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19_speciation.pptx

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Recall that selection acts on individuals, but only populations evolve

Mutation and sexual reproduction produce the genetic variation that contributes to differences within a population (the “gene pool”)

Variation in individual genotypes leads to variation in individual phenotypes

However, recall not all phenotypic variation is heritable (Why is this important?)

Population Genetics

Godfrey Hardy: English mathematician Wilhelm Weinberg: German physician concluded INDEPENDENTLY that: 1) In the absence of evolutionary pressure, allele and genotype frequencies (from 0 to 1) will remain constant across generations. 2) If mating is random, gene frequencies in the population relate to phenotypic frequencies by a simple mathematical formula. I.e., if you know one, you can calculate the other.

Hardy-Weinberg Equilibrium (1908)

Frequencies calculated using a binomial expansion:

(p+q)2 = p2 + 2pq + q2

p = frequency of first (e.g., dominant) allele

q = frequency of second (e.g., recessive) allele

p2 = individuals homozygous for first allele

2pq = individuals heterozygous for both alleles

q2 = individuals homozygous for second allele

The sum of allele frequencies will always be 1.0 (= 100%). Although the chapter keeps going, limit your study to the simplest case of two alleles.

Five forces can cause gene frequencies to change:

Mutation

Gene flow (migration)

Non-random mating

Genetic drift

Natural Selection

We can monitor phenotypes or genotypes to see if the equation predicts accurately across generations. Often it does not, and a population not in H-W equilibrium indicates that one or more of the 5 forces are operating. Thus, the population is evolving!

Mutagen

DNA

Self-fertilization

Mutation

Gene Flow

Nonrandom Mating

Genetic Drift

Selection

a. The ultimate source of

variation. Mutation

alone usually does not

alter overall allele

frequencies.

b. A very potent agent of

change. Individuals or

gametes move from one

population to another.

c. Inbreeding is the most

common form. Doesn’t

always alter allele

frequency but changes

genotype frequency.

d. Statistical accidents.

The random fluctuation

in allele frequencies

increases as population

size decreases.

e. The only agent that

produces adaptive

evolutionary changes.

Natural Selection: A Closer Look

Natural selection is the only of the five forces that consistently results in adaptation

Selection brings about this match between organisms and their environment by acting on phenotypes, not genotypes. Nature cannot directly “see” genes inside an organism.

Relative fitness is a measure of the genetic contribution to subsequent generations compared to the most fit phenotype (usually set at 1.0)

Thus, nature indirectly favors, or selects, certain genotypes and their alleles based on relative fitness of their corresponding phenotypes

There are three modes of natural selection:

Directional selection favors individuals at one end of the phenotypic range

Disruptive selection favors individuals at both extremes of the phenotypic range

Stabilizing selection favors intermediate variants and acts against extremes of the phenotypic range

Fig. 23-13

Original population

(b) Disruptive selection

(a) Directional selection

Phenotypes (fur color)

Frequency of individuals

Original

population

Evolved

population

Figure 23.13 Modes of selection

Directional selection for negative phototropism in Drosophila lab experiment

Disruptive selection on beak size in black-bellied seedcracker finch in west Africa

Stabilizing selection for birth weight in humans

Why can’t selection produce perfect organisms (e.g., why don’t we have wheels)?

Selection can act only on existing variations

Evolution is limited by historical constraints

Adaptations are often compromises across multiple traits and environments

Chance, selection, and the environment interact so that maximum fitness is very much a moving target across space and time

In the Galápagos, Darwin discovered species of plants and animals found nowhere else on Earth.

Species is a Latin word meaning “kind” or “appearance”

Biologists compare morphology, physiology, biochemistry, and molecular sequences when grouping organisms

Speciation, the process by which new species originate, is at the nexus of evolution

What is a species?

