How Populations Evolve

How Populations Evolve Chapter 13

Biology and Society: Persistent Pests • Mosquitoes and malaria - in the 1960s, the World Health Organization (WHO) began a campaign to eradicate the mosquitoes that transmit malaria - the chemical DDT was used with promising results - but some mosquitoes with inherent resistance to the poison survived and were free to reproduce - their offspring may also inherit the genes for pesticide resistance - in each subsequent generation, the proportion of pesticide-resistant insects increases

Figure 13.1

Charles Darwin and The Origin of Species • On Nov 24,1859 published On the Origin of Species by Means of Natural Selection • Darwin (1809 – 1882) argued that contemporary species arose from ancestors - through a process of ‘descent with modification,’ with natural selection as the mechanism • Basic idea of natural selection – organisms can change over generations - individuals with certain heritable traits leave more offspring than others

Result of natural selection is evolutionary adaptation - increase in frequency of traits that are suited to the environment • Genetic composition of a population changes over time – evolution - Darwin’s book focused attention on the great diversity of organisms, including - their origins and relationships, their similarities and differences, their geographic distribution, and - most importantly on their adaptations to their surrounding environment

Camouflage – example of evolutionary adaptation - related species of insects called mantids have diverse shapes and colors that evolved in different environments Figure 13.2

Darwin’s Cultural and Scientific Context • Prevailing thought during Darwin’s lifetime was that the Earth was relatively young - and populated by a huge number of unrelated species • The Origin of Species challenged this theory

Historical context of Darwin’s life and ideas Figure 13.3

The Idea of Fixed Species • 2500 years ago, Greek philosopher Anaximander promoted the idea that life arose in water - and that simpler forms preceded more complex forms • Aristole (324-322 BCE) held the belief that species are fixed and did not evolve - the Judeo-Christian culture fortified this idea - suggested that the Earth is only about 6,000 years old

Lamarck and Evolutionary Adaptations • In the mid-1700s, the study of fossils began to take form as a branch of science • French naturalist Georges Buffon suggested the Earth might be much older than 6,000 years - observed similarities between fossils and living species - in 1766 proposed that certain fossil forms might be ancient versions of similar living species

In the early 1800s, French naturalist Jean-Baptiste de Lamarck suggested an explanation - organisms evolved by the process of adaptation - the refinement of characteristics that equipped organisms to be successful in their environments - also suggested some erroneous ideas, such as the inheritance of acquired characteristics - strong beaks of seed-cracking birds, result of ancestors exercising their beaks and passing beak power to offspring

The Voyage of the Beagle • Dec 1831, Darwin left Great Britain on the HMS Beagle to explore the world - main mission was to chart poorly known stretches of the South American coastline - collected 1000s of specimens - observed various adaptations in organisms from - diverse environments from Brazilian jungles, Argentina grasslands, desolate and from frigid lands at the southern tip of South America

Figure 13.4

Darwin was intrigued by - the geographic distribution of organisms on the Galapagos Islands - most of the animals live nowhere else in the world, but - the fact that Galápagos organisms resembled those in South America - those living in temperate regions of SA, seemed more closely related to species living in tropical regions of SA then species living in Europe’s temperate regions

Figure 13.5

The New Geology • Darwin was strongly influenced by the writings of geologist Charles Lyell - Principles of Geology - an ancient Earth sculpted by gradual geologic processes that continue today - earthquakes occurring sporadically over millions of years can eventually give rise to a mountain range • Darwin applied this principle of gradualism to the evolution of Earth’s life - rocks bearing marine fossils were gradually lifted from the seafloor

Descent with Modification • Darwin made 2 main points in The Origin of Species: - organisms inhabiting Earth today descended from ancestral species - natural selection was the mechanism for descent with modification • Darwin postulated, as descendants of the earliest organisms spread into various habitats - over millions of years they accumulated different modifications, or adaptations, to diverse ways of life

Descent with Modification • Evolutionary tree based mainly on evidence from fossils – anatomy, order of appearance in geologic time and distribution • Genetic analyses suggest that African elephants are two separate species Figure 13.6

