaea.

Historical Context for Evolution

▪ Linnaeus – Taxonomy or naming organisms, developed binomial nomenclature, ex., Homo sapiens

▪ Cuvier-studied fossils and strata, came up with catastrophism

▪ James Hutton-geologist came up with gradualism – Earth molded by forces still at work today

▪ Lyell-also a gradualist –thought Earth to be very old –this influenced Darwin

▪ Lamarck-also developed a theory of evolution. Based on inheritance of acquired characteristics and use and disuse

▪ Wallace-came up with Natural Selection identical to Darwin

Darwin

▪ Naturalist left England aboard the HMS Beagle – visited the Galapagos Islands, etc

▪ Worked out theory of natural selection or descent with modification as the mechanism for evolution

▪ Published “On the Origin of the Species”

Natural Selection Main Points

▪ Populations tend to grow exponentially, overpopulate, and exceed their resources – idea came about after reading Malthus’s work

▪ Overpopulation results in competition & a struggle for existence

▪ In any population, there is variation and an unequal ability of individuals to survive & reproduce (Darwin didn’t know how variation took place, it took Mendel’s work to help here)

▪ Only the best-fit individuals survive & get to pass on their traits

▪ Evolution occurs as advantageous traits accumulate in a population. No individual organism changes, rather, the frequency of an allele within a population changes

How Insects “Become” Resistant to Pesticides

▪ Insects do not actually become resistant to pesticides

▪ Some insects are naturally resistant to the insecticide (think variability here)

▪ When the poison is spread, these insects have a selective advantage

▪ They survive to breed – this is directional selection

Before Poison After Poison

Creating Variation in a Population

▪ Balanced Polymorphism –think dark/light mice and what surface they live on – two or more phenotypic distinct forms in a single population

▪ Geographic Variation-think rabbits in the north vs the south

▪ Sexual Reproduction– Independent assortment of

chromosomes

– Crossing over

– Random fertilization

▪ Outbreeding-mating with non-closely related individuals within a population

▪ Diploidy-2n condition, keeps a huge pool of alleles

Causes of Evolution

▪ Genetic Drift

▪ Gene Flow

▪ Mutations

▪ Nonrandom Mating

▪ Natural Selection

Genetic Drift

▪ Change in gene pool by chance– Bottleneck effect: something

takes place that reduces size of population

– Founder effect: a small population breaks away from a bigger one –think the Amish and polydactyly here

▪ The increase or decrease in allelic freq. of a trait results from genetic drift or from the trait being advantageous or disadvantageous.

Gene Flow

▪ The movement of alleles into or out of a population. It can occur as a result of the migration of fertile individuals or gametes between populations

▪ For example, pollen from one valley can be carried by the wind across a mountain to another valley

▪ Will increase diversity

Mutations & Nonrandom Mating

▪ Mutations are changes in genetic material and are the raw material for evolutionary change

▪ Nonrandom mating is that individuals choose their mates for a specific reason – this serves to eliminate the less-fit individuals

Hardy-Weinberg Equilibrium –Characteristics of Stable Populations

1. Population must be very large

2. Population must be isolated from other populations

3. There must be no mutations in the population

4. Mating must be random

5. No natural selection

The Hardy-Weinberg Equation

▪ HW equation enables us to calculate frequencies of alleles in a population.

Speciation

▪ The def. of a species is a population whose members have the potential to interbreed in nature and produce viable, fertile offspring

▪ If one group of genes in a population gets separated from another, speciation could occur.

▪ Anything that fragments a population and isolates small groups of individuals may cause speciation

▪ Allopatric speciation-caused by geographic isolation (mountains, rivers, lakes, etc.)

▪ Sympatric speciation-no geographic isolation takes place– Polyploidy

– Habitat isolation

– Behavioral isolation

– Temporal isolation

– Reproductive isolation

Polyploidy

▪ Where a cell has more than two complete sets of chromosomes (4n,8n, etc.)

▪ Common in plants

▪ Results from nondisjunction during meiosis when gametes with the 2n number are fertilized by another abnormal (2n) gamete, thus the daughter cell has 4n

▪ Polyploid organisms can only breed with each other now – so are isolated

Reproductive Isolation

▪ Prezygotic barriers – things that prevent mating

▪ Postzygotic barriers – things that prevent the production of fertile offspring, once mating has occurred

Post or Pre?

Patterns of Evolution

▪ Divergent Evolution: when a population becomes isolated from the rest of the species and evolves into a new species – allopatric & sympatric speciation are examples of divergent evolution

▪ Convergent Evolution: When unrelated species occupy the same environment and subjected to similar selective pressure – they will show similar adaptations (good example, whales and fish – similar shape due to same environment

▪ Parallel Evolution: two related species that made similar evolutionary adaptations after their divergence from a common ancestor

▪ Coevolution: a reciprocal evolutionary set of adaptations of two interacting species. Think predator-prey relationships here

Adaptive Radiation

▪ This is the emergence of numerous species from a common ancestor introduced into an environment

▪ Each newly emerging form specializes to fill an available ecological niche

▪ 14 species of Darwin’s finches on the Galapagos Islands diverged from a single ancestral species

Modern Theory of Evolution

▪ Gradualism-theory that organisms descend from a common ancestor gradually, over a long period of time, in a linear or branching fashion. Fossils should exist as evidence of every stage in the process with no missing links. However, the fossil record is at odds with this theory because scientists rarely fin transitional forms or missing links.

▪ Punctuated Equilibrium-developed by Stephen J. Gould and Niles Eldridge after they observed that the gradualism theory was not supported by fossil record. The theory proposes that new species appear suddenly after long periods of stasis.

Gradualism vs Punctuated Evolution

Origin of Life

▪ Classic experiments to recreate the early Earth atmosphere to determine how the first organic molecules and earliest life developed. The big names here:– A.I. Oparin & Haldane: hypothesized separately that under the conditions of

early Earth, organic molecules could form

– Stanley Miller & Urey: Tested Oparin/Haldane’s hypothesis – created a variety of organic molecules, including amino acids

– Sidney Fox: started with organic molecules and was able to produce membrane bound, cell-like structures he called proteinoid microspheres

Heterotroph Hypothesis & Theory of Endosymbiosis

▪ 1st cells on earth were anaerobic heterotrophic prokaryotes

▪ Evolved around 3.5 bya, while eukaryotes did not evolve until 1.5 bya

▪ Lynn Margulis proposed that endosymbiosis allowed the evolution of eukaryotes

▪ Endosymbiosis states that mitochondria and chloroplasts were once free-living prokaryotes that took up residence inside larger prokaryotic cells

Evidence for Endosymbionts

▪ Chloroplasts and mitochondria have their own DNA

▪ This DNA is more like prokaryotic DNA than euk DNA (no histones, single strand, circular, etc)

▪ These organelles have a double membrane. The biggie here is that the organelle’s inner membrane belongs to it, while the outer membrane belongs to the host cell’s plasma membrane

Phylogenetic Trees

▪ All living things evolved rom a common ancestor almost 4 bya, which is why the principles of biology apply to all organisms

▪ The evolutionary history of these relationships is known as phylogeny

▪ A phylogenetic tree or cladogram is a diagrammatic reconstruction of that history

▪ To make a cladogram, scientists narrow down the possibilities by using the principle of maximum parsimony, which states that one should follow the simplest explanation that coincides with the facts

Each group is a clade as each has a trait that is not shared with their ancestors