EVOLUTION Chapter 18 Definition descent with modification This

EVOLUTION Chapter 18

• Definition- descent with modification • This refers to when one group of organisms gives rise to another group • Evolution occurs on the population or species level, individual organisms can not evolve!

• Specifically – evolution refers to changes in allele frequencies in populations over time • When one generation of organisms reproduces and creates the next, the frequencies of the alleles for the various genes represented in the population may be different from what they were in the parent generation • Remember – all of the allele frequencies for a particular gene must add up to 1

• Frequencies can change so much that some alleles can be lost completely in a population • Other alleles can become fixed (all individuals have the same allele for that character) • Over many generations the species can change so much that it becomes very different from the ancestral species • Or a population can branch off and become a new species (this is speciation)

• Allele frequencies may change because of random factors or due to natural selection • 4 major modes of evolution: 1)Genetic drift – change in allele frequencies that is due to chance events • Bottleneck – when genetic drift dramatically reduces population size 2)Gene flow- change in allele frequencies as genes from one population are incorporated into another

• Gene flow is sometimes referred to as migration when the individuals are actively relocating • This is random with respect to which organisms succeed but there are certainly cases when migration is not random

3) Mutation – the third random event that can change allele frequencies • Always random with respect to which genes are affected • But – the changes in allele frequencies that occur as a result of mutation may not be random • Mutation is the basis of variation

• Up until now, all of these modes of evolution act RANDOMLY with respect to the alleles in the population • Which alleles increase and which decrease are due to chance events (not because some are better than others) • The 4 th mode is natural selection- the only nonrandom process!

4) Natural Selection – • Natural selection is not synonymous with evolution • It is just one process by which evolution can occur • Natural selection helps us to explain why organisms look and behave the way they do

N. S. based on 3 conditions: 1) Variation – for natural selection to occur, a population must exhibit phenotypic variation (there must be differences between individuals) 2) Heritability – parents must be able to pass on the traits that are under natural selection. If a trait can not be inherited, it can not be selected for or against

3) Differential Reproductive Success: Reproductive success measures how many offspring you produce that survive relative to how many the other individuals in your population produce. There must be variation between parents in how many offspring they produce as a result of the different traits that the parents have

• Those traits that allow an individual to survive longest (and produce the most offspring) are favored • Those are the traits that will increase in frequency in the next generation • This is NOT random • b/c allele freq always add up to 1 = 100% reproductive success is relative to the other organisms in the population

Why aren’t all organisms perfectly adapted to their env’t? • Different alleles confer diff. advantages in different env’ts • Also, the environment is constantly changing • Therefore, species are also constantly changing as the traits that give them an advantage also change • If there is a case where a trait becomes unconditionally advantageous over MANY generations, then that allele may become fixed

• Ex. Of a fixed allele – all spiders have 8 legs • But –where there are heritable characters that both vary and confer fitness advantages (or disadvantages) on their host organisms natural selection can occur

Jean Baptiste Lamarck • Proposed that evolution occurs by the inheritance of acquired characteristics • Ex. Giraffe necks – according to Lamarck, giraffes have evolved long necks because they are constantly reaching for leaves at the tops of trees • He said a giraffe’s neck lengthened during its lifetime and then that giraffe’s offspring would have long necks as a result

• This made sense because at the time no one knew about genes and hereditary info • But we now know that this is not the case because we can not change our DNA during our lifetime by using or disusing certain traits

Charles Darwin • His idea was consistent with Mendelian genetics (although it is not known if Darwin even knew of Mendel’s theory at the time) • Darwin suggested theory of natural selection and coined the term “survival of the fittest” • He proposed a hypothetical unit of heredity which could be passed from parent to offspring

Adaptations • Adaptation- trait that if altered, affects the fitness of an organism • Adaptations are the result of natural selection!!!!! • Can include physical and intangible adaptations • Ex. Of adaptations – arm length, eye color, mating behavior, lifespan length, age at reproductive maturation

Types of Selection 1) Directional Selection – occurs when members of a population at one end of a spectrum are selected against, while those at the other end are selected for ex. – elephants in an area where the only food available is on the tops of tall trees -- in this case those elephants with long trunks will be selected for and short will be selected against

2) Stabilizing Selection – selection for the mean of a population for a given allele. Ex – human birth weight – babies that are too small or too large will be at a disadvantage -- this has the effect of reducing variation in a population for a particular gene

3) Disruptive selection – (aka diversifying selection) – the opposite of stabilizing selection - when individuals at the two extremes of a spectrum of variation do better than the more common forms in the middle - ex. snail shell color – very light and very dark shells camouflage but those that are in between will stick out

