WHAT INFLUENCES NATURAL SELECTION

EVOLUTION

REVIEW OF NATURAL SELECTION PRINCIPLESPrinciples of Natural SelectionA. Observations about populations:1. Natural populations have an inherent

reproductive capacity to increase.2. As a population expands, resources that

are used by its individuals (such as food and living space) eventually become limited.

3. When resources are limited, individuals of a population compete for them.

PRINCIPLES OF NATURAL SELECTIONB. Observation about genetics1. Individuals of a species share certain

traits.2. Individuals of a natural population vary in

the details of their shared traits.3. Traits have a heritable basis, in genes.

Alleles (slightly different forms a gene) arise by mutation.

PRINCIPLES OF NATURAL SELECTIONC. Inferences1. A certain form of a shared trait may make

its bearer more competitive at securing a limited resource.

2. Individuals better able to secure a limited resource tend to have more offspring than others of a population.

3. Thus, an allele associated with an adaptive trait tends to become more common in a population over time.

What Influences Natural Selection?There are factors that influence natural selection?1. Genetic variety > Within a species (the fact that not all organisms of a

given kind contain the same genes) is brought about by mutations (changes to genes) and genetic recombination (the mixing of gene combinations that occurs during sexual reproduction).

2. Gene expressionThe degree to which genes determine a trait.3. Excess reproduction> the ability of most species to reproduce excess

offspring.

What Influences Natural Selection?So genetic variety, gene expression and

excess reproduction all exert influence on the process of natural selection.

1. GENETIC VARIETYIf all individuals were identical genetically, it

would not matter which ones reproduce – the same genes would be passed to the next generation and natural selection would not occur.

Under genetic variety are two processes that make genes vary in an organism.

1. Mutation 2. Genetic Recombination

GENETIC VARIETY1. Mutation – it is a change in either the

amount or the arrangement of the genetic material in a cell.

It can happen to any type of cell.If it occurs in a gamete, the resulting

characteristic could be inherited.Mutations occur in the normal body cells as

well.If mutations occur on these normal body cells

(not on gametes), these are called somatic mutations and they cannot be passed on to the next generation.

GENETIC VARIETY – 1. MutationChromosome mutations involve large scale

changes and may involve parts, or even, whole chromosomes.

Point mutation (Gene mutation) on the other hand are on a much smaller scale.

It involves a change to relatively small number of bases in the genetic material.

GENETIC VARIETY – 1. MutationThe rate at which mutation takes place can

be increased by exposure to various mutagens.

1. Ultraviolet radiation2. X-rays3. Organic chemicals

GENETIC VARIETY – 1. MutationGenes of a particular trait originated as a

result of mutations sometime in the past and have been maintained with in the species as result of sexual reproduction.

Many mutations are unfavorable since they originate from a random change in a gene or genetic combination that has already proven valuable to the organism. (These mutation is called lethal mutation).

GENETIC VARIETY – 1. MutationBut it is quite possible that a new beneficial

gene could come about as a result of random mutation from over thousands of generations from populations of millions of individuals, each of whom has thousand of genes.

We can consider this type of mutation a beneficial mutation or neutral mutation.

GENETIC VARIETY – 1. MutationEvery gene originated from an existing gene.As an example we could say:1. That the blue eye gene is a mutation of the

brown eye gene.2. That the blond hair gene is a mutation of

the brown hair gene.

EvolutionPresentation