Patterns of Evolution

Year 13 Biology

Revision of Yr 12 Work

Darwin’s Theory Sources of Variation Agents that change gene frequencies

Darwin’s Theories

Believed that species evolved from ancient forms

Species did not remain unchanged throughout time

Darwin’s theories

REASONING Species have high reproductive rates but

selection pressures act to create a struggle for existence

There is variation in offspring, some better adapted than other.

Those best adapted will survive longer and therefore have an increased chance of passing on their favourable characteristics

Fittest will survive Natural Selection

Sources of Variation

Three main sources of variation Crossing over Sexual reproduction/meiosis Mutations

The actions of these sources create variation by ‘mixing’ alleles into new combination or creating new alleles

Agents that change allele frequencies

Genetic Drift Mutations Gene flow Founder Effect Natural Selection

Genetic Drift

A change in allele frequencies of a population as a result of chance – random events

This happens in small populations where chance alone may play a considerable role

Bottleneck and founder effects are two situations where genetic drift can be significant.

Mutations

The ultimate source of variation Can change equilibrium in populations

by adding or taking away alleles therefore changing possible combinations

Also provides new material for Natural Selection

Gene Flow

Immigration/emigration Introduce (or take away) alleles into a

gene pool When immigrants arrive from another

population possessing a different gene pool, new alleles are introduced.

Can both increase or decrease genetic variation

Natural Selection

Sorts genetic variability and accumulates and maintains favourable genotypes in a population

Reduces genetic diversity within the gene pool but increases differences between populations

There are three types of Natural Selection Stabilising, directional and disruptive

Types of Selection

Stabilising selection - favours the average

Directional selection – one extreme favoured over the other and average

Disruptive selection – favours both extremes

The Species Concept

“A species is a group of actually or potentially interbreeding natural populations that is reproductively isolated from other such groups”

Boundaries between a species gene pool can be somewhat unclear. This leads to a continuous increase or decrease in characteristics as you move through a range of a species.

Cline

A cline is the pattern of variation of characteristics between adjacent populations.

Example: The NZ Tomtit shows a clinal variation in

the length of the tail and wing. North Island tomtits have shorter tails and wings than the southern tomtits

Ring Species

When two apparently different species are joined by a series of geographical and structural intermediate types.

Circular cline that results in two ends of the cline overlapping. E.g. herring gull and lesser black-backed

gull. Adjacent populations can interbreed but

not where the arms of the loop overlap.

Speciation

The formation of a new species is called speciation. Species definition – a group of organisms

that normally interbreed in nature to produce fertile offspring, and belong to the same gene pool.

An important part of forming a new species is separation of the gene pool of a species from the gene pool of the parent population.

Speciation

Speciation can be separated into two types:

1. Allopatric Is when a species population is

separated by a geographical barrier. Populations may experience geographic

isolation due to such events as mountain building, changes in sea level, changes in river courses.

Complete pg 251

Speciation2. Sympatric Occurs when a sub-population becomes

reproductively isolated in the midst of the parent population. Populations are said to be sympatric if their ranges overlap. This can arise from a change in niche or

lifestyle, where mating can be only between those who have adopted a new lifestyle or by the sudden appearance of a new species

Complete pg 253

HOMEWORK

Complete Biozone pages

245- 246 “The species concept”

Reproductive Isolating Mechanisms

These prevent populations of related species in the same area from interbreeding.

They may operate before fertilisation (prezygotic) or after fertilisation (postzygotic).

Prezygotic (before fertilisation)

Temporal Behavioural Structural Spatial Ecological Gamete mortality

See page 243 of Biozone for details

Postzygotic (after fertilisation)

Although the sperm and egg may fuse, there may be other problems along the path to the development of a fertile adult.

Hybrid Inviability Hybrid Sterility Hybrid Disadvantage

Stages in Species Development See page 250 Biozone

Possible sequence of events in the development of two new species.

As time passes the amount of genetic variation increases and each group becomes more isolated from the other.

Geographical barriers, prezygotic and postzygotic mechanisms act to keep the two species isolated

Patterns of Evolution

Evolution may cause related species to become different (diverge) or unrelated species to become similar (converge)

Divergent Evolution – when one species evolves into two species

Convergent Evolution – occurs when largely unrelated species evolve similar features

Divergent Evolution

When an ancestral species diverges into two or more species that eventually occupy different ecological niches

New speciesAncestral species

New Species

When many new species are formed it is called Adaptive Radiation

Adaptive Radiation

Like divergent evolution but is RAPID and gives rise to a large number of species that occupy different niches

Examples Galapagos Finches, NZ Hebe

Homologous Structures

Structural similarities that suggest organisms descended from a common ancestor.

Used to indicate evolutionary relationships

Examples Pentadactyl limb: bones of forelimb of air

breathing vertebrates have similar bones arranged in a comparable pattern eg, bird’s wings, bat’s wings, dog’s leg, human arm

Convergent Evolution

Ancestors are different but different species come to resemble each other due to similar ecological niches and natural selection

Analogous Structures

Arise as a result of convergent evolution. Structures are used for the same

purpose in unrelated species, but have evolved from different origins.

Examples Streamlined body shape in the unrelated

vertebrate groups: fish (sharks), mammal (dolphin) and bird (penguin)

The eye in humans and the octupus

Other Ideas

Punctuated Equilibrium Instead of a gradual change, species stay

the same for periods of time – most of a species existence is spent in stasis and little time is spent in active evolutionary change.

Other Ideas

Gradualism Assumes populations slowly diverge from

one another by accumulating adaptive characteristics in response to different selective pressures.

Gradualism vs. Punctuated

Co-evolution

Each party in a co-evolutionary relationship exerts selection pressures on the other and, over time, the species become dependent on each other.

Examples Predator-prey relationships, parasite-host

relationships, plants and their pollinators