Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Gene expression programming wikipedia , lookup
The Selfish Gene wikipedia , lookup
Mate choice wikipedia , lookup
Hologenome theory of evolution wikipedia , lookup
Evolutionary landscape wikipedia , lookup
Evolution of sexual reproduction wikipedia , lookup
Saltation (biology) wikipedia , lookup
Sexual selection wikipedia , lookup
Natural selection wikipedia , lookup
Chapter 23 The Evolution of Populations Overview: The Smallest Unit of Evolution • One misconception is that organisms evolve during their lifetimes • Natural selection acts on individuals, but only populations evolve • Consider, for example, a population of medium ground finches on Daphne Major Island – During a drought, large-beaked birds were more likely to crack large seeds and survive – The finch population evolved by natural selection Overview: The Smallest Unit of Evolution • One misconception is that organisms evolve during their lifetimes • Natural selection acts on individuals, but only populations evolve • Consider, for example, a population of medium ground finches on Daphne Major Island – During a drought, large-beaked birds were more likely to crack large seeds and survive – The finch population evolved by natural selection Gene flow is the movement of alleles between populations • Gene flow: movement of alleles from one pop. to another • Occurs when individuals join new populations and reproduce. • Gene flow keeps neighboring populations similar. • Low gene flow increases the chance that two populations will evolve into different species. bald eagle migration Adaptive radiation 10% of population natural disaster kills five green frogs 20% of population Hardy-Weinberg • The Hardy-Weinberg principle: – Allele frequencies in a population will remain constant assuming: • No Mutations • No Gene Flow • Random Mating • No Genetic Drift • No Selection Outline • Population genetics – Variations in terms of allele differences. • Microevolution – Hardy-Weinberg – Causes of Microevolution • Natural Selection – Types of Selection • Macroevolution Population Genetics • Population – All members of a single species – Occupying a particular area at the same time. HapMap Project • People inherit patterns of sequence differences, called haplotypes – If one haplotype of a person has an A rather than a G at a particular location in a chromosome, there are probably other particular base differences near the A – Genetic data from African, Asian, and European populations will be analyzed • A HapMap is a catalog common sequence differences that occur in a species – The goal of the project is to link haplotypes to risk for specific illnesses – May lead to new methods of preventing, diagnosing, and treating disease haplotype map (haploid genotype) SNP = Single Nucleotide Polymorphism HapMap Project Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (Top left, center, right; Bottom 1, 3, 4): © Vol. 105/PhotoDisc/Getty; (Bottom 2): © Vol. 42/PhotoDisc/Getty; (Bottom 5): © Vol. 116/PhotoDisc/Getty Microevolution • In 1930s population geneticists described variations in a population in terms of alleles • Microevolution pertains to changes within a population. evolutionary – Various alleles at all the gene loci in all individuals make up the gene pool of the population. – Gene pool of a population: • Genotype • Allele frequencies Frequency of Gametes Calculation • From genotype frequencies, the allele and gamete frequencies can be calculated Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. genotypes frequency of genotypes in the population frequency of alleles and gametes in the population DD Dd dd 0.04 0.32 0.64 0.04 + 0.16 0.16 + 0.64 0.20 D 0.80 d Hardy-Weinberg • The Hardy-Weinberg principle: – Allele frequencies in a population will remain constant assuming: • No Mutations • No Gene Flow • Random Mating • No Genetic Drift • No Selection Hardy-Weinberg Equilibrium F1generation DD Dd dd Genotypes: Genotype frequencies: 0.04 0.32 0.64 Allele and gamete frequencies: D = 0.20 d = 0.80 eggs sperm F2generation 0.20 D 0.80 d 0.20 D 0.04 DD 0.16 Dd 0.80 d 0.16 Dd 0.64 dd Offspring Genotype frequencies: 0.04 DD + 0.32 Dd + 0.64 dd = 1 2 2 p + 2pq + q = 1 p2 = frequency of DD genotype (dark-colored) = (0.20)2 2pq = frequency of Dd genotype (dark-colored) = 2(0.20)(0.80) q2 = frequency of dd genotype (light-colored) = (0.80)2 =0.04 =0.32 =0.64 1.00 Industrial Melanism and Microevolution Early observation 36% dark-colored phenotype Later observation 64% dark-colored phenotype Peppered moth (Biston betularia) Hardy-Weinberg • Required conditions are rarely (if ever) met – Changes in gene pool frequencies are likely – When gene pool frequencies change, microevolution has occurred • Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place • The founding of a small population can lead to genetic drift. – It occurs when a few individuals start a new population. – The founder effect is genetic drift that occurs after start of new population. The current population is thought to have descended from only seven females and eight males. One of the early colonists apparently carried a recessive allele for retinitis pigmentosa, a progressive form of blindness that afflicts homozygous individuals. The frequency of the allele that causes this disease is ten times higher on Tristan da Cunha than in the populations from which the founders came. Causes of Microevolution • Genetic Mutations – The raw material for evolutionary change – Provides new combinations of alleles – Some might be more adaptive than others