Chapter 15: Nonadaptive Evolution and Speciation

How can speciation occur?

*Allopatric Speciation*: the physical separation of two populations of the same species and the separate adaptations to the environment that each one undergoes that results in reproductive isolation. Mechanisms include land forms separated as islands, formation of barriers such as canyons or mountain ranges, bodies of water isolated from the ocean (Great Salt Lake) Allos: means neighbor There are both *pre-zygotic* and *post-zygotic* barriers that result in reproductive isolation. These may include behavioral isolation, temporal isolation, mechanical isolation, and genetic isolation

What is a species?

*Biological Species Concept* - A population of individuals whose members can interbreed and produce fertile offspring - Different species cannot mate because they are *reproductively isolated*

Population Genetics (Figure)

*Gene Pool*: total collection of alleles in a population Identify *allele frequency* Figure: Population geneticists study the gene pools of populations. If a gene pool changes (that is, if the allele frequencies have changed) over the course of generations, then evolution has occurred. Each mouse has two alleles for each gene. If there are 15 mice, there are 30 alleles for each gene. > A change in allele frequency means evolution has occurred.

Nonadaptive Evolution

*Genetic Drift* - Change in allele frequencies between generations that occurs purely by chance - Subset of population reproduces - Subset of alleles represented in next generation - Decreases genetic diversity of a population

What causes change in allele frequency?

*Natural Selection*: population is better adapted *Nonadaptive Evolution*: caused by mutation, genetic drift, and gene flow

What is a species? (3)

*Reproductive Isolation* - Mechanical isolation - Gametic isolation - Hybrid inviability - Hybrid infertility

What is a species? (2)

*Reproductive isolation* - Ecological isolation - Temporal isolation - Behavioral isolation

Hardy-Weinberg Equilibrium: Five Necessary Conditions

1. No mutation introducing new alleles into the population 2. No natural selection favoring some alleles over others 3. An infinitely large population size (and, therefore, no genetic drift) 4. No influx of allies from neighboring populations (i.e., no gene flow) 5. Random mating of individuals

Summary

1. Population is identified by the particular collection of alleles in its gene pool 2. Genetic diversity is reflected by the number of different allele in a population's gene pool 3. Evolution is a change in allele frequencies in a population over time. Evolution can be adaptive or nonadaptive. Mutation, genetic drift, and gene flow are nonadaptive forms. 4. Founder and bottleneck effects are types of genetic drift 5. Gene flow is the movement of alleles between different populations of the same species 6. Inbreeding of closely related individuals may occur in small, isolated populations 7. Hardy-Weinberg equilibrium describes the frequency of genotypes in a non evolving population 8. Speciation can occur when gene pools are separated, gene flow is restricted, and populations diverge genetically over time

Hardy-Weinberg Equilibrium Requirements

1. The population is large 2. There is no natural selection 3. There are no mutations 4. There is random mating 5. There is no immigration or emigration *A population needs to meet all these criteria to not evolve. No population does this, so all populations evolve*

Driving Questions

1. What is a gene pool (and can you swim in it?) 2. How do different evolutionary mechanisms influence the composition of a gene pool? 3. How does the gene pool of an evolving population compare to the gene pool of a nonvoting population? 4. How do new species arise, and how can we recognize them?

Evolution (Definition 4)

A fourth area of study called 'evolution' is *abiogenesis*. *Abiogenesis* is the study of how living, reproducing ('bio') systems might have started ('genesis') from non-living chemicals ('abio-').

Evolution (Definition 3)

A third definition of evolution goes something like *"the adaptation of a population to its environment"*. This area of study is part of *Ecology*.

Evolution (Definition 2)

Another definition of 'evolution' is *"how new species form"*. A more technical word to define this area of study is *"speciation"*.

Biological Speciation

Based on the concept of reproductive isolation. Two organisms are from the same species if they can produce fertile offspring (dogs). If not, they are not of the same species (horses plus donkeys = mules)

Morphological Speciation

Based on the phenotypic differences that we can observe and measure. We use this category for fossils of extinct species on which we cannot observe their reproduction

Hardy-Weinberg Equilibrium (cont.)

Baseline to judge if a population is evolving *Hardy-Weinberg Equation*: p^2 + 2 pq + q^2 = 1 p^2 is the frequency of homozygous dominants 2 pq is the frequency of heterozygotes q^2 is the frequency of homozygous recessives

But how does time create species?