(a) Similarity between different species

(b) Diversity within a species

The Biological Species Concept

The BSC defines a species as a group whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with members of other species

It is used as a decision rule: Members of the same species? Yes or No

Because gene flow holds the phenotype of a population together, the BSC focuses on factors that restrict the flow of genetic material

Reproductive Isolation

Reproductive isolation is the existence of biological barriers that impede two species from producing viable, fertile offspring

Hybrids are the offspring of crosses between different species, thus apparently conflicting with our species definition (BSC)

To resolve this apparent conflict, biologists focused on the isolating mechanisms that keep closely related species separate despite overlapping ranges

Prezygotic barriers block fertilization from occurring by:

Impeding different species from attempting to mate

Preventing the successful completion of mating

Hindering fertilization if mating is successful

Habitat Isolation

Temporal Isolation

Prezygotic barriers

Behavioral Isolation

Mating

attempt

Mechanical Isolation

(f)

(e)

(c)

(a)

(b)

(d)

Individuals

different

species

Habitat isolation: Two species do not encounter each other because they occupy different habitats, even though they are not isolated by physical barriers or geography.

Aquatic Thamnophis

Terrestrial Thamnophis

Temporal isolation: Species that breed at different times do not mix their gametes. Eastern spotted skunks breed only in spring while the western species breeds only in autumn. Their ranges overlap, but their breeding seasons do not.

Eastern spotted skunk

(Spilogale putorius)

Western spotted skunk

(Spilogale gracilis)

Behavioral isolation: Courtship rituals and other behaviors unique to a species can be effective barriers to reproduction. The dance of these blue footed boobies (think of clowns or fools, not breasts) allows them to identify potential mates of their own species. Like a passcode or secret handshake, mating will not proceed unless they get it right. (I usually do it in person, but instead see the video at the end of this PPT)

Mechanical isolation: Morphological differences can prevent successful mating. These two snail species have shells coiled in the opposite direction, causing their genitals to be on opposite sides when they try to copulate.

Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species. Compatibility is checked by unique proteins of the gamete cell membranes.

Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult (hybrids have lower fitness than parental species, but not necessarily zero fitness):

Reduced hybrid viability

Reduced hybrid fertility

Hybrid breakdown

Prezygotic barriers

Gametic Isolation

Fertilization

Reduced Hybrid Viability

Postzygotic barriers

Reduced Hybrid Fertility

Hybrid Breakdown

Viable,

fertile

offspring

(g)

(h)

(i)

(j)

(l)

(k)

Reduced hybrid viability: Genes of the different parent species may interact to impair a hybrid’s development. There is no healthy parent here for comparison, but this hybrid salamander is thin and slightly deformed.

Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile. E.g., mules, which are hybrids between horses and donkeys, represent an evolutionary dead end.

Hybrid breakdown: First-generation hybrids seem fine, but offspring of subsequent generations are feeble or sterile. Lower fitness of these rice plant hybrids (middle) takes 2 generations to reveal itself.

Prezygotic barriers

Habitat Isolation

Individuals

different

species

Temporal Isolation

Behavioral Isolation

Mating

attempt

Mechanical Isolation

Gametic Isolation

Fertilization

Reduced Hybrid Viability

Reduced Hybrid Fertility

Postzygotic barriers

Hybrid Breakdown

Viable,

fertile

offspring

(a)

(b)

(d)

(c)

(e)

(f)

(g)

(h)

(i)

(j)

(l)

(k)

Fig. 24-4

Other Definitions of Species

The biological species concept cannot be applied to fossils or asexual organisms including all prokaryotes

“Potential to interbreed in nature” can be practically impossible to assess

Other species concepts emphasize unity within a species, rather than restricted gene flow between species

The morphological species concept defines a species by anatomical features

The ecological species concept emphasizes a species’ ecological niche (environmental role)

The phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree

They all usually agree on what is the same vs. separate species, but conflicts can arise.

Despite limitations, BSC is preferred for its objective criterion: ability to successfully reproduce. The other definitions are subjective.

