General Notes on Natural Selection and Mendelian Genetics

 

Four Underlying Premises to Darwinís Theory of Evolution by Natural Selection

 

 

 

1.       Variation - members within a species exhibit individual differences, and these differences are heritable. They vary in any one generation. Some variations are genetic.

 

2.       Overproduction - natural populations increase geometrically, producing more offspring than will survive. More individuals are produced than will live to grow up and reproduce.

 

3.       Competition - individuals compete for limited resources: struggle for existence.

 

4.    Survival to reproduce - only those individuals that are better suited to the environment survive and reproduce (survival of the fittest) passing on to a proportion of their offspring the advantageous characteristics.

 

Offspring that inherit the advantageous traits are selected for.

(Nature, in a sense, selects or favors them. These organisms live and reproduce.

a.    Their chances of survival are enhanced.

b.   Many live to reproductive age

c.    Through inheritance, they pass on the desirable attributes to their progeny.

 

It follows, then, that those hereditary traits that make their owners more likely to grow up and reproduce will become increasingly more common in a population from one generation to the next.

 

Natural selection example - population changes in Biston betularia, the peppered moth of England.

 

The change in a population, over time, from gray-mottled moths to charcoal black moths is, itself, evolution. It refers to a population shift in genotypes.

 

Natural selection over many generations has produced populations of the peppered moth that are well-adapted to survive in their environments, populations that have characteristics that have changed as the environment changes, or that are particularly advantageous to the prevailing environment.

 

A trait that promotes the survival and reproductive success of an organism in a particular environment is an adaptation.

 

Whereas natural selection refers to an organism's selective advantage in nature, artificial selection refers to the particular selection a breeder places upon an organism. In artificial selection, it is the breeder or farmer that determines which members of the population shall reproduce, and which will not.

 

Mendelian Genetics

 

Mendel was the first person to recognize that genetic traits are inherited as separate particles (discrete units). This was particularly important from the standpoint of evolution because many hereditary characteristics - human height, intelligence, skin color - are continuous over a broad range. Prior to 1900, biologists believed that the inheritable characteristic of parents blended in their children.

The theory of evolution by natural selection requires that inherited variations be maintained from

generation to generation, giving natural selection a "handle" or different traits to "choose" from. If these variations blended with each other in each generation, they would eventually merge in to some great average, and differences among individuals would disappear.

 

Genes - particles of inheritance that are part of the DNA molecule.

 

In sexual reproduction, a new individual receives half its genes from its mother's egg, and half from its father's sperm.

 

The plants Mendel chose (with care) to study the transmission of traits were particularly good for his inquiry:

1.        they were available in many varieties.

2.        each bred true to type: tall from tall, dwarf from dwarf.

3.        pea flowers are perfect flowers, and the modified petal, the keel, is particularly protective in ensuring successful self-pollination.

 

By crossing 2 varieties with contrasting traits - such as tall and dwarf varieties - Mendel could trace the inheritance of traits.

Example: crossing a red-flowered pea plant with a white-flowered pea plant.

First filial generation - genetically mixed offspring(hybrids).

Second filial generation - produce genotypic offspring in a 1:2:1 ratio.

 

Gene pairs - Mendel saw that these results could be explained if an inherited trait, such as flower color, is governed by 2 "factors", which we now call, genes.

 

Each organism receives 2 genes for each trait, 1 form each parent. During reproduction, each offspring receives, at random, 1 or these 2 genes.

 

A genetic trait may occur in 2 different forms: e.g. flower color may be red or white, plants may be tall or dwarfed. The genes that govern this trait must come in alternative forms: an alternate expression of a gene is referred to as an allele.

Any one plant may have 2 alleles for red flowers or 2 alleles for white flowers. An individual with 2 of the same alleles is said to be homozygous for that allele. In this instance, homozygous for flower color.

An individual with 2 different alleles at a locus, say 1 red and 1 white, is said to be heterozygous for flower color.

 

Dominant-Recessive

We saw (above) that the plants of the F1 generation were all genetically mixed (different genotypes from either of their parents), but were all red-flowered. What happened to the white-flowered alleles?

When the F1 generation reproduced, the F2 generation revealed red-flowered plants and white-flowered plants in a 1:2:1 genotype ratio and a 3:1 phenotype ration (3 red:1 white).

Mendel concluded that:

1.        One allele of a gene may express itself (appear as an observable trait in an organism) and mask the presence of the other allele.

2.        The allele that expresses itself is the dominant allele.

3.        The allele that is masked is the recessive allele.

 

Homozygous and heterozygous red-flowered plants cannot be told apart just by looking at them, because of the nature of dominant alleles.

The recessive allele can only be detected in the homozygous condition, when the dominant allele is not present.

 

Coding - geneticists often use a short-hand in which genes are designated by letters of the alphabet: capital (upper case) letters stand for dominance; lower case letters for recessive qualities.

E.g. "RR" refers to the dominant red-flowered plants; "rr" refers to the recessive white-flowered plants.

 

Genotype-Phenotype

Genotype refers to the genetic make-up of an individual.

Phenotype refers to the expression of the genes, how they manifest themselves.

 

An individual with a dominant phenotype may have a genotype that is either homozygous dominant or heterozygous.

An individual with a recessive phenotype (white flowers, blue eyes) must have a homozygous recessive genotype.

 

An individual's genotype is fixed at fertilization.

An individual's phenotype, however, results from the interaction of all its genes with one another, and with factors of the environment.

 

Monohybrid cross - a genetic cross in which only one trait of the parents (flower color, for example) is of interest.

Principle of segregation - when gametes are formed, the gene pairs become separated so that each sex cell (egg or sperm) only receives one of each kind of gene.

When the genotypes of parents are known, Punnett squares can be used to predict the genotypes of the offspring and in their expected ratios.

 

Incomplete dominance

The pairs of alleles Mendel studied all exhibited a dominant-recessive relationship. Since Mendel, geneticists have discovered many allelic pairs that exhibit incomplete dominance.

1.        Neither allele masks the presence of the other.

2.        The heterozygote, then, ahs a different phenotype and genotype, from either homozygote.

3.        E.g. the pink flowers of snapdragons.

 

Codominance

In a dominant relationship, 1 allele is expressed.

With incomplete dominance, no allele is expressed; there is anew phenotype.

In codominance, both alleles are expressed in the hetrozygote's phenotype.

E.g. sickle-cell anemia.

 

Mutations - changes in DNA. Once the DNA sequence is changed, DNA replication copies the altered sequence, and passes it along to future generations of that cell line. Mutations may occur in any cell. If they occur in cells that do lead to gametes, they are called somatic mutations. Somatic mutations occur in cells of leaves, stems, and roots of plants and are usually not passed on to the offspring.

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