Biologists largely take it as an article of faith that the separation of two populations will eventually lead to two species, but what the mechanisms are or how long is usually takes are not well known. For most, time is the 'miracle' explanation which needs no defense

Population Genetics (2)

Changes in allele frequency over time Population evolves Good, bad, or neutral consequences result in the population becoming more adapted to its environment - natural selection results in a population adapting

Inbreeding Depression

Closely related individuals are more likely to share the same alleles Negative reproductive consequences for a population Associated with high frequency of homozygous individuals possessing harmful recessive alleles

Affects of Urbanization (Figure)

Country mice and urban mice differ most often in genes associated with immunity and response to stress. These genes may be responding to selective pressures in the urban environment, such as exposure to pollution and competition for food and mates. Alleles for 'regular gene' X: Country mice and city mice share similar allele diversity for most genes Alleles for 'stress gene' 1 & 2: Country ice have more allele diversity than city mice in those genes associated with response to stress <> Country mice and city mice differ in their allele diversity for genes associated with immunity and response to stress <> Alleles for 'stress gene' 1 & 2: City mice share more alleles of genes associated with response to stress (that is, they have less allele diversity). Presumably, these alleles have been selected because they help mice cope with the unique stresses of city living.

Ecological Isolation

Different environments The Arctic Fox and the Desert Fox live in such different places, they never encounter each other

Behavioral Isolation

Different mating activities The Prairie Chicken is not attracted to the mating display of the Ring-Necked Pheasant

Why is genetic diversity important?

Diverse gene pool gives a population more flexibility to survive in a changing environment The more genetically diverse a population, the more ways it has to adapt

Theodosius Dobzhansky

Dobzhansky hypothesized that if a way to distinguish species arose inside the hybrid zone, natural selection would heavily select for those genes in the hybrid zone. Then the genes would move outside the zone until all members of the species would avoid hybridization. He called this process *reinforcement*. It would drive the two species apart more quickly than random development of adaptive differences. However, no one has managed to measure Reinforcement occurring, despite many hybrid zone studies. The exact mechanism of rapid speciation is still a mystery.

Adaptive Radiation

Evidence for speciation can be found in the pattern of *adaptive radiation* - the emergence of numerous species from a common ancestor Ex: Evolution of mammals from a mammal-like reptilian ancestor. There is a lot of evidence in the fossil record, especially in jaw and tooth structures, and gait differences. *The progression is clear*

Hardy-Weinberg Equilibrium (Figure)

Figure: The Hardy-Weinberg Equation In a nonliving population mice will be mating randomly, with no mutation, natural selection, or genetic drift to change allele frequencies When we add up the genotype frequencies in this non evolving population, we get the Hardy-Weinberg equation: p^2+2pq+q^2=1

Evolution

From the beginning, evolutionary theory has been plagued by all sorts of people who want to use it as a justification for their social/political theories. Evolutionary theory has been plagued by those who oppose it strictly because they want to justify some other social/political theory.

GH Hardy (Figure)

GH Hardy set up two allele 'frequencies', p and q, where p is how often 'A' occurs and 'q' is how often 'a' occurs p + q = 1 In the example before, p = 90% (or 0.9) and q = 10% (or 0.1) > p + q = 0.9 + 0.1 = 1 Since 1 x 1 = 1 then: (p+q)x(p+q)=1=p^2+2xpxq+q^2 P+q=1, (p+q)x(p+q)=1

Gametic Isolation

Gametes cannot unite The games from a dog and a cat cannot unite to form a zygote

Hybrid Inviability

Gametes unite but viable offspring cannot form. The goat and sheep can mate, but the zygote formed does not survive

Nonadaptive Evolution: Bottleneck Effect

Genetic Drift - Bottleneck Effect *Bottleneck Effect*: Genetic bottlenecks occur when a population loses a large proportion of its members. If the original population is large, the reduced population is likely to retain the same alleles present in the original population. But in a small starting population, bottlenecks are more consequential: the loss of individuals is more likely to result in the loss of alleles from the population. A more diverse, original population > A bottleneck event, like rapid habitat loss, eliminates a large percentage of the original population. By chance, the surviving mice have a different (reduced) allele frequency that the original population > A less diverse bottleneck population

Nonadaptive Evolution (Figure)

Genetic Drift - For example, populations get separated Figure: Researchers collected tail DNA from 312 mice at 15 locations in NYC. Once they analyzed each mouse's DNA, the researchers wanted to get a sense of how related the populations were. They assigned mice with similar genotypes particular colors and sorted all the mice by location. They found that mice within a population shared more alleles with one another than they did with mice from other populations. Each vertical bar represents the genetic data of 1 mouse > Mice sharing similar genetic sequences are shown in the same color > Mice in one population share more colors {alleles} with each other than they do with mice in another population.