Causes of Microevolution • Gene Flow – Movement of alleles between populations when: • Gametes or seeds (in plants) are carried into another population • Breeding individuals migrate into or out of population – Continual gene flow reduces genetic divergence between populations Gene Flow Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. selfpollination stigma stamen Pisum sativum Causes of Microevolution • Nonrandom Mating – When individuals do not choose mates randomly • Assortative mating: – Individuals select mates with their phenotype – Individuals reject mates with differing phenotype • Sexual selection: – Males compete for the right to reproduce – Females choose with males possessing a particular phenotype – Both of these cause an increase in homozygotes Causes of Microevolution • Genetic Drift – Occurs by disproportionate random sampling from population – Can cause the gene pools of two isolated populations to become dissimilar – Some alleles are lost and others become fixed (unopposed) – Likely to occur: • After a bottleneck • When severe inbreeding occurs, or • When founders start a new population – Stronger effect in small populations Genetic Drift Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10% of population natural disaster kills five green frogs 20% of population Genetic Drift • Bottleneck Effect – A random event prevents a majority of individuals from entering the next generation – Next generation composed of alleles that just happened to make it Genetic Drift • Founder Effect – When a new population is started from just a few individuals – The alleles carried by population founders are dictated by chance – Formerly rare alleles will either: • Occur at a higher frequency in the new population, or • Be absent in new population Figure 23.1 Average beak depth (mm) Figure 23.2 10 9 8 0 1978 1976 (similar to the (after prior 3 years) drought) • Microevolution is a change in allele frequencies in a population over generations • Three mechanisms cause allele frequency change: – Natural selection – Genetic drift – Gene flow • Only natural selection causes adaptive evolution Concept 23.1: Genetic variation makes evolution possible • Variation in heritable traits is a prerequisite for evolution • Mendel’s work on pea plants provided evidence of discrete heritable units (genes) Genetic Variation • Genetic variation among individuals is caused by differences in genes or other DNA segments • Phenotype is the product of inherited genotype and environmental influences • Natural selection can only act on variation with a genetic component Humans appeared late in Earth’s history. Humans share a common ancestor with other primates. • Primates are mammals with flexible hands and feet, forward-looking eyes and enlarged brains. • Primates evolved into prosimians and anthropoids. – Prosimians are the oldest living primates. – They are mostly small and nocturnal. – Anthropoids are humanlike primates. – They are subdivided into the New World monkeys, Old World monkeys, and hominoids. – Homonoids are divided into hominids, great apes, and lesser apes. – Hominids include living and extinct humans. • Bipedal means walking on two legs. – foraging – carrying infants and food – using tools • Walking upright has important adaptive advantages. There are many fossils of extinct hominids. • Most hominids are either the genus Australopithecus or Homo. • Australopithecines were a successful genus. • The Homo genus first evolved 2.4 million years ago. Modern humans arose about 200,000 years ago. • Homo sapiens fossils date to 200,000 years ago. • Human evolution is influenced by a tool-based culture. • There is a trend toward increased brain size in hominids. Australopithecus afarensis Lucy – 4 mybp Homo habilis Homo neanderthalensis 1.5 mybp Homo sapiens 196,000 thousand ybp • Many species evolve from one species during adaptive radiation. – ancestral species diversifies into many descendent species – descendent species usually adapted to wide range of environments Evolutionary biology today Evolution unites all fields of biology "Nothing in Biology Makes Sense Except in the Light of Evolution" -Theodosius Dobzhansky 1973 What is the study of Paleontology? • Heritability is the ability of a trait to be passed down. • There is a struggle for survival due to overpopulation and limited resources. Populations would grow geometrically if resources were unlimited. • Darwin proposed that adaptations arose over many generations. • Natural selection is a mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals. Natural selection explains how evolution can occur. • There are four main principles to the theory of natural selection. – Variation: the heritable differences in each population are the basis of natural selection. – Overproduction: having many offspring increases the chances of survival but creates competition – Adaptation: a certain variation that allows an organism to survive better than organisms it competes against. – Descent with modification: heritability of adaptations. • Fitness is the measure of survival ability and ability to produce more offspring. A population shares a common gene pool. Variation Between Populations • Most species exhibit geographic variation, differences between gene pools of separate populations • For example, Madeira is home to several isolated populations of mice – Chromosomal variation among populations is due to drift, not natural selection Genetic variation comes from several sources. • Mutation is a random change in the DNA of a gene. – can form new allele – can be