Checkpoint • What is gradualism? How did Darwin apply that idea to the evolution of life? • What were the 2 main points in Darwin’s The Origin of Species? • Darwin’s phrase for evolution, ____ with ____, captured the idea that an acesteral species could diversify into many descendant species by the accumulation of different ____ to various environments

Answers • Gradualism, the idea that large changes on Earth can result from the accumulation of small changes over a very long time; Darwin applied this idea to suggest that species evolve through the slow accumulation of small changes over time • Descent of diverse species from common ancestors and natural selection as the mechanism of evolution • Descent; modification; adaptations

Evidence of Evolution • Biological evolution leaves observable signs - clues to the past include the 5 lines of evidence in support of evolution • Fossils – a historical record - biogeography - comparative anatomy - comparative embryology - molecular biology

The Fossil Record • Fossils – preserved remnants or impressions left by organisms that lived in the past - most are found in sedimentary rocks • Sedimentation – sand and silt eroded from the land carried by rivers to seas and swamps - particles settle to the bottom over millions of years - deposits pile up and compress older sediments below into rock - varying rates of sedimentation and types of particles lead to identifiable layers, or strata, of rock

Formation of sedimentary rock - deposition of fossils from different time periods Each stratum, or layer, represents a particular time in Earth’s history and is characterized by a collection of fossils of local organisms that lived at that time Figure 13.7

The Fossil Record • The ordered sequence of fossils as they appear in rock layers, mark the passing of geologic time - each rock layer bears a unique set of fossils - represents a local sampling of organisms that lived when the sediment was deposited -reveals the appearance of organisms in a historical sequence - prokaryotes, oldest known fossils, dating from ~3.5 bya - fits with molecular and cellular evidence that prokaryotes are the ancestors of all life

Strata of Sedimentary Rock at the Grand Canyon • Colorado River – cut through over 2,000m of rock - exposed sedimentary layers (like pages from the book of life) enable you to look back through 100s of millions of years - each layer entombs fossils that represent some of the organisms from that period of Earth’s history Figure 13.8

Fossils in younger layers of rock reveal evolution - of various groups of eukaryotic organisms - successive appearance of the different classes vertebrates (animals with backbones), oldest - fishlike fossils, amphibians, reptiles, then mammals • Paleontologists – scientist that study fossils - have discovered many transitional forms that link past and present; for example birds - a series of fossils provides evidence that birds descended from one branch of dinosaurs or that a - series of fossilized whales connect these aquatic mammals to their land-dwelling ancestors

Transitional fossil linking past and present – hypothesis that whales evolved from terrestrial ancestors predicts a 4-limbed beginning for whales • Paleontologists digging in Egypt and Pakistan have identified extinct whales that had hind limbs – fossilized leg bones of Basilosaurus Figure 13.9

Biogeography • The study of the geographic distribution of species - first suggested to Darwin that today’s organisms evolved from ancestral forms - noted that the Galápagos animals resembled species of the SA mainland more than animals on similar but distant islands - suggested that Galápagos species evolved from SA immigrants • Many examples from biogeography support evolutionary theory- marsupials in Australia - in the Darwinian view, we find species where they are because they evolved from ancestors that inhabited those regions

Figure 13.10 • Biogeography – Australia is home to many unique plants and animals, such as marsupials, mammals that evolved in isolation from other continents where placental mammals diversified

Comparative Anatomy • Comparison of body structures between different species - confirms that evolution is a remodeling process in which - ancestral structures that functioned in one capacity become modified as they take on new functions or - descent with modification • Homology – the similarity in structures due to common ancestry - forelimbs of diverse mammals are homologous structures - same skeletal elements make up the forelimbs of humans, cats, whales, and bats - vestigial organs, remnants of structures that served important functions in the organism’s ancestors

Homologous structures: anatomical signs of descent with modification • Forelimbs of all mammals are constructed by the same homologous bones • Hypothesis that all mammals descended from a common ancestor predicts that their forelimbs, though diversely adapted, would be variations on a common anatomical theme Fig 13.11

How Populations Evolve