Two Main Types of Speciation

Depending on whether or not groups are geographically separated when they diverge genetically:

Allopatric speciation

Sympatric speciation

Fig. 24-5

(a) Allopatric speciation

(b) Sympatric speciation

Allopatric (“Other Country”) Speciation

Gene flow is interrupted or reduced when a species is divided into geographically isolated subpopulations

What constitutes a barrier depends on the ability of individuals to disperse. For example, not even the Grand Canyon will affect birds that routinely fly long distances (e.g., condors).

Separated populations will then evolve independently through mutation, natural selection, genetic drift, etc. They will diverge.

Fig. 24-6

A. harrisi

A. leucurus

Sympatric (“Same Country”) Speciation

Sympatric speciation takes place in geographically overlapping populations

This can occur through several mechanisms, including:

- Polyploidy: chromosome issues (see text)

- Sexual Selection: non-random mating (see text and will come up again)

- Habitat Differentiation: e.g., insects living on different host plants

This fly species is in the process of speciation due to introduction of apple trees. Encounter rates and mating preferences are higher among individuals that grew up on the same kind of host fruit.

Hybrid zones provide opportunities to study speciation

A hybrid zone is a region in which members of different species mate and produce hybrids

Can occur in a single band or more complex patterns (e.g., a mosaic of parents and hybrids)

Hybrids often have reduced fitness compared with parent species, causing selection against hybridization

Fig. 24-13

EUROPE

Fire-bellied

toad range

Hybrid zone

Yellow-bellied

toad range

Yellow-bellied toad,

Bombina variegata

Fire-bellied toad,

Bombina bombina

Allele frequency (log scale)

Distance from hybrid zone center (km)

0.01

0.1

0.5

0.9

0.99

Hybrid Zones over Time (3 possible outcomes)

Reinforcement of reproductive barriers continues to split 2 species until no hybrids; should occur when hybrids are less fit than parents

Weakening of reproductive barriers leads to fusion back into 1 species (original or new?); should occur when hybrids are as fit, or more fit, than parents

Stability of the zone with continued formation of hybrids (3 groups, species status debatable); occurs when something prevents other outcomes

Fig. 24-14-1

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Fig. 24-14-2

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Isolated population

diverges

Fig. 24-14-3

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Isolated population

diverges

Hybrid

zone

Hybrid

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Isolated population

diverges

Hybrid

zone

Hybrid

Possible

outcomes:

Reinforcement

Fusion

Stability

Speciation Rates and Genetics

(?) Hybrid zone studies indicate that much of our difficulty defining species comes from the fact that speciation is a process. We are sometimes asking for the outcome before it has been determined.

Questions remain concerning how long it takes for new species to form or how many genes need to differ between species

Broad patterns of speciation can be studied using the fossil record, morphological data, and molecular data

Patterns in the Fossil Record

There are many examples of species that appear suddenly, persist essentially unchanged for some time, and then disappear

Eldredge and Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change

This contrasts with gradualism, the traditional view of slow and steady changes over time

Debate has highlighted some interesting issues, but I would say the punctuated pattern is seen more often when looking across traits.

Fig. 24-17

(a) Punctuated pattern

(b) Gradual pattern

Time

Speciation Rates

Patterns in the fossil record and other studies suggest that speciation can be rapid or very slow

A large review shows the interval between speciation events ranges from about 4,000 years (some cichlids) to about 40,000,000 years. (some beetles), with a mean of 6.5 million years.

Note that speciation by polyploidy (very common among plants) can occur almost instantaneously. This type of sympatric speciation was not included in the analysis.

The Genetics of Speciation

The explosion of genomics has led to identification of specific genes involved in some cases of speciation

Speciation might require differences in a single, a few, or thousands of alleles

E.g., Japanese water snail: one gene change causes the shell to spiral in the opposite direction, leading to sympatric speciation by mechanical isolation

Fig. 24-19

The dance of the blue footed booby is a classic example of behavioral isolation:

https://www.youtube.com/watch?v=lcPHFQP9GN0