• Directional, disruptive and stabilizing selection describe the way in which allele frequencies can change as a result of the forces of natural selection • There also 2 other types of selection that complement natural selection:

1) Sexual Selection – occurs because individuals differ in mating success - since not all individuals will have the maximum number of possible offspring, there must be some reason why some individuals have greater reproductive success than others - this is purely about access to mating opportunities

- sexual selection occurs by 2 primary processes: a) within-sex competition b) Choice

• In mammals (and some other species), females are limited in the number of offspring they can produce in their lifetimes (because of internal gestation) • Males however are not limited (b/c sperm are cheap to produce and few males participate in offspring care) • As a result, in mammals, males compete and females choose

• Males compete because females are a limiting resource • Females are choosy because they invest a lot in every reproductive effort

• This behavior leads to the evolution of characters for 2 main functions: 1) As weaponry or other tools for male competition (ie. Large testes for sperm competition) 2) As traits that increase mating opportunities because females prefer to mate with males who have them (ie. Colorful feathers in many birds)

• We are not entirely sure on what basis females make their mate choices • There are theories that certain characteristics are an indicator of “good genes” which would be beneficial for a female’s offspring • Honest indicators – sexually selected traits that are the result of female choice

• Keep in mind that all of this choosing is not a conscious decision! (except for humans) • But those females who choose those with certain traits have better reproductive success and so the frequency of that choice will increase in the next generation

2)Artificial selection – when humans become the agents of natural selection • Specifically select certain individuals to breed while restraining others from doing so • Has resulted in the domestication of a wide range of plant and animal species as well as specific traits (like cattle with lean meat or flowers with specific colors)

4 Basic Patterns of Evolution 1) Coevolution – mutual evolution between 2 species. - Perfect example are predator-prey relationships

2) Convergent evolution – 2 unrelated species evolve in a way that makes them more similar - this is b/c they are both responding in the same way to some environmental challenge - convergent characters are those that are similar in 2 species even through they do not share a common ancestor Ex. – bird and insect wings

3) Divergent evolution – 2 related species evolve in a way that makes them less similar - this can lead to speciation (allopatric or sympatric)

4) Parallel evolution – Similar evolutionary changes occurring in 2 species that can be related or unrelated - they are simply responding in a similar manner to a similar environmental condition

3 Sources of Variation • One of the conditions for evolution is variation, here’s where variation within a population comes from: 1) Mutation – random changes in the DNA of an individual can introduce new alleles into a population

2) Sexual Reproduction – this is due to crossover, independent assortment of homologous pairs and the fact that all sperm and ova are unique and when joined will form a unique individual

3) Balanced Polymorphism – when characters have one or more phenotypic variants - ex. Color of tulip petals - If one phenotypic variant leads to increased reproductive success, we expect directional selection to eventually eliminate all other varieties

• BUT – there also many examples where variation is prominent and one allele is not uniformly better than the others • There are 3 ways that balanced polymorphism is maintained in populations:

a)Heterozygote advantage – ex. Sickle cell b)Hybrid vigor and outbreeding – 2 unrelated individuals are less likely to have the same recessive deleterious alleles, also outbreeding increases the number of heterozygous alleles and thus increasing heterozygote advantage ex. Artificially selected plants are outbred to increase hybrid vigor

c)Frequency dependent selection – the least common phenotype is selected for while the common phenotypes have a disadvantage ex. In some fruitflies, females choose to mate with males that have the rarer phenotype, resulting in selection against the more common variants

Speciation • A species is a group of interbreeding (or potentially interbreeding) organisms • Speciation is the process by which new species evolve • There are 2 main forms of speciation:

1) Allopatric Speciation • In this case interbreeding ceases because of some barrier that separates a single population into two. • Ex. An area with no food or a mountain range • The two populations evolve independently and if they change enough, then even if the barrier is removed, they will not be able to interbreed

2) Sympatric Speciation • This occurs when interbreeding ceases even though no physical barrier prevents it • This can occur due to polyploidy or balanced polymorphism

• Polyploidy – when an individual has more than the normal number of sets of chromosomes • the individual may be healthy, but it can not reproduce with nonpolyploidic members of its species • in some plants it has resulted in new species because polyploidic individuals are only able to mate with each other

• Balanced Polymorphism – can also lead to speciation if 2 variants diverge enough to no longer be able to interbreed (ie. If potential mates no longer recognize each other as possible partners