Nonadaptive Evolution: Founder Effect

Genetic Drift - Founder Effect Figure: Allele frequencies can change from one generation to the next purely as a result of change: this is genetic drift. Drift has more dramatic effects in smaller populations than in larger ones. *Founder Effect*: the founder effect is a type of genetic drift that occurs when a small group of 'founders' leaves a population and establishes a new one. If, by chance, the original population are absent from the founder, they will also be absent from the new population. A large, diverse original population > A few founders migrate to previously uninhabited territory. By chance, these mice have a reduced frequency for some alleles compared to the original population > A less-diverse founding population

Speciation (Figure)

Genetic divergence of populations, leading over time to reproductive isolation and the formation of new species Figure: The Galapagos archipelago is a series of islands off the coast of South America. Finches first came to the Galapagos from a population on the mainland of South America. As they spread from island to island, they encountered different environments, including available food sources, which influenced bill size and shape in each new island population. As separated finch populations evolved in different food environments, they diverged from they ancestral population to have smaller, pointed bills for insects, longer bills for cactus fruit and flowers, or thick, strong bills for hard seeds. In addition, they evolved such that the separated populations could not interbreed. At least 13 finch species have diverged from the original South American species.

Affects of Urbanization

High diversity in stress alleles may mean that country mice have to deal with a wider variety of stresses High diversity in stress alleles may mean that city mice deal with a lower variety of stresses, or that the stresses they deal with are more intense and so only a few alleles survive Or that city mice are the result of a small founder population that rapidly filled the nice. More than one explanation is possible.

Hardy-Weinberg Principle

How can we predict when changes in allele frequency might occur? Hardy-Weinberg argued they will stay the same unless something causes them to change - natural selection. Allele frequencies will stay the same if the following *all* occur, i.e. the population *will not evolve* if these happen

Hardy-Weinberg Equilibrium

In a non evolving population, allele and genotype frequencies do not change over time *Hardy-Weinberg Equilibrium* Use to identify genes that have changed because of evolutionary mechanisms

Temporal Isolation

Mating behavior or fertility at different times. The Leopard Frog mates in early spring and the Bullfrog mates in early summer.

Inbreeding

Mating between closely related individuals Does not change the allele frequency within a population Increases the proportion of homozygous individuals to heterozygotes

Mechanical Isolation

Mating organs are incompatible Plants pollinated by the hummingbird do not receive pollen from plants pollinated by the Black Bee

Mechanisms of Evolution: Adaptive and Nonadaptive Mechanisms of Evolution (Figure)

Mechanisms of Evolution, How Allele Frequencies Change, Adaptive or Nonadaptive, How Genetic Diversity is Affected Natural Selection - Individuals with favorable alleles reproduce preferentially, increasing the frequency of these alleles - Adaptive - Usually decreases; unfavorable alleles may be eliminated from the population Mutation - New alleles are created randomly - Nonadaptive - Increases; new alleles are introduced into the population Genetic Drift - Allele frequencies change due to chance events - Nonadaptive - Usually decreases; alleles may be eliminated from the population Gene Flow - Alleles move form one population to another - Nonadaptive - Increases; new alleles are added to the population

Gene Flow (Figure)

Migration and interbreeding of individuals move alleles between populations. Populations that can interbreed with other populations have higher alleles diversity than isolated populations. Population A - Red and Green Alleles > Random migration and interbreeding increase the frequency of green alleles and introduce new blue allele to Population A <> random migration and interbreeding introduce red alleles to Population B > Population B - Blue and Green Alleles

Reintroducing Genetic Diversity

Must bring in new alleles *Mutation* *Gene Flow*: movement of alleles from one population to another

Evolution (Definition 1)

Some define 'evolution' as *the genetic changes in populations over time*. This is more technically referred to as *"population genetics"*. Figure: Source Population Founder Effect Bottleneck Expansion

Population Genetics

Study of the genetic makeup of populations and how genetic composition changes Understand how a group of organisms is coping with environmental changes

Urban Evolution

Studying evolution in Manhattan

Sympatric Speciation

The begging of a new species within the distribution of a parent population of the original species. This requires the ability of the organisms to undergo polyploidy, or the presence of more than two sets of chromosomes in the cells. This is rare or non-existent in animals but is fairly common in plants

Gene Flow

Urbanization can prevent gene flow, can lead to interbreeding

Hybrid Infertility

Viable hybrid offspring cannot reproduce. Zebras and horses are different species because their hybrid offspring, zebroids, cannot produce offspring of their own