passed on to offspring if in reproductive cells • Recombination forms new combinations of alleles. – usually occurs during meiosis – parents’ alleles arranged in new ways in gametes Figure 23.5 Ldh-Bb allele frequency 1.0 0.8 0.6 0.4 0.2 0 46 44 42 Maine Cold (6°C) 40 38 36 Latitude (ºN) 34 32 Georgia Warm (21ºC) 30 Sources of Genetic Variation • New genes and alleles can arise by mutation or gene duplication Formation of New Alleles • A mutation is a change in nucleotide sequence of DNA • Only mutations in cells that produce gametes can be passed to offspring • A point mutation is a change in one base in a gene • The effects of point mutations can vary: – Mutations in noncoding regions of DNA are often harmless – Mutations in a genes can be neutral because of redundancy in the genetic code Rapid Reproduction • Mutation rates are low in animals and plants • The average is about one mutation in every 100,000 genes per generation • Mutations rates are often lower in prokaryotes and higher in viruses Natural selection acts on distributions of traits. • A normal distribution graphs as a bell-shaped curve. – highest frequency near mean value – frequencies decrease toward each extreme value • Traits not undergoing natural selection have a normal distribution. Natural selection can change the distribution of a trait in one of three ways. • Microevolution is evolution within a population. – observable change in the allele frequencies – can result from natural selection Hardy-Weinberg • Required conditions are rarely (if ever) met – Changes in gene pool frequencies are likely – When gene pool frequencies change, microevolution has occurred • Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place Genetic variation in a population increases the chance that some individuals will survive. Where does that variation occur? Hardy-Weinberg Equilibrium F1generation DD Dd dd Genotypes: Genotype frequencies: 0.04 0.32 0.64 Allele and gamete frequencies: D = 0.20 d = 0.80 eggs sperm F2generation 0.20 D 0.80 d 0.20 D 0.04 DD 0.16 Dd 0.80 d 0.16 Dd 0.64 dd Offspring Genotype frequencies: 0.04 DD + 0.32 Dd + 0.64 dd = 1 2 2 p + 2pq + q = 1 p2 = frequency of DD genotype (dark-colored) = (0.20)2 2pq = frequency of Dd genotype (dark-colored) = 2(0.20)(0.80) q2 = frequency of dd genotype (light-colored) = (0.80)2 =0.04 =0.32 =0.64 1.00 Industrial Melanism and Microevolution Early observation 36% dark-colored phenotype Later observation 64% dark-colored phenotype Peppered moth (Biston betularia) • Genetic variation leads to phenotypic variation. • Phenotypic variation is necessary for natural selection. • Genetic variation is stored in a population’s gene pool. – made up of all alleles in a population – allele combinations form when organisms have offspring • Allele frequencies measure genetic variation. – measures how common allele is in population – can be calculated for each allele in gene pool Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population that is not evolving • In real populations, allele and genotype frequencies do change over time • The five conditions for non-evolving populations are rarely met in nature: 1. 2. 3. 4. 5. No mutations Random mating No natural selection Extremely large population size No gene flow Genetic variation comes from several sources. • Hybridization is the crossing of two different species. – occurs when individuals can’t find mate of own species – topic of current scientific research A zonkey Populations, not individuals, evolve. In which of the following scenarios will natural selection (N.S.) most likely occur? a) Very little genetic variation is present within a species b) Harsh environmental conditions result in competition for survival c) No reproductive isolation barriers exist within a species living in an area. d) A geographical area has plenty of food to support individuals within the species living in that area. Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population • Three major factors alter allele frequencies and bring about most evolutionary change: – Natural selection – Genetic drift – Gene flow Natural Selection • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions • For example, an allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture Genetic Drift • The smaller a sample, the greater the chance of deviation from a predicted result • Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next • Genetic drift tends to reduce genetic variation through losses of alleles Figure 23.9-1 CRCR CRCR CRCW CWCW CRCR CRCW CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Figure 23.9-2 CRCR CRCR CRCW CWCW 5 plants leave offspring CRCR CWCW CRCW CRCR CWCW CRCR CRCW CRCW CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCW CRCR CRCW Generation 2 p = 0.5 q = 0.5 Figure 23.9-3 CRCR CRCR CRCW CWCW 5 plants leave offspring CRCR CWCW CRCW CRCR CWCW CRCR CRCW CRCW CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCW 2 plants leave offspring CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCW Generation 2 p = 0.5 q = 0.5 CRCR CRCR CRCR CRCR Generation 3 p = 1.0 q = 0.0 Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations 2. Genetic drift causes allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed Gene Flow • Gene flow consists of the movement of alleles among populations • Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) • Gene flow tends to reduce variation among populations over time • Gene flow can decrease the fitness of a population • Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland – Mating causes gene flow between