• Another imp term related to evolution – • Adaptive radiation – a rapid series of events that occur when one or more ancestral species invades a new env’t • Perfect ex. – Darwin’s Finches • If there are many ecological niches, several species will evolve because each can fill a different niche

When evolution is not occurring: Hardy-Weinberg Equilibrium • Evolution is constantly happening in humans and other species • Hardy-Weinberg is a theoretical concept to describe those special cases where a population is in stasis (not evolving)

• 1) 2) 3) 4) 5) In order for a population to be in H-W equilibrium the following must be true: No mutations No gene flow No genetic drift (the pop must be large) No natural selection (so the traits are neutral; none give an advantage or disadvantage) Random Mating

The equation: p+q=1 - This equation is used to determine if a population is in H-W - Symbol p is the frequency of allele 1 (often the dominant allele) - q is the frequency of allele 2 (often the recessive - The frequency of p + q will always add to 1 if the population is in H-W

Easy example • If 60% of the alleles for a given trait are dominant • Then p = 0. 6 • Then you can figure out that q is 0. 4 (40%) • b/c 1 - 0. 6 = 0. 4

The other equation: p 2 + 2 pq + q 2 = 1 - p 2 represents the freq of homozygous dominant (AA) - q 2 represents the freq of homozygous recessive (aa) - 2 pq (pq+qp) represents the freq of the heterozygotes (Aa and a. A)

More complex example • Population of acacia trees is 16% short (which is a, recessive) • And 84% tall (which is A, dominant) • What are the frequencies of the two alleles? • (it is NOT just. 16+. 84 because you have to consider the heterozygotes!)

• You must determine the value of q first! • We know that q 2 = 0. 16 so we find q by taking the square root q = 0. 4 • Since p + q = 1 we plug in…p + 0. 4 = 1 • p = 0. 6

A step further - If they ask you for the percentages of the homozygous dominant and heterozygous conditions just plug in what you know - We already know q 2 = 0. 16 or 16% - 2 pq = (2) (0. 6) (0. 4) = 0. 48 or 48% - p 2 = (0. 6) = 0. 36 or 36% 16 + 48 + 36 = 100

• H-W equilibrium rarely if ever occurs in real populations or organisms • But we use the equations as a tool to determine whether a pop is evolving or not • If the freq do not add up to 1 (or 100%) then we need to look to see why not • ie. The pop is too small so genetic drift is a factor or maybe one of the traits is advantageous so it is being selected for

Evidence for Evolution 1) Homologous characters – traits are homologous if they are similar because their host organisms arose from a common ancestor (this implies evolution) - ex. Bone structure in bird wings is homologous in all bird species

2) Embryology – the study of embryos reveals similarities between organisms at the earliest stages of life, although adults (or even at birth) the species look completely different - ex. Human embryos have gills for a short period of time (this hints at our aquatic ancestry)

Embryology continued… • “Ontogeny recapitulates phylogeny” – Ernst Haeckel • Ontogeny is an individual’s development • Phylogeny is a species’ evolutionary history • This means that during an organism’s embryonic development, it will at some point resemble the adult form of all its ancestors before it

• Ex. At certain points human and fish embryos look very similar • Haeckel thought that embryonic similarity between developing individuals could be used to deduce phylogenetic relationships • Turns out this rarely holds true

3) Vestigial characters – most organisms carry characteristics that are no longer useful, although they once were - the env’t has changed enough so that the trait is no longer useful, but it is not deleterious so it is not selected against - appendix

• The three main types of evidence for evolution can often be found in the fossil record • The fossil record is the physical manifestation of species that have gone extinct (this includes things like bones and imprints)

Micro and macroevolution Microevolution – evolution at the level of populations and species - so far everything we have been talking about is microevolution Macroevolution – the big picture, the study of evolution of groups of species over very long periods of time

• There are disagreements as to the pattern of macroevolution • Some believe in gradualism – evolutionary change is a steady slow process • Others believe in punctuated equilibrium – change occurs in rapid bursts separated by large periods of stasis • Because the fossil record is incomplete it is very hard to test the two theories!

How life probably emerged • Know the steps of the heterotroph theory • This states that the first organisms were heterotrophs • It is important to note that in this theory there was no free oxygen “in the beginning”

• Alexander Oparin proposed his theory (the Oparin theory) that oxygen would have prevented the formation of simple molecules because it is too reactive and would have taken place of any other element in chemical reactions • Miller and Urey tested this hypothesis by simulating a primordial env’t and were able to form organic molecules (including amino acids) in the absence of oxygen!