the central and eastern populations – Immigration from the mainland introduces alleles that decrease fitness – Natural selection selects for alleles that increase fitness – Birds in the central region with high immigration have a lower fitness; birds in the east with low immigration have a higher fitness Figure 23.12 60 Survival rate (%) 50 Population in which the surviving females eventually bred Central Eastern Central population NORTH SEA Eastern population Vlieland, the Netherlands 40 2 km 30 20 10 0 Females born in central population Females born in eastern population Parus major Figure 23.12a Parus major • Gene flow can increase the fitness of a population • Consider, for example, the spread of alleles for resistance to insecticides – Insecticides have been used to target mosquitoes that carry West Nile virus and malaria – Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes – The flow of insecticide resistance alleles into a population can cause an increase in fitness • Gene flow is an important agent of evolutionary change in human populations Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution • Evolution by natural selection involves both change and “sorting” – New genetic variations arise by chance – Beneficial alleles are “sorted” and favored by natural selection • Only natural selection consistently results in adaptive evolution Types of Selection • Most traits are polygenic - variations in the trait result in a bell-shaped curve • Three types of selection occur: – (1) Directional Selection • The curve shifts in one direction – Bacteria become resistant to antibiotics – Guppies become more colorful in the absence of predation • Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Selection favors certain genotypes by acting on the phenotypes of certain organisms Three Type of Natural Selection Number of Individuals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phenotype Range Phenotype Range Phenotype Range stabilizing selection directional selection disruptive selection Number of Individuals Peak narrows. a. Peak shifts. b. Two peaks result. c. • Natural selection can take one of three paths. – Directional selection favors phenotypes at one extreme. • Natural selection can take one of three paths. – Stabilizing selection favors the intermediate phenotype. Directional Selection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. No predation All guppies are drab and small Amount of Color Low predation above waterfall High predation 0 below waterfall Experimental site Result © Helen Rodd 4 Months 8 12 Types of Selection • Three types of selection occur (cont): – (2) Stabilizing Selection • The peak of the curve increases and tails decrease • Ex - when human babies with low or high birth weight are less likely to survive – (3) Disruptive • The curve has two peaks • Ex – When Cepaea snails vary because a wide geographic range causes selection to vary Stabilizing Selection Due to stabilizing selection, the average human birth weight stays steady. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 100 20 70 50 15 30 20 10 10 7 5 5 3 2 .9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5 Birth Weight (in kilograms) Percent Infant Mortality Percent of Births in Population Disruptive Selection Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Forested areas Low-lying vegetatio © Bob Evans/Peter Arnold, Inc. Natural selection is not the only mechanism through which populations evolve. Gene flow is the movement of alleles between populations • Gene flow: movement of alleles from one pop. to another • Occurs when individuals join new populations and reproduce. • Gene flow keeps neighboring populations similar. • Low gene flow increases the chance that two populations will evolve into different species. bald eagle migration Genetic drift is a change in allele frequencies due to chance • Genetic drift: change in allele frequency due to chance • Common in small populations, some alleles may increase in frequency, while others decrease or disappear • A population bottleneck can lead to genetic drift. – It occurs when an event drastically reduces population size. – The bottleneck effect is genetic drift that occurs after an event drastically reduces the population. • The founding of a small population can lead to genetic drift. – It occurs when a few individuals start a new population. – The founder effect is genetic drift that occurs after start of new population. The current population is thought to have descended from only seven females and eight males. One of the early colonists apparently carried a recessive allele for retinitis pigmentosa, a progressive form of blindness that afflicts homozygous individuals. The frequency of the allele that causes this disease is ten times higher on Tristan da Cunha than in the populations from which the founders came. • Genetic drift has negative effects on a population. (bottle neck and founder effect) – less likely to have some individuals that can adapt – harmful alleles can become more common due to chance Sexual Selection • Sexual selection is natural selection for mating success • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Figure 23.15 • Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex • Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates • Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival • How do female preferences evolve? • The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should increase in frequency © 2011 Pearson Education, Inc. Sexual selection occurs when certain traits increase mating success. • Sexual selection: processes in which certain traits increase mating success and therefore become more common in the population. – males produce many sperm continuously – females are more limited in potential offspring each cycle • Females preferentially mate with males that display certain traits, so those traits get passed on to offspring and become more exaggerated each generation. https://www.youtube.com/watch?v=W7QZnwKqopo Bird of Paradise mating dance • There are two types of sexual selection. – intrasexual selection: competition among males – intersexual selection: males display certain traits to females Sexual Selection • Female Choice – Choice of a mate is serious consideration • Good genes hypothesis: Females choose mates on the basis of traits that improve the chance of survival. • Runaway hypothesis: Females choose mates on the basis of traits that improve male appearance. • Male Competition – Can father many offspring because they continuously produce sperm in great quantity. – Compete to inseminate as many females as possible. Sexual Selection • Sexual selection adaptive changes in males and females to increase ability to secure a mate. – Males - ability to compete – Females choose to select a male with the best fitness (ability to produce surviving offspring). Sexual Selection • The drab females tend to choose flamboyant males as mates. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sexual Selection: Competition Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. a: © Y. Arthus-Bertrand/Peter Arnold, Inc.; b: © Neil McIntre/Getty Images New species can arise when populations are isolated. The isolation of populations can lead to speciation. • Populations become isolated when there is no gene flow. – Isolated populations adapt to their own environments. – Genetic differences can add up over generations. • Reproductive isolation can occur between isolated populations. – members of different populations cannot mate successfully – final step to becoming separate species • Speciation is the rise of two or more species from one existing species. Populations can become isolated in several ways. • Behavioral barriers can cause isolation. – called behavioral isolation – includes differences in courtship or mating behaviors • Geographic barriers can cause isolation. – called geographic isolation – physical barriers divide population • Temporal barriers can cause isolation. – called temporal isolation – timing of reproductive periods prevents mating Evolution occurs in patterns. Evolution through natural selection is not random. • Natural selection can have direction. • The effects of natural selection add up over time. Heterozygote Advantage • Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes • Natural selection will tend to maintain two or more alleles at that locus • The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance Figure 23.17 Key Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Frequency-Dependent Selection • In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population • Selection can favor whichever phenotype is less common in a population • For example, frequency-dependent selection selects for approximately equal numbers of “rightmouthed” and “left-mouthed” scale-eating fish Why Natural Selection Cannot Fashion Perfect Organisms 1. 2. 3. 4. Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection, and the environment interact • Convergent evolution describes evolution toward similar traits in unrelated species. • Convergent evolution describes the evolution of traits toward similar features. • Divergent evolution describes evolution toward different traits in closely related species. red fox kit fox ancestor How do convergent and divergent evolution illustrate the directional nature of natural selection? Species can shape each other over time. • Two or more species can evolve together through coevolution. – evolutionary paths become connected – species evolve in response to changes in each other • Coevolution can occur in beneficial relationships. • Coevolution can occur in competitive relationships, sometimes called evolutionary. Species can become extinct. • Extinction is the elimination of a species from Earth. • Background extinctions occur continuously at a very low rate. – occur at roughly the same rate as speciation – usually affects a few species in a small area – caused by local changes in environment • Background extinctions occur continuously at a very low rate. – occur at roughly the same rate as speciation – usually affects a few species in a small area – caused by local changes in environment • Mass extinctions are rare but much more intense. – destroy many species at global level – thought to be caused by catastrophic events – at least five mass extinctions in last 600 million years A small portion of a population becomes geographically isolated from the rest of the population. As such, it runs the risk of decreased… a) b) c) d) Genetic drift Mutation rate Natural selection Genetic variation Can you define these words: Endosymbiosis – Primate – Prosimian – Anthropoid – Hominid – Bipedal – Evolution through natural selection is not random. Explain and justify this statement. REMEMBER! Speciation often occurs in patterns. • A pattern of punctuated equilibrium exists in the fossil record. – – – – theory proposed by Eldredge and Gould in 1972. episodes of speciation occur suddenly in geologic time. followed by long periods of little evolutionary change. revised Darwin’s idea that species arose through